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Early Ordovician continental break-up in Variscan Europe: NdSr isotope and trace element evidence from bimodal igneous associations of the Southern Massif Central, France
Abstract
The high-grade Marvejols Group and the low-grade Albigeois-Cévennes sedimentary sequence contain bimodal igneous rocks of Early Ordovician age which are representative of a widespread thermal event in the European Variscides. Comparison of their NdSr isotope and trace-element characteristics provides additional evidence for their origin in an ensialic extensional setting. As an alternative to the back-arc model proposed by previous authors, we propose that these associations record a continental break-up episode unrelated to contemporaneous subduction. In this model, the widespread Early Paleozoic bimodal magmatism marks the birth of a Mid-European oceanic arm. We propose that the break-up was controlled by both transtensional processes and mantle-plume activity.High grade associations, such as the Marvejols Group, and low-grade bimodal associations, such as the Albigeois-Cévennes, might be regarded as representing opposite rifted passive margins. The “northern”, Marvejols-type margin was involved in a Late Ordovician-Silurian subduction-like process, with HP-HT metamorphism. In contrast, the “southern” Albigeois/Cévennes margin remained largely unaffected, possibly as part of the overriding plate of the subduction zone.“Pseudo-calc-alkaline” signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break-up episodes. In this case, negative Nb anomalies are produced by the addition of crustal components enriched in Th and LREE relative to Nb. This inferred alternative origin of Nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data.
... In general, bimodal volcanic rocks are mainly composed of basalt and rhyolite, which are commonly considered to be generated in extensional environments, such as continental rifts, back-arc basins, and post-orogenic extensional settings [44][45][46][47][48][49][50]. In addition, slab break-off [51], lithospheric delamination [52][53][54], and asthenospheric mantle convection erosion [55] can induce partial melting of the lithospheric mantle and/or asthenosphere mantle and crust to form bimodal volcanic rocks. ...
... Generally, the bimodal volcanic suites are associated with extensional tectonic regimes and were generated in different tectonic settings, such as continental rift, a postcollisional setting, and back-arc basins [44][45][46][126][127][128]. The bimodal volcanic rocks are mainly composed of basalts and meta-alkaline felsic volcanic rocks in the post-collisional setting, which generally have A-type granite characteristics [89,129]. ...
... Generally, the bimodal volcanic suites are associated with extensional tectonic regimes and were generated in different tectonic settings, such as continental rift, a post-collisional setting, and back-arc basins [44][45][46][126][127][128]. The bimodal volcanic rocks are mainly composed of basalts and meta-alkaline felsic volcanic rocks in the post-collisional setting, which generally have A-type granite characteristics [89,129]. ...
The transition from the Proto- to the Paleo-Tethys is still a controversial issue. This study reports a new petrology, zircon U–Pb geochronology, and whole-rock geochemistry of volcanic rocks from the Maoniushan Formation in the Nankeke area, northern Qaidam (NQ) of the Tibetan Plateau, to provide new evidence for the transition from the Proto- to the Paleo-Tethys oceans. The volcanic suite consists mainly of rhyolitic crystal lithic tuff lavas and minor basalts. Zircon U–Pb data indicate that the bimodal volcanic rocks were formed during the Early Devonian (ca. 410–409 Ma). Geochemically, the basalts have low contents of SiO2 (48.92 wt.%–51.19 wt.%) and relatively high contents of MgO (8.94 wt.%–9.99 wt.%), TiO2 (1.05 wt.%–1.29 wt.%), K2O (2.35 wt.%–4.17 wt.%), and K2O/Na2O ratios (1.04–2.56), showing the characteristics of calc-alkaline basalts. Their rare earth element (REE) patterns and trace element spider diagrams are characterized by enrichments in LREEs (LREE/HREE = 18.31–21.34) and large ion lithophile elements (LILEs; Rb, Th, and K) and depletion in high-field-strength elements (HFSEs; Nb, Ta, P, and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.82–0.86), which are similar to Etendeka continental flood basalts (CFB). These features suggest that the basalts were most likely derived from low degree (1%–5%) partial melting of the asthenospheric mantle, contaminated by small volumes of continental crust. In contrast, the felsic volcanics have high SiO2 (68.41 wt.%–77.12 wt.%), variable Al2O3 (9.56 wt.%–12.62 wt.%), low MgO, and A/CNK ratios mostly between 1.08 and 1.15, defining their peraluminous and medium-K calc-alkaline signatures. Their trace element signatures show enrichments of LREEs and LILEs (e.g., Rb, Th, U, K, and Pb), depletion of HFSEs (e.g., Nb, Ti, Ta, and P), and negative Eu anomalies (Eu/Eu* = 0.22–0.66). These features suggest that the felsic volcanics were derived from partial melting of the middle crust, without interaction with mantle melts. Considering all the previous data and geochemical features, the Maoniushan Formation volcanic rocks in NQ formed in a post-collisional extensional setting associated with asthenospheric mantle upwelling and delamination in the Early Devonian. Together with the regional data, this study proposed that the Proto-Tethys Ocean had closed and evolved to the continental subduction/collision orogeny stage during the Middle to Late Ordovician, evolved to the post-collisional extensional stage in the Early Devonian, and finally formed the Zongwulong Ocean (branches of the Paleo-Tethys Ocean) in the Late Carboniferous, forming the tectonic framework of the Paleo-Tethys Archipelagic Ocean in the northern margin of the Tibetan Plateau.
... l'océan de Chamrousse, Guillot et al., 1992;Ménot et al., 1988;, et aboutità l'Ordovicien inférieurà l'ouverture de l'océan Rhéique et a la séparation du microcontinent Avalonia (Linnemann et al., 2007;Nance et al., 2010), linsà tholéiitiques et de rhyolites alcalinesà peralcalines (e.g. Crowley et al., 2000;Pin et Marini, 1993). Ces associations sont communément associéesà des corps ultramafiques, et pourraient correspondreà des anciens bassins océaniques ouà des zones de lithosphère hyperamincie de la transition océan-continent. ...
... 450-480 Ma, Bussy et al., 2011;Rubatto et al., 2001;Schaltegger & Gebauer, 1999). Similar Ordovician tholeiitic magmatism is commonly observed in all parts of the Variscan basement (Pin & Marini, 1993), and is generally attributed to a widespread extensional event which affected the northern part of Gondwana and resulted in the rifting of the Rheic and Saxo-Thuringian Oceans during the early-mid Ordovician (Linnemann et al., 2007). ...
... This Ordovician age is consistent with the regional record in the ECM, where most of igneous mafic protoliths yield ages between 450 and 480 Ma (Jacob et al., 2021a;Paquette et al., 1989;Rubatto et al., 2010). Cambrian-Ordovician magmatism is widespread in the Variscan basement of Europe (Pin et Marini, 1993), and is generally attributed to a regional extensional event that affected all the northern part of Gondwana and resulted in the rifting of the Rheic and Saxo-Thuringian Oceans during the Early-mid-Ordovician. ...
Les massifs cristallins externes (MCE) des Alpes occidentales forment les portions affleurantes du socle Paléozoïque impliqué dans la collision alpine. A l’instar d’une grande partie du socle continental ouest-européen, ces massifs ont été structurés par l’orogenèse varisque au cours du Dévonien et du Carbonifère (~380–300 Ma). Cependant, plusieurs points d’ombre demeurent concernant l’âge des déformations et le degré de métamorphisme associé aux événements varisques dans les MCE. En particulier, des reliques de haute pression (éclogites et granulites HP) affleurent dans les MCE, mais l’âge et les conditions P-T de ce métamorphisme restent mal contraints. Par ailleurs, ces massifs ont été affectés par des grands décrochements au cours du Carbonifère supérieur et du Permien, et ont ensuite été remaniés par la tectonique alpine, ce qui complique leur intégration au sein du système varisque. Ainsi, la signification des MCE par rapport aux grands blocs tectoniques et aux sutures varisques en Europe reste mal définie.Cette thèse vise à apporter de nouvelles contraintes géochronologiques et thermobarométriques sur les événements tectono-métamorphiques et magmatiques varisques dans les MCE, afin de proposer un modèle géodynamique cohérent. Ce travail a été mené sur les massifs de Belledonne et de l’Oisans, en parallèle d’une seconde thèse (Jonas Vanardois) réalisée sur le socle des massifs du Mont Blanc et des Aiguilles-Rouges, dans le cadre du chantier RGF Alpes piloté par le BRGM.L’étude pétrochronologique des reliques de haute pression (HP) a permis de mettre en évidence un stade métamorphique HP d’âge Viséen, avec des estimations d’âge et de conditions P-T cohérentes sur l’ensemble des MCE (~330-340 Ma, 1.4-1.8 GPa et 650-750°C). La préservation de reliques progrades marque un stade métamorphique précoce à 460-550 °C et 0.4-1.0 GPa, incompatible avec un contexte de subduction froide, et marquant plutôt un contexte d’épaississement crustal par sous-charriage de la plaque inférieure.L’exhumation des unités métamorphiques profondes issues de la croûte inférieure est contrôlée par un grand système décrochant tardi-varisque (325-300 Ma) parcourant l’ensemble des MCE. La comparaison reliques HP des massifs de Belledonne et de l’Oisans révèle des chemins d’exhumation contrastés. Dans le massif de Belledonne, les unités profondes ont été exhumées de manière relativement isotherme (pic de température ~750 °C) au cœur de la zone de décrochement, alors que les unités du massif de l’Oisans, situées en marge de ce système, ont été exhumées plus tardivement et ont subi un échauffement important (800–870 °C) lors de l’exhumation.Un second volet de la thèse concerne l’étude pétrographique et géochimique d’enclaves de péridotites découvertes dans le massif de l’Oisans, emballés dans des migmatites. Ces péridotites portent les traces d’un métasomatisme mantellique d’origine continentale, caractérisé par la cristallisation de phlogopite/pargasite, un enrichissement marqué en éléments incompatibles (LILE, LREE), et des signatures en Nd peu radiogéniques (-8.12<EpsNdi< 0.59). Un lien a pu être établi entre ce métasomatisme et la présence d’un magmatisme ultrapotassique généralisé sur l’ensemble des MCE, qui résulte probablement de la fusion partielle du manteau métasomatisé. Des nouvelles datations U-Pb sur zircon réalisées sur une intrusion ultrapotassique en Oisans, ainsi que les relations avec les formations encaissantes suggèrent un âge de ce magmatisme étalé entre ~340 et 320 Ma, qui coïncide avec les phases précoces de la collision varisque dans les MCE.A l’échelle régionale, nous proposons que les MCE enregistrent une évolution géodynamique similaire à celle du massif de Bohème et des Vosges, liée à la convergence vers le SE du système Saxothuringien, et nous amène à regrouper ces massifs en un domaine varisque oriental, séparé du domaine occidental (Bretagne Sud, MCF) par la faille de Bray, qui agirait comme une zone transformante.
... Bimodal volcanic rocks were first discovered in a rift environment (Wilson, 1989). Later, it was found that they can form in many tectonic environments, e.g., ocean island, island arc, back-arc basin, continental rift and post-orogenic extension (e.g., Li et al., 2002;Pin and Marini, 1993;Shinjo and Kato, 2000;Zhang et al., 2008). Although the lithology, eruption scale, and petrogeochemical and isotopic characteristics of bimodal volcanic rock in different tectonic environments are significantly different (Pin and Marini, 1993;Wilson, 1989;Zhang et al., 2008), there is a consensus on the relationship between bimodal volcanic rocks and the extensional environment. ...
... Later, it was found that they can form in many tectonic environments, e.g., ocean island, island arc, back-arc basin, continental rift and post-orogenic extension (e.g., Li et al., 2002;Pin and Marini, 1993;Shinjo and Kato, 2000;Zhang et al., 2008). Although the lithology, eruption scale, and petrogeochemical and isotopic characteristics of bimodal volcanic rock in different tectonic environments are significantly different (Pin and Marini, 1993;Wilson, 1989;Zhang et al., 2008), there is a consensus on the relationship between bimodal volcanic rocks and the extensional environment. The East African Rift is a major representative area of bimodal volcanic rocks, characterized by the occurrence of alkali-rich basalts, SiO 2 -unsaturated peralkaline rocks (e.g., carbonatites), trachytes, phonolites, and pantellerites (Wilson, 1989). ...
We take two typical Northeast Asia bimodal volcanoes as examples to explain the general features of Cenozoic bimodal ocean island basalt (OIB)-type volcanism in Northeast Asia. We present mineralogical, petrological, geochemical, isotopic, and full-waveform seismic tomographic evidence for the existence of two-layer magma chambers of Late Cenozoic volcanic activity beneath Ulleung Island and Mt. Changbai (Paektu). Ulleung Island volcanic rocks, which are composed of alkaline basalt, phonotephrite, trachyte, and phonolite, belong to the alkaline magma series and display enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs), slight depletion of heavy rare earth elements (HREEs), enriched ⁸⁷Sr/⁸⁶Sr (0.70475–0.70507) and ¹⁴³Nd/¹⁴⁴Nd (0.51250–0.51255) isotopic values, and enriched ²⁰⁷Pb/²⁰⁴Pb (15.544–15.626) and ²⁰⁸Pb/²⁰⁴Pb (38.750–38.954) values, similar to the geochemistry of OIB. Ulleung Island felsic volcanic rocks are characterized by significant negative Ba, Sr, P, Eu, and Ti anomalies (δEu = 0.14–0.35) and positive Pb anomalies, slightly higher ⁸⁷Sr/⁸⁶Sr isotopic ratios relative to those of mafic volcanic rocks, although the mafic and felsic samples have similar Sr, Nd, and Pb isotope compositions without significant differences. Ulleung Island and Mt. Changbai volcanic activities are likely related to the involvement of subduction-related compositions, but parts of the Mt. Changbai samples have been contaminated by crustal components to a certain extent. Mafic volcanic rocks of Ulleung Island are segregated from a deeper mantle source within a pressure range of 10.1–21.2 kbar compared with felsic volcanic rocks, which exhibit fractional crystallization of clinopyxene, spinel, plagioclase, and olivine. To explain this, a two-layer magma chamber beneath Ulleung Island with depths of 30–40 and 60–80 km is proposed, which is supported by mineral crystallization pressure and a three-dimensional full-waveform seismic tomography model. We also suggest that similar magma eruption processes and a two-layer magma chamber with depths of ~10 and 40–60 km also exist beneath Mt. Changbai. Taking the typical Cenozoic bimodal samples from Ulleung Island and Mt. Changbai as examples, we argue that two-layer magma chambers exist beneath Cenozoic bimodal OIB-type volcanic activities in Northeast Asia.
... Vestiges of the Rheic Ocean include rift-related complexes associated with its formation and arc-related high-to low-pressure (eclogite, amphibolite, and greenschist facies) complexes associated with its subduction. These suites, which were widely dispersed by the subsequent breakup of Pangea, are well preserved along the contact between the Central Iberian and Ossa Morena zones in Iberia (e.g., Quesada, 1990Quesada, , 1997Sanchez-Garcia et al., 2003), in the Massif Central (Pin and Marini, 1993;Pin and Paquette, 1997), in an isolated occurrence in southern Britain (Lizard Complex; Davies, 1984), and in the Acatlán Complex of Mexico (Keppie and Ramos, 1999;. Unfortunately Sm-Nd isotopic data from these complexes are scarce. ...
... The best data for the rifting stage are from the Massif Central, where ε Nd values (t = 480 Ma) show evidence of mixing between a LREE-depleted mantle component with values of ca. +6.0, and a crustal rhyolite with ε Nd of -6.3 ( Fig. 4; Pin and Marini, 1993). The mantle source for these basalts is similar to the depleted mantle value at t = 480 (ca. ...
... Bimodal volcanism typically characterizes an extensional environment, which can occur within various tectonic settings, including continental rifts [1,2], within-plate extensional settings [3][4][5], intraoceanic islands [6], ocean island arcs [7,8], incipient back-arc depressions [9], mature island/active continental margins [7][8][9], and back arcs [9][10][11][12]. In each of these modern tectonic environments, the volcanic activity may give rise to specific features, such as lithological assemblage, geochemical signature, and type of associated ore deposits [12,13]. ...
... In this paper, we present U-Pb zircon dating and geochemical and Sr-Nd-Hf isotopic composition for Karadaban bimodal volcanic rocks from the north Altyn region. Our aims are (1) to constrain petrogenesis and magma sources of bimodal volcanic rocks in the subduction setting and (2) to discuss the early Paleozoic evolution of the Altyn Tagh region. ...
Detailed geochronological, geochemical, and Sr-Nd-Hf isotopic data are presented for early Paleozoic volcanic rocks in the Karadaban area from the northern Altyn region, NW China, with the aim to constrain their petrogenesis and tectonic implications. The Karadaban volcanic rocks show a bimodal distribution in composition, with rhyolite and basalt. The LA-ICP-MS zircon U-Pb age indicates that the volcanic rocks were erupted at 512 Ma. The mafic rocks are calc-alkaline, enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; Ba and U) and depleted in high-field strength elements (HFSE; Nb and Ta). These features together with their depleted isotopic signature (initial 87Sr/86Sr=0.70413–0.70817 , εNdt=2.7 to 3.7) suggest that they were likely derived from a depleted mantle source but mixed with crustal components while upwelling. The felsic rocks show an A-type affinity, with high alkalis and Rb/Sr and Ga/Al ratios; enriched in LILE (e.g., Rb, K, Th, U, and REE) and depleted in Ba, Sr, Nb, P, and Ti; and with fractionated REE patterns with strong negative Eu anomalies. The combination of the decoupling of εNdt values (−2.5 to −6.3) and εHft values (+5.5 to +14.7) in the setting of subduction indicates that the felsic rocks were generated by partial melting of the juvenile crustal as a result of magma upwelling. The geochemical and Sr-Nd-Hf isotopic characteristics, coupled with regional geology, indicate that the formation of the Karadaban bimodal volcanic rocks involves an extensional regime associated with a subduction-related environment. The rifting of the back arc in response to the retreat of the subducting northern Altyn oceanic lithosphere may account for the Karadaban bimodal volcanic rocks.
... 21 suites were emplaced. Most back-arc basins usually display very large amounts of contemporaneous arc-derived magmatic rocks, with distinct island-arc-tholeiitic and/or calc-alkaline affinities (Ikeda and Yuasa, 1989;Stern et al., 1990;Pin and Marini, 1993). No conspicuous evidence of Mid-Permian arc-derived magmatic rocks has been found so far in Central China (e.g., Dai et al., 2013;Yang et al., 2016;Cheng et al., 2017;Wang et al., 2017;Yu et al., 2017; 1a. ...
... 24 present-day Gulf of California (Kelts, 1981;Einsele, 1986) and the Variscan Europe (Pin and Marini, 1993). China (modified from Zhang, 2001Zhang, , 2002Xu et al., 2006 andSong et al., 2013). ...
Little has been known regarding ~200-Myr-long geologic history of Central China during the Late Paleozoic between two major continental collisional events marked by ~480–440-Ma North Qinling–North Qaidam and ~230-Ma Dabie–Sulu ultrahigh-pressure (UHP) belts, respectively. The Gonghe basin is juxtaposed with major Chinese continental blocks across several outstanding Tethyan sutures in Central China and may hold answers to many critical questions about the tectonic amalgamation of China. In this paper, geochronological (laser ablation–inductively coupled plasma–mass spectrometry, LA–ICP–MS), mineral chemistry, major and trace element, and Sr–Nd–Hf isotopic data are presented for newly discovered Mid-Permian magmatic rocks in the Gonghe basin on the northeastern Qinghai–Tibetan Plateau. The data indicate that magmatism was active at ~270 Ma and shows a bimodal composition including gabbro, diabase and granite. Major oxides and trace elements are obviously correlated with SiO2 between the mafic and felsic rocks. They are both characterized by enrichments in light-rare-earth elements (LREEs) and large-ion-lithophile elements (LILEs), negative Eu anomalies and depletion in high-field-strength elements (HFSEs), together with similar Sr–Nd–Hf isotopic signature, suggesting that they likely shared the same magma chamber, and that the felsic rocks were produced by fractional crystallization of the mafic rocks. The high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7061 to 0.7094) and low εNd(t) values (−6.4 to −3.7), as well as Nb–Ta deletions indicate that they were likely derived from enriched mantle metasomatized by subduction-related fluids during the Early Paleozoic. In view of absence of coeval arc-related magmatism in contrast to broad seamount mafic magmatism in the Gonghe basin, the bimodal magmatism is ascribed to an intracontinental rifting event that could have been caused by transtension in Central China since the Mid-Permian, which could have been responsible for the initiation of the Gonghe and Songpan–Ganzi basins.
... Ballèvre et al., 2012] Ordovician felsic volcanic-sedimentary "porphyroïd" formations are recognized in S. Brittany, Vendée, S. Limousin, Albigeois, Rouergue, Cévennes [Pouclet et al., 2017, Cousinié et al., 2022. Alkaline basalts, dolerite, and gabbro are also locally exposed [Pin and Marini, 1993]. The Ordovician rifting is responsible for the development of "leptyno-amphibolite complexes" [e.g. ...
... The metasediments were deposited during the Ediacaran . The base of the UGU is composed of a bimodal igneous association referred to as the Leptyno-Amphibolitic Complex (LAC; Forestier, 1961, Santallier et al., 1988 typically considered to record a Cambro-Ordovician rifting event (e.g., Lotout et al., 2017;Pin & Lancelot, 1982;Pin & Marini, 1993). The mafic rocks, locally associated with garnet peridotites (e.g., Gardien et al., 1988Gardien et al., , 1990Lasnier, 1968Lasnier, , 1971, are metamorphosed in the eclogite facies at high temperature (e.g., Lardeaux et al., 2001;Mercier et al., 1989;Nicollet, 1977;Nicollet & Leyreloup, 1978). ...
Two eclogite samples from the Haut-Allier record a prograde evolution from ~20 kbar, 650 °C to 750 °C, 22–23 kbar followed by heating up to 850–875 °C and partial melting. Incipient decompression in high-pressure granulite facies conditions (19.5 kbar, 875 °C) was followed by exhumation to high-temperature amphibolite facies conditions (<9 kbar, 750–850 °C). Following a detailed geochemical, petrological and geochronological investigation using trace-element data and LA-ICP-MS U-Pb dating of zircon, apatite and rutile, the eclogites reveal an Ordovician (c. 490 Ma) rifting event followed by Devonian (c. 370–360 Ma) subduction and Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central (FMC). The previously proposed Silurian age for the subduction, that strongly influenced many tectonic models, is definitively rejected. In the light of other geological data from the FMC, including the lithological and geochemical zoning of calc-alkaline Devonian volcanism, we propose a southward polarity of the subduction and question the very existence of the so-called Massif Central Ocean. Furthermore, we infer that following subduction, the eclogites were relaminated to the upper plate and exhumed at the rear of the magmatic arc pointing to similarities with the geodynamics of the Bohemian Massif.
The petrochronological record of zircon is particularly complex. Metamorphic zircon with clear eclogitic REE patterns (no Eu anomaly, flat HREE) and inclusions (garnet, rutile, omphacite) shows concordant apparent ages that spread from c. 380 down to c. 310 Ma. This apparent age pattern strongly contrasts with the well-defined age of apatite and rutile of c. 350 Ma. Apparent zircon ages younger than 350 Ma unequivocally testify that zircon can recrystallize outside the conditions of the eclogite facies, which resets the U–Pb while preserving an apparent eclogitic signature. Local fractures filled by analcite, thomsonite, plagioclase and biotite testify to late interaction of the eclogites with alkaline fluids at relatively low temperatures. This interaction, possibly at c. 310 Ma or later, could lead to the recrystallisation of zircon while leaving apatite unaffected.
... Ledru et al. (1994b) (Pin et Peucat, 1986). C'est un épisode d'extension avec formation de dépôts de plate-forme continentale accompagné d'un volcanisme bimodal (Pin et Marini, 1993). L'extension conduit à la formation de croûte océanique comme en attestent les restes d'ophiolites observés dans les différentes unités lithotectoniques (Dubuisson et al., 1988 ;Couturier et al., 1992 ;Ledru et al., 1994a). ...
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... The Early Palaeozoic rocks represent important relics of the northern Gondwana margin, from where the central European crust is mainly derived (Linnemann et al. 2004;Nance et al. 2008;von Raumer et al. 2013). A widely accepted assumption is that the northern Gondwana margin was in a state of extension in the Early Palaeozoic (Franke et al. 2000;Franke et al. 2017) and was affected by high heat flow and rift-related magmatism (Pin and Marini 1993). However, the existence of Ordovician Barrow-type metamorphism in Alpine basement units (e.g., Biino 1995;Zurbriggen et al. 1997;Faryad et al. 2002;Franz and Romer 2007) implies that subduction processes must have played a role as well, at least locally. ...
Results of a combined petrological, geochemical and geochronological study suggest that metasedimentary rock units in the Krndija region of the Slavonian Mountains, Croatia, were affected by at least three major tectonometamorphic imprints: during the Middle Ordovician (Sardic event), the early Carboniferous (Variscan event), and the Cretaceous (Alpine event). All three metamorphic phases are established by electron microprobe-based in-situ U–Th–Pb dating of monazite grains. The Sardic metamorphic event is additionally confirmed by a precise Lu–Hf garnet-whole-rock isochron age of 466.0 ± 2.3 Ma. Taken together, the data unveil a relatively large and well-preserved piece of the cryptic Sardic orogen in central Krndija, that we name the Kutjevo Zone. A Sardic subduction-related metamorphic event (ca. 540-580 ℃, 8–11 kbar) at ca. 466 Ma is manifested in the mineral paragenesis Ca-rich garnet plus rutile. A low degree of retrograde reequilibration suggests a subsequent fast exhumation. Low-Ca cores in some garnets and staurolite relics record a pre-HP metamorphic event that involves isobaric heating from 570 to 610 ℃ at ~ 7 kbar. We attribute this (so far undated) event to mid-crustal contact metamorphism caused by early Sardic magmatism. Southern parts of Krndija (the Gradište Zone) experienced an (additional?) clockwise PT evolution in Variscan times at ca. 350 Ma. Garnet formed with ilmenite during a PT increase from 580 ℃/5 kbar to 600 ℃/6 kbar and underwent later strong retrograde resorption. Slow Variscan exhumation resulted in andalusite formation at < 550 ℃/ < 3.8 kbar. Penetrative Alpine metamorphism was observed in low-grade phyllites in the north. The lithology and metamorphic history of the Kutjevo Zone is similar to what has been reported from the Sardic Strona-Ceneri Zone in the western Alps. Both areas expose metapelitic (metagreywacke) rocks with a pre-middle Ordovician formation age. These metasedimentary rocks are inter-layered with numerous small amphibolitic units as well as metagranitoids and were likely deposited along the active Gondwana margin, perhaps in a fore-arc position, prior to their subduction during the middle Ordovician. According to recent palaeogeographic reconstructions, both the Kutjevo Zone and the Strona-Ceneri Zone have once resided in an eastern sector of the northern Gondwana margin (i.e., in E-Armorica). We conclude that in the Middle Ordovician, important subduction activities took place in this E-Armorican segment of north Gondwana, which is today exposed in the Alps. The W-Armorican segment of north Gondwana (now exposed in the French, German, and Czech Variscides) had probably already mutated from a (Cadomian) subduction setting to an extensional (transtensional–transpressional) setting by the late Cambrian.
... Geochemically, the late Carboniferous volcanic rocks display typical bimodal features in the Hala'alate Mountain, the so-called 'Daly Gap' (Chayes 1963(Chayes , 1977. The occurrence of bimodal associations is typically thought to reflect the extensional tectonic regime (Davies and Macdonald 1987;Lightfoot et al. 1987;Shinjo and Kato 2000), but recent researches show that BVR can also be found in other tectonic settings, such as ocean Island (Geist et al. 1995), continental rifting (Duncan et al. 1984;Pin and Marini 1993;Garland et al. 1995;) intra-oceanic Island arc (Brouxel et al. 1987), back-arc basin (Hochstaedter et al. 1990a(Hochstaedter et al. , 1990b, mature Island arc (Frey 1984;Pin and Zalduegui 1997), and postcollisional setting (Coulon et al. 1986). ...
The West Junggar terrain (WJT), as a crucial part of the Central Asian Orogenic Belt (CAOB), is distributed with numerous igneous rocks, which provide critical information for crustal growth. However, the closure of the Junggar Ocean (JO) and the beginning of the postcollisional tectonic stage of the WJT have been controversial. This study delimited the regional lithologic units based on remote sensing geological mapping and recognized a series of bimodal volcanic rocks (BVR) in Hala’alate Mountain of the WJT. LA-ICP‒MS zircon U‒Pb geochronology and geochemistry were used to discuss the petrogenesis of the BVR and determine the stage of regional tectonic evolution. Geochronological results yield crystallization ages of 302 ± 4 Ma, 298 ± 2 Ma, 304 ± 1 Ma, and 303 Ma± 2 Ma for the basaltic andesite, basalt, and two rhyolitic samples, respectively. Basalts and basaltic andesites are calc-alkaline, and display enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). Notably, basaltic andesites in this area were once misjudged as sanukitoids owing to their low contents of Mg#, Ni, Cr, and other characteristics that are inconsistent with the typical definition of sanukitoids. The rhyolites are A2-type granitoids with high SiO2 contents and are depleted in Nb, Ta, P, Ti, and Sr, showing enriched LREE patterns with negative Eu anomalies. These features indicate that the magma of the mafic end-member of the BVR is derived from the partial melting of the depleted lithospheric mantle, whereas the felsic end-member magma can be associated with the remelting of the lower crust due to the upwelling and underplating of mafic magma. In combination with previous studies of simultaneous basic dikes and felsic rocks, a postcollisional tectonic stage was proposed for the WTJ during the late Carboniferous, suggesting that the JO was closed.
... Before the Variscan orogeny, the terranes forming the future Variscan domains were located in the Northern margin of the Gondwana and were westerly bounded by the Iapetus and Tornquist oceans ( Fig. 2a) (von Stampfli et al., 2013). The Cambrian-Ordovician period is characterized by a broad extensional setting inducing rifting and subsidence in the northern Gondwana (von Raumer and Stampfli, 2008) and resulting in a thin margin associated to a bimodal magmatism, with numerous intrusions of alkaline and tholeiitic mafic rocks, that was associated with alkaline and peralkaline rhyolite (e.g., Pin and Marini, 1993;Crowley et al., 2000). Between ca. ...
A l’échelle des orogénèses, la déformation et la rhéologie de la lithosphère continentale sont fortement contrôlées par les processus de migmatisation et les phénomènes magmatiques au cours de l’évolution des grandes chaines continentales. La déformation de la croûte moyenne assistée par les mécanismes de fusion partielle et la présence de liquides silicatés joue un rôle majeur dans l’accommodation de grands déplacements horizontaux et verticaux au sein de l’orogenèse en construction. Ces fluages de la croûte partiellement fondue sont décrits dans les grands systèmes orogéniques actuels (Himalaya et Andes) et anciens (Chaîne Varisque). Dans le détail, la déformation syn-orogénique, les processus de ségrégation des liquides silicatés et leur migration dans la croûte orogénique sont des phénomènes inter-opérant de manière complexe, à plusieurs échelles de temps et d’espace.Par l’étude structurale, pétrologique et géochronologique des massifs Varisques de l’Agly (Pyrénées) et des Aiguilles-Rouges (Alpes), nous abordons cette problématique en ciblant les interactions entre partitionnement de la déformation et fusion partielle dans une croûte orogénique à haute-température. En parallèle, nous proposons de repositionner ces massifs dans leur cadre tectonique Varisque afin d’améliorer la compréhension des processus de fluage à l’origine de leur construction et leur structuration.L’analyse structurale du massif de l’Agly et la datation des déformations indiquent que la fusion partielle commence dès 325 Ma et se termine aux alentours de 300 Ma. Cette migmatisation est contemporaine du développement d’une fabrique planaire subhorizontale interprétée comme le fluage horizontal gravitaire de la croûte partiellement fondue. Ce fluage est localement perturbé par une zone de cisaillement dextre kilométrique drainant les liquides magmatiques et permettant le transfert et l’emplacement de plutons dans la croûte supérieure. Cette déformation dextre est interprétée comme une des branches nord d’une zone de cisaillement d’échelle crustale représentée par la zone axiale des Pyrénées.Dans les Aiguilles-Rouges, la reconstitution du trajet P-T-t-D des éclogites et de métapélites de moyen grade indiquent un enfouissement en conditions de MP/MT jusqu’à des conditions de haut grade atteignant des conditions de pression-température maximales de l’ordre de 1.75 GPa et 710 °C à 340-330 Ma. L’existence d’une ancienne zone de subduction océanique dans les Massifs Cristallins Externes (ECMs) potentiellement associée à ces conditions de HP-HT est questionnée et discutée. Nos données indiquent qu’entre 340 et 330 Ma, la croûte inférieure est exhumée à la faveur d’un fluage horizontal de la croûte partiellement fondue pendant que la croûte moyenne/supérieure subit un enfouissement, dans un régime global en transpression dextre. Ces deux unités sont ensuite juxtaposées et exhumées par les mouvements transcurrents dextres le long de l’Eastern Variscan Shear Zone (EVSZ). Nos données suggèrent que la nucléation des zones de cisaillement verticales composant l’EVSZ s’opère dès les premiers stades de migmatisation et préférentiellement dans les orthogneiss subissant une fusion hydratée. La EVSZ évolue ensuite sous la forme d’un large réseau de zones de cisaillement anastomosées favorisant le drainage des fluides, augmentant la production de liquides à l’origine de la formation de plutons syn-cinématiques, mis en place dans les zones de dilatance d’échelle crustale.La synthèse de nos résultats nous permet de proposer une évolution géodynamique des ECMs s’intégrant dans l’évolution de la chaîne Varisque Européenne et nous conduisent à présenter une nouvelle reconstitution paléogéographique de la branche Sud-Est Varisque à la fin du Carbonifère. Cette nouvelle vision du positionnement des ECMs, et la comparaison avec le massif de l’Agly nous permettent de discuter de l’évolution spatiale et temporelle du fluage de la croûte fondue au cours du Carbonifère.
... The development of Cambro-Ordovician rift basins across the north Gondwa n a shelf was associated with bimodal and perialkaline magmatism between ca. 520 -500 Ma within the western segment and calc-alkaline and peraluminous magmatism in the eastern segment of the Gondwana shelf during the Lower Ordovician (El Attari et al., 2019;Ikenne et al., 2017;Pin and Marini, 1993;Soulaimani et al., 2003;Stephan et al., 2019b). The rifting of Avalonia from the north Gondwana margin at the Cambro-Ordovician boundary is associated with calc-alkaline magmatism, which has been recorded in the detrital zircon record of the Moroccan Meseta, were a lower Ordovician greywacke records a unimodal a zircon age spectra of 488 Ma (Letsch et al., 2017). ...
Over 10,000 published detrital zircon ages have been reprocessed (applying a +10% normal and reverse concordance range) and analysed to understand the evolution of the detrital zircon record of North‐West Africa during the Phanerozoic. Using dissimilarity and clustering analysis, shifts in detrital zircon populations allow interpretation of the evolution of source regions and source to sink systems throughout the Phanerozoic within the West Gondwana superfan. Previous thermochronology and field studies conducted across North‐West Africa indicate significant and sustained shifts in source regions in Meso‐Cenozoic times which are not recorded in the detrital zircon geochronology record. This discrepancy is most notable for Mesozoic to modern source to sink studies focused on the evolution of the Atlasic rift and opening of the Atlantic and Tethyan Oceans to the west and north respectively. Our results indicate a high degree of similarity between samples from Cambrian times onwards due to successive phases of sediment recycling. This highlights the need to integrate detrital zircon analysis with other techniques to provide confident reconstruction of sediment routing systems across Morocco. This systematic review also reveals the ubiquitous occurrence of Mesoproterozoic zircons within Moroccan sediment. No basement of this age is known from Northwest Africa – often described as the ’Mesoproterozoic Gap’, which was thought to be a diagnostic feature of sediment derived from the West African Craton. However, zircons of this age form 7% of all analysed zircons and are present in sediments from at least 700 Ma. The presence of this population is interpreted as strongly diagnostic of provenance from either the Amazonian Craton or the Eastern Gondwana Orogen within Central Africa. Their presence in the Moroccan detrital record from the Neoproterozoic onwards raises questions about the position of the West African Craton in the Proterozoic, and for the spatial extent of Mesoproterozoic orogeny within north Africa.
... Therefore, the LAC is often compared to a similar complex in the French Massif Central (complexe leptyno-amphibolique C.L.A. representing the felsic/mafic gneisses with a tholeiitic signature, defined as remnants of ophiolites, and ductile deformed in deep-seated shear-zones e.g. Santallier et al. 1988;Pin and Marini 1993). Geochemistry indicates that the LAC protolith was N-MORB or undifferentiated IAT basaltic magma produced in the arc or back-arc geodynamic setting with possible slight crustal material contamination showing by I Sr(500) = 0.7052-0.7088, ...
The Western Carpathians crystalline basement recorded common indications of the Cadomian/Avalonian basement precursors. There the pre-Mesozoic basement of the Tatric Unit was studied with respect to its lithological, structural, metamorphic and age characteristics. The obtained data proves that this basement is made of two different rock complexes—the older Lower Étage (Cambrian to Silurian) showing high-grade metamorphic evolution, and the younger Upper Étage (Upper Silurian to Devonian) presenting low-grade metamorphism and younger stage of development. First representative U–Th–Pb detrital zircon data from metamorphosed siliciclastics from the Tatric Unit of the Western Carpathians indicate a general dominance of Neoproterozoic—mainly Ediacaran source rocks. The latter are interpreted to have been located at the Cadomian arc supplied mostly from the Saharan Metacraton. Combined with less frequent Archean and Paleoproterozoic zircon populations a common Gondwanan provenance derived from reworked West African Craton is proposed for part of the samples. However, some samples contain Mesoproterozoic detrital zircons what is typical for the Avalonian microcontinent. The studied metamorphic rocks are interpreted to originate from the Rheic Ocean and also partly from the Rheno-Hercynian basin. Indeed, significant differences in the metamorphic evolution of both metamorphic rock assemblages suggest their present juxtaposition and or their thrusting during the Late Devonian to Carboniferous (Visean) Variscan subduction/collision processes.
... A-type magmatic rocks in NW Iberia (Díez Fernández et al., 2012, and references therein), in the Maures massif (Seyler, 1986;Briand et al., 2002), in the Armorican Massif and in the southern Massif Central (Albigeois area, Pin and Marini, 1993); (iii) the lack of regional metamorphism related to crust thickening (e.g., Montero et al., 2007); and (iv) the coeval opening of the Rheic ocean (Díez Montes et al., 2010;Nance et al., 2010). The ultimate origin of this rifting event remains debated. ...
It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian–Tremadocian igneous bodies. In this study, we document the case of the “Cézarenque–Joyeuse” gneisses in the Cévennes parautochton domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to grauwackes. Collectively, it is inferred that the bulk of the Cézarenque–Joyeuse gneisses represent former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer-grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA–ICP–MS U–Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1±5.5 Ma and 483.0±5.5 Ma which unambiguously ties the Cézarenque–Joyeuse gneisses to the Furongian–Tremadocian volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750–820 °C of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron–wustite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian–Tremadocian igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an extensional environment and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences.
... AZFA49 AMFA148 AZ100 AZFA49 AMFA148 AZ100 AZFA49 AMFA148 AZ100 AZFA49 AMFA148 wt% ppm ppm ppm SiO 2 60,11 70,29 68,82 Ba 832 1034 687 Zr 129,5 91,8 170,6 La 10,5 7,3 28,6 Al 2 O 3 18,24 16,15 15,68 Be 3 2 3 Y 9,1 7,3 14,1 Ce 20,9 The association of continental crustal melts with mafic and felsic rocks derived from enriched to greatly depleted mantle is comparable with the development of magma-rift margins formed in response to lithospheric thinning above an abnormally hot asthenosphere (Pin and Marini, 1993, and references therein). The emplacement of Cambrian bimodal magmatism marks the transition from a convergent setting to intra-continental rifting in the northern margin of Gondwana. ...
This study presents geochronological and geochemical data of intermediate-felsic plutonic rocks from the Paleozoic Western High Atlas (Moroccan Meseta). Samples were taken from Medinet diorite emplaced in Cambrian Taroudant Group volcanic and sedimentary rocks and Tiniskt microgranite intrusive in Ediacaran Ouarzazate Group volcano-sedimentary rocks and Tighardine granodiorite. Based on U–Pb analysis of zircon, two well-constrained ages of 514 ± 3 Ma and 521 ± 5 Ma (Cambrian Series 2), were determined for Medinet diorite and Tiniskt microgranite, respectively. It is suggested that both Cambrian Series 2 plutons may represent the deep-seated magma chambers of Terreneuvian-Cambrian Series 2 eruptions recognized in the Paleozoic Western High Atlas. These granitic and dioritic rocks are peraluminous and show compositional affinity with subalkaline calc-alkaline series comparable with the oldest Terreneuvian V2 volcanic rocks. These findings provide evidence of a protracted period of production of calc-alkaline granitic and dioritic magmas during the transition from an Ediacaran convergent setting to Cambrian intra-continental rifting in the northern Gondwana margin
... Before the Variscan orogeny, the terranes forming the future Variscan domains were located in the Northern margin of the Gondwana and were westerly bounded by the Iapetus and Tornquist oceans ( Fig. 2a) (von Stampfli et al., 2013). The Cambrian-Ordovician period is characterized by a broad extensional setting inducing rifting and subsidence in the northern Gondwana (von Raumer and Stampfli, 2008) and resulting in a thin margin associated to a bimodal magmatism, with numerous intrusions of alkaline and tholeiitic mafic rocks, that was associated with alkaline and peralkaline rhyolite (e.g., Pin and Marini, 1993;Crowley et al., 2000). Between ca. ...
Partial melting, melt migration and strain partitioning during the Variscan orogeny.
Examples of the Aiguilles-Rouges (Alpes) and Agly (Pyrenees) massifs.
... This Ordovician age is consistent with the regional record in the ECM, where most of igneous mafic protoliths yield ages between 450 and 480 Ma (Jacob et al., 2021;Paquette et al., 1989;Rubatto et al., 2010). Cambrian-Ordovician magmatism is widespread in the Variscan basement of Europe (Pin and Marini, 1993), and is generally attributed to a regional extensional event that affected all the northern part of Gondwana and resulted in the rifting of the Rheic and Saxo-Thuringian Oceans during the Early-mid-Ordovician. ...
The Oisans–Pelvoux massif belongs to the Paleozoic basement of the External western Alps and records high temperature (HT) metamorphism associated with intense migmatization during the syn to post-collisional stages of the Variscan orogeny. Metamorphic assemblages related to the early collision stages have been obliterated making it difficult to constrain the earlier evolution of the unit. In this study, we report the finding of new bodies of high pressure (HP) mafic granulites, which preserve relics of the prograde evolution. Relics of Mn-rich garnet cores containing inclusions of epidote, titanite, chlorite and rare white micas constrain an early prograde stage at 460–550 °C and 0.4–1.0 GPa. The HP assemblage consists in garnet + clinopyroxene+ quartz + rutile ± plagioclase ± amphibole ± biotite and yield peak-P conditions at 650–730 °C and 1.5–1.7 GPa. Decompression was associated with heating to HT granulite-facies conditions of 800–870 °C and 0.6–0.9 GPa, which led to the development of granoblastic polycrystalline mosaics over the resorbing HP assemblage. The U-Pb dating of magmatic zircon cores constrains the emplacement age of the mafic protoliths at 479 ± 5 Ma. Timing of the prograde to HP evolution is estimated around 345–330 Ma based on rutile U-Pb dating, which is contemporaneous with HP recorded in the other crystalline massifs of the western Alps. Zircon metamorphic rims yields U-Pb dates scattering between 337 and 294 Ma, which are interpreted to record crystallization during decompression and heating to granulite facies. Metamorphic conditions during the early prograde stage precludes scenarios involving subduction of a cold passive margin and are better reconciled with thickening of an orogenic wedge during collision. In contrast with other Variscan HP relics of the external Alps, the HP rocks of the Oisans–Pelvoux massif record heating to HT granulitic conditions during decompression, which may result from longer residence time in the lower crust before exhumation.
... Bimodal volcanic rocks are generally related to extensional regimes and can be formed in various geodynamic settings (Wang et al. 2000), such as continental rifts (Pin and Marini 1993;Wilson 1989), oceanic islands (Geist et al. 1995), incipient back-arc spreading (Hochstaedter et al. 1990a, b), post-orogenic extensional settings (Coulon et al. 1986), intra-oceanic arcs (Brouxel et al. 1987) and mature islands/active continental margins (Frey et al. 1984;Pin and Paquette 1997). Considering that the Wusun Mountain magmatic belt was built inside the Yili-Central Tianshan Block with widespread continental crustal basement (Hu et al. 2000), the bimodal volcanic rocks are unlikely to have formed in oceanic islands or intra-oceanic arc settings. ...
In Central Asia, the Carboniferous is a crucial period in the formation of the Tianshan Belt and associated bending of the Kazakhstan tectonic collage. In order to reveal Carboniferous magmatic events of the region and their tectonic implications, we conducted field investigations, zircon U–Pb dating, whole-rock geochemical and Sr–Nd isotopic studies on the Early Carboniferous Dahalajunshan Formation and Late Carboniferous Yishijilike Formation volcanic rocks of the Wusun Mountain Range (southern Yili-Central Tianshan Block). Volcanic rocks of the Dahalajunshan Formation consist of calc-alkaline basalt, andesite and dacite, yielding new zircon U–Pb ages of ~ 350 Ma. They have positive whole-rock εNd(t) values (+ 0.5 to + 1.6). In contrast, the Yishijilike Formation volcanic rocks dominantly comprise alkaline and calc-alkaline bimodal suites that erupted at ~ 337 Ma to 313 Ma and have higher whole-rock εNd(t) values (+ 2.3 to + 4.3). These two episodes of Carboniferous magmatism were correlated with partial melting of depleted mantle that metasomatized by slab-derived fluids. The late Carboniferous Wusun Mountain magmatic belt shows characteristics of a back-arc system that evolved due to trench retreat relative to the southern margin of the Yili-Central Tianshan Block. This mechanism induced an extensional regime with gradually depleting magma sources. The asymmetric retreat of the paleo-subduction zones of the South Tianshan Ocean and Junggar Ocean relative to the Yili-Central Tianshan Block was hence a vital driving force for the bending of the Kazakhstan Orocline.
... Although bimodal volcanic suites were traditionally believed to occur in a continental rifting setting (Wilson, 1989), many studies suggest that they could be generated in various tectonic environments, such as oceanic island, active continental margin, back-arc basin and post-collision extensional settings (Pin and Marini, 1993;Qian and Wang, 1999;Shinjo and Kato, 2000;Zhang and others, 2008). This study reveals that the bimodal volcanic sequence from the Daqingshan Complex can be divided into Group 1 tholeiitic suites and Group 2 high-K calc-alkaline suites. ...
... Bimodal volcanism typifies extensional environments in a range of geodynamic environments. It occurs in continental rifts (Garland et al., 1995;Pin & Marini, 1993;Tian et al., 2019), oceanic islands (Frey et al., 1984;Pin & Paquette, 1997), island arcs (Frey et al., 1984;Hochstaedter et al., 1990;Manikyamba et al., 2016), backarcs (Bagas et al., 2008;Elliott et al., 1997;Ewart et al., 1998;Hochstaedter et al., 1990;Pearce et al., , 1999Terentiev et al., 2017), in postcollisional extensional settings (Zhang et al., 2008) and within-plate environments (Bonin, 2004;Frost et al., 1999). The differing geodynamic environments have characteristic, but overlapping, lithogeochemical and isotopic signatures, distinct lithological assemblages, and associated ore deposit types. ...
Bimodal volcanism occurs in a range of extensional environments that are characterized by distinctive but overlapping, lithogeochemical and isotopic signatures and lithological assemblages. Where basaltic magmatism is associated with ferroan (A‐type) rhyolites, it is typically the lithogeochemistry and isotopic systematics of the basaltic rocks that are the most beneficial in constraining the tectonic evolution. This study presents geochemical and isotopic data from mafic volcanic rocks of the bimodal Aillik Group, that formed during the assembly of the Paleoproterozoic supercontinent Nuna. Lithogeochemical signatures define two suites. Suite 1 samples have N‐MORB (Normal‐Mid‐Ocean Ridge Basalt) chemical affinities such as flat HREE, and smooth but flat LREE and have εNd(t = 1860 Ma) values between +2.8 and +4.8. Suite 2 samples typically have compositions between N‐MORB and IAT (Island Arc Tholeiites) basalts, with variable REE patterns ranging from IAT to OIB (Ocean Island Basalt). The basalts display moderately developed subduction zone signatures, such as negative Nb and Ti, and have εNd(t = 1860 Ma) ranging from −3.4 to +2.2. Geochemical variability within the Aillik Group mafic rocks is explained by processes active in arc settings and involves mixing between depleted mantle components with variable contributions from crustal and subducted‐slab sources. Coupled with field evidence, the geochemical and isotopic data support emplacement of the basalt in an extensional, backarc basin setting. This backarc basin formed due to localized extension during Nuna assembly. The Makkovik Province preserves a complex orogen characterized by multiple diachronous, compressional, and extensional events along a long‐lived active margin of the Archean North Atlantic Craton.
... Ordovician tholeiitic magmatism is commonly observed in all parts of the Variscan basement (Pin & Marini, 1993), and is generally attributed to a widespread extensional event which affected the northern part of Gondwana and resulted in the rifting of the Rheic and Saxo-Thuringian Oceans during the Early-Mid-Ordovician (Linnemann et al., 2007). ...
The age and P‐T conditions of Variscan high pressure (HP) metamorphism in the Paleozoic basement of the western Alps remain poorly constrained, but is nevertheless crucial to build a consistent tectonic scenario for the southeastern domain of the Variscan Belt. We report here the results of a structural, petrological, thermobarometric and geochronological investigation of an eclogite‐bearing unit exposed in the northeastern part of the Belledonne Massif (France). This unit is mostly composed of metasediments, that are locally migmatized and contain decameter‐to‐hectometer‐scale lenses of orthogneiss and amphibolites. SIMS U‐Pb dating of magmatic zircon cores in two retrogressed ecogites yields ages at 456±4 Ma and 448±6 Ma, which are intepreted to date the emplacement of the magmatic protoliths. The peak pressure stage in the retrogressed eclogites is estimated to be >1.4 GPa at 690–740°C, and was followed by decompression from 1.4 to ca. 1.0 GPa at 700–800 °C. By contrast, the investigated migmatitic metasediment does not present any trace of HP metamorphism, but instead preserves prograde evolution from sub‐solidus conditions (ca. 0.8–1.1 GPa and 600–700 °C) to supra‐solidus conditions (1.1‐1.4 GPa and 700–780 °C). A later stage of retrogression below ca. 0.5–0.8 GPa and 570–610 °C is recorded in both lithologies, and is taken to indicate cooling and exhumation to upper crustal levels. Metamorphism was roughly coeval in the retrogressed eclogites and in the migmatitic metasediment. Metamorphic zircon rims yield U‐Pb dates scattering between 340–310 Ma in both lithologies. In the migmatitic metasediment, a distinct younger age at 306±3 Ma is interpreted to represent late stages of melt crystallization. In the retrogressed eclogites, zircon zoning and chemical composition (Th/U and REE) indicate initial crystallization of the rims during the HP stage followed by protracted growth during decompression to granulitic / amphibolitic conditions. Rutile U‐Pb dating in one eclogite sample yields an age of 340±11 Ma similar to the oldest zircon ages and is interpreted to approximate the age of the peak pressure metamorphism. Retrogression in the amphibolite facies is correlated with the development of a penetrative, N30° subvertical mylonitic S2 foliation. Regionally, this deformation occurs in a dextral transpressive corridor interpreted to represent a crustal‐scale shear zone active during the mid‐late Carboniferous. We therefore suggest that this structure has driven the exhumation of eclogites from the lower crust and their mixing with mid‐crustal felsic lithologies devoid of HP assemblages.
... It is widely accepted that the evolution of the Variscan belt started after the Neoproterozoic Cadomian orogeny with subsequent Cambro-Ordovician rifting, differential opening of the Rheic Ocean along the Gondwana margin and drifting of peri-Gondwanan terranes (e.g. Pin and Marini, 1993;Murphy et al., 2006;von Raumer and Stampfli, 2008;Linnemann et al., 2008;Nance et al., 2010;von Raumer et al., 2013). A series of peri-Gondwanan terranes along the north of the Gondwana supercontinent provide important records on the tectonic nature of the processes that operated during the Early Palaeozoic. ...
Zircon petrochronology from amphibolites and retrogressed eclogites from the basement of the Western Tatra Mountains (Central Western Carpathians) reveals a complex rock evolution. An island-arc related basaltic amphibolite from Žiarska Valley shows three distinct zircon forming events: igneous zircon growth at ca. 498 Ma (Middle/Late Cambrian) and two phases of amphibolite-facies metamorphism at ca. 470 Ma (Early Ordovician) and at ca. 344 Ma (Early Carboniferous). A retrogressed eclogite from Baranèc Mountain records two zircon forming events: metamorphic zircon growth under eclogite-facies conditions at ca. 367 Ma (Late Devonian) and amphibolite-facies metamorphism at ca. 349 Ma (Early Carboniferous). These data contribute towards understanding and correlating major tectonothermal events that shaped the eastern margin of Gondwana in the Early Palaeozoic and its subsequent Variscan evolution. The metabasites record vestiges of two completely independent oceanic domains preserved within the Central Western Carpathians: (1) An Ediacaran to Cambrian oceanic arc related to the proto-Rheic - Qaidam oceans and metamorphosed to amphibolite-facies in the Early Ordovician subduction of the proto-Rheic - Qaidam arc during the Cenerian orogeny (ca. 470 Ma) and (2) Late Devonian oceanic crust related to a back-arc basin (Pernek-type), formed by the opening of the Paleotethys and metamorphosed to eclogite-facies during Devonian subduction (ca. 367 Ma). The common Variscan and later evolution of these oceanic remnants commenced with amphibolite-facies metamorphic overprinting in the Early Carboniferous (amphibolite: ca. 344 Ma; retrogressed eclogite: ca. 349 Ma) related to an Early Variscan consolidation and the formation of Pangea. None of the investigated rocks of the Central Western Carpathians show any evidence of being chronologically or palaeogeographically related to the Rheic Ocean, therefore any prolongation of the Rheic suture from the Sudetes into the Alpine-Carpathian realm is highly problematic. Instead, the Southern and Central Alpine Cenerian orogeny can be traced into the Central Western Carpathians.
... Black arrows are the proposed paleocurrent directions according to Lin et al. (2016). Modified from Lin et al. (2016) Studies concluded therefore that large granite and orthogneiss bodies represent a Cadomian basement that is overlain by lower Paleozoic sedimentary cover (Autran and Guitard 1966;Autran and Guitard 1969;Guitard 1970;Vitrac-Michard and Allègre 1975 (Gebauer and Grünenfelder, 1976;Roger et al., 2004;Padel, 2016) reflects the separation of these formations from the north Gondwana margin (Pin and Marini 1993;Stampfli 1996;Neubauer 2002;von Raumer, et al., 2003). ...
La dynamique de formation d’une chaîne de montagne peut être reconstruite à partir de l’étude des bassins réceptacles des produits d’érosion. Les travaux présentés ont été réalisés dans la partie orientale du Bassin d’Aquitaine (Corbières), à partir de l’étude de la série de Palassou. Ces sédiments, en majorité conglomératiques, traduisent une phase d’érosion majeure des reliefs, accompagnant la continentalisation des bassins au cours de l’Yprésien et la période de raccourcissement principal de l’orogenèse pyrénéenne. Trois unités tectono-stratigraphiques ont été défini dans cette série : l’unité 1 - Yprésien supérieur-Lutétien – caractérisée par la présence de galets Méso-Cénozoïques, l’unité 2 – Bartonien – caractérisée par la présence des galets de socle et l’unité 3 – Priabonien- caractérisée par la présence à nouveau de galets Méso-Cénozoïques. L’objectif de cette thèse est de comprendre le mode de remplissage du bassin d’avant-pays nord pyrénéen et son évolution au cours de l’Éocène. Ceci permettra d’appréhender la formation des reliefs à l’intérieur de la chaîne et de caractériser les différentes sources d’apport. Trois approches ont été utilisées pour reconstruire le routage des sédiments dont la première est l’étude sédimentaire et la caractérisation des environnements de dépôts des deux premières unités. Les résultats obtenus montrent l’identification de quatre séquences de remplissage sédimentaire dans l’unité 1, dont l’essentiel s’est déposé dans le synclinal de Talairan jusqu’à son débordement au cours du dépôt de la troisième séquence. Ceci amène à interpréter le synclinal de Talairan comme un bassin en piggy back. La deuxième approche est la thermochronologie basse température appliquée sur des clastes de granites issus de l’unité 2. Les résultats de traces de fission sur apatite montrent des âges plus jeunes d’est en ouest alors que les résultats de (U-Th-Sm)/He sur apatite montrent une dispersion des âges. La modélisation thermique de ces résultats indique un réchauffement post-Bartonien, traduisant une série sédimentaire plus épaisse au moment du dépôt puis partiellement érodée. La phase d’érosion est estimée comme pré-Langhienne suite aux résultats de modélisation thermique et des contraintes stratigraphiques. La troisième approche utilisée au cours de cette thèse est la datation U/Pb sur zircon. Des échantillons de matrice de conglomérat ainsi que des galets de granite issus des trois unités ont fait l’objet d’étude par cette approche. Les résultats obtenus montrent un signal Varisque majeur. Ils ont été couplés avec des analyses Raman et des directions de paléo-courant mesurés afin de caractériser les sources des sédiments pour chaque unité. Les dépôts de l’unité 1 sont issues de la Zone Nord Pyrénéenne. Les dépôts de l’unité 2 ont comme source la Zone Axiale des Pyrénées alors que les dépôts de l’unité 3 présentent un spectre d’âges assez large dont la source est la partie orientale et centrale des Pyrénées. Ces résultats ont permis de reconstruire le cheminement des sédiments dans les Corbières au cours de l’Éocène.
... A widely accepted view is that the evolution of the Variscan belt started after the Neoproterozoic Cadomian orogeny with Cambro-Ordovician rifting at the northern margin of Gondwana and the opening of the Rheic Ocean (e.g. Pin and Marini, 1993;Murphy et al., 2006;von Raumer and Stampfli, 2008;Linnemann et al., 2008;Nance et al., 2010;Nance et al., 2012). One of the major problems in studies on the evolution of the Mid-European Variscan belt is to better understand the role of these pre-Variscan crustal events that preceded the structurally dominant, mainly Carboniferous Variscan tectono-metamorphism and magmatism. ...
Zircon U-Pb SIMS dating combined with in-context (in thin section) monazite and xenotime U + Th-total Pb dating was used to clarify the Palaeozoic evolution of the ‘cold’ Chopok granite (Nízke Tatry Mountains, Slovakia). Four distinct zircon, monazite and xenotime age domains testify to a prolonged evolution from igneous formation to multi-stage metasomatism and hydrothermal overprinting. The geological interpretation of age patterns from ‘cold’ granites, expected to have low zircon saturation temperatures (<800 °C) and relatively high amounts of zircon inheritance, requires special care, especially for what concerns proper attribution of zircon inheritance and igneous growth ages. These issues can be resolved using zircon saturation temperatures (TZrn) as proxy for the amount of zircon inheritance in combination with the temperature differences between TZrn and the granite solidus. In this respect, the Chopok granite is an atypical ‘cold’ granite. Due to TZrn being substantially lower (ca. 80 °C ± 50 °C) than the granite solidus temperature, practically no zircon inheritance was found. The zircon age data indicates that the Chopok granite is a product of an Early Ordovician (475.8 ± 3.3 Ma) magmatic event, corresponding with the widespread Early Palaeozoic magmatism recorded throughout the European Variscan belt. This is further corroborated by the phosphate mineral ages. The post-magmatic activity recorded in the U-Pb systematics of the zircon and phosphates overgrowths can be related to the different phases of the evolution of the Variscan orogen. Early Carboniferous (ca. 352 Ma) metasomatism documents the main Variscan orogenic event, whereas the Permo-Triassic age (ca. 255 Ma) reflects thermo-tectonic activity associated with large-scale crustal extension, contemporaneous with the initial continental leading to the break-up of Pangea.
... Nance et al. 1991Nance et al. , 2008Nance et al. , 2010Stern 1994;Tait et al. 1997Tait et al. , 2000Murphy et al. 2004;Garfunkel 2015;Henderson et al. 2016;Honarmand et al. 2016), and then underwent an episode of extension and rifting related to the opening of the Rheic Ocean at the end of the Cambrian and Early Ordovician (e.g. Pin and Marini 1993;Prigmore et al. 1997;Crowley et al. 2000;Etxebarria et al. 2006;Murphy et al. 2006;Linnemann et al. 2008;von Raumer and Stampfli 2008;Peřestý et al. 2017). In turn, these continental fragments, often referred to as ʽterranesʼ, have been incorporated into, and variously reworked within, the Variscan and Alpine orogens in Europe and the former now extend from southwest Iberia (Spain) through the Armorican Massif in France to the Bohemian Massif in the east (Figure 1(a); e.g. ...
In this paper, laser ablation ICP-MS U–Pb detrital zircon ages are used to discuss provenance and early Palaeozoic palaeogeography of continental fragments that originated in the Cadomian–Avalonian active margin of Gondwana at the end of Precambrian, were subsequently extended during late Cambrian to Early Ordovician opening of the Rheic Ocean, and finally were incorporated into and reworked within the European Variscan belt. The U–Pb detrital zircon age spectra in the analysed samples, taken across a late Neproterozoic (Ediacaran) to Early/Middle Devonian metasedimentary
succession of the southeastern Teplá–Barrandian unit, Bohemian Massif, are almost
identical and exhibit a bimodal age distribution with significant peaks at about 2.1–1.9 Ga and 650–550 Ma. We interpret the source area as an active margin comprising a cratonic (Eburnean) hinterland rimmed by Cadomian volcanic arcs and we suggest that this source was available at all times during deposition. The new detrital zircon ages also corroborate the West African provenance of the Teplá–Barrandian and correlative Saxothuringian and Moldanubian units, questioned in some palaeogeographic reconstructions. Finally, at variance with the still popular concept of the Cadomian basement units as far-travelled terranes, we propose that early Palaeozoic basins, developed upon the Cadomian active margin, were always part of a wide Gondwana shelf and drifted northwards together before involvement in the Variscan collisional belt.
... Namely, Ediacaran sediments intruded by Cambrian granitic plutons represent a Cadomian basement for the thick detrital sedimentary sequences of lower Paleozoic age deposited during the post-Pan-African dislocation of the Gondwana continent (Duthou et al., 1981(Duthou et al., , 1984R'Kha Chaham et al., 1990;Alexandrov et al., 2001;Chelle-Michou et al., 2017;Couzinié et al., 2017Couzinié et al., , 2019. Other orthogneisses with intrusive contacts (dyke networks, contact metamorphism) of Ordovician age, represent former laccoliths emplaced during extension along the northern margin of the Gondwana continent (Lasnier, 1968;Bernard-Griffiths, 1975;Bernard-Griffiths et al., 1977;Duthou et al., 1981Duthou et al., , 1984R'Kha Chaham et al., 1990;Pin and Marini, 1993;Barbey et al., 2001;Deloule et al., 2002;Roger et al., 2004;Cocherie et al., 2005;Castiñeiras et al., 2008;Melleton et al., 2010;Lotout et al., 2017). The calc-alkaline to tholeiitic chemical signature of the LAC also suggests an emplacement of this bimodal magmatic suite in a continental to oceanic rift environment during the Cambrian-Ordovician (Pin and Lancelot, 1982;Bodinier et al., 1986;Briand et al., 1995;Chelle-Michou et al., 2017). ...
We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. These features reflect the horizontal growth of the Variscan belt and the formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleothethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that correspond to post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust.
... In the Variscan orogenic belt, contrasted geodynamic settings have been proposed to comprehend the Late Ediacaran-Early Paleozoic evolution of North Gondwana (Pereira et al., 2006;Chelle-Michou et al., 2017;Álvaro et al., 2018 and reference therein; Arboit et al., 2019;Couzinié et al., 2019). A controversy persists regarding the geodynamic meaning of the geochemistry signature of the Cambrian-Ordovician magmatism that is characteristic of the Iberian (Sánchez García et al., 2008Abati et al., 2010;Díez Fernández et al., 2015), Armorican (Pin and Marini, 1993) and Bohemian (Linnemann et al., 2007(Linnemann et al., , 2014 massifs, in the Sardinia (Carmignani et al., 1994;Cocco and Funedda, 2019) and pre-Alpine basements (von Raumer et al., 2003), and to the east in Saudi Arabia and Iran (von Raumer et al., 2015). Actually, it is admitted that the Ediacaran magmatic activity was related to the development of a peri-North Gondwanan active margin (i.e. ...
... This can indicate a contribution of a 'subduction component' in the source or crustal contamination (e.g. Pin & Marini, 1993). However, assimilation of crustal material did not occur on a large scale, as manifested by low large-ion lithopile elements and SiO 2 contents (Krzemiński, 2004). ...
The age of granophyric diorite from the Sosnowiec IG-1 borehole (Brunovistulia Terrane) was studied by means of U–Pb single-grain zircon analysis performed on a SHRIMP (sensitive high-resolution ion microprobe) IIe device. The isotope ages and provenance of zircons from the Emsian tuffs cropping out in the southern part of the Holy Cross Mountains (Małopolska Terrane) were also investigated using the same method. The age of the diorite intrusion (420 ± 2 Ma) is comparable with the combined Ar–Ar/magnetostratigraphic age of the Bardo diabase intrusion from the northern part of the Małopolska Terrane. These intrusions were emplaced during the same event of regional tectonic extension associated with the Rheic Ocean closure and the onset of processes creating the Rheno-Hecynian Basin near the Silurian/Devonian boundary. A negative Nb anomaly characteristic of both intrusions could be linked with the subduction of the Rheic oceanic crust under the SE margin of the Old Red Continent. Emsian magmatic activity in the distant Rheno-Hercynian Zone provided several tuff layers in the northern part of the Małopolska Terrane. As can be inferred from zircon ages, these tuffs were derived from mafic eruptions that cut sedimentary rocks containing detrital zircons transported from Baltica. This interpretation fits the existing models of development of the Rheno-Hercynian Basin in the Emsian.
... Linnemann et al., 2007;Nance et al., 2010;Ilnicki et al., 2013;Hajná et al., 2018), the Cambro-Ordovician extension along the northern shelf of Gondwana is associated with locally important bimodal magmatism and the formation of graben systems (e.g. Pin and Marini, 1993;Crowley et al., 2000;Timmerman, 2008). Intracontinental extension resulted in the separation of segments of thickened Cadomian crust by segments of thinner continental crust along northern Peri-Gondwana (Kroner and Romer, 2013). ...
The Late Paleozoic Variscan Orogen of Europe and North Africa comprises reworked Neoproterozoic to Early Paleozoic crust of the northern Gondwanan shelf that collided with Laurussia. The orogen is characterized by an arcuate trend of the Rheic suture along two orthogonal orogenic arcs and an apparently arbitrary juxtaposition of contrasting paleogeographic proxies to the south of the suture. The comparison of the sedimentary provenance, paleontological, lithostratigraphic, tectonic, and magmatic record demonstrates a contiguous but bipartite, i.e. a western and an eastern, shelf to the south of the Rheic Ocean. Here we reconstruct the development and architecture of the Paleozoic shelf of northern Gondwana preceding the formation of Pangea. In the early Paleozoic both shelf segments were affected by a heterogeneous extension whereby age and composition of extension-related magmatic rocks varies systematically from Cambrian alkaline and tholeiitic rocks in the western shelf to Ordovician calc-alkaline and peraluminous rocks in the eastern shelf. The regional variation in age and composition of the magmatic rocks reflects an eastward decreasing rate of extension along northern Gondwana. The higher extension in the western shelf culminated in the formation of the Armorican Spur. The subsequent intra-Ordovician compressional event, i.e. the “Sardic phase” and the “Cenerian orogeny”, exclusively affected the eastern shelf. Early Devonian collision of the Armorican Spur with Laurussia initiated the subduction accretion stage of the Variscan orogeny resulting in the formation of the Rheno-Hercynian–Moravo-Silesian Arc. At that time, the eastern shelf remained in a passive margin setting. Triggered by Late Devonian rifting along the eastern margin of Arabia, the eastern shelf decoupled from the Gondwanan plate and was displaced eastward, parallel to the northern margin of remaining mainland Gondwana. Early Carboniferous collision of the eastern shelf with the western shelf resulted in orogen wide transpressional tectonics and the formation of the Ibero-Armorican Arc. The tectonic interplay between the two Gondwanan shelf segments is the underlying cause of the final patchwork pattern of paleogeographic markers and the arcuate shape of the Variscan orogenic belt.
... Linnemann et al. (2007) interpreted this high magmatic activity as a reaction to a slab break off caused by the subduction of a heavier oceanic plate. Due to a general change of the geological situation from an arc to a transform margin comparable to the present-day Basin and Range Province in Western USA, the opening of the Rheic Ocean was possible (e.g., Pin and Marini 1993;Nance and Murphy 1996;Kryza and Pin 1997;Nance et al. 2002). ...
The post orogenic evolution of Variscan Central Europe is characterized by the formation of numerous basins. The early Permian Döhlen Basin is located in the Elbe Zone (Germany) and is bordered by metamorphic rocks of the Erzgebirge and numerous Variscan magmatic complexes. The NW-SE-oriented basin is evidence for a major rearrangement of stress fields during the post-Variscan reactivation of fault zones in Central Europe. Eleven samples of magmatic rocks and sediments have been analyzed with respect to their U-Th-Pb isotope ratios and geochemical composition. Of three magmatic samples (two tuffs, one trachyandesite), we analyzed 170 zircon grains. The Unkersdorf Tuff of the Unkersdorf Formation gave an age of 294 ± 3 Ma (Upper Asselian to Sakmarian) whereas a trachyandesite of the same formation was dated at 293 ± 5 Ma (Lower Artinskian to Lower Asselian). The Wachtelberg Ignimbrite (Upper Bannewitz Formation) showed an age of 286 ± 4 Ma (Artinskian to Lower Kungurian). As first study, we also analyzed 984 detrital zircon grains of nine Late Paleozoic Central European sandstone and conglomerate samples of the Niederhäslich Formation and the Bannewitz Formation with respect to their U-Pb age composition. All sediments but two yielded two distinct age groups between 295-340 Ma and 530-750 Ma, as well as a minor amount of Precambrian zircon ages. Geochemical data points to an active margin setting with developing strike-slip basins. The data suggests a c. 10 Ma lasting basin formation during the second culmination of volcano-tectonic activity with basic to intermediate melts. The second youngest formation (Niederhäslich Formation) consists predominantly of pre-Permian basement material, which implies only minor volcanic activity and erosion from adjacent basement blocks. On the contrary, the uppermost and youngest Bannewitz Formation features strong evidence for volcanic activity in the neighboring area of the basin. The present study strongly suggests a rapid basin development and further shows how the evolution of the Döhlen Basin is proof for several post-Variscan tectonic reactivation phases in Sakmarian and Lower Kungurian of Central Europe. Finally, our results exemplarily show how basin evolution may be characterized by radiometric data of detrital zircon grains.
... The formation and internal evolution of the pegmatites have been given the credit to the complex interplay between silicate melt and hydrous fluid (Simmons and Webber, 2008;Thomas et al., 2012). Strontium is generally considered to be significantly mobile in aqueous fluids (Lucassen et al., 2011;Pin and Marini, 1993), thus Rb\ \Sr isotope system can be easily disturbed by complex fluid-rock reaction (Buettner et al., 2005;Hegner et al., 1998;Zhu et al., 2013). High Rb/Sr ratios of some samples and the post-crystallization migration of radiogenic 87 Sr at low temperature also introduce large uncertainties in calculated are-corrected ( 87 Sr/ 86 Sr) i and the interpretation of pegmatite source (Clark and Černý, 1987;Jahn, 2004;Wu et al., 2002). ...
Evidence for open-system magmatic processes related to wallrock assimilation accompanied by fractional crystallization (AFC) is present in the Guangshigou biotite pegmatites, North Qinling Orogen. The biotite pegmatite-gneiss contacts generally coincide with the greatest enrichment of U and Th. Zircon UPb dating constrains the crystallization ages of the biotite pegmatite (rim zone-415 ± 2.6 Ma; internal zone-413.5 ± 2.5 Ma), in line with a pyrite PbPb isochron age (413 ± 22 Ma). Metamict areas in zircon show generally elevated concentrations of trace elements and expulsion of radioactive Pb. Internal zone samples, representing uncontaminated magma, have negative to positive zircon (~ 413 Ma) εHf(t) (− 1.53 − + 3.24), low εNd(t) values (− 2.4), and old Hf and Nd model ages (tDM2 = 1.5–1.19 Ga, T2DM = 1.35 Ga, respectively), indicating a dominantly recycled Mesoproterozoic lower crustal material with involvement of some juvenile materials in the source region. The magmatic oxygen fugacity (fO2) and crystallization temperatures ranges from − 24.81 to − 13.34 of log fO2 and 570 °C to 793 °C, respectively. Compared to the internal zone, pegmatite rim samples display a variable and lower εNd(t) values (− 3.9 to − 2.8) and T2DM (1.47–1.37 Ga), but similar Hf isotopic compositions, favouring a three-component isotopic mixing model (recycled Mesoproterozoic lower crust materials, juvenile materials, and host gneiss). Pronounced variations of Ti, Y, U, Th, Hf, and REE concentrations in zircon from grain to grain in individual samples and from area to area within individual grains suggest a fluctuating crystallization environment in hybridized magma from which the rim-hosted zircons crystallized. Variable and high radiogenic Pb ratios of pyrites forming in the hybridized magma were inherited from the matrix. Zircons from both zones exhibit similar Hf isotope patterns, indicating the rim-hosted zircons crystallized during the early stage of hybridization of magma. However, the heterogeneous Nd isotopic composition in individual pegmatites imply an incorporation of wallrock-derived melt with less radiogenic Nd.
... The Cambro-Ordovician event produced volumetrically significant igneous rocks that were generated during rifting at the northern margin of the Gondwana continent and the resultant opening of the Rheic Ocean after the Neoproterozoic Cadomian orogeny (e.g. Pin & Marini, 1993;Murphy et al. 2006;Pin et al. 2007;von Raumer & Stampfli, 2008; †Author for correspondence: mjast@interia.pl Nance et al. 2012). ...
The pressure–temperature–deformation–time ( P–T–d–t ) record of metagranitic rocks and adjacent diverse rocks of the metavolcano-sedimentary group from the Orlica–Śnieżnik Dome (OSD) in SW Poland is examined. The study aims to better understand the course of the break-up of northern Gondwana and the overprinting Variscan tectonometamorphism in the NE Bohemian Massif. We test the existing hypotheses that explain the Cambro-Ordovician thermal event recorded in the meta-supracrustal group by (i) syn-deformational regional metamorphism or (ii) the contact metamorphism of the (meta)sedimentary rocks around the intruding ~490–500 Ma granitic magmas. In addition, we check the extent and timing of the Variscan prograde and retrograde medium-pressure metamorphism in the OSD. The results imply that Early Palaeozoic monazites, rarely preserved in both rock groups, document ~490–500 Ma volcanic and plutonic events related to the Gondwana's break-up and following disturbance of the Th–U–Pb system during younger, Variscan events. The monazite geochronology reveals no distinct Cambro-Ordovician thermal aureole around the post-granitic orthogneisses. However, no large-scale Variscan juxtaposition is evident between the two main OSD rock groups or within the meta-supracrustal rocks. Consistent P–T–d–t results for the meta-supracrustal rocks and the orthogneisses suggest that their precursors contacted before the Variscan tectonometamorphism. The directly contiguous ortho- and paragneisses together experienced tectonometamorphic processes at maximum depths that correspond to 7.5–8.0 kbar and maximum temperatures of ~600–620°C, as a result of the Variscan collision of Gondwana and Euramerica. The continental collision-related events intensified at ~360 Ma and ~330–340 Ma.
... 530-500 Ma), an asymmetric rift basin comparable to the current Basin and Range Province developed (Nance and Murphy 1996;Nance et al. 2002). The former active margin evolved into a transform margin, which was the beginning of the opening of the Rheic Ocean (Pin and Marini 1993;Nance and Murphy 1996;Kryza and Pin 1997;Nance et al. 2002). During the transition from a collisional to a rift-related setting along the northern margin of Gondwana a second pulse of high magmatism occurred and led to the intrusion of the Rumburk granite and the Izera-Kowary unit of the Karkonosze-Izera Massif (Borkowska et al. 1980;Kröner et al. 1994;Gehmlich et al. 1997;Pin et al. 2007;Oberc-Dziedzic et al. 2009;Białek et al. 2014; Fig. 1). ...
The basement of the Saxo-Thuringian Zone consists
of Upper Neoproterozoic (c. 650-570 Ma) Cadomian
arc sediments (Lusatian greywackes) and voluminous intrusions
of Early Cambrian granitoids with ages of c. 540 Ma
(Lausitz Block and Karkonosze–Izera Massif). The latter
basement complexes comprise several c. 505 Ma granites,
granodiorites, and gneisses emplaced during the change
from a collisional tectonic setting to rift-related geotectonics.
We present a new age for the Rumburk granite of
504 ± 3 Ma linking Late Cambrian plutonism at the northern
margin of Gondwana with the initial phase of a Cambro–
Ordovician rift event. Trace element analysis points
to a linkage of the Rumburk granite with other Late Cambrian
aged rocks of the Karkonosze–Izera Massif. Furthermore,
geochemical data also provide evidence of a melting
and recycling of Lusatian greywackes by the intrusion of
the Rumburk granite. The youngest age peak of the Rumburk
granite at c. 504 Ma is considered to be the age of
emplacement. Older inherited age populations at c. 540
and c. 610 Ma are present and likely the result of a melting
and recycling of Lusatian granitoids and greywackes.
The appearance of Neoproterozoic inheritance and Lu–Hf
similarities with the Rumburk granite strongly suggest the
Lusatian greywackes as source rocks. There is a significant
age gap of c. 35 Ma between Cambrian plutonic and
volcanic rocks in Saxo-Thuringia. Hence, we consider two distinct pulses of magmatic activity during the transition
from the Cadomian orogeny to the opening of the Rheic
Ocean.
... The Para-autochthonous Unit (PAU), well exposed in the Cé vennes, consists of a thrust sheet imbrication of greenschist facies metapelites, quartzites, and metagrauwackes with subordinate layers of conglomerate, felsic and mafic lava, and rare intrusions (Caron, 1994;Pin and Marini, 1993). In the Albigeois area, the PAU has been subdivided into the Saint-Sernin-sur-Rance and the Saint-Salvi-de-Carcavè s nappes (Gué rangé -Lozes, 1987; Guérangé -Lozes and Burg, 1990;Gué rangé -Lozes and Guérangé , 1984). ...
In the Variscan French Massif Central and Armorican Massif, the tectonic significance of a widespread NW–SE-trending stretching lineation, coeval with medium pressure–medium temperature metamorphism, is an open question. Based on a structural analysis in the southern part of the Massif Central, we show that this top-to-the-NW shearing is a deformation event, referred to as D2, which followed a D1 top-to-the-south shearing Devonian phase, and was itself re-deformed by a Late D3 Visean–Serpukhovian southward-thrusting event. We date the D2 phase at 360 Ma (Famennian–Tournaisian boundary). In the Armorican Massif, D2 is the “Bretonian phase” recorded in the metamorphic series and sedimentary basins. Geodynamically, D2 is related to a general northwestward shearing during the Laurussia–Gondwana collision, which occurred after the closure of the Rheic Ocean, as indicated by the emplacement of the Lizard ophiolitic nappe in Britain. The left-lateral Nort-sur-Erdre fault accommodated the absence of ductile shearing in Central Armorica.
... The conspicuous presence of volcanic rocks (meta-tuffs, meta-lavas) throughout the stratigraphic pile exposed in low-grade units (Alvaro et al., 2014;Marini, 1987;Pouclet et al., 2017) and the existence of several orthogneiss massifs of similar ages within the high-grade domains (Alexandre, 2007;Duthou et al., 1984;Melleton et al., 2010;Roger et al., 2015) testify for a protracted magmatic activity from the late Ediacaran to the late Ordovician. Petrogenetic studies attest to the tapping of two contrasting reservoirs throughout this period: (i) the asthenospheric/lithospheric mantle, source of the metabasites from the low-grade units and the LAC (Briand et al., 1992;Marini, 1987;Pin and Marini, 1993;Pouclet et al., 2017); (ii) preexisting crustal lithologies (Alexandre, 2007;Melleton et al., 2010) as a source of felsic magmas. Coeval sedimentation, crust-and mantle-derived magmatism have been attributed to long-lived continental extension ending up in the formation of a hyperextended margin during the Ordovician (Lardeaux et al., 2014). ...
From the Neoproterozoic to the early Paleozoic, the northern Gondwana margin was sequentially shaped by the Cadomian accretionary and the Variscan collisional orogens which offers the opportunity to investigate the relative extent of crust production/reworking in both geodynamic settings. In the eastern part of the Variscan French Massif Central (FMC), the Velay Orthogneiss Formation (VOF) represents a consistent lithological unit of the pre-Variscan basement and comprises augen gneisses and leucogneisses. Such rocks constitute a unique record of the pre-Variscan magmatic history and bear critical information on the crustal evolution of the northern Gondwana margin.
... Nevertheless, the Rheic suture zone is now recognized in southern England and south of the Rhenish Massif, while south of this suture, many relicts of mafic-ultramafic complexes, MORB-type eclogites, alkalic magmas and contemporaneous crust-derived acidic magmatic rocks of Upper Cambrian-Lower Ordovician ages are found (see compilations in: Berger et al. 2006;von Raumer et al. 2013von Raumer et al. , 2015Villaseca et al. 2015). Despite some disagreement about the source and geodynamic implication of such magmatism, it is generally agreed that at this period the northern Gondwana margin was under widespread extensional tectonics that locally led to the formation of slow-spreading oceanic crust, distinct from that of the Rheic Ocean (Berger et al. 2006;Bouchardon et al. 1989;Briand et al. 1991;Díez Fernández et al. 2012;Matte 2001;Montes et al. 2010;Pin and Marini 1993;von Raumer and Stampfli 2008). In the Massif Central, this "oceanic" domain is often referred to as the Galicia-Brittany Ocean (Matte 2001) or Massif Central-Moldanubian Ocean (Tait et al. 1997). ...
The East Massif Central (EMC), France, is part of the internal zone of the Variscan belt where late Carboniferous crustal melting and orogenic collapse have largely obliterated the pre- to early-Variscan geological record. Nevertheless, parts of this history can be reconstructed by using in-situ U-Th-Pb-Lu-Hf isotopic data of texturally well-defined zircon grains from different lithological units. All the main rock units commonly described in the EMC are present in the area of Tournon and include meta-sedimentary and meta-igneous rocks of the Upper Gneiss Unit (UGU) and of the Lower Gneiss Unit (LGU), as well as cross-cutting Variscan granitoid dikes and a heterogeneous granite coring the major Velay dome. Herein we demonstrate that the UGU and the LGU have markedly distinct zircon records. The results of this study are consistent with deposition of the protoliths of the paragneisses within a back-arc basin that was located adjacent to the Arabian-Nubian shield and/or the Saharan Metacraton during the late Ediacaran and collected detritus from the Gondwana continent. At ~ 545 Ma some of these sedimentary rocks were affected by a first melting event that formed the protoliths of the LGU orthogneisses, those of which subsequently remelted at ca. 308 Ma to form the Velay granite-migmatite dome. Protoliths of the UGU result mainly from a bimodal rift-related magmatism at ~ 480 Ma, corresponding to melting of the Ediacaran sediments and depleted mantle. Zircon rims from the UGU additionally provide evidence for a metamorphic/migmatitic overprint during the Lower Carboniferous (~ 350–340 Ma). Finally, several generations of granite dikes of which inherited zircons display characteristics of both the UGU and the LGU were protractedly emplaced from ~ 322 Ma to ~ 308 Ma, the youngest of which being coeval with the formation of the Velay dome. Our data further show that the vast majority of crustal material ultimately involved in the Variscan orogeny, which forms the present-day basement in the EMC, was derived from a sedimentary mixture of various components from the Gondwana continent deposited in Ediacaran times, with no evidence for the involvement of an older autochthonous crust.
... Its geodynamic history is marked by substantial extension and rifting of the north Gondwanan margin (Murphy & Nance, 1991;Nance et al., 2010) and migration of detached Gondwanan microcontinents to the north where they progressively amalgamated with Laurussia, forming the Variscan orogenic belt and ultimately the Pangea supercontinent (Ziegler, 1982;Stampfli et al., 2013). Indeed, the Early Palaeozoic rifting event is recorded in the whole European Variscan belt by the formation of large Cambro-Ordovician sedimentary basins Linnemann et al., 2004), extensive felsic and mafic magmatism (Pin & Marini, 1993;Crowley et al., 2000;S anchez-Garc ıa et al., 2003S anchez-Garc ıa et al., , 2008Jammes et al., 2009;Ball evre et al., 2012), LP-HT granulite facies metamorphism ( St ıpsk a et al., 2001) and faunal data (Fatka & Mergl, 2009;Torsvik & Cocks, 2009). The onset of the Variscan orogeny is constrained by the timing of eclogite facies metamorphism, suggesting the existence of Late Silurian subduction in the French Massif Central (Lardeaux et al., 2001Faure et al., 2009) and early-mid Devonian subduction in Iberia (Casado et al., 2001) and the Bohemian Massif (Stosch & Lugmair, 1990;Beard et al., 1995). ...
Rift-related regional metamorphism of passive margins is usually difficult to observe on the surface, mainly due to its strong metamorphic overprint during the subsequent orogenic processes that cause its exposure. However, recognition of such a pre-orogenic evolution is achievable by careful characterization of the polyphase tectono-metamorphic record of the orogenic upper plate. A multidisciplinary approach, involving metamorphic petrology, P–T modelling, structural geology and in situ U-Pb monazite geochronology using laser-ablation split-stream inductively coupled plasma mass spectrometry, was applied to unravel the polyphase tectono-metamorphic record of metapelites at the western margin of the Teplá-Barrandian domain in the Bohemian Massif. The study resulted in discovery of three tectono-metamorphic events. The oldest event M1 is LP–HT regional metamorphism with a geothermal gradient between 30 and 50 °C km−1, peak temperatures up to 650 °C and of Cambro-Ordovician age (c. 485 Ma). The M1 event was followed by M2-D2, which is characterized by a Barrovian sequence of minerals from biotite to kyanite and a geothermal gradient of 20–25 °C km−1. D2-M2 is associated with a vertical fabric S2 and was dated as Devonian (c. 375 Ma). Finally, the vertical fabric S2 was overprinted by a D3-M3 event that formed sillimanite to chlorite bearing gently inclined fabric S3 also of Devonian age. The high geothermal gradient of the M1 event can be explained as the result of an extensional, rift-related tectonic setting. In addition, restoration of the deep architecture and polarity of the extended domain before the Devonian history – together with the supracrustal sedimentary and magmatic record – lead us to propose a model for formation of an Ordovician passive continental margin. The subsequent Devonian evolution is interpreted as horizontal shortening of the passive margin at the beginning of Variscan convergence, followed by detachment-accommodated exhumation of lower-crustal rocks. Both Devonian shortening and detachment occurred in the upper plate of a Devonian subduction zone. The tectonic evolution presented in this article modifies previous models of the tectonic history of the western margin of the Teplá-Barrandian domain, and also put constraints on the evolution of the southern margin of the Rheic ocean from the passive margin formation to the early phases of Variscan orogeny.
... The occurrence of basic rocks also shows a mantle contribution (Pin and Marini, 1993;Giacomini et al., 2006). In some Variscan plutons, mafic enclaves and plutonic dykes, point to an input of asthenospheric mantle source injected in the continental crust (Pin and Duthou, 1990;Pin, 1991;Bussy et al., 2000). ...
Detrital zircons from Cretaceous micaschist, Late Eocene-Earliest Oligocene sandstone and Early Oligocene siltstone of the Western Alps fall into three main separable age clusters at 610-540 Ma, 490-430 Ma, and 340-280 Ma that correspond to the Cadomian (Neoproterozoic), Ordovician and Variscan (Carboniferous) events widespread in Western and Central Europe. Hf isotopic results indicate that these three magmatic and tectonic episodes did not give rise to significant production of juvenile crust. A distinguishable group of Triassic zircons, around 250-200 Ma which is considered to derive from the Southern Alps, has been detected in the Early Oligocene "Schistes à Blocs" formation and the Brianconnais “Flysch Noir”. In contrast, this age group is absent in Late Eocene-Earliest Oligocene sandstones. In agreement with sedimentological studies, our results show that the main source areas of the Eocene sandstone were probably located in the European continent. The arrival of detritus from the Internal Zone occurred in Early Oligocene, coeval with the tectonic rotation from northwestward to westward in the propagation of allochthonous units. Based on previous studies and our new data, we argue that the the Brianconnais Zone was likely a paleorelief since the Middle Eocene that accounts for the lack of detritus from the Adriatic units. Contemporary sediments were accumulated in the foredeep of the Adriatic Plate. From Oligocene time onwards, the blockage was cut through after a regional uplifting, and thus the Internal Zone started to provide detritus into the western flexural basins.
... This magmatic activity is commonly interpreted as the incipient break-up of the northern Gondwana margin and the associated opening of oceanic basins bounded by microcontinental blocks (e.g. Pin & Marini 1993;Crowley et al. 2000;Schätz et al. 2002). All these arguments confirm the Gondwana derivation of the entire Vosges Mountains. ...
A geological synthesis of the Palaeozoic Vosges Mountains (NE France) is presented using existing observations and new data. The geodynamic evolution involves: (1) Early Palaeozoic sedimentation and magmatism; (2) Late Devonian subduction triggering back-arc spreading; (3) early Lower Carboniferous continental subduction, continent–continent collision and polyphase deformation and metamorphism of the orogenic root; and (4) late Lower Carboniferous orogenic collapse driven by thermal weakening of the middle crust. The evolution is integrated within the framework of the European Variscan Belt. The Northern Vosges comprise sediments of Rhenohercynian affinity separated from Teplá-Barrandian metasediments by a Lower Carboniferous magmatic arc. The latter is correlated with the Mid-German Crystalline Rise, and is ascribed to the south-directed subduction of the Rhenohercynian Basin. The Saxothuringian–Moldanubian suture is thought to be obliterated by the magmatic arc, while the Lalaye–Lubine Fault is interpreted as the Teplá-Barrandian–Moldanubian boundary. The Central Vosges are paralleled with the Moldanubian domain of the Bohemian Massif where identical lithologies record the Devonian–Carboniferous SE-directed subduction of the Saxothuringian passive margin below the Moldanubian upper plate. The Southern Vosges represent the upper Moldanubian crust and are linked to the southern Black Forest. The presence of an oceanic domain to the south of the Vosges–Black Forest remains unclear.
... All these interpretations agree, however, on the fact that the Variscan belt resulted from the final collage of two supercontinents, Laurussia to the north and Gondwana to the south (Fig. 1). The Early Paleozoic pre-orogenic extensional event is recorded across the entire Variscan belt by the formation of large Cambro-Ordovician sedimentary basins (e.g., Young 1990;Linnemann et al. 2004) and massive bimodal magmatism (e.g., Pin 1990;Pin and Marini 1993;Abati et al. 1999;Crowley et al. 2000;Sánchez-García et al. 2003. Among the most typical examples are the porphyritic granitic and rhyolitic orthogneisses with emplacement ages clustering between 490 and 450 Ma (e.g., Helbing and Tiepolo 2005;Solá et al. 2008;Montero et al. 2009;Ballèvre et al. 2012;Talavera et al. 2013;Del Greco et al. 2016 and references therein). ...
New U-Pb dating on zircon yielded ca. 470 Ma ages for the granitoids from the Lévézou massif in the southern French Massif Central. These new ages do not support the previous interpretation of these granitoids as syn-tectonic intrusions emplaced during the late Devonian-early Carboniferous thrusting. The geochemical and isotopic nature of this magmatism is linked to a major magmatic Ordovician event recorded throughout the European Variscan belt and related to extreme thinning of continental margins during a rifting event or a back arc extension. The comparable isotopic signatures of these granitoids on each side of the eclogite-bearing leptyno-amphibolitic complex in the Lévézou massif, together with the fact that they were emplaced at the same time, strongly suggest that these granitoids were originally part of a single unit, tectonically duplicated by either isoclinal folding or thrusting during the Variscan tectonics.
We provide a thorough review of the literature on peraluminous magmatism of Late Neoproterozoic and Early Palaeozoic (mostly Late Cambrian-Middle Ordovician) age cropping out in many places around the world (SW South Africa, NE Patagonia, NW Argentina, Colombia, SE Mexico and Guatemala, the European Variscan Massifs and from Turkey to northern Burma through Tibet). Petrographically, these volcanic and plutonic rocks contain K-feldspar phenocrysts and sometimes smaller bluish-quartz phenocrysts in a glassy/fine-grained (volcanic/subvolcanic) or medium- to coarse-grained (plutonic) matrix of quartz, plagioclase, K-feldspar and biotite, with other Al-bearing phases such as muscovite and garnet as minor phases. Notably, amphibole is conspicuously absent. Geochemically, these dacitic (tonalitic) to rhyolitic (granitic) rocks are silica-rich, peraluminous and with a strongly crustal Sr-Nd isotopic signature, pointing to S-type magmatism, but they also show characteristics of I-type subduction (a trace element signature typical of continental-arc magmatism) and A-type (enrichment in Ga) magmatism. A prominent geochemical feature is a marked depletion in Sr, resulting in low to very low Sr/Y ratios (usually <5). This, together with flat HREE slopes, suggests melting at low pressures. The arc signature is inherited from their crustal sources, which may comprise an old crustal basement and sediments derived from Pan-African and from Andean-type orogenic belts. Coeval, volumetrically minor mafic rocks are also common in many outcrops and are part of a bimodal sequence. Researchers have mostly attributed this magmatism to extensional tectonics in a back-arc setting, where the upwelling of the asthenospheric mantle triggered the high-temperature-low-pressure partial melting of a largely metasedimentary (upper continental) crust with little or no contribution from the mantle. In a reconstruction of Early Palaeozoic Gondwana, all outcrops are situated in peri-Gondwanan terranes, implying that they are related to (and the consequence of) rifting processes that led to the opening or aborted opening of several oceans (Rheic, proto-Tethys), reflecting a common evolution of the margin of Gondwana during the Cambrian and Ordovician. Given the similarities in petrography and geochemistry (major and trace elements and Sr-Nd isotopes) and the very large volume, several silicic Large Igneous Provinces have been proposed for some sectors, and the possibility that the entire magmatism comprises a single LIP is evaluated. Although correlations of this magmatism in different regions have been established previously, to our knowledge, this is the first study to integrate detailed petrographic, geochemical and geochronological data from all outcrops and to conclude that the peraluminous porphyritic magmatism reviewed here is the main magmatic expression of extension in the peri-Gondwanan area during the Early Palaeozoic.
A large number of Late Silurian–Early Devonian intrusive rocks are distributed in the central Beishan orogenic belt (BOB). Tectonic setting of these intrusive rocks is of great significance to the study of the subduction and accretion of the Paleo-Asian Ocean. Previous studies show that most of the intrusive rocks in this region are S-type or A-type granitoids. In this study, we firstly reported the Late Silurian–Early Devoniandia bases, granodiorites on the southside of the Baiyunshan ophiolitic mélanges belt, as a part of Hongliuhe-Xichangjing ophiolitic mélanges belt (HXOMB). Zircon LA-ICP-MS U-Pb dating yields emplacement ages between 418 and 397 Ma, REE distribution patterns exhibit enriched LREE and flat HREE in the diabases, the discriminant diagrams show that the diabases have geochemical characteristics of intraplate basalt. The granodiorites in this paper present more like S- and A-type granitoids reported, showing the geochemical characteristics of syn/post-collision granites. Actually, the bimodal magmatic rocks are developed during Late Silurian–Early Devonian on both sides of the HXOMB, which are related to the tectonic background of the post orogeny extension. The diabases are tholeiitic with relative strong depleted εHf(t) (+8.1 to +13.0), which are mainly from relative depleted mantle. The granodiorites are calc-alkaline with relative slightly depleted εHf(t) (+0.7 to +5.6) and the lower Mg# and MgO contents (34.6–36.9, 0.50 wt.%–1.19 wt.% respectively), reflecting the source characteristics of meta-basalt. Therefore, the remelting of juvenile crust may be the main way of continental crust accretion during Late Silurian–Early Devonian in the central BOB.
Unravelling the pre-Variscan evolution of the French Massif Central (FMC) demands an accurate assessment of the origin and emplacement age of the numerous meta-igneous rocks present throughout the metamorphic nappe pile. Among those, the Montredon–Labessonnié orthogneiss is a metagranite body exposed in the low-grade domain of the northern Montagne Noire. New results of LA–ICP–MS zircon U–Pb dating demonstrate that its granitic protolith was emplaced at 544.0±6.2 Ma. Whole-rock major element geochemistry, zircon crystal morphologies and the inherited zircon date distribution collectively indicate that the protolith formed by melting of Ediacaran sedimentary rocks. This orthogneiss thus represents a newly identified marker of the latest Ediacaran magmatic event well-recorded in the FMC
Khondalite Belt is located in the northwestern part of North China Craton, and is a typical continent-to-continent
collisional orogen, along which the Yinshan Block collided with the Ordos Block at 1.95–1.85 Ga. However, this
model was mainly relied on high-grade metamorphic records in the Khondalite Belt, whether the extensively
exposed low-grade rocks were also involved in such collision remains poorly constrained. Erdaowa Group is an
important lithological unit of the belt and consists of extensive low-grade metamorphic rock assemblages (i.e.
greenschist to amphibolite facies), and provide crucial insights into understanding the issue. Based on petro�logical and geochemical characteristics, low-grade metamorphic rocks of Erdaowa Group show a large affinity to
bimodal volcanic sequence characterized by basaltic and acidic endmembers. Meta-mafic rocks were derived
from partial melting of lithospheric mantle with minor contamination of crustal materials, whereas meta-acidic
rocks were generated from the middle-to-lower crust. Geochemical data of both rock assemblages show de�pletion of HFSE and enrichment of LILE, indicative of a subduction-related magmatic arc environment. U-Pb
zircon dating results indicated that these rocks erupted at 2465 ± 42 Ma, and experienced metamorphism at
1940–1861 Ma. In combination with available petrological, geochemical and geochronological data, we infer
that arc magmatism was developed at a back-arc basin area of a ~2.45 Ga active continental margin along
southern Yinshan Block. Together with those high-grade metamorphic rocks, these low-grade rocks were also
incorporated into the final collisional event at ~1.95–1.90 Ga.
To better understand the Neoproterozoic tectonic evolution along the northern margin of Yangtze Block, we have determined the geochronological and geochemical compositions of newly recognized bimodal volcanic suite and coeval granites from the western Dabie terrain. LA-ICP-MS zircon U-Pb dating reveals that the felsic and mafic volcanics from the Hong’an unit have crystallization ages of 730 ± 4Ma and 735 ± 5Ma, respectively, indicating that the bimodal suite was erupted during the Neoproterozoic. The Xuantan, Xiaoluoshan, and Wuchenhe granites yield U-Pb ages of 742 ± 4 Ma, 738 ± 4 Ma, and 736 ± 4 Ma, respectively. The felsic volcanic rocks show peraluminous characteristics, and have a close affinity to S-type granite. The mafic volcanic rocks are basalt in compositions, and are likely generated from a depleted mantle source. The granites belong to high-K calc-alkaline and calc-alkaline series, display metaluminous to peraluminous, and are mainly highly fractionated I-type and A-type granite. The granites and felsic volcanics have zircon εHf(t) values of −16.4 to + 5.6 and two-stage Hf model ages (TDM2) of 1.28 to 2.40 Ga, suggesting that they were partial melting of varying Mesoproterozoic–early-Neoproterozoic crust. The granites have εNd(t) of -14.7 to -1.5, and the two-stage Nd model ages (TDM2) values of 1.54 to 2.61 Ga, also implying the Yangtze crustal contribution. These Neoproterozoic bimodal suite and coeval granites were most likely generated in a rifting extensional setting, triggered by the mantle upwelling, associated with crust–mantle interaction. Intensive magmatic rocks are widespread throughout the South Qingling, Suizhao, western Dabie and eastern Dabie areas during 810–720 Ma, and show peak ages at ~ 740 Ma. Combining regional geology, we support a continental rifting extensional setting for the north margin of the Yangtze Block during the break-up of the supercontinent Rodinia.
The East European Craton has a thick sequence of volcano-sedimentary rocks preserved in the Losevo belt that developed along the junction between Sarmatian and Volgo-Uralian microcontinents. The major lithologies of the Losevo terrain (LT) are a dominant bimodal volcanic suite and a basalt–andesite–dacite–rhyolite assemblages (BADR). The LT rocks have been divided from lower to upper sequences into the Terrigene, Strelitsa and Podgornoye Formations, but the stratigraphic subdivisions have not been geochronologically tested. Here we present geochemistry and SHRIMP zircon geochronology of volcanic rocks from the LT. The volcanic suite from the Terrigene Formation has tholeiitic and calc-alkaline affinites, significant enrichment in LILE and LREE and strong depletion in HFSE with εNd(t)=+ 2.6, whereas the felsic dikes display an A-type affinity, with typical enrichment in Zr, Nb, Y, and depletion in Sr and Ti, fractionated REE patterns, and strong negative Eu anomalies with εNd(t) in the range of -0.5 to 2.6. The bimodal volcanic suite of the Strelitsa Formation is composed of tholeiites displaying minor depletion in LREE, slight enrichment of LILE, no or weak depletion of Nb resembling transition MORB with εNd(t)=+3.0 to +3.6) and rhyolites with high LREE/HREE, high Sr/Y, no Eu anomaly, and strong depletion in Nb and Ti (εNd(t)=+1.8 to +2.9) resembling slab-derived high pressure adakitic melts. The volcanic rocks of the Podgornoye Formation are bimodal with tholeiitic chemistry, lack enrichment in LILE and LREE and have a slight depletion in HFSE (εNd(t)=+3.7) together with rhyolites having high LREE/HREE, moderate Sr/Y, no Eu anomaly, and strong depletion in Nb and Ti (εNd(t)=+2.1 to +2.6) resembling slab derived relatively low-pressure adakite-like melts. The BADR assemblage has significant enrichment in LILE and LREE and strong depletion in HFSE, similar to arc-like volcanics. Geochronological data indicate that the early LT volcanic rocks were formed during the early (Terrigene Formation) stage of intra-continental arc with a continental basement whereas the Strelitsa bimodal volcanic rocks were formed during a middle stage of back-arc extension and the Podgornoye bimodal volcanic rocks and BADR were formed during a later stage intra-oceanic arc. The identification of a 2170 to 2120 Ma back-arc basin in the East Sarmatian Orogen together with broadly coeval arcs indicate that the eastern margin of the Sarmatia was active with an arc–back-arc environment. Our new data suggest that the initial melts of the bimodal suite were adakitic derived by slab melting, followed by mantle metasomatism, whereas the basaltic magmas formed in an island arc setting. The LT and similar-aged volcanic belts in other terrains are considered to represent the initial (2.1–2.0 Ga), subduction-related growth of the Paleoproterozoic Columbia supercontinent.
High pressure trondjemitic rocks interbedded with eclogites and high pressure amphibolites were recognized in the leptyno-amphibolitic series of the Rouergue and Marvejols metamorphic terranes. These rocks appear to have been derived by partial melting of the surrounding amphibolites. The partial melting took place before the well-known Barovian metamorphism that has affected the whole crystalline basement of the French Massif Central. This older metamorphic event is considered to be equivalent to the high pressure granulite facies metamorphism recognized in the Variscan of Europe. This acid-basic high pressure bimodel association argues for a close similarity between Archean greenstone belts and the leptyno-amphibolitic series.
The early-Paleozoic magmatic series in the Cholet area is composed of three different units: a western acidic volcanic sequence, and an eastern microgranitic massif with associated gabbroic rocks. The trace element geochemistry shows that part of the acidic volcanic sequence is identical to the plutonic microganite and may originate from crustal anatexis. Parts of the gabbroic rocks have been more or less extensively contaminated by acidic magmas. The microgabbros have trace element patterns characteristic of continental tholeiites. The whole magmatic series in the Cholet area may be attributed to Ordovician-Silurian extensional tectonics.-from Authors
The data show the chemical variations for 51 samples of basalt, and are used to infer the compositional differences in the mantle sources for these basalts. -K.A.R.
The Lower Palaeozoic bimodal spilite-keratophyre suites of the Variscan orogenic belt in Polish W Sudetes include the Upper Cambrian Kaczawskie Mts and the Upper Silurian Leszczyniec formations. The Upper Cambrian basic rocks are metamorphosed intra-plate tholeiitic and subordinate alkaline basalts emplaced in an ensialic environment; they were probably related to the Cambro-Ordovician rifting and thinning of the continental crust in western and central Europe. The Silurian basic rocks have compositional characteristics of immature island-arc tholeiites (including REE patterns depleted in light REE, and low contents of Ti, Zr and Nb) and were related to the subduction of the oceanic lithosphere. The data imply a succession of extensional and compressional periods during the evolution of the Variscan belt in the Polish area and the existence of at least small ocean basins across the European plate. -R.A.H.
Basaltic lavas from fracture zones and ridge segments were sampled systematically along the Southwest Indian Ridge from the Bouvet triple junction to 11oE. Major- and trace-element analyses and isotopic variations serve to distinguish three compositional groups: 1) a group with typical 'depleted' N-type MORB characteristics (Zr/Nb > 20, Y/Nb > 8 and LREE depletion with La/YbN 0.35-1.1), 2) a relatively enriched group with typical 'plume' characteristics or P-type MORB (Zr/Nb 5.8-6.8, Y/Nb 0.9- 1.2 and strong LREE enrichment with La/YbN 4.8-6.8), and 3) an intermediate group with a transitional composition or T-type MORB (Zr/Nb 7.7-11.0, Y/Nb 1.3-3.0 and slight LREE enrichment with La/YbN 1.7-4.3). N- and T-type lavas are the most abundant.-R.A.H.
The vast extent of sedimentation deposits making up the Grès Armoricain and its equivalents over the north Gondwanan domain is used in this study to test the hypothesis of continuity between the Armorica and Gondwana plates during the Early Ordovician. The distribution of continental and marine environments, the large volume of transported elastic material, and the consistent current directions all indicate that the source of sediment supply on the north Gondwanan margin was to the south of the Sahara. Although there were local signs of incipient rifting between the two plates, Armorica was not yet detached from Gondwana by Arenigian time.
Major and trace element variations in an Ordovician alkaline orthogneiss (ortholeptynites) series of the Maures massif, France,
suggest that the pre-metamorphic rocks were alkaline and peralkaline rhyolites derived by fractional crystallization from
trachy-rhyolitic magma. Differentiation proceeded under low oxygen fugacity conditions, with the production of high-silica
comendites. By comparison with unaltered alkaline and peralkaline rhyolites, it is possible to prove that the parental magma
was not similar to the metatholeiites that are associated with the orthogneisses in the field. Nor were the metarhyolites
derived from the alkaline to transitional metabasalts of the same age which occur elsewhere in the Maures massif. The orthogneisses
have geochemical features typical of alkaline and peralkaline rhyolites occurring with basalts in continental, anorogenic,
bimodal associations. This hypothesis is also supported by zircon morphology and mineral chemistry. The Maures massif was
a within-plate continental volcanic province, whose crust underwent doming and thinning, but not break up, during lower Palaeozoic
times.
An unusually thick basic formation of presumed Ordovician age appears in the Albigeois 'Schistes et Roches Vertes' series belonging to the 'Schistes Peripheriques' belt of the French Massic Central. Chemical analysis of relict clinopyroxenes allows us to encompass the main geochemical features of this volcanism: transitional to tholeiitic nature, weakly differentiated in a distensive tectonic environment. Other criteria imply an ensialic distension in a thinned continental crust.-English summary
The unconformity between Lower Ordovician and Brioverian formations in central Brittany is associated with a layer wedging, 2 types of basal conglomerates according to the angle of unconformity, and volcanic flows. These datas are integrated into a model of Ordovician tectonics inducing tilted blocks in the Brioverian.-English summary
The Albigeois metabasites belong to the Initial Rifting Tholeiites (IRT): transitional-to-tholeiitic basalts related to continental splitting and aborted rifts. These volcanics remain very akin to TAU type MORB, and can be regarded as by-products of hotspot activity which probably drove the pre-Caradoc distension of the South Variscan realm, ie the Sardinian Phase s.l. Abridged English version is included.-English summary
The French concept of the so-called leptynitic-amphibolitic complexes was created by Forestier in 1961. Originally, it was purely descriptive and applied to bimodal acid-basic associations in high grade metamorphic series of the Massif central, commonly associated with minor amounts of carbonates and garnet peridotites. Since then, several aspects have been emphasized by successive authors: 1) the associated basic and ortho- or paraderived acid rocks must contain relict high-pressure assemblages such as eclogites and/or HP granulites; 2) the L.A.C. are bimodal magmatic series affected by at least one metamorphic event of variable intensity; 3) the L.A.C. are typical of orogenic mobile zones; 4) the L.A.C. are litho-tectonic formations emphasizing major ductile deep-seated shear-zones. Consequently, the L.A.C. are no longer precisely defined and any attempt to do using a single criterion can lead to erroneous geological interpretations. -from English summary
Early Archaean and Proterozoic dyke suites, which may be the ancient analogues of Phanerozoic continental flood basalts, have similar chemical characteristics. The nature of processes involved in the generation of flood-basalt magmas may therefore not have changed significantly since early in the history of the Earth. The chemistry of Proterozoic dykes places tight constraints on the degree of possible crustal contamination of these magmas and stresses the heterogeneity within sub-continental margin source-regions. Much of the isotopic and trace-element evidence used to support crustal contamination in Phanerozoic flood basalts is ambiguous. Although crustal contamination can never be entirely ruled out, primary mantle-derived mafic magmas can clearly have chemical characteristics which might otherwise be considered as typical of a crustal 'imprint'.-R.A.H.
Quite apart from isotope systematics, the elemental compositions of these basaltic lavas and associated dykes can contribute substantially to our understanding of their petrogenesis. Major- and trace-element analyses of worldwide flood-basalt occurrences (including 15 new analyses) are discussed in detail. When their incompatible elements are plotted on chondrite-normalized diagrams ('spidergrams' - hence arachnids), a few show the comparatively smooth downward-facing curves which characterize ocean-island tholeiites and both oceanic and continental alkalic basic lavas, but most show distinctive features which may be modelled by the addition of a few per cent of various types of fusible continental crust to oceanic magmas.-R.A.H.
Enrichment patterns involving K, Rb, Ba, Sr, Zr, Nb, P, Ti and Y are presented for Karoo lavas relative to MORB. The most enriched types (picrite basalts from SE Zimbabwe and N Lebombo) are enriched by a factor of 80 in Ba, 10-40 for Sr, Rb and K, and factors of approx 5-6 in Zr, Nb and P; Ti is only enriched by a factor of 2 and Y by = or <1.5. In contrast, the least enriched rocks (basaltic lavas and dykes) are enriched in K, Rb and Ba by factors of only 5-10. This great range cannot be explained by varying degrees of fractionation en route to the surface; nor is crustal contamination a major factor in generating the observed patterns. Detailed studies of the picrite basalts indicate that two end-member primitive magmas are required; these may be identified as basalt-depleted/trace-element-enriched and basalt-less-depleted/-trace-element-less-enriched. Thus it is suggested that trace-element heterogeneity in the mantle is the principal controlling factor in determining trace-element levels in the lavas.-R.A.H.
Two types of high-pressure granulites can be recognized within the leptyno-amphibolitic group of the Marvejols area: type 1 granulites are plagiopyrigarnites which contain relics of primary plagioclase showing doleritic textures; they occur as tectonic lenses with sharp contacts within the enclosing amphibolites. Type II granuites are plagiopyrigarnites in which the plagioclase has a secondary origin; they result from the recrystallization of kyanite and phlogopite-bearing ecologites and occur as tectonic lenses progressively grading into the enclosing amphibolite at their margins. The two types of granulites have undergone similar tectono-metamorphic histories. Trace element geochemistry indicates distinct magmatic affinities for the two types of granulites: type I resembles mid ocean ridge basalts but displays significant enrichment in Th and Rb suggesting a genesis in a narrow marginal basin. Type II granulites resemble the western Pacific high-Mg andesites (boninites) for several minor and trace elements including Sr, Hr, Zr, P, Ti, HREE, V and Cr. The amphibolites have geochemical characteristics very similar to the type II granulites but they have suffered an early stage of alteration, possibly in oceanic conditions. The association of these two types of magma has been frequently described in ophiolites, especially in the eastern Mediterranean. It is believed to represent an original setting in a marginal or back-arc basin, above a subduction zone. -from English summary
Detailed stratigraphy based on whole-rock geochemistry is presented for a 1200 m sequence of basaltic lava flows in the Western Ghats escarpment near Mahabaleshwar. Five separate sections are used to define a regional dip of approximately 0-5° to the SW. From the base upwards the following formations are described: Bushe, Lower Poladpur, Upper Poladpur, Ambenali, and Mahabaleshwar. Inter-formation boundaries, with the exception of the Upper Poladpur-Ambenali, are sharp, and are particularly well defined by breaks in Sr-isotopic composition. Two of the formation bases are marked by abnormally mafic flows- the Kamshedi picrite horizon at the base of the Upper Poladpur, and the Kelghar mafic unit at the base of the Mahabaleshwar. Major element compositions are controlled throughout largely by the degree of gabbro fractionation. Intense crustal contamination further modifies compositions in the lower part of the sequence (Bushe-Upper Poladpur) and has strong effects on trace elements and Sr-isotopes. Contamination decreases up-sequence leading to the comparatively uniform Ambenali rocks. The Mahabaleshwar Formation represents a change towards magmatism generated in an enriched mantle with many characteristics similar to those of oceanic island basalts. The geochemical discussion deals mainly with two well-developed mixing lines, one between Ambenali magmas and granitic crust, the other between ambenali magmas and the products of the postulated enriched mantle source. The detailed stratigraphic sequences strongly support the RTF (replenished, tapped, fractionated) magma chamber model of O'Hara &Mathews (1981) and the idea of periodical replenishment by picritic magmas (e. g. Huppert & Sparks, 1980b). This is believed to be the first demonstration of such processes operating on a large scale in a continental basalt province.
Simple arguments show that ascending thermal plumes will entrain their surroundings as the result of coupling between conduction of heat and laminar stirring driven by the plume motion. In the initial stages of ascent of a plume fed by a continuous buoyancy flux (a starting plume) the plume consists of a large buoyant head followed by a narrow vertical conduit. Laboratory experiments reported here show that the spherical head entrains ambient material as it rises, while the axial conduit carries hot source material to the stagnation point at the cap of the plume, from where it spreads laterally into thin laminae. We develop an analysis of the effects of entrainment on the structure and dynamics of starting plumes. The analysis predicts that under conditions appropriate to the earth's mantle large volumes of cooler lower mantle will be stirred into the head of a plume by the time it reaches the top of the mantle if it originates at the core-mantle boundary. The result is a major cooling and enlargement of the head. Source material ascending in the trailing conduit will undergo little contamination or cooling until the conduit is deflected from the vertical by large scale shear associated with plate motion. This plume structure explains the close association of high-temperature melts (komatiites or picrites) with more voluminous, lower temperature basalts in Archaean greenstones and modern continental flood basalt provinces: the picrites can be produced by melting in the hot axial conduit and the basalts from the cooler bulk of the head. More generally, we put forward stirring in plumes as one plausible mechanism contributing to compositional heterogeneity in hotspot melts.The predicted diameter of plume heads originating at the core-mantle boundary is ∼ 1000 km and this is expected to enlarge to ∼ 2000 km when the plume collapses beneath the lithosphere. This result is in excellent agreement with the observed extent of volcanism and uplift associated with continental flood volcanism. It also provides support for the hypothesis that at least some plumes originate at the core-mantle boundary.
A convergent margin magma series with characteristic low Nb and Ta abundances and enrichments in alkalis and alkaline earths is intercalated with typical intraplate alkalic basalts in a back-arc setting, 200-250 km above the Wadati-Benioff zone on the North Island, New Zealand. These two contrasting magma types, together with late-stage K-rich maflc lavas, were erupted over a short time period (160-274 Ma) and constitute the Alexandra Volcanics. Field relationships indicate that these diverse magma types were contemporaneous, and thus their mantle source regions coexisted, in a single tectonic environment.The convergent margin magma series forms a linear chain of stratovolcanoes aligned at right angles to the present subduction zone. Closed-system polybaric fractional crystallization models can explain the evolution from ankaramites to transitional olivine basalts to olivine tholeiites to high-Al basalts to medium- and high-K andesites. The most primitive lavas have geochemical (high LIL/LREE and LIL/HFS element ratios) and Sr, Nd, and Pb isotopic compositions typical of convergent margin magmas. Calculated source compositions suggest that three components are involved: a MORB component, a component derived from subducted oceanic crust, and a contribution from subducted sediments.The alkalic basalts occur as dispersed monogenetic volcanoes and are intercalated with the larger convergent margin stratovolcanocs. These basalts are enriched in LILE, LREE, Nb, and Ta, and have low Ba/Nb and Ba/La ratios, all of which are characteristic of ocean island (intraplate) basalts (OIBs). Their relatively high ε Nd (+55 and low 87Sr/ 86Sr(0703l-07036) are also typical of OIBs. These alkalic magmas were derived from the underlying continental lithospheric mantle that has been enriched by upward-migrating silica-undersaturated melts, probably including volatiles, from the low- velocity zone. A subducted slab component is not required to account for their incompatible element enriched character.The K-rich mafic lavas, basanites, and absarokites are volumetrically minor and cap the largest of the stratovolcanoes, Pirongia. The basanites have geochemical and isotopic compositions which suggest they are mixtures of multiple source components, including the alkalic and convergent margin region.
The depleted chemical signature of MORB has been commonly thought to be a consequence of the progressive extraction of continental
crust from the upper mantle, although few attempts have been made to evaluate the exact nature of the depletion process. The
spectrum of MORB compositions, at least in terms of Ba, Th, K, Pb, U, Ce and Nb data, cannot be accommodated by extraction
of continental crust alone. Development of the MORB reservoir (Depleted MORB Mantle or DMM) began early in Earth history by
extraction of incompatible elements from the upper mantle into the oceanic crust. This resulted in a chemically-zoned upper
mantle, with strong depletion of the highly-incompatible elements. However, the necessary fractionation of Nb from K and other
incompatible elements was achieved by removal at subduction zones of a large-ion lithophile element (LILE: K, Ba, Rb, etc.)
rich and Nb-depleted fluid (here termed the Slab Derived Component (SDC)), from the Residual Slab Component (RSC). The eclogitic
LILE-depleted RSC was variably remixed in the lower mantle, possibly with primordial mantle and minor volumes of SDC (as continental
crust or lithosphere) to produce a niobium-rich reservoir, the complement to the Nb-deficient continental crust. Remixing
of this material with DMM in the upper mantle is better able to account for the spectrum of MORB compositions. The presently
available trace element and isotope data preclude significant recycling of continental crust into the MORB mantle, although
a component of such material appears to be necessary to produce the spectrum of ocean island basalt compositions.
The elemental compositions of terrestrial igneous rocks are reviewed with special emphasis on those elements that partition strongly into the liquids in mafic and ultramafic systems. Published data are supplemented by 79 new major- and trace-element analyses. The magmatism of ocean basins is considered in terms of a model that has the following main features: (i) density layering in the sub-lithospheric upper mantle, so that the more fertile source of ocean-island basalts (o.i.b.) underlies the less fertile source of mid-ocean ridge basalts (m.o.r.b.); (ii) the genesis of all mantle-derived magmas restricted to very small degrees of partial fusion; (iii) genesis of m.o.r.b. source mantle as residuum from the loss of a melt fraction (forming o.i.b. magmas and lithospheric veins) from o.i.b.-source mantle; (iv) subduction of o.i.b.-veined lithosphere, with a thin veneer of m.o.r.b. and sediments, to the 670 km seismic discontinuity, followed by re-heating of these components and their buoyant upwelling into the o.i.b.-source reservoir; (v) very little chemical communication across the 670 km discontinuity. All continental anorogenic magmatism (distant from subduction zones in space and time) seems to be related ultimately to the o.i.b.-source mantle reservoir, which therefore must extend beneath the lithospheric roots of continents. The minor sodic-alkalic magmatism of continents is effectively identical in composition to o.i.b. Some continental flood basalts are similar but the majority contain minor contamination (rarely more than 15%) from fusible sialic rocks. Although substantial amounts of sediments appear to be subducted, only a small proportion of them seems to re-appear in the products of island-arc and Cordilleran magmatism. Much larger sediment fractions enter the sparse ultrapotassic magmatism that occurs far behind some subcontinental subduction zones and also characteristically follows the subduction related magmatism of collisional orogenies. The remaining subducted sediments finally pass into the o.i.b.-mantle source reservoir. It is well established that, during and immediately after collisional orogeny, the fusion of sialic crust contributes substantially (or even occasionally exclusively) to batholithic magmatism. Nevertheless, the elemental variation in such magmas implies that the role of fractional crystallization in their genesis has tended to be underestimated in recent years. Mantle-derived mafic to ultramafic magmas appear to be directly or indirectly (as heat sources) involved at deep crustal levels in the parentage of most batholithic intermediate and acid magams. These mantle-derived liquids are subduction-related before continental collisions and then change to o.i.b., several million years after subduction ceases. Enhanced subduction of terrigenous sediments during the final stages of ocean closure leads to the large subducted sialic fractions which re-emerge in the ultrapotassic mafic magmas that characteristically appear immediately after a continental collision.
In the Münchberg Massif in the Variscan foldbelt of southern Germany two varieties of eclogite are known which are intercalated with amphibolite-facies meta-igneous and meta-sedimentary rocks: a dark kyanite-free and a lighter colored kyanite-bearing type. Kyanite-free eclogites, which are discussed here, have a major and trace element composition which suggests derivation from ocean-floor basalts with melt to cumulate compositions. Internal Sm-Nd isochrons (clinopyroxene-amphibole-garnet) and one Rb-Sr isochron (clinopyroxene-amphibole-mica) yield eclogitization ages in the range of 380 to 395 Ma. Thus, the age of eclogitization is only marginally higher ( < 15 Ma) than the age of amphibolite-facies metamorphism in the Münchberg Massif as derived from K-Ar ages of amphiboles and micas from metasediments and meta-igneous rocks. A seven point whole-rock Sm-Nd isochron for one eclogite body results in an age of 480 +/- 23 Ma with an initial εNd of 8.7 +/- 0.6 and is likely to record the age of igneous formation of the eclogite protoliths. Sr isotopic compositions back-calculated to that time are anomalously high and variable if compared to Nd isotopes. This can be explained by alteration with an aqueous or fluid phase with high 87Sr/86Sr, most likely seawater, either during igneous formation in an oceanic rift environment or subduction-related eclogitization. In addition, some eclogites show a marked enrichment of incompatible, immobile elements and plot far below the whole-rock Sm-Nd isochron. These features are ascribed to the presence of an evolved crustal component, probably acquired during extrusion of the basaltic protoliths by mixing with country-rock gneisses.
Half of the northern part of the French Massif Central has been shown to be allochthonous. In the allochthonous rocks, preserved high-pressure granulites represent an early tectonometamorphic event which took place before thrusting. During the southward-directed overthrusting of this high-grade metamorphic slab a plurifacial inverted metamorphism was developed in the underlying sediments. Adjacent areas show different types of metamorphism in the autochthonous and parautochthonous pelites. The climax of the metamorphism is shown to have been reached by the end of the thrusting episode. This was followed by the retrogression of the high-grade rocks and the establishment of a low-pressure metamorphism in both the thrust unit and the autochthonous-parautochthonous terranes beneath. We describe this example of inverted metamorphic zonation associated with the large-scale thrusting of an already metamorphosed series.
Two bimodal meta-igneous complexes from the southern Massif Central provide important tectonic constraints on the initial stage of the Palaeozoic orogen. The metabasites may be subdivided into two groups according to the distribution of the Large Ion Lithophile (LIL) and High Field Strength (HFS) elements: N-type MORBs with low LIL/HFS ratios and subduction-type basalts with high LIL/HFS ratios. These associations suggest a complex tectonic setting such as an extensional zone or a back-arc basin above a subduction zone. The great volume of felsic rocks with low incompatible trace element contents indicates partial melting of an underlying continental crust. The association of terrigeneous sediments with N-type MORBs, subduction-type basalts and high-silica peraluminous rhyolites argue against an intra-oceanic setting, and suggest that the area was one of ensialic crustal tension during middle Ordovician. The two complexes are identified as remnants of ensialic back-arc basins rather than relicts of major oceanic sutures. Such marginal basins could have been located in a mainly continental domain, such as the Armorican microplate.
Subaerial plateau basalts, initially c . 1 km thick, overlie Cretaceous or older formations and are locally separated from pre-Tertiary rocks by ( a ) a quartzitic conglomerate and ( b ) a (younger) sequence of tuffaceous sediments and hyaloclastites. The plateau lavas are divisible into a Lower (typically microphyric, quartz tholeiitic) Series and an Upper (typically porphyritic, olivine tholeiitic) Series. N–S and NE–SW trending antithetic faults give rise to ocean-facing fault-line scarps and landward-facing dip slopes. Dolerite sills, abundant beneath the lava pile, occasionally cut the Lower Series lavas. While dykes trend in various directions, a very prominent NE–SW swarm (mainly quartz tholeiitic) traverses Hold with Hope, deflecting to more nearly N–S across Gauss Halvø.
A large volcanic centre (Myggbukta Complex) lies across the main dyke swarm in the vicinity of the deflection and may be genetically related to the swarm. The complex involves propylitized basaltic, rhyolitic and intermediate lavas and pyroclastic rocks, together with volcanogenic sediments, cut by a profusion of basic to acid intrusive sheets. Formation of this complex at a late stage in the volcanic history of the region was attended by subsidence and crustal down-sagging. The main dyke swarm, whose trend roughly parallels that of the spreading axes N and S of the Jan Mayen Fracture Zone, may reflect subordinate rifting to the W of the principal rift zones along which spreading was taking place.
30 km E of Myggbukta, the Kap Broer Ruys granophyre/felsite is a partially unroofed intrusion around which a broad metamorphic aureole has been superimposed on the Mesozoic and Tertiary sediments and lavas. The southern coastal region of Hold with Hope, connecting the Myggbukta and Kap Broer Ruys centres, may indicate a zone of crustal weakness which determined the initiation of the Jan Mayen Fracture Zone.
The E Greenland continental margin differs from typical margins in having a superimposed regional uplift which has exposed the early rift products for direct examination. Most of the igneous rocks in this region find counterparts in the oceanic setting with the proviso that the area is part of an aseismic ridge. Thus, picrites, basalts, sheeted dyke swarms and layered gabbros are present and these can be used to shed light on oceanic magmatic events. Nephelinitic rocks are products of off-axis volcanism and find no counterparts in the present oceanic rift system, although they occur in marginal regions of the ocean basins and were generated in distal regions at the initiation of rifting in Icelandic anomaly. Voluminous salic rocks in the area owe their generation largely to continental crust which has undergone extensive melting. They find no direct counterparts in oceanic areas. The outstanding degree of exposure in this area combined with its transitional nature and wide spectrum of rock types make it a unique laboratory for the elucidation of a variety of problems regarding both oceanic and continental volcanism together with the nature of rifted continental margins in general.
The central European basement is divided into linear domelike zones of crystalline rocks that are separated by sedimentary troughs. Analyses of clasts in conglomerates and of radiometric dates indicate maximum intensities of magmatic and metamorphic processes in the Proterozoic, at the Cambrian-Ordovician boundary, and in the Late Devonian and Carboniferous. The geosynclinal evolution took place in an intracontinental area. There is no indication of the existence of an oceanic crust in central Europe. The orogenic events were caused ultimately by vertical rise of light buoyant basic magma produced by gravitative differentiation in the upper mantle. Synkinematically intruded granite masses, which originated by anatexis of the crust and produced basement uplifts, are surrounded by broad metamorphic aureoles, whereas postkinematic intrusions are characterized by local zones of contact metamorphism. As granitic melts rose, mobile troughs developed in front of the steep-flanked plutons. Shallow-water deposits, disconformities, and converging strata are typical of the sedimentary cover of the plutons. The flanks of the basement uplifts are shown by conglomeratic sediments and by outward directions of sediment transport. The basins are shown by uninterrupted sequences of thick pelagic sediments. Deformation of the troughs was caused by vertical movements connected with granitic intrusion. At the rims of diapiric plutons, gravitative sliding of the sedimentary cover and deep-seated faulting took place, and tholeiitic basalt magmas of continental origin altered during processes of spilitization ascended. Horizontal shortening is only of subordinate importance, mainly in connection with gravity tectonics. The genesis of the central European basement cannot be explained in the sense of "the new global tectonics."
A simple model is developed which explains the occurrence of volcanic continental margins and flood basalts as a consequence of their association with nearby plumes that were active at the time of rifting. In the model, asthenosphere temperatures are increased by 100-150 C over large regions of the earth by heat advected upward in mantle plumes. The amount of partial melt generated by the asthenosphere as it wells up beneath rifts in these hot areas is calculated. Observational constraints from all known examples of volcanic continental margins are reviewed and the model is used to explain these observations.
Nine samples of blueschist facies metabasic rocks and four samples of associated metasedimentary lithologies were analysed for REE contents and Nd isotopic composition. Sampling includes the main rock types exposed on Groix, concentrating on those metabasic rocks with least dispersed RbSr systems. The REE abundance patterns appear not to have suffered significant alteration since emplacement of the protolith magma despite subsequent blueschist facies metamorphism. The data, when integrated into a wider geochemical data-base, allow the identification and characterization of both tholeiitic and alkaline magma-types in the original igneous suite which was differentiated from depleted mantle sources. The basalts probably originated in an oceanic context. Some sediments are predominantly volcaniclastic, in which a continental crustal component may be recognized. Other sediments are closer in composition to continental crust with variable additions of tholeiitic or alkali-basalt debris. The volcanic-sedimentary assemblage on Groix is suggestive of ocean-island magmatic activity associated with a sedimentary sequence derived from a nearby continent.
The Keweenawan Mamainse Point Formation of Ontario consists of a sequence of tholeiitic lavas and transitional basalt dykes, with trace-element characteristics similar to other proto-oceanic rift sequences. Stratigraphic variation of geochemistry allows the subdivision of the sequence into five series, each of which has suffered shallow-level fractional crystallization. Differences in incompatible element ratios and REE-patterns, however, can only be explained by variations in the melting process. REE-modelling suggests that parental magmas may have been derived from a heterogeneous mantle with flat chondritic and LREE-depleted sources. However, the systematic evolution of REE-patterns and other trace-elements are better explained by dynamic partial melting within two subsequent rising mantle-diapirs.
A review of the magmatic associations providing evidence for the existence of extensional regimes during the Palaeozoic evolution of Variscan Europe is presented. The available geochronological data allow recognition of two major rifting events, one in the Early Ordovician and the other in the Middle-Late Devonian. Scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage. It is suggested that the Variscan Belt resulted from a complex, two-stage collision process, through successive suturing (in the Silurian and the Early Carboniferous) of these oceanic domains. The superposition of two distinct opening-closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the Variscan orogen.
In rock analysis laboratories, sample preparation is a serious problem, or even an enormous bottleneck. Because this laboratory is production-oriented, this problem was attacked by automating progressively, different steps in rock analysis for major, minor and trace elements. This effort has been considerably eased by the fact that all sample preparation schemes in this laboratory for the past three decades have been based on an initial lithium borate fusion of rock samples and all analytical methods based on multi-element atomic emission spectrometry, with switch-over from solid analysis by arc/spark excitation to solution analysis by plasma excitation in 1974. The sample preparation steps which have been automated are: weighing of samples and fluxes, lithium borate fusion, dissolution and dilution of fusion products and ion-exchange separation of difficult trace elements such as rare earth elements (REE). During 1985 and 1986, these different unit operations have been assembled together as peripheral units in the form of a workstation, called LabRobStation. A travelling robot is the master of LabRobStation, with all peripheral units at its reach in 10 m2 workspace. As an example of real application, the automated determination of REE, based on more than 8000 samples analysed during 1982 and 1986, is presented.
The Gulf of California is a region of active ensialic basin development and transform faulting Legs 64 and 65 of the Deep Sea Drilling Project have demonstrated the diversity of basement stratigraphy: drilling at Sites 474, 482, 483 and 485 at the mouth of the Gulf recovered massive basaltic flows and occasional pillow lavas and sills, whereas drilling in the central part of the Gulf, in the Guaymas Basin, recovered a sequence of sediments and massive doleritic sills. Very high sedimentation rates (over 1000 m/Ma) are characteristic of the Guaymas Basin and prevent extrusion of the basaltic magma onto the seafloor, thus resulting in at least partial intercalation of oceanic layers 1 and 2. Such a process of ocean crust formation could be locally important during the early stages of ocean and marginal basin development, when sedimentation rates may be very high
Over 200 samples of basalts and dolerites from the Gulf of California have been analysed by X-ray fluorescence and neutron activation techniques. Basalts from the mouth of the Gulf are tholeiitic and contain very low abundances of the more-hygromagmatophile (more-HYG) elements (K, Rb, Ba, Th, Ta, Nb and light REE) relative to the HYG elements (Y, Zr, Ti and heavy REE), thus resembling depleted N-type MORB from elsewhere along the E Pacific Rise (EPR). Dolerites from the Guaymas Basin also have a tholeiitic chemistry and some EPR-basalt characteristics (e.g. La N ,/Sm N , < l), but they have higher La/Yb, Sr/Zr, Zr/Ti and Th/Hf ratios than the basalts from the Gulf mouth region. It is suggested that these differences are due in part to variations in mantle source composition, the mantle underlying the central and northern parts of the Gulf containing a minor sub-continental (residual calc-alkaline) component.
The Thorsmörk ignimbrite, southern Iceland, contains a suite of granophyre xenoliths displaying magmatic or high-temperature sub-solidus mineral assemblages. These granophyres are consanguineous with the erupting comenditic magma. Four types of mineral assemblages are distinguished: 1.(A) oligoclase, edenitic hornblende, salitic pyroxene, magnesian biotite, magnetite and sphene;2.(B) oligoclase, manganoan to sodic ferro-augite, fayalite, richterite, ilmenite and magnetite;3.(C) anorthoclase, ferrohedenbergite to aegirine hedenbergite, ilmenite, magnetite and (riebeckite);4.(D) cryptoperthite, aegirine hedenbergite to (aegirine), aenigmatite, arfvedsonite, ilmenite and magnetite.Geothermometry shows that the xenoliths have crystallized between 900°C and 500°C at moderate oxygen fugacities, just above the FMQ buffer. It is further demonstrated that a hot vapour phase heavily charged with sodium and halogens, played a major role in the late sub-solidus crystallization of the different types.
The ThHfTa diagram, recently proposed [1] as a means of discriminating basalts erupted in various tectonic environments and of detecting crustal contamination in such magmas, is shown to be unsatisfactory for both of its stated purposes. Data are presented for Th, Hf, Ta, Sr and Sr- and Pb-isotopes in basic lavas and a dolerite sill from the ensialic British Tertiary Volcanic Province (BTVP). Taken in conjunction with published results, the isotopic ratios show that some of the BTVP basic magmas are essentially uncontaminated by continental crust, except for the selective introduction of small variable amounts of unradiogenic Pb. Those BTVP magmas which show appreciable isotopic contamination have interacted with either upper or lower crust, or both. Th/(Hf + Ta) is insensitive to contamination with the Th-poor lower crust of cratons but rises during gross contamination with Th-rich upper crust. BTVP basic magmas containing negligible to moderate crustal isotopic components plot in the field of the Th-Hf-Ta diagram occupied by “normal” mid-ocean ridge basalts. Other volcanic provinces of known tectonic setting which plot wholly or partially outside their appropriate fields on the ThHfTa diagram are: the Hawaii-Emperor Seamount Chain, the Snake River Plain (Idaho, U.S.A.), the Azores and the Gregory Rift (Kenya).