Article

Zircon U-Pb geochronology of paragneisses and biotite granites from the SW Iberian Massif (Portugal): Evidence for apalaeogeographical link between the Ossa-Morena Ediacaran basins and the West African craton

May 2008
Geological Society London Special Publications 297(1):385-408

May 2008
297(1):385-408

DOI:10.1144/SP297.18

Authors:

Manuel Francisco Pereira

Escola de Ciências e Tecnologia, Universidade de Évora

Martim Chichorro

New University of Lisbon. Faculdade de Ciências e Tecnologia

José Brandão Silva

University of Lisbon

Sensitive high-resolution ion microprobe U-Th-Pb age determinations on detrital and inherited zircon from the Évora Massif (SW Iberian Massif, Portugal) provide direct evidence for the provenance of the Ossa-Morena Ediacaran basins (Série Negra) and a palaeogeographical link with the West African craton. Three samples of the Série Negra paragneisses contain large components of Cryogenian and Ediacaran (c. 700-540 Ma) detrital zircon, but have a marked lack of zircon of Mesoproterozoic (c. 1.8-0.9 Ga) age. Older inherited zircons are of Palaeoproterozoic (c. 2.4-1.8 Ga) and Archaean (c. 3.5-2.5 Ga) age. The same age pattern is also found in the Arraiolos biotite granite, which was formed by partial melting of the Série Negra and overlying Cambrian rocks. These results are consistent with substantial denudation of a continental region that supplied sediments to the Ediacaran Ossa-Morena basins during the final stages of the Cadomian-Avalonian orogeny (peri-Gondwanan margin with principal zircon-forming events at c. 575 Ma and c. 615 Ma). Combined with the detrital zircon ages reported for rocks of the same age from Portugal, Spain, Germany and Algeria, our data suggest that the sediment supply to the Ediacaran-Early Palaeozoic siliciclastic sequences preserved in all these peri-Gondwanan regions was similar. The lack of Grenvillian-aged (c. 1.1-0.9 Ga) zircon in the Ossa-Morena and Saxo-Thuringia Ediacaran sediments suggests that the sediment in these peri-Gondwanan basins was derived from the West African craton.

Cadomian/Pan-African consolidation of the Iberian Massif assessed by its detrital and inherited zircon populations: is the ~610Ma age peak a persistent Cadomian magmatic inheritance or the key to unravel its Pan-African basement?

Article

Full-text available

Nov 2022
GEOL ACTA

This work assessed the age distribution of Cadomian/Pan-African orogenic events (550-590 and 605-790Ma, respectively) in several zones of Iberian Massif by means of detrital and inherited zircon analysis compilation. Detrital zircon age spectra show that throughout the late Neoproterozoic-to-Early Ordovician era (~120Ma sedimentary record), the main systematic peak occurs at ~610Ma, followed by peaks at typical Cadomian ages (~590-550Ma). Inherited zircons incorporated in Cambrian-to-Lower Ordovician igneous rocks show typical Cadomian ages (~590-550Ma) but, once again, a remarkably consistent Pan-African ~610Ma peak occurs. In accordance with compiled zircon data and taking into account the evidence of North African peri-cratonic inliers, Ediacaran (~610Ma) zircons incorporated in Paleozoic magmas provide indirect evidence of Pan-African magmatism, suggesting that these magmas and synorogenic sediments are likely to constitute the cryptic stratigraphic infrastructure of most of the Iberian Massif. The main source of ~610Ma inherited zircons may be the lateral chrono-equivalents of the Saghro and Bou Salda-M`Gouna Groups (Anti-Atlas, Morocco) and/or coeval igneous rocks from West African Craton or Trans-Sahara Belt, emplaced at a stratigraphic level below the late Ediacaran sediments of the Ossa Morena Zone and the Central Iberian Zone. Assuming that the Iberian crust is a fragment of the Pan-African orogen, a relative paleoposition situated between the West African Craton and the Trans-Saharan Belt during the Late Neoproterozoic is proposed. The closed-system behaviour of Stenian-Tonian detrital zircon ages in the Trans-Sahara Belt suggests that this mega-cordillera acted as a barrier, in paleogeographic terms, to separating the Sahara Metacraton from Iberia. In Iberia, the opening of the system to Stenian-Tonian detrital zircon during the Ordovician indicates that, at that time, the Trans-Saharan Belt had already become a vast peneplain, which favoured a large drainage system with a long-distance transport mechanism that fed the passive continental margins.

Tracking the Late Devonian high-P metamorphic belt in the Variscan Orogen: New constraints on the PT evolution of eclogites from the Cubito-Moura Unit (SW Iberian Massif)

Article

Feb 2021
LITHOS

The Cubito-Moura Unit is a high-P metamorphic succession that occurs in the southern part of the Ossa-Morena Complex (SW Iberian Massif). It includes a series of metasedimentary, metafelsic and metamafic rocks affected by a high-P, low to intermediate-T metamorphic event during Late Devonian times. Geochemistry of the metabasic rocks reveals that the generation of the protoliths occurred in a supra-subduction zone setting during Late Ediacaran to Early Ordovician times, either in a back-arc or fore-arc context. The eclogites contain atoll-like garnets, omphacite, amphibole, phengite (up to Si = 3.38 apfu), paragonite, rutile and quartz. Thermodynamic modelling in the MnNCKFMASTH system indicates a high-P metamorphic event at ~24 kbar and ~ 585 °C followed by a thermal peak at ~19 kbar and ~ 630 °C, and a subsequent exhumation to ~15 kbar. This P-T path indicates deep subduction at c. 370 Ma of this arc-related section. The lithostratigraphy and tectonothermal evolution of the Cubito-Moura Unit are equivalent to that of a number of units along the Iberian, Armorican and Bohemian massifs that can be correlated as a part of the same Basal Allochthonous Terrane. These units define a single Late Devonian high-P, low- to intermediate-T metamorphic belt developed during the first stages of the Variscan Orogeny.

Detrital zircon provenance of Triassic sandstone of the Algarve Basin (SW Iberia): evidence of Gondwanan- and Laurussian-type sources of sediment

Article

Full-text available

May 2020

Detrital zircon populations from six samples of upper Triassic sandstone (Algarve Basin) were analysed, yielding mostly Precambrian ages. zircon age populations of the Triassic sandstone sampled from the western and central sectors of the basin are distinct, suggesting local recycling and/or lateral changes in their sources. Our findings and the available detrital zircon ages from the Palaeozoic terranes of SW Iberia, Nova Scotia and NW Morocco were jointly examined using the Kolmogorov–Smirnov test and multidimensional scaling diagrams. The obtained results enable direct discrimination of competing Laurussian-type and Gondwanan-type sediment sources, involving recycling and mixing relationships. The detrital zircon populations of the Algarve Triassic sandstone are very different from those of the lower–upper Carboniferous Mértola and Mira formations (South Portuguese Zone), upper Devonian – lower Carboniferous Horta da Torre, Represa and Santa Iria formations (Pulo do Lobo Zone), and the late Carboniferous Santa Susana and early Permian Viar basins, which are ruled out as potential sources. The detrital zircon populations of Triassic sandstone from the central sector and those from the Ossa–Morena Zone Ediacaran–Cambrian siliciclastic rocks, upper Devonian – Carboniferous Ronquillo, Tercenas, Phyllite-Quartzite and Brejeira formations (South Portuguese Zone), and Frasnian siliciclastic rocks of the Pulo do Lobo Zone are not statistically distinguishable. Thus, sedimentation in the central sector was influenced by Gondwanan- and Laurussian-type putative sources exposed in SW Iberia, in contrast to the western sector, where Meguma Terrane and Sehoul Block Cambrian siliciclastic rocks allegedly constituted the main (Laurussian-type) sources. These findings provide insights into the denudation of distinctive source terranes distributed along the late Palaeozoic suture zone that juxtaposed the Laurussian and Gondwanan margins.

The Cambrian-Early Ordovician Rift Stage in the Gondwanan Units of the Iberian Massif

Chapter

Aug 2019

A rifting stage initiated the Variscan cycle in NW Gondwana, lasted from Terreneuvian to Early Ordovician times and culminated in opening of the Rheic Ocean. The result of lithospheric stretching was the development of a horst-and-graben structure in the upper crust and formation of basins with sharp variations in thickness and facies of the sedimentary infill. Emplacement of large volumes of igneous rocks, both plutonic and volcanic, accompanied this stage in three different intervals: (i) Early Igneous Event (Terreneuvian), exclusively composed of felsic peraluminous rocks associated with the formation of core complexes in the mid-upper crust; (ii) Main Igneous Event (Cambrian Series 2 to Furongian), displaying bimodal character; and iii) Late Event (Tremadocian-Floian), with mixed characteristics of the other two events and abundant peralkaline rocks. The rifting axis was initially located close to the Cadomian suture that fringed the Ossa Morena Zone. For about 60 m.y. the rifting processes initially propagated “zip-like” along the axis and then widened cratonward to affect inner parts of Gondwana, such as the Central Iberian Zone. The rift/drift transition was diachronous, starting in Iberia (Ossa Morena Zone) in the Furongian.

Recycling of the Proterozoic crystalline basement in the Coastal Block (Moroccan Meseta): New insights for understanding the geodynamic evolution of the northern peri-Gondwanan realm

Article

Full-text available

Mar 2018
PRECAMBRIAN RES

Recycling of the Proterozoic crystalline basement in the Coastal Block (Moroccan Meseta): New insights for understanding the geodynamic evolution of the northern peri-Gondwanan realm

Article

Mar 2018
PRECAMBRIAN RES

Detrital zircon age spectra from the siliciclastic rocks of the Lalla Mouchaa Calcschists and El Jadida Dolomitic formations (the Coastal Block of the Moroccan Meseta) are dominated by Paleoproterozoic and Ediacaran ages. The provenance of these two formations is a composite Proterozoic crystalline basement. El Jadida rhyolite (584.2 ± 4.8 Ma) represents the Ediacaran crystalline basement of the El Jadida dome. El Jadida rhyolite is unconformably overlain by the microbreccia, arkosic sandstone and dolostone of the El Jadida Dolomitic Formation with a maximum depositional age of ca. 539 Ma (Lower Cambrian). Detrital zircon-age spectra from El Jadida Dolomitic Formation (ca. 583-582 Ma) suggest direct recycling of El Jadida rhyolite as an exclusive original primary source. However, in the Western Rehamna massif, detrital zircon-age spectra from pre-Middle Cambrian microbreccia and arkosic sandstone of the Lalla Mouchaa Calcschists Formation (ca. 2.05-2.03 Ga) indicate exclusive recycling of the ca. 2.05 Ga-aged crystalline basement rocks (original primary source). Detrital zircon contents of the siliciciclastic rocks from these two formations of the Coastal Block are consistent with derivation from either Eburnian (Paleoproterozoic) or Cadomian/Pan-African (Ediacaran) igneous rocks. The discovery of this composite Proterozoic crystalline basement in the Moroccan Meseta stresses that Cadomian/Pan-African magmatic arcs were built on an Eburnian basement in a paleoposition close to the West African craton, as part of the northern peri-Gondwanan realm.

Birth and demise of the Rheic Ocean magmatic arc(s): Combined U–Pb and Hf isotope analyses in detrital zircon from SW Iberia siliciclastic strata

Article

May 2017
LITHOS

Paleozoic continental reconstructions indicate that subduction of Rheic oceanic lithosphere led to collision between Laurussia and Gondwana which was a major event in the formation of the Ouachita-Appalachian-Variscan orogenic belt and the amalgamation of Pangea. However, arc systems which record that subduction are poorly preserved. The preservation of Devonian detrital zircon in late Devonian-early Carboniferous siliciclastic rocks of SW Iberia, rather than arc-related igneous rocks indicates that direct evidence of the arc system may have been largely destroyed by erosion. Here we report in-situ detrital zircon U–Pb isotopic analyses of late Devonian-early Carboniferous siliciclastic rocks from the Pulo do Lobo Zone, which is a reworked Late Paleozoic suture zone located between Laurussia and Gondwana. Detrital zircon age spectra from the Pulo do Lobo Zone Frasnian formations show striking similarities, revealing a wide range of ages dominated by Neoproterozoic and Paleoproterozoic grains sourced from rocks typical of peri-Gondwanan terranes, such as Avalonia, the Meguma terrane and the Ossa-Morena Zone. Pulo do Lobo rocks also include representative populations of Mesoproterozoic and early Silurian zircons that are typical of Avalonia and the Meguma terrane which are absent in the Ossa-Morena Zone. The Famennian-Tournaisian formations from the Pulo do Lobo Zone, however, contain more abundant middle-late Devonian zircon indicating the contribution from a previously unrecognized source probably related to the Rheic Ocean magmatic arc(s). The middle-late Devonian to early Carboniferous zircon ages from the siliciclastic rocks of SW Iberia (South Portuguese, Pulo do Lobo and Ossa-Morena zones) have a wide range in εHfT values (− 8.2 to + 8.3) indicating the likely crystallization from magmas formed in a convergent setting. The missing Rheic Ocean arc was probably built on a Meguma/Avalonia type basement. We propose for the Pulo do Lobo Zone that the Frasnian sedimentation occurred through the opening of a back-arc basin formed along the Laurussian active margin during Rheic Ocean subduction, as has been recently proposed for the Rhenohercynian Zone in Central Europe. Detrital zircon ages in the Frasnian siliciclastic rocks indicate provenance in the Meguma terrane, Avalonia and Devonian Rheic Ocean arc(s). As result of back-arc basin inversion, the Frasnian formations underwent deformation, metamorphism and denudation and were unconformably overlain by Famennian to Visean siliciclastic strata (including the Phyllite-Quartzite Formation of the South Portuguese Zone). The latest Devonian-early Carboniferous detritus were probably shed to the Pulo do Lobo Zone (Represa and Santa Iria formations) by recycling of Devonian siliciclastic rocks, from the South Portuguese Zone (Meguma terrane) and from a new distinct source with Baltica /Laurentia derivation (preserved in the Horta da Torre Formation and Alajar Mélange).

Geochemistry and tectonostratigraphy of the basal allochthonous units of SW Iberia (Évora Massif, Portugal): Keys to the reconstruction of pre-Pangean paleogeography in southern Europe

Article

Jan 2017
LITHOS

The basal allochthonous units of NW and SW Iberia are members of an intra-Gondwana suture zone that spreads across the Iberian Massif and was formed during the collision of Gondwana and Laurussia in the late Paleozoic. This suture zone is made of allochthonous terranes and is currently preserved as a tectonically dismembered ensemble. A multi-proxy analysis is applied to the basal allochthonous units of Iberia to test their affinity and potential usage for tracing a suture zone. A comparison of the lithostratigraphy, tectonometamorphic evolution, geochronology, and geochemical characteristics of the Ediacaran series of these units reveals striking affinities. They derive from rather similar immature sedimentary successions, deposited along the same continental margin, and in relation to a Cadomian magmatic arc. Sm-Nd systematics indicates that the isotopic sources are among the oldest of the Iberian Massif (ca. 2.15–1.5 Ga), suggesting a very strong contribution from the West African Craton. These Ediacaran series were affected by high-P and low- to medium-T metamorphism (blueschist to eclogite facies) during the Late Devonian (ca. 370 Ma). They occur below allochthonous ophiolitic sequences, and on top of autochthonous or parautochthonous domains lacking of high-P and low- to medium-T Devonian metamorphism, i.e., tectonically sandwiched between lithosphere-scale thrusts. The combination of all these characteristics makes these particular Ediacaran series different from the rest of the terranes of the Iberian Massif. Such singularity could be useful for tracing more occurrences of the same suture zone along the Variscan orogen, particularly in cases where its preservation and recognition may be cryptic. It also contributes to improve the paleogeographic reconstruction of the margin of Gondwana during the Ediacaran.

Provenance of upper Triassic sandstone, southwest Iberia (Alentejo and Algarve basins): tracing variability in the sources

Article

Jan 2016

Laser ablation ICP-MS U–Pb analyses have been conducted on detrital zircon of Upper Triassic sandstone from the Alentejo and Algarve basins in southwest Iberia. The predominance of Neoproterozoic, Devonian, Paleoproterozoic and Carboniferous detrital zircon ages confirms previous studies that indicate the locus of the sediment source of the late Triassic Alentejo Basin in the pre-Mesozoic basement of the South Portuguese and Ossa-Morena zones. Suitable sources for the Upper Triassic Algarve sandstone are the Upper Devonian–Lower Carboniferous of the South Portuguese Zone (Phyllite–Quartzite and Tercenas formations) and the Meguma Terrane (present-day in Nova Scotia). Spatial variations of the sediment sources of both Upper Triassic basins suggest a more complex history of drainage than previously documented involving other source rocks located outside present-day Iberia. The two Triassic basins were isolated from each other with the detrital transport being controlled by two independent drainage systems. This study is important for the reconstruction of the late Triassic paleogeography in a place where, later, the opening of the Central Atlantic Ocean took place separating Europe from North America.

The significance of changes in the source areas during the Carboniferous turbiditic deposition (SW Iberia)

Article

Full-text available

Jan 2014

U–Pb dating of detrital zircons from the Carboniferous turbidites of southwestern Iberia (the Cabrela, Mértola, Mira, and Brejeira formations) shows that synorogenic sedimentation during the Carboniferous was marked by significant variations in the source areas, involving the denudation of different crustal blocks and a break in synorogenic volcanism. The Visean is characterized by the accumulation of immature turbidites (the Cabrela and Mértola formations and the base of the Mira Formation). These turbidites were probably formed in relation to sources (magmatic arcs) of Mid–Late Devonian age poorly influenced by sedimentary recycling, as indicated by the near-absence of pre-Devonian zircons, ages that are typical of the Gondwana basement. The presence of Carboniferous grains in Visean turbidites indicates that volcanism was active at this time. Later, Serpukhovian to Moscovian turbiditic sedimentation (the Mira and Brejeira formations) included sedimentary detritus derived from mature felsic source rocks situated far from active magmatism. The abundance of Proterozoic and Palaeozoic zircons reveals strong recycling of the pre-Carboniferous basement. A peri-Gondwanan provenance is indicated by zircon populations with Neoproterozoic (the Cadomian–Avalonian and Pan-African zircon-forming events), Palaeoproterozoic, and Archean (the West African Craton zircon-forming events) ages. The presence of Late Ordovician and Silurian detrital zircons in the Brejeira turbidites, which do not correspond to the Gondwana basement of southwestern Iberia, indicates an external source (Laurussia?).

Evidence for multi-cycle sedimentation and provenance constraints from detrital zircon U–Pb ages: Triassic strata of the Lusitanian basin (western Iberia)

Article

Full-text available

Jun 2016
TECTONOPHYSICS

Laser ablation ICP-MS U-Pb analyses were conducted on detrital zircons of Triassic sandstone and conglomerate from the Lusitanian basin in order to: i) document the age spectra of detrital zircon; ii) compare U-Pb detrital zircon ages with previous published data obtained from Upper Carboniferous, Ordovician, Cambrian and Ediacaran sedimentary rocks of the pre-Mesozoic basement of western Iberia; iii) discuss potential sources; and iv) test the hypothesis of sedimentary recycling. U-Pb dating of zircons established a maximum depositional age for this deposit as Permian (ca. 296. Ma), which is about sixty million years older compared to the fossil content recognized in previous studies (Upper Triassic). The distribution of detrital zircon ages obtained points to common source areas: the Ossa-Morena and Central Iberian zones that outcrop in and close to the Porto-Tomar fault zone. The high degree of immaturity and evidence of little transport of the Triassic sediment suggests that granite may constitute primary crystalline sources. The Carboniferous age of ca. 330. Ma for the best estimate of crystallization for a granite pebble in a Triassic conglomerate and the Permian-Carboniferous ages (<. ca. 315. Ma) found in detrital zircons provide evidence of the denudation of Variscan and Cimmerian granites during the infilling of continental rift basins in western Iberia. The zircon age spectra found in Triassic strata are also the result of recycling from the Upper Carboniferous Buçaco basin, which probably acted as an intermediate sediment repository. U-Pb data in this study suggest that the detritus from the Triassic sandstone and conglomerate of the Lusitanian basin is derived from local source areas with features typical of Gondwana, with no sediment from external sources from Laurussia or southwestern Iberia.

Changing Carboniferous Arc Magmatism in the Ossa-Morena Zone (Southwest Iberia): Implications for the Variscan Belt

Article

Full-text available

May 2022

Carboniferous magmatism in southwestern Iberia was continuously active for more than 60 m.y. during the development of the Appalachian-Variscan belt of North America, North Africa and Western-Central Europe. This collisional orogen that records the closure of the Rheic Ocean is essential to understanding the late Paleozoic amalgamation of the Pangea supercontinent. However, the oblique convergence between Laurussia and Gondwana that lasted from the Devonian to the Carboniferous was likely more complex. Recently, a new tectonic model has regarded the Iberia Variscan belt as the site of coeval collisional and accretionary orogenic processes. Early Carboniferous plutonic rocks of southwest Iberia indicate arc magmatism in Gondwana. The Ossa-Morena Zone (OMZ) acted as the upper plate in relation to the geometry of the Paleotethys subduction. This active accretionary-extensional margin was progressively involved in a collisional phase during the Late Carboniferous. Together, the Évora Massif and the Beja Igneous Complex include three successive stages of bimodal magmatism, with a chemical composition indicative of a long-lived subduction process lasting from the Tournaisian to the Moscovian in the OMZ. The earliest stage of arc magmatism includes the Tournaisian Beja and Torrão gabbro-dioritic rocks of the Layered Gabbroic Sequence. We present new geochemical and Nd isotopic and U-Pb geochronological data for magmatic rocks from the Main (Visean-Serpukhovian) and Latest (Bashkirian-Moscovian) stages of arc magmatism. Visean Toca da Moura trachyandesite and rhyolites and Bashkirian Baleizão porphyries and Alcáçovas quartz diorite share enriched, continental-crust like characteristics, as indicated by major and trace elements, mainly suggesting the addition of calc-alkaline magma extracted from various mantle sources in a subduction-related setting (i.e., Paleotethys subduction). New U-Pb zircon geochronology data have allowed us to establish a crystallization age of 317 ± 3 Ma (Bashkirian) for Alcáçovas quartz diorite that confirms a temporal link with Baleizão porphyry. Positive εNd(t) values for the Carboniferous igneous rocks of the Beja Igneous Complex and the Évora gneiss dome indicate production of new juvenile crust, whereas negative εNd(t) values also suggest different grades of magma evolution involving crustal contamination. The production and evolution of Carboniferous continental crust in the OMZ was most likely associated with the development of an active continental margin during the convergence of the Paleotethys Ocean with Gondwana, involving juvenile materials and different grades of crustal contamination.

Detrital zircon ages and provenance of a Cambrian succession in the Sierra Albarrana Domain (SW Iberian Massif)

Article

Nov 2021
LITHOS

This study presents the first U-Pb geochronological data on detrital zircon grains from the metasedimentary successions of the Sierra Albarrana Domain (SW Iberian Massif) obtained by SHRIMP and LA-ICP-MS. We provide new information on the Maximum Depositional Ages (MDA) during the middle Cambrian through a systematic study, together with age comparisons of the successions, using Kernel Density Estimates (KDE) diagrams, Cumulative Age Distributions (CADs) and the Kolmogorov–Smirnov (K-S) test. On the other hand, the U-Pb zircon data presented in this study have been compared with all the existing data from detrital zircon grains in Neoproterozoic and Cambrian rocks of the Iberian Massif (20 samples, 2706 U–Pb zircon ages). For the comparison, in addition to the KDEs, CADs and K-S test, we apply 3D multidimensional scaling techniques (3D-MDS). The results, together with the geochemical and isotopic characterisation from previous studies, suggest that this domain is likely to be part of the autochthonous section of the Central Iberian Zone (CIZ). Zircon age patterns indicate a probable sedimentary provenance from the Saharan Metacraton with, or without, minor input from the Tuareg Shield. The most important local detrital source corresponds to Cadomian magmatism developed during a magmatic event (535–515 Ma) followed by back-arc extension and early Paleozoic rifting (535–460 Ma). The remaining Mesoproterozoic, Paleoproterozoic and Archean zircon grains would have been provided by the Paleoproterozoic basement and/or the older continental crust recycled in the western sections of the Gondwana margin.

Isotope geochemistry evidence for Laurussian-type sources of South Portuguese Zone Carboniferous turbidites (Variscan Orogeny)

Article

Apr 2020

New Rb–Sr isotopic data from South Portuguese Zone (SPZ) turbidites show that the 87Sr/86Sr(t) ratio increases from the basal Mértola Formation (Visean–Serpukhovian: 0.706–0.707), through the Mira Formation (Serpukhovian–Bashkirian: 0.706–0.712) to the uppermost Brejeira Formation (Bashkirian–Moscovian: 0.713–0.715). In addition, estimated Nd TDM model ages for the Mértola (1.29–1.09 Ga), Mira (1.58–1.1 Ga) and Brejeira (1.73–1.37 Ga) formations indicate inverted stratigraphy for their isotopic sources. The isotope geochemical data indicate significant changes in the sources from which the SPZ Carboniferous turbidites are derived, consistent with the progressive denudation of a continental magmatic arc built on the Laurussian margin. Mértola turbidites inherited their geochemical and isotopic characteristics from an adjacent dissected Middle–Late Devonian continental magmatic arc with an intermediate–felsic composition: that is a Laurussian (Rheic magmatic arc)-type source. The progressive erosion of its plutonic roots and older host continental basement rocks are indicated in the Mira and Brejeira formations by the increasing contribution of recycled ancient continental crust. The pronounced similarity between the Nd TDM model ages and the detrital zircon populations of the Mira and Brejeira formations (SW Iberia) suggest that they share a common Laurussian (West Avalonia/Meguma terrane)-type source but a contribution from Gondwanan (Ossa-Morena)-type sources cannot be discarded.

Identifying late Neoproterozoic–early Paleozoic sediments in the South Qilian Belt, China: A peri-Gondwana connection in the northern Tibetan Plateau

Article

Jul 2019
GONDWANA RES

The provenance and maximum depositional age of Neoproterozoic to early Paleozoic sedimentary rocks from the Balonggonggaer Formation (BF) in the South Qilian belt (northern Tibetan Plateau) is established using LA-ICP-MS UPb age determinations on detrital zircons taken from fifteen metasedimentary samples. The BF comprises two tectonically juxtaposed metasedimentary sequences that were derived from different source regions. Unit A is characterized by turbiditic facies, thick greywackes, and has zircons ages older than 0.7 Ga and is dominated by 2.2–1.8 Ga and 0.8–0.7 Ga populations that are compatible with a source region within the Quanji massif. Unit A might be deposited after the mid-Neoproterozoic and represent passive margin deposits that developed along the Quanji massif margin during Neoproterozoic continental break-up. Unit B is a highly deformed and metamorphosed sedimentary sequence, showing a distinct provenance dominated by age peaks at about 0.56–0.68 Ga, 1.2–0.9 Ga and 1.60 Ga. These populations bear resemblance to those found in peri-Gondwana terranes. These results favor the placement of Unit B alongside northern peri-Gondwanan terranes. During the early Cambrian, the Qilian-Qaidam basement accreted to the northern margin of Gondwana along the Proto-Tethys. These two distinct sequences of the BF were juxtaposed along the northern margin of Gondwana during the Ordovician to middle Devonian.

Age and characteristics of the Loma del Aire unit (SW Iberia): Implications for the regional correlation of the Ossa-Morena Zone

Article

Mar 2016
TECTONOPHYSICS

Finding of ca. 620 Ma old zircons in a volcanic rock of the Loma del Aire unit, one of many structural divisions in the Ossa Morena Zone, some years ago, attracted much attention to this unit, which would contain, if proven, the oldest rocks so far dated not only in Ossa Morena but also in the entire Iberian Massif. In this paper, new field, petrographic, whole-rock geochemical and Sm-Nd isotope data as well as new TIMS U-Pb zircon ages are presented, which collectively allow a much better characterization and dating as Cambrian of the rock sequence in the Loma del Aire unit; the previously found ca. 620 Ma old zircon population is now interpreted as xenocrysts incorporated in the Cambrian magma. Our data together with recently published provenance studies, bear important implications concerning the nature and evolution of the underlying lithospheric basement of the Ossa Morena Zone, which shares many characteristics with the Neoproterozoic peri-West-African craton arc systems currently exposed in the Precambrian inliers of the Moroccan Anti-Atlas, the North Armorican Zone of Brittany, Normandy and the Channel Islands (Cadomian Arc), and the Saxo-Thuringian and Teplá-Barrandian Zones of the Bohemian Massif. A correlation of the Ossa Morena zone with these arc systems in the Neoproterozoic and Early Paleozoic is thus proposed.

Discussion on Detrital zircon geochronology of the Carboniferous Baixo Alentejo Flysch Group (South Portugal); constraints on the provenance and geodynamic evolution of the South Portuguese Zone, Journal , Vol. 172, 294–308

Article

Oct 2015

Manuel Francisco Pereira

Multidisciplinary re-assessment of the Ediacaran–Cambrian boundary interval in south-western Europe

Article

Full-text available

May 2024
NEWSL STRATIGR

Provenance and source-to-sink of lower-middle Jurassic sediments from hinterland mounts to NW-Gondwana hyper-extended passive margin (Prerif sub-domain, External Rif, Morocco): First evidence from sedimentary petrology and detrital Zircon geochronology

Article

Sep 2023
MAR PETROL GEOL

Visean high-K mafic-intermediate plutonic rocks of the Ossa-Morena Zone (SW Iberia): implications for regional extensional tectonics

Article

Oct 2022

Field relationships and new U-Pb geochronology data indicate a temporal link among the diverse high-K mafic-intermediate magmas of the Ossa-Morena Zone (OMZ). Ages of ca. 338-335 Ma for the Vale de Maceiras gabbro and the Campo Maior microdiorite and quartz-diorite indicate that plutonism took place during Variscan extensional D2 deformation event in the OMZ. The syn-tectonic nature of the Vale de Maceiras pluton is attested by the orientation of intrusive contacts, magmatic foliation, and growth of contact metamorphic minerals in relation to Variscan extensional D2 foliation. The Campo Maior microdiorite, quartz-diorite, and orthomigmatites are temporally linked to high-temperature mylonitic gneisses formed simultaneously with the Variscan extensional D2 deformation event. The geochemical features of the Vale de Maceiras and Campo Maior mafic-intermediate rocks show an affinity with the sanukitoid series. This finding suggests that the observed geochemical variability, from tholeiitic to calc-alkaline and sanukitoid, in the Visean OMZ plutonic rocks (ca. 349-335 Ma) may have been inherited from partially melted mantle domains that were previously contaminated by crustal melts during subduction. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6243822

Late Jurassic syn-rift deposition in the Flemish Pass basin, offshore Newfoundland: Evidence for Tithonian magmatism and Appalachian-Variscan sediment sources from quantitative mineral and detrital zircon U–Pb-Hf isotope studies of Mizzen discovery strata

Article

Oct 2022
MAR PETROL GEOL

Quantitative mineral and detrital zircon U–Pb-Hf isotope studies of syn-rift sandstone units in the Mizzen F-09 delineation well, northern Flemish Pass basin, were conducted to test competing models for Grand Banks paleogeography and Late Jurassic development of the Newfoundland-Iberia magma-poor rift system. Bodhrán formation (informal nomenclature) strata yield ca. 145–155 Ma detrital zircon grain populations that indicate a late Tithonian (146 ± 1 Ma) maximum depositional age for syn-rift fluvial to tide-modulated fluvial strata in Mizzen F-09 and evidence for Grand Banks rift magmatism to have occurred throughout the Late Jurassic. Late Tithonian tectonic subsidence and magmatism in the northern Flemish Pass basin region were coincident with the onset of necking processes and extreme crustal thinning within the Newfoundland-Iberia rift system. The chondritic to superchondritic Hf isotope compositions of Late Jurassic detrital zircon grains indicate repeated partial melting of lithospheric mantle during rift development, perhaps along master detachment faults or transfer zone systems. Bodhrán formation strata have chemically unstable minerals (plagioclase, potassium feldspar, garnet, muscovite, rutile, staurolite) and late Paleozoic to Paleoarchean detrital zircon age populations that together show provenance from Appalachian-Variscan basement and cover assemblages. A positive correlation between the abundance of metamorphic minerals and unique-aged, mid-Permian (ca. 271–280 Ma) detrital zircon grains from post-Variscan igneous rocks indicate Iberian provenance contributions to Bodhrán formation strata. The new results from Mizzen F-09 are consistent with models for WNW-directed fluvial discharge into the northern Flemish Pass basin and sediment derivation from upper Paleozoic foreland basin or cover assemblages located west of Variscan front, some which may have been stripped off Beothuk Knoll, Flemish Cap, or adjacent highlands.

New insights on the Escoural Orogenic gold district (Ossa-Morena Zone, SW Iberia): Geochemistry, fluid inclusions and stable isotope constraints from the Monfurado gold prospect

Article

Full-text available

Mar 2022
ORE GEOL REV

The Escoural gold district belongs to the Montemor-Ficalho metallogenic belt which is part of the Portuguese section of Ossa-Morena Zone (OMZ), at the SW of Iberia. The Escoural gold district includes twelve gold prospects and/or deposits largely controlled by the NW-SE Montemor-o-Novo Shear Zone (MNSZ) and associated fault zones, extending for approximately 30 km. Ubiquitously, gold-arsenopyrite-loellingite assemblages hosted in quartz-sericite-chlorite veins are found in most deposits, although, in the Monfurado prospect, the gold-bearing assemblages are more complex. This prospect is located in the vicinity of a Cambrian SEDEX-VMS iron deposit, from which massive and disseminated iron-ores hosted in marbles and calcsilicate rocks, were exploited. The interplay of the gold mineralizing processes with the iron-rich host rocks has favored gold deposition at the Monfurado prospect. Selected samples from six drill cores allowed to define two mineralizing events: the pre-ore and ore stages. Two gold mineralization styles characterize the ore-stage: i) massive sulfide horizons in which gold (Au= 85.6 - 86.3 wt. %; Ag= 13.1 - 13.6 wt.%) is hosted in arsenopyrite and pyrite or, seldomly, gold particles (Au= 91.8 wt.%; Ag= 7.1 wt.%) found in an arsenopyrite-rich layer; and ii) quartz-chlorite-pyrite veins crosscutting acid metavolcanic rocks with rhyolite-rhyodacite affinities, in which gold (Au= 80.5 - 82.9 wt.%; Ag= 16.8 - 18.7 wt.%) is found as fracture filling in pyrite, sometimes accompanied by Bi-Te phases. Arsenopyrite geothermometer suggests that for type i the overall deposition temperature falls within the range of 188°C to 372°C. Type ii mineralization lacks arsenopyrite, and for this reason, thermodynamic constraints were gathered from fluid inclusions and chlorite geothermometer. CH4-rich fluid inclusions are ubiquitous in transgranular fluid inclusion planes, suggesting that reduced fluids percolated the rocks that host type ii mineralization. The reduced fluids support the transport of gold in sulfide complexes, such as AuHS- and Au(HS)-2. Furthermore, secondary H2O-NaCl fluid inclusions (Lw2) were found, with mean salinities of 6.0 eq. w(NaCl) and mean homogenization temperature of 226°C, with corresponding pressures of 3.0 MPa, thus suggesting late hydrostatic regimes. Chlorite geothermometer results are in the range of 229 °C and 309 °C, agreeing with the fluid inclusion homogenization temperatures for Lw2 fluids. Sulfur isotope (δ34S) analysis of representative sulfide phases collected from both mineralization types, revealed signatures ranging from 8.5 ‰ and 10.6 ‰, indicating a single sulfur source. The gathered results suggest that although fluid transport is structurally controlled by MNSZ activity, the sulfidation reactions promoted by fluid-rock interactions are the main control on gold deposition from type i mineralization. It is further suggested that a coeval gold-event can lead to the deposition of two different types of mineralization, related to distinct gold deposition mechanisms. The tectonic and geodynamic settings in which the Escoural gold district developed correlate it to worldwide Palaeozoic orogenic gold deposits, with the Monfurado prospect being an example of the complexity of such geological settings.

In situ LA-ICP-MS trace element analysis of magnetite as a vector towards mineral exploration: A comparative case study of Fe-skarn deposits from SW Iberia (Ossa-Morena Zone)

Article

Mar 2022
J GEOCHEM EXPLOR

The Azenhas and Alvito Fe-deposits are located at SW of the Iberian Variscan belt, in a wide Fe-Zn ore district (Montemor-Ficalho Belt), part of the Ossa-Morena Zone. Both deposits are dominantly composed of magnetite ores that display distinct ore formation processes and, at the Azenhas deposit, massive magnetite ores are mainly hosted in amphibolites (Middle Cambrian – Ordovician), with a genesis arguably associated with metamorphic-metasomatic reactions promoted by fluid circulation through several thrust faults that led to and expressive tectonic pilling. Contrastingly, the Alvito massive magnetite ores are formed by the emplacement of a gabbro-dioritic suite in contact with calcite-dolomite marbles, constituting a typical calcic exoskarn Fe-skarn deposit. Primary and secondary magnetite were identified and characterized, and further selected for in situ laser ablation coupled inductively mass spectroscopy (LA-ICP-MS) trace element analysis. Results show that primary magnetite (Mag I) from the Azenhas deposit is discriminated by higher concentrations of Mg, Cr, Mn, Zn, Co, and Sn, whereas secondary magnetite (Mag II) is depleted in most trace elements, although displaying relatively higher concentrations of V, Ga, Mo, and Pb. The application of TMg-Mag geothermometer reinforced the discriminatory indexes, with Mag I displaying higher-temperature estimations (ca. 770 °C) when compared to Mag II (ca. 420 °C), thus suggesting that primary ores are associated to higher temperature processes than previously assumed. Characterization of the ores from Alvito deposit revealed primary magnetite characterized by abundant ilmenite and Al-spinel oxy-exsolutions which led to high measured Al concentrations in magnetite. The trace element content indicates high-temperature hydrothermal magnetite, underlining the contribution of hot hydrothermal fluids from the igneous body emplacement, also supported by the TMg-Mag geothermometer (ca. 685 °C). The anomalous presence of Co (max. 156 ppm) and Ni (max. 100 ppm) concentrations in Mag I from Alvito, along with pentlandite-magnetite assemblages, suggest that prone conditions were sustained for the development of Ni-bearing ores and that magnetite composition could be an interesting geochemical proxy for the exploration of such mineralization. The Mn + Al versus Ti + V discriminant diagram shows that primary ores from both deposits plot in the skarn field. The combination of magnetite analyses with whole-rock geochemistry and field geology, allowed us to define criteria that can be applied in the discrimination of Fe-deposits and contribute to improve the geological models of the studied ore deposits, thus beneficiating future exploration in SW Iberia.

Variscan intracrustal recycling by melting of Carboniferous arc-like igneous protoliths (Évora Massif, Iberian Variscan belt)

Article

Sep 2021

Bulk rock geochemistry and sensitive high-resolution ion microprobe zircon geochronology of igneous and metaigneous rocks of the Évora gneiss dome, located to the north of the reworked Rheic Ocean suture zone in the southwest Iberian Variscan belt, reveal a succession of magmatic and melting events lasting ∼30 m.y. between ca. 341−314 Ma. The study of detailed field relationships of orthomigmatites (i.e., migmatites from igneous protoliths) and host granitic rocks proved to be crucial to reconstruct the complex sequence of tectono-thermal events of the Évora gneiss dome. The older igneous protoliths, with marked geochemical arc-like signatures, are represented by 338 ± 3 Ma tonalites and 336 ± 3 Ma diorites. These tonalites and diorites appear as mesosomes of igneous orthomigmatites containing new melts (leucosomes) of monzogranite composition and silica-poor trondhjemites formed in a melting episode at 329 ± 4/6 to 327 ± 3 Ma. The absence of peritectic phases (e.g., pyroxene), together with shearing associated with migmatization, imply the existence of water-rich fluids during melting of the older igneous rocks of the Évora gneiss dome. This melting event is coeval with the second magmatic event of the Évora gneiss dome represented by the neighboring Pavia pluton. A porphyritic monzogranite dated at 314 ± 4 Ma defines a later magmatic event. The porphyritic monzogranite encloses large blocks of the orthomigmatites and contains magmatic mafic enclaves (autoliths) dated at 337 ± 4 Ma that are ∼23 m.y. older than the host rock. All studied rocks of the Évora gneiss dome show arc-like, calc-alkaline geochemical signatures. Our results support recycling of intermediate-mafic plutonic rocks, representing the root of an early magmatic arc that formed at the time of Gondwana-Laurussia convergence (after the closure of the Rheic Ocean) and coeval subduction of the Paleotethys. A geodynamic model involving ridge subduction is proposed to explain the Early Carboniferous intra-orogenic crustal extension, dome formation, exhumation of high-grade rocks, compositional variations of magmatism and formation of new granitic magmatism in which, arc-like signatures were inherited from the crustal source.

Petrogenesis of a late-Variscan rhyodacite at the Ossa Morena-Central Iberian zones boundary, Iberian Massif, Central Portugal: Evidence for the involvement of lithospheric mantle and meta-igneous lower crust

Article

Jun 2016
CHEM ERDE-GEOCHEM

A late-Variscan rhyodacite is exposed at the contact between the Ossa Morena Zone and the CentralIberian Zone of the Iberian Massif, Central Portugal. Dykes of rhyodacite intruded the Série Negra Unitand the Sardoal Complex that are part of the Cadomian basement. The igneous crystallization age of therhyodacite (308 ± 1 Ma) was obtained on igneous monazite by the ID-TIMS U-Pb method. It is broadly coeval with the emplacement of late-Variscan granitoids during the last deformation phase of the VariscanOrogeny (ca. 304–314 Ma) and with the development of the large late-Variscan strike-slip shear zones(ca. 307 Ma). The rhyodacite samples are calc-alkaline, show identical composition and belong to thesame magmatic sequence. The rhyodacite isotopic signatures (Sm-Nd and �18O) are consistent withdepleted-mantle juvenile sources and the contribution of the meta-igneous lower crust. The input of mantle juvenile sources is related to Variscan reactivation of lithospheric fractures. The inherited Neo-proterozoic (ca. 619 Ma) and Mesoproterozoic (ca. 1054 Ma) zircon ages, are similar to those of the Central Iberian Zone. This suggests that lower crust of the Central Iberian Zone was involved in the magma generation of the rhyodacite. Coeval late-Variscan magmatic rocks display a larger contribution from ancientcrustal components, which may be attributed to the smaller volume and faster cooling rate of the rhy-odacite and consequent lower melting of the crust. Mixing of juvenile mantle-derived melts with meltsfrom the lower continental crust was followed by fractional crystallization of garnet and amphibole thatremained in the source. Fractional crystallization of plagioclase, biotite, quartz and zircon occurred inshallower magma chambers. Fractional crystallization of zircon was not significant.

Transcurrent displacement of the Cadomian magmatic arc

Article

Apr 2021

The Ossa-Morena Zone in Iberia, and equivalent terranes in northern France and Central Europe, are thought to be paleogeographically linked to the West African Craton at Ediacaran to early Paleozoic times. Evidence is mainly based on metasedimentary rock detrital zircon age spectra, characterized by two dominant populations of Paleoproterozoic and Cryogenian-Ediacaran ages, with a systematic lack of late Stenian-early Tonian zircon grains. We report here U-Pb-Hf results on detrital zircon grains from six Ordovician-Devonian metasedimentary rocks from the Ossa-Morena Zone. In addition to the Cryogenian-Ediacaran and Paleoproterozoic populations already recognized in previous studies on uppermost Ediacaran and lower Cambrian rocks, the samples ubiquitously show a late Stenian-early Tonian population centered at ≈1 Ga and representing ≈20% of the concordant dates. The εHf versus age plot of the studied samples mainly depicts two vertical arrays corresponding to the Cryogenian-Ediacaran and Stenian-Tonian detrital zircon populations, which spread from εHf values of ≈10 down to ≈− 20. The detrital zircon age distribution and Hf isotope signature of the studied rocks point to the Sahara metacraton as the most plausible sediment source. The eastward translation of the continental ribbon represented by the Ossa-Morena Zone and equivalent domains of the Variscides from an original position close to the West African Craton to an Ordovician-Devonian location close to the Sahara metacraton probably occurred at latest Ediacaran-earliest Cambrian times in a dextral strike-slip tectonic setting that postdated Pan-African collision and Cadomian subduction.

Revisiting the Intermediate Sediment Repository Concept Applied to the Provenance of Zircon

Article

Full-text available

Feb 2021

This paper revisits the intermediate sediment repository (ISR) concept applied to provenance, using a comparison of the detrital zircon population of Holocene beach sand from the southwest Portuguese coast with populations from their potential source rocks. The U–Pb age of detrital zircon grains in siliciclastic rocks allows for the interpretation of provenance by matching them with the crystallization ages of igneous source (protosource) rocks in which this mineral originally crystallized or which was subsequently recycled from it, acting as ISRs. The comparative analysis of the Precambrian, Paleozoic, and Cretaceous ages using recent statistical tools (e.g., kernel density estimator (KDE), cumulative age distribution (CAD), and multidimensional scaling (MDS)) suggests that the zircon age groups of Carboniferous, Triassic, and Pliocene-Pleistocene ISRs are reproduced faithfully in Holocene sand. Furthermore, the recycling of a protosource (Cretaceous syenite) in a sedimentary system dominated by ISRs is evaluated. It is argued that the ISR concept, which is not always taken into account, is required for a better understanding of the inherent complexity of local provenance and to differentiate sediment recycling from first- cycle erosion of an igneous rock.

U–Pb zircon geochronology and phase equilibria modelling of HP‑LT rocks in the Ossa‑Morena Zone, Portugal

Article

Full-text available

Sep 2020

The Ossa-Morena Zone (OMZ) has a complex geological history including both Cadomian and Variscan orogenic events. Therefore, the OMZ plays an important role in understanding the geodynamic evolution of Iberia. However, the P–T–t evolution of the OMZ is poorly documented. Here, we combine structural and metamorphic analyses with new geochronological data and geochemical analyses of mafic bodies in Ediacaran metasediments (in Iberia known as Série Negra) to constrain the geodynamic evolution of the OMZ. In the studied mafic rocks, two metamorphic stages were obtained by phase equilibria modelling: (1) a high-pressure/low-temperature event of 1.0 ± 0.1 GPa and 470–510 °C, and (2) a medium-pressure/higher-temperature event of 0.6 ± 0.2 GPa and 550–600 °C. The increase in metamorphic temperature is attributed to the intrusion of the Beja Igneous Complex (around 350 Ma) and/or the Évora Massif (around 318 Ma). New U–Pb dating on zircons from the mafic rocks with tholeiitic affinity yields an age between 815 and 790 Ma. If the zircons crystallised from the tholeiitic magma, their age would set a minimum age for the pre-Cadomian basement. The ca. 800 Ma protolith age of HP-LT tholeiitic dykes with a different metamorphic history than the host Série Negra lead us to conclude that: (1) the HP-LT mafic rocks and HP-LT marbles with dykes were included in the Ediacaran metasediments as olistoliths; (2) the blueschist metamorphism is older than 550 Ma (between ca. 790 Ma and ca. 550 Ma, e.g., Cadomian).

A unique blueschist facies metapelite with Mg-rich chloritoid from the Badajoz-Córdoba Unit (SW Iberian Massif): correlation of Late Devonian high-pressure belts along the Variscan Orogen

Article

Aug 2021

The Badajoz-Córdoba Unit (BCU, SW Iberian Massif) is a Variscan high-P unit mainly constituted by metapelites, metagreywackes, orthogneisses, Grt-amphibolites, and retrogressed eclogites (high-P metamorphism at c. 377 Ma). Discovery of rare metapelites with well-preserved high-P mineral assemblages, including large garnets up to 1 cm in diameter with abundant inclusions, chloritoid (up to XMg = 0.32), kyanite, staurolite, chlorite, phengite (up to Si = 3.16 apfu), paragonite, margarite, and rutile, allows detailed determination of the tectonothermal evolution of the unit. Pseudosection modelling of representative samples indicates that this mineral assemblage formed in blueschist facies (near eclogite facies transition) at P conditions higher than 20 kbar at c. 525°C and that it underwent a subsequent severe exhumation and moderate heating. The lithological composition of the BCU, the age of high-P metamorphism and the characteristics of the high-P mineral assemblages are similar to those found in other high-P and low to intermediate-T units of the Variscan Orogen. All these units form part of a single blueschist-eclogite facies metamorphic belt formed during Late Devonian subduction of the external margin of Gondwana.

Chronostratigraphic framework and provenance of the Ossa-Morena Zone Carboniferous basins (southwest Iberia)

Article

Full-text available

Jul 2020
SE

Carboniferous siliciclastic and silicic magmatic rocks from the Santa Susana–São Cristovão and Cabrela regions contain valuable information regarding the timing of synorogenic processes in SW Iberia. In this region of the Ossa-Morena Zone (OMZ), late Carboniferous terrigenous strata (i.e., the Santa Susana Formation) unconformably overlie early Carboniferous marine siliciclastic deposits alternating with volcanic rocks (i.e., the Toca da Moura volcano-sedimentary complex). Lying below this intra-Carboniferous unconformity, the Toca da Moura volcano-sedimentary complex is intruded and overlain by the Baleizão porphyry. Original sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon are presented in this paper, providing chronostratigraphic and provenance constraints since available geochronological information is scarce and only biostratigraphic ages are currently available for the Santa Susana–São Cristovão region. Our findings and the currently available detrital zircon ages from Paleozoic terranes of SW Iberia (Pulo do Lobo Zone – PLZ – South Portuguese Zone – SPZ – and OMZ) were jointly analyzed using the K–S test and multidimensional scaling (MDS) diagrams to investigate provenance. The marine deposition is constrained to the age range of ca. 335–331 Ma (Visean) by new U–Pb data for silicic tuffs from the Toca da Moura and Cabrela volcano-sedimentary complexes. The Baleizão porphyry, intrusive in the Toca da Moura volcano-sedimentary complex, yielded a crystallization age of ca. 318 Ma (Bashkirian), providing the minimum age for the overlying intra-Carboniferous unconformity. A comparison of detrital zircon populations from siliciclastic rocks of the Cabrela and Toca de Moura volcano-sedimentary complexes of the OMZ suggests that they are derived from distinct sources more closely associated with the SPZ and PLZ than the OMZ. Above the intra-Carboniferous unconformity, the Santa Susana Formation is the result of the recycling of distinct sources located either on the Laurussian side (SPZ and PLZ) or Gondwanan side (OMZ) of the Rheic suture zone. The best estimate of the crystallization age of a granite cobble which was found in a conglomerate from the Santa Susana Formation yielded ca. 303 Ma (Kasimovian–Gzhelian), representing the maximum depositional age for the terrestrial strata. The intra-Carboniferous unconformity seems to represent a stratigraphic gap of approximately 12–14 Myr, providing evidence of the rapid post-accretion and collision uplift of the Variscan orogenic belt in SW Iberia (i.e., the OMZ, PLZ, and SPZ).

Geochemical and isotopic (Sm Nd) provenance of Ediacaran-Cambrian metasedimentary series from the Iberian Massif. Paleoreconstruction of the North Gondwana margin

Article

Dec 2019
EARTH-SCI REV

The geochemical and isotopic (Sm-Nd) features of the Ediacaran-Cambrian siliciclastic series from the Central Iberian Zone and the Iberian Allochthonous Domains indicate a significant paleogeographic change at the northern margin of Gondwana, associated with the development of the Avalonian-Cadomian arc. During the Neoproterozoic, the opening of a peri-Gondwanan back-arc basin and its subsequent widening during the Cambrian led to a progressive separation of the sedimentary basins from the main focus of the probably waning Cadomian magmatic arc activity. This work presents a paleogeographic reconstruction of the distribution of the Iberian terranes along the North Gondwanan margin during Ediacaran and Cambrian times, based on the geochemical and Nd-isotope data from the Iberian Allochthonous and Autochthonous Domains. These results suggest a location close to the West Africa Craton for both the allochthonous and autochthonous terranes, but they would be distributed laterally along the northern margin of Gondwana, occupying arc- or continental-ward positions within a wide back-arc basin. In this context, the peri-Gondwanan volcanic arc acted as the main supplier of the abundant juvenile material, as reflected in positive values of Nd and young TDM model ages (720-1215 Ma) of the Middle Cambrian siliciclastic series from NW Iberia Upper Allochthonous Units. On the other hand, the rather equivalent geochemical and isotopic features of the sedimentary series of the NW and SW Iberia Basal Allochthonous Units suggest a common paleolocation for those series, yet within the back-arc basin but closer to the mainland and older isotope sources. The sedimentary series of these terranes present the oldest TDM ages (1499-2156 Ma), clearly greater than the TDM ages that characterize coeval sedimentary series of the Iberian Autochthonous Domain (1256-1334 Ma). This finding allows us to place Iberian Basal Allochthonous Units very close to the West Africa Craton, from where they received a dominant contribution of old crustal materials, whereas the Iberian Autochthonous Domain occupied a more eastern paleoposition closer to the Sahara Metacraton.

The Finisterra-Léon-Mid German Cristalline Rise Domain; Proposal of a New Terrane in the Variscan Chain

Chapter

Sep 2019

This chapter characterizes the Finisterra Terrane, enhancing its differences from the neighbouring Iberian Terrane. The contact between these terranes is the Porto-Tomar-Ferreira do Alentejo Shear Zone, a major lithospheric structure whose complex Variscan evolution remains debatable. The lithostratigraphic, tectonometamorphic and magmatic features observed in the Finisterra Terrane show that it was an independent terrane during the Devonian. This situation changed during the Mississippian, when the main features of the Finisterra and the Iberian Terranes became similar, which indicates that both terranes evolved together since the Carboniferous times. The similarities of the Finisterra Terrane with the Central European Variscan domains, namely the Léon Block and the Mid-German Crystalline Rise, enable us to propose a new tectono-stratigraphic terrane (Finisterra-León-MGCR Terrane), which defines an arcuate pattern compatible with the Ibero-Armorican Arc.

Evidence of a Paleoproterozoic basement in the Moroccan Variscan Belt (Rehamna Massif,Western Meseta)

Article

Jul 2015
PRECAMBRIAN RES

Cadomian metasediments and Ordovician sandstone from Corsica: detrital zircon U–Pb–Hf constrains on their provenance and paleogeography

Article

Full-text available

Jul 2018

The island of Corsica, which belongs to the southern Variscan realm, was detached from southern France in the Tertiary. Alongside Alpine and Variscan edifices, it carries outliers of Neoproterozoic metasediments and Lower Paleozoic siliciclastics including Ordovician sandstone and conglomerates as well as Hirnantian diamictites in its NW sector. We investigated the U–Pb–Hf of detrital zircons of basement metasediments and overlying Ordovician sandstone and channeling conglomerate to constrain their provenance, the timing of their deposition, and to deduce the late Ediacaran to Ordovician paleogeography. The youngest detrital zircons in the metasediments are 0.55–0.53 Ga indicating their maximum age of deposition is Late Ediacaran to Early Cambrian, thus classifying the Corsica basement metasediments as Cadomian. The U Pb analyses revealed that a preponderance of the detrital zircons in the basement micaschist and quartzite portray Neoproterozoic ages concentrating between 0.55 and 0.65 Ga. This is partly consistent with derivation from Pan-African terranes of north Africa yet the presence of detrital zircons younger than ~ 0.6 Ga indicates significant input from Cadomian magmatic arcs that resided within or at the margin of the Cadomian basin in which the metasediments of Corsica were deposited. εHf values of the Ediacaran zircons varies between samples, indicating the provenance comprised both, juvenile arcs and magmatic arcs that involved various degrees of mixing with old crustal components. The Hf-TDM ages of many of the Ediacaran-aged zircons point to a plausible involvement of Meso-Paleoproterozoic crust in the generation of these Cadomian arcs. The presence of small but fairly distinguished populations of Mesoproterozoic aged (1.0–1.6 Ga) as well as 2.0–2.2 Ga and 2.4–2.6 Ga detrital zircons in the Cadomian metasediments farther indicates the presence of such crust in the provenance. Although Mesoproterozoic detrital zircons are usually considered the hallmark of Avalonian terranes, the presence of Hirnantian glacial sediments at the Corsican sequence indicates it resided in the vicinity of Gondwana. We therefore postulate that the Pre Neoproterozoic zircons have sourced from exotic crustal vestiges that were entrained and accreted within the Cadomian realm itself. The transition into the overlying Ordovician sandstone and conglomerate marks a major change in the provenance, possibly pointing to lateral motions along the strike of the peripheral Cadomian domain. The youngest concordant detrital zircon in the Ordovician (“Ciuttone”) sandstone yielded an age of 0.48 Ga. The detrital zircon ages define an overwhelming concentration at 0.55 Ga, indicating the source of the Ordovician sandstone was cut off from the Gondwana hinterland and that sand was exclusively derived from a latest Ediacaran arc. In view of the sharp detrital zircon age peak, the Corsica Ordovician sandstone cannot be straightforwardly correlated with Armorican sandstone because the detrital zircon spectra of the latter are generally broader, indicating derivation from various sectors of the North Gondwana crust. εHf(t) values of the 0.55 Ga zircons are mostly positive and the corresponding TDM ages at 0.7–1.2 Ga indicating derivation from a juvenile island arc. While TDM ages of this type are common for late Ediacaran Avalonian rocks, the presence of Hirnantian diamictite in Corsica further substantiates that the aforementioned 0.55 Ga island arc evolved in the peripheral Cadomian realm. As a whole, the U–Pb–Hf zircon data from the Corsica sequences reveal the presence of juvenile Cadomian arcs alongside Cadomian arcs that recycled ancient (pre-Neoproterozoic) crust. Along strike variations of this type are known from the Japanese islands, in line with the peripheral Cadomian orogeny being an ancient analog of a Western-Pacific type plate boundary.

Using U-Pb detrital zircon systematics to constrain the architecture and provenance of the Ronda peridotites’ crustal hanging wall, S Spain

Article

Jun 2024
GONDWANA RES

Contribution of Remote Sensing and Structural Geology in the Mapping of Tectonic Fractures in the Zat Region (Western High Atlas, Morocco)

Article

Jun 2024

The Zat region has a Precambrian basement that is partially covered by deformed Phanerozoic terrains. The last three orogeneses that Morocco experienced, from the Precambrian to the Quaternary, shaped this region. We used optical imagery from Landsat 8 OLI and ASTER DEM to map the tectonic fractures in this region. First, radiometric and geometric corrections were taken into account. Then, during the automatic extraction of lineaments, directional filters were used. Many approaches were used in the validation procedure, including the creation of false colour images, principal component analysis, and the removal of artificial lineaments by superimposing them on geological and topographic maps, Google Earth data, and field measurements. The listed lineaments have four major directional ranges: N–S, NW–SE, E–W, and NE–SW. The Hercynian and Alpine fractures are designated by the N–S and NE–SW directions, respectively. However, Precambrian filled fractures are distinguished by lineaments that fluctuate in the WNW–ESE direction. The geographical distribution of lineaments demonstrates the presence of two hard nuclei (Ourika gneissic massif and Afra ignimbritic massif) having controlled the region’s deformation. The region’s tectonic intensity decreases at the level of these nuclei and increases at the level of the surrounding terrains, which may include mineralising indices. This study highlights a region that is likely to be mined because of its geological and structural heritage.

Paleogeography of the Gondwana passive margin fragments involved in the Variscan and Alpine collisions: Perspectives from metavolcanic-sedimentary basement of the Western Carpathians

Article

Apr 2024
EARTH-SCI REV

The general configuration of the main continental blocks in the Gondwana supercontinent and the Ediacaran–early Paleozoic tectonic evolution of its northern margin are widely accepted. However, reconstruction of the original positions and the question of potential separation of the Gondwana-derived crustal segments that are now included in the Variscan and Alpine orogenic belts remain controversial. The Western Carpathians, part of the Alpine–Carpathian belt, represents an important crustal segment broken-off from northern Gondwana and later incorporated into both the Variscan and Alpine collisional orogens. The earliest tectonic evolution and paleogeography of the pre-Variscan basement of the Western Carpathians remains poorly known, due to insufficient age data and intense polyphase tectonometamorphic overprints, both Variscan and Alpine. This paper provides new results of Usingle bondPb dating and Hf isotopic analysis of detrital zircons as well as a whole-rock geochemical study from metavolcanic-sedimentary basement units of the Western Carpathians. The obtained age spectra suggest that the sedimentary succession was supplied dominantly by Ediacaran (c. 600 Ma) zircons, with a relatively minor role for Stenian–Tonian (c. 1.2–0.9 Ga) and Paleoproterozoic cratonic (c. 2.2–1.8 Ga) zircons. The mixed Hf isotopic signature (εHf(t) values ranging from −20 to +12) of the Ediacaran zircons indicates substantial mixing of mantle-derived magmas with mature crustal material, typical of continental magmatic arcs. In contrast, the mostly negative εHf(t) values (−15 to +4) of the cratonic zircons suggest recycling of an older continental crust. The presumably youngest part of the sequence is also characterised by high proportion of early Paleozoic zircons with generally negative εHf(t) values (−10 to −2). The zircon Usingle bondPb age spectra, Hf isotopic patterns and whole-rock geochemical signatures of the studied Western Carpathians sequences are interpreted as reflecting deposition at a progressively developing Cambrian–Silurian passive margin setting. The West Carpathian data have been correlated with a comprehensive detrital zircon Usingle bondPb age and Hf isotope data set compiled from possible source areas and other Gondwana-derived units to test the possibility of their primary linkages. These correlations indicate strong similarity in both zircon Usingle bondPb age spectra and Hf isotopic compositions to other parts of the Ediacaran (Cadomian) continental magmatic arc. Older, cratonic sources are linked to the Saharan or East African parts of northern Gondwana, whereas the early Paleozoic detritus must represent a local volcanic source. Taken together, our new data from the Western Carpathians provide constraints for a new paleogeographic model of the northern African part of the Gondwana passive margin.

Tales from xenoliths: the link between the Papôa Volcanic Breccia and the Berlengas Archipelago (West Portugal)—geodynamic implicationsCuentos de xenolitos - El vínculo entre la Brecha Volcánica de Papôa y el Archipiélago de las Berlengas (Oeste de Portugal): implicaciones geodinámicas

Article

Nov 2023

The Papôa Volcanic Breccia (Peniche, West Portugal) is a Mesozoic pyroclastic deposit that includes numerous igneous and metamorphic xenoliths belonging to the Variscan basement of the Lusitanian Basin, in which this unit outcrops. These rocks correspond mostly to syenogranites and diatexites affected by intense pyrometamorphism and, despite their strong secondary alteration by weathering, they still preserve several original petrographic features that are traceable to the lithologies that comprise the Berlengas Archipelago, located 15 km offshore the Peniche coast. New geochemical and mineral chemistry data now indicate a genetic relationship between these basement xenoliths and the Late Devonian migmatites outcropping in the Farilhões Anatectic Complex. Such evidence strongly suggests the physical continuity of these high-grade metamorphic terrains at least to the Peniche region, thus constraining the nature and composition of the crustal block beneath the sedimentary sequences of the Lusitanian Basin in the studied location. In addition, in situ LA–ICP–MS U–Pb dating of rutile formed during the pyrometamorphic event endured by the xenoliths provided an age of 92.3 ± 1.4 Ma for the Papôa volcanic eruption, placing this occurrence within the third magmatic cycle related to the opening and spreading of the North Atlantic Ocean in the West Iberian Margin.

Coupled U-Hf isotopes and trace elements of detrital zircon grains from the lower Ediacaran turbidites in the Skoura inlier (Central High Atlas, Morocco): Implications for crustal evolution of the north-western Gondwana margin

Article

Jan 2023
PRECAMBRIAN RES

Geochemical and Sr–Nd isotopic constraints on the petrogenesis of the Tiflet granitoids (Northwestern Moroccan Meseta): geological implications

Article

Jan 2021

The Moroccan Variscan orogen contains numerous small felsic magmatic bodies from the Upper Neoproterozoic. The Tiflet granitoids (Northwestern Moroccan Meseta) consist of two lenticular bodies (Taïcha and Sidi BouJemaa) with granodioritic composition and a zircon U–Pb age of 609 Ma, which outcrop in the BouRegreg corridor of the northern Moroccan Meseta. Both granitoids are magnesian with a calc-alkaline trend, mostly showing a peraluminous affinity but with a mineral association typical of I-type granitoids. Major element variations reflect a higher degree of evolution for Sidi BouJemaa than for the Taïcha granitoid, but there is no clear evidence that they are related by fractionation processes. A predominantly metaigneous source in the lower crust or mixtures of continental crust and subcontinental lithosphere, presumably from the Paleoproterozoic basement, is possible for these granitoids judging from their low values of Al2O3/(FeO + MgO + TiO2) and (Na2O + K2O)/(FeO + MgO + TiO2), high (CaO/Na2O) values, in conjunction with relatively low initial 87Sr/86Sr ratios (0.7054–0.7108) and negative εNd(t) values (− 6.52 to − 8.45). REE patterns with elevated LREE/HREE fractionation, slight Eu anomalies and flat patterns from Er to Lu are typical of syn-collisional granitoids. In contrast, post-collisional HKCA granitoids are related to a transtensional tectonic regime, with low 87Sr/86Sr ratios and high 143Nd/142Nd values that indicate an important mantle contribution.

Tectonic setting and isotopic sources (Sm–Nd) of the SW Iberian Autochthon (Variscan Orogen)

Article

Jan 2021

Metasedimentary rock successions in Sierra Albarrana Group (SW Iberia Autochthonous Domain) were deposited during the Early Paleozoic influenced by the evolution of a peri-Gondwanan active margin. Their geochemical composition indicates dominant contributions from felsic igneous sources and an upper continental crust provenance. The high mineralogical and geochemical maturity, together with negative εNd(530), ranging from − 11.3 to − 4.5, and old Nd model ages (TDM: 1388–1897 Ma) imply reworked materials from old continental source areas with a limited juvenile contribution for the siliciclastic sedimentary rocks in Sierra Albarrana. Both, geochemical and isotope features were caused by the progressive denudation of rocks bearing the isotopic imprint from an old basement exposed along the Gondwanan margin. The paleobasin for the SW Iberia Autochthon probably occupied an outboard position across the peri-Gondwanan margin during Ediacaran–Cambrian times. In a convergent scenario, the interaction between the peri-Gondwanan trench and the external part of the continent generated tectonic uplift, giving way to exposure of crust formed during earlier stages of the Cadomian Orogeny and erosion. Sediment redistribution through Early Paleozoic times supplied recycled detrital materials, which likely filled a retro-arc basin formed after the switch from an extensional to a compressional regime in the upper plate of the Cadomian Orogen that is recorded throughout the peri-Gondwanan domain of Iberia. A subsequent extensional stage led to a progressive widening of its marginal basins, thus bringing the onset of a passive margin from the Cambro–Ordovician onwards. Nd model ages of the Sierra Albarrana Group overlap those of Early Cambrian series from the southernmost Central Iberian Zone, and are considered an indication for the paleogeographic closeness between all these sequences during Cambrian times, occupying eastern positions closer to Tuareg Shield and the Sahara Metacraton sections along the Gondwanan margin.

Early Paleozoic accretionary orogens along the Western Gondwana margin

Article

Full-text available

Aug 2020

Early Paleozoic accretionary orogens dominated the Western Gondwana margin and were characterized by nearly continuous subduction associated with crustal extension and back-arc basin development. The southwestern margin is represented by Famatinian and Pampean basement realms exposed in South America, both related to the protracted Paleozoic evolution of the Terra Australis Orogen, whereas the northwestern margin is mainly recorded in Cadomian domains of Europe and adjacent regions. However, no clear relationships between these regions were so far established. Based on a compilation and reevaluation of geological, paleomagnetic, petrological, geochronological and isotopic evidence, this contribution focuses on crustal-scale tectonic and geodynamic processes occurring in Western Gondwana accretionary orogens, aiming at disentangling their common Early Paleozoic evolution. Data show that accretionary orogens were dominated by high-temperature/low-pressure metamorphism and relatively high geothermal gradients, resulting from the development of extended/hyperextended margins and bulk transtensional deformation. In this sense, retreating-mode accretionary orogens characterized the Early Paleozoic Gondwana margin, though short-lived pulses of compression/transpression also occurred. The existence of retreating subduction zones favoured mantle-derived magmatism and mixing with relatively young (meta)sedimentary sources in a thin continental crust. Crustal reworking of previous forearc sequences due to trenchward arc migration thus took place through assimilation and anatexis in the arc/back-arc regions. Therefore, retreating-mode accretionary orogens were the locus of Early Paleozoic crustal growth in Western Gondwana, intimately associated with major flare-up events, such as those related to the Cadomian and Famatian arcs. Slab roll back, probably resulting from decreasing convergence rates and plate velocities after Gondwana assembly, was a key factor for orogen-scale geodynamic processes. Coupled with synchronous oblique subduction and crustal-scale dextral deformation, slab roll back might trigger toroidal mantle flow, thus accounting for bulk dextral transtension, back-arc extension/transtension and a large-scale anticlockwise rotation of Gondwana mainland.

Pétrographie et géochimie des granites Tardi-panafricains de la region de Tiflet (Meseta occidentale, Maroc).

Conference Paper

Jul 2017

Les granites Ediacariens de Tiflet: apport de la geochimie isotopique du neodyme

Conference Paper

Apr 2019

Geochemical and isotopic data of Ediacaran granitic rocks of Tiflet (Western Meseta, Morocco).

Conference Paper

Full-text available

Apr 2018

Chronostratigraphic framework and provenance of the Ossa-Morena Zone Carboniferous basins (SW Iberia)

Preprint

Full-text available

Mar 2020

Abstract. Carboniferous siliciclastic and silicic magmatic rocks from the Santa Susana-São Cristovão region contain valuable information regarding the timing of synorogenic processes in SW Iberia. In this region of the Ossa-Morena Zone (OMZ), Late Carboniferous terrigenous strata (i.e. the Santa Susana Formation) unconformably overlie Early Carboniferous marine siliciclastic deposits alternating with volcanic rocks (i.e. the Toca da Moura volcano-sedimentary complex). Lying below this intra-Carboniferous unconformity, the Toca da Moura volcano-sedimentary complex is intruded and overlain by the Baleizão porphyry. Original SHRIMP and LA-ICP-MS U-Pb zircon are presented in this paper, providing chronostratigraphic and provenance constraints, since available geochronological information is scarce and only biostratigraphic ages are currently available for the Santa Susana-São Cristovão region. Our findings and the currently-available detrital zircon ages from Paleozoic terranes of SW Iberia (Pulo do Lobo Zone- PLZ, South-Portuguese Zone- SPZ, and OMZ), were jointly analyzed using the K-S test and MDS diagrams to investigate provenance. The marine deposition is constrained to the age interval of c. 335–331 Ma (Visean) by new U-Pb data for silicic tuffs from the Toca da Moura volcano-sedimentary complex. The Baleizão porphyry, intrusive in the Toca da Moura volcano-sedimentary complex, yielded a crystallization age of c. 317 Ma (Bashkirian), providing the minimum age for the overlying intra-Carboniferous unconformity. A comparison of detrital zircon populations from siliciclastic rocks of the Cabrela and Toca de Moura volcano-sedimentary complexes of the OMZ suggests that they derived from distinct sources more closely associated with the SPZ and PLZ than the OMZ. Above the intra-Carboniferous unconformity, the Santa Susana Formation is either the result of the recycling of distinct sources located in the Laurussian-side (SPZ and PLZ) and Gondwanan-side (OMZ) of the Rheic suture zone. The best estimate of the crystallization age of a granite cobble found in a conglomerate from the Santa Susana Formation yielded c. 303 Ma (Kasimovian-Gzhelian), representing the maximum depositional age for the terrestrial strata. The intra-Carboniferous unconformity seems to represent a stratigraphic gap of approximately 12–14 Ma, providing evidence of the rapid post-accretion/collision uplift of the Variscan orogenic belt in SW Iberia (i.e. the OMZ, PLZ and SPZ).

Comment on Baltic provenance of top-Famennian siliciclastic material of the northern Rhenish Massif, Rhenohercynian zone of the Variscan orogen, by Koltonik et al., International Journal of Earth Sciences (2018) 107:2645–2669

Article

Mar 2019

Koltonik et al. (Int J Earth Sci 107:2645–2669, 2018) evidence that the Late Devonian siliciclastic rocks from the Rheno-Hercynian Zone, in Germany, derived from Baltica and Scandinavian Caledonides. This finding together with what is known about the provenance of the Pulo do Lobo and South Portuguese zones, in Portugal and Spain, reinforces the probability that Late Devonian basins may have been sourced from distinct terranes placed along the Variscan suture. Our comment intends to underline changes in the provenance of the Late Devonian basins along the active margin of Laurussia, and also, to improve the correlation model for the Variscan tectonic units from SW Iberia and Germany.

Palaeogeography and crustal evolution of the Ossa–Morena Zone, southwest Iberia, and the North Gondwana margin during the Cambro-Ordovician: a review of isotopic evidence

Article

Sep 2016
INT GEOL REV

Cambro-Ordovician palaeogeography and fragmentation of the North Gondwana margin is still not very well understood. Here we address this question using isotopic data to consider the crustal evolution and palaeogeographic position of the, North Gondwana, Iberian Massif Ossa–Morena Zone (OMZ). The OMZ preserves a complex tectonomagmatic history: late Neoproterozoic Cadomian orogenesis (ca. 650–550 Ma); Cambro-Ordovician rifting (ca. 540–450 Ma); and Variscan orogenesis (ca. 390–305 Ma). We place this evolution in the context of recent North Gondwana Cambro-Ordovician palaeogeographic reconstructions that suggest more easterly positions, adjacent to the Sahara Metacraton, for other Iberian Massif zones. To do this we compiled an extensive new database of published late Proterozoic–Palaeozoic Nd model ages and detrital and magmatic zircon age data for (i) the Iberian Massif and (ii) North Gondwana Anti-Atlas West African Craton, Tuareg Shield, and Sahara Metacraton. The Nd model ages of OMZ Cambro-Ordovician crustal-derived magmatism and Ediacaran-Ordovician sedimentary rocks range from ca. 1.9 to 1.6 Ga, with a mode ca. 1.7 Ga. They show the greatest affinity with the Tuareg Shield, with limited contribution of more juvenile material from the Anti-Atlas West African Craton. This association is supported by detrital zircons that have Archaean, Palaeoproterozic, and Neoproterozoic radiometric ages similar to the aforementioned Iberian Massif zones. However, an OMZ Mesoproterozoic gap, with no ca. 1.0 Ga cluster, is different from other zones but, once more, similar to the westerly Tuareg Shield distribution. This places the OMZ in a more easterly position than previously thought but still further west than other Iberian zones. It has been proposed that in the Cambro-Ordovician the North Gondwana margin rifted as the Rheic Ocean opened diachronously from west to east. Thus, the more extensive rift-related magmatism in the westerly OMZ than in other, more easterly, Iberian Massif zones fits our new proposed palaeogeographic reconstruction.

Provenance Analysis of the Late Ediacaran Basins from Southwestern Iberia (Série Negra Succession and Beiras Group): Evidence for a Common Neoproterozoic Evolution

Chapter

Jan 2014

This study makes a comparison of detrital-zircon age populations from upper Ediacaran greywackes of the Ossa–Morena Zone (OMZ) with those from the southern domains of the Central Iberian Zone (S-CIZ). The results reveal that the main difference between the age spectra of both populations of detrital zircon is the Neoproterozoic, in particular the Cryogenian grains. Our new data suggest that deposition in the CIZ and OMZ Ediacaran basins was coeval, indicating a long-lived magmatic event typical of the northern Gondwana margin (Avalonian–Cadomian belt and Pan-African belt). Overall, southwestern Iberia shows the following sequence of Cryogenian and Ediacaran zircon-forming events: (1) ca. 850–700 Ma, Pan-African suture (well represented in the Beiras Group and in the Mares Formation of the Série Negra succession); (2) ca. 700–635 Ma, early Cadomian arc (dominant in the Beiras Group and in the Mares Formation of the Série Negra succession); and (3) ca. 635–545 Ma, late Cadomian arc (predominant in the Mosteiros and Escoural formations of the Série Negra succession). The results emphasise that the late Ediacaran basins of southwestern Iberia evolved together in the active margin of North Gondwana in the same palaeogeographical scenario but were sufficiently separated to justify the differences identified in their Neoproterozoic detrital zircon contents. This finding shows that there is no basis for considering that the boundary between the OMZ and the S-CIZ marks a Cadomian suture.

Reply to Discussion on ‘Detrital zircon geochronology of the Carboniferous Baixo Alentejo Flysch Group (South Portugal); constraints on the provenance and geodynamic evolution of the South Portuguese Zone, Journal of the Geological Society , 172, 294–308

Article

Full-text available

Jan 2015

Metamorphism

Chapter

Full-text available

Jan 1990

The regional metamorphic pattern within the Ossa-Morena Zone is characterized by an alternation of low and medium-to high-grade belts which retain more or less homogeneous features along the strike (Fig. 1). As already stated by Quesada et al. (this Vol.) it is now unanimously recognized that this metamorphic framework developed during at least two main tectonothermal cycles: Late Precambrian and Hercynian. Nevertheless, major uncertainties still exist concerning which event is to be held responsible for the metamorphic evolution of specific areas; this is mainly due to our still poor knowledge of the relationships pertaining to recrystallization-deformation and the lack of geochronological constraints on these events. Moreover, the few available absolute ages on these (sometimes) highly complex metamorphic rocks are not amenable to straightforward interpretation. The great majority of the data was obtained by the K-Ar or 40Ar-39Ar methods; thus, we are dealing with cooling ages, which means that the ages determined for the metamorphic rocks represent the timing when those rocks were brought up to shallower crustal levels and/or to P-T conditions below the closing exchange temperature of their respective isotopic systems. Therefore, what the data is telling us is the ages of the main uplift tectonic events and not necessarily the timing of either their metamorphic recrystallization and/or deformation.

Main steps of the Ossa Morena tectono-stratigraphic evolution in Portugal[Tracos gerais da evolucao tectono-estratigrafica da Zona de Ossa- Morena, em Portugal]

Article

Full-text available

Jan 1991

In the Portuguese part of the Ossa Morena Zone several sectors with distinct stratigraphy and structure are identified. They represent the continuation into Portugal of the well established Spanish tectono-stratigraphic domains. The stratigraphic sequences and interbedded volcanics of the distinct sectors allow the visualization of the main steps of the OMZ tectono-stratigraphic evolution. -from English summary

Crustal growth and deformational processes in the northern Gondwana margin: Constraints from the Évora Massif (Ossa-Morena zone, southwest Iberia, Portugal)

Chapter

Full-text available

Jan 2007

The aim of this article is to present a compilation of available information on the évora Massif based on structural mapping, whole-rock geochemistry, recognition of metamorphic mineral assemblages, and geothermobarometry. In our view, trans- current movements responsible for strong orogen-parallel stretching were dominant and had a major role in the geodynamic evolution of this part of Ossa-Morena zone (southwest Iberian Massif). Cadomian and Variscan orogenic events separated by a period of intense rifting were the cause for the composite distribution of zones with contrasting metamorphic paths, the structural complexity, the variety of lithological associations, and the sequence of deformation events and magmatism. The proposed geodynamic reconstruction for this segment of the northern Gondwana continental margin includes three main stages in chronological order: (1) Neoproterozoic accretion and continental magmatic arc developing, dismantling, and reworking, followed by late-"orogenic" magmatismi; (2) Lower Paleozoic crustal thinning, block tilting, and mantle upwelling, induced by generalized rifting, leading to the formation of marine basins with carbonate platform sediments and thick accumulations of vol- caniclastic and terrigenous sediments, contemporaneous with normal and enriched mid-oceanic ridge basalt-type magmatism; and (3) Upper Paleozoic transpressional orogenesis resulting from obliquity of convergence and the geometry of the involved blocks. The third stage includes the tectonic inversion of Lower Paleozoic basins, crustal thickening, the exhumation of high- to medium-pressure rocks and partial exhumation of high-grade metamorphic lithologies (controlled by local transtension and major detachments), the formation of synorogenic basins filled with volcanic- sedimentary sequences, and finally, the emplacement of late Variscan granodiorites and granites.

Origin of the Rheic ocean: rifting along a Neoproterozoic suture?

Conference Paper

Full-text available

Apr 2005

Superposed Hercynian and Cadomian orogenic cycles in the Ossa-Morena zone and related areas of the Iberian Massif

Article

Full-text available

Jan 2000

Benito Ábalos

Polyphase ductile deformation and metamorphism in the Ossa-Morena zone of the Iberian Massif are related to a complex evolution including two major tectonothermal episodes of Cadomian (Late Proterozoic to Early Cambrian) and Hercynian age (middle to late Paleozoic), Petrological, structural, and geochronological data attest to a number of tectonic episodes in the 620-480 Ma interval, The model presented for this orogeny comprises stages of volcanic are generation, backarc extension, tectonic inversion, crustal thickening, and cratonization in an Andean-type continental margin, The Hercynian cycle began in the early Paleozoic with episodes of continental rifting. Orogenic events occurred in the 390-300 Ma interval and affected both the Cadomian basement, which was heterogeneously reworked and overprinted, and the Paleozoic cover. Hercynian regional metamorphism was generally of low grade. High-pressure assemblages and thermal domes developed locally, and were related to subduction and late extensional events. The geodynamic model proposed for the Cadomian and Hercynian orogenies and the correlation with comparable areas from pre-Mesozoic massifs elsewhere in western Europe account for the geodynamic scenarios that drove the fate of Cadomian terranes.

Provenance of Neoproterozoic and lower Paleozoic siliciclastic rocks of the central Ross Orogen, Antarctica: Detrital record of rift-, passive- and active-margin sedimentation

Article

Full-text available

Sep 2004

Siliciclastic rocks in the Transantarctic Mountains record the tectonic transforma- tion from a Neoproterozoic rift-margin set- ting to a passive-margin and ultimately to an active early Paleozoic orogenic setting along the paleo-Pacifi c margin of East Antarctica. New U-Pb detrital-zircon ages constrain both the depositional age and sedimentary prov- enance of these strata. In the central Trans- antarctic Mountains, mature quartz arenites of the late Neoproterozoic Beardmore Group contain Archean and Proterozoic zircons, refl ecting distal input from the adjacent East Antarctic shield, Mesoproterozoic igneous provinces, and Grenville-age parts of East Gondwana. Similarly, basal sandstones of the Lower Cambrian Shackleton Limestone (lower Byrd Group) contain zircons refl ect- ing a dominantly cratonic shield source; the autochthonous Shackleton was deposited during early Ross orogenesis, yet its basal sandstone indicates that the inner shelf was locally quiescent. Detrital zircons from the Koettlitz Group in southern Victoria Land show a similar age signature and constrain its depositional age to be ≤670 Ma. Signifi cant populations (up to 22%) of ca. 1.4 Ga zircons in these Neoproterozoic and Lower Cambrian sandstone deposits suggest a unique source of Mesoproterozoic igneous material in the East Antarctic craton; comparison with the trans- Laurentian igneous province of this age sug- gests paleogeographic linkage between East Antarctica and Laurentia prior to ca. 1.0 Ga. In strong contrast, detrital zircons from upper Byrd Group sandstones are dominated by young components derived from proxi- mal igneous and metamorphic rocks of the emerging Ross orogen. Zircon ages restrict deposition of this syn- to late-orogenic suc- cession to ≤520 Ma (Early Cambrian or younger). Sandstone samples in the Pensacola Mountains are dominated by Grenville and Pan-African zircon ages, suggesting a source in western Dronning Maud Land equivalents of the East African orogen. When integrated with stratigraphic relationships, the detri- tal-zircon age patterns can be explained by a tectonic model involving Neoproterozoic rifting and development of a passive-margin platform, followed by a rapid transition in the late Early Cambrian (Botomian) to an active continental-margin arc and forearc setting. Large volumes of molassic sediment were shed to forearc marginal basins between Mid- dle Cambrian and Ordovician time, primar- ily by erosion of volcanic rocks in the early Ross magmatic arc. The forearc deposits were themselves intruded by late-orogenic plutons as the locus of magmatism shifted trenchward during trench retreat. Profound syntectonic denudation, followed by Devonian penepla- nation, removed the entire volcanic carapace and exposed the plutonic roots of the arc.

U-Pb geochronological constraints on Precambrian stratified units in the Avalon Composite Terrane of Nova Scotia, Canada: tectonic implications

Article

Full-text available

Feb 2011

Concordant U-Pb ages of detrital zircons in several Proterozoic units in the Avalon Composite Terrane of Nova Scotia, along with previously published data, indicate that deposition of low- and high-grade stratified units in the Cobequid Highlands (Gamble Brook Formation) and central Cape Breton Island (variously assigned to the George River Group or George River Metamorphic Suite and the Bras d'Or Gneiss) started after ~977 Ma, extended through a period of volcanic activity between ~694 and 637 Ma, and ended before the ~629-612-600 Ma volcanic activity associated with deposition of the Georgeville and Jeffers groups in the Antigonish and Cobequid highlands, and the East Bay Hills and Coxheath groups in southern Cape Breton Island. These age data, along with U-Pb analyses of metamorphic and igneous zircon and monazite, also constrain three Late Proterozoic tectonothermal events: (i) oblique sinistral ductile shear deformation and greenschist-amphibolite-facies metamorphism probably occurred between ~694 and 629 Ma; (ii) oblique dextral deformation and greenschist-facies metamorphism is bracketed between ~612 and 607 ± 2 Ma; and (iii) low-angle, normal shear deformation associated with low-pressure, high-temperature metamorphism took place at ~553-550 Ma. These three events may record (i) opening and (ii) closing of the Antigonish-Cobequid back-arc basin, followed by (iii) gravitational collapse of the central Cape Breton magmatic arc following cessation of subduction, respectively. The combination of ~700-550 Ma arc magmatism and ~1.3-1.0 Ga concordant detrital zircon ages in the Avalon Composite Terrane suggests provenance in the Amazon craton or Oaxaquia in Mexico.

Proterozoic history of the Borborema Province (NE Brazil): Correlations with neighboring cratons and Pan-African belts and implications for the evolution of western Gondwana

Data

Full-text available

Aug 2003

Sérgio P. Neves

Geological and geochronological correlations between Borborema province (NE Brazil) and neighboring cratons and Brasiliano/Pan-African belts indicate that the Amazonian, West African, and São Francisco/Congo cratons and the basement of the Araguaia, Borborema, Nigerian, and Cameroon provinces were part of the Atlantica supercontinent. This continent was established at the end of the Transamazonian/Eburnean cycle (∼2.0 Ga) and, apart from ubiquitous taphrogenesis in the 1.8-1.7 Ga interval, remained largely unaffected for the following 1 Ga. Around 1 Ga an important magmatic event in Borborema province correlates with rifting episodes and anorogenic magmatism in the São Francisco, Congo, and Amazonian cratons. These events are interpreted as failed attempts to break up Atlantica, which at this time may have been part of the larger Rodinia supercontinent. Renewed extensional conditions in Borborema province during the middle and late Neoproterozoic are attributed to far-field stresses transmitted to the interior of Atlantica by outwardly dipping subduction zones that encircled its northern (present day coordinates) portion. The rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the Borborema province essentially includes reworked intracontinental domains. Regional deformation and metamorphism, starting at 650-640 Ma, and shear zone development, beginning at 590-595 Ma, were continuously developed through time and were synchronous throughout most of the Borborema, Araguaia, Cameroon, and Nigerian provinces. Postorogenic conditions were reached 540-530 Myr ago, while active deformation was still occurring in other belts that accreted around Atlantica to form western Gondwana.

Precambrian successions in SW Iberia: Their relationship to 'Cadomian' orogenic events

Chapter

Full-text available

May 1990

Cecilio Quesada

Despite extensive reworking in Devonian and Carboniferous times, two major deformational events taking place in late Precambrian times may be recognized in the SW Iberian Massif. These have been collectively referred to as ‘Cadomian’ or ‘Pan-African’. The Precambrian successions may be subdivided into pre-orogenic and syn-orogenic groups on the basis of their tectonostratigraphic evolution with respect to the Cadomian orogeny. The pre-orogenic successions (roughly mid-upper Riphean) represent different parts of a miogeoclinal wedge, the basement of which is not known as yet. Polyphase deformation and variable degrees of metamorphism accompanied the first Cadomian event. The syn-orogenic successions (roughly upper Riphean-Vendian) were laid down during and after the first Cadomian event and were in turn deformed and slightly metamorphosed by the second single-phased deformation. These correspond to two distinct types: calc-alkaline igneous successions and flysch-like deposits, that can be interpreted respectively as an orogenic continental arc suite and a foreland basin fill, on the basis of their geochemical and sedimentological characteristics.

Basement isotopic signatures and Neoproterozoic paleogeography of Avalonian-Cadomian and related terranes in the Circum-North Atlantic

Article

Full-text available

Jan 1996

Avalonian-Cadomian and related terranes of the circum-North Atlantic constitute portions of an accretionary orogen that developed on an active Gondwanan margin at the periphery of the Neoproterozoic supercontinent Vendia. As a peripheral orogenic belt lying oblique to adjacent cratonic age provinces and the interior collisional orogenic sutures associated with supercontinent assembly, its component terranes are likely to have evolved upon different cratonic basements. Available Nd isotopic data and U-Pb detrital zircon ages support this contention and suggest that West Avalonia, Carolina, and Florida lay adjacent to the Amazonian craton during the Neoproterozoic, whereas Cadomia, Spain, and portions of East Avalonia occupied positions adjacent to the West African craton. Neodymium isotopic compositions for crustally derived felsic rocks from each of these peri-Gondwanan terranes are typical of the upper crust (Sm/Nd ∼ 0.19). However, initial eNd values for West Avalonia are strongly positive with 1.1-0.8 Ga depleted mantle model ages, whereas those for Cadomia are predominantly negative and yield crustal residence ages of 1.9-1.0 Ga. Nd evolution diagrams for West Avalonia show little overlap with those for Cadomia (suggesting basements of contrasting isotopic characteristics) but closely match those of the Tocantins Province that borders the Amazonian craton in Central Brazil. A peri-Amazonian position for West Avalonia is also suggested by detrital zircon data from Neoproterozoic metasedimentary units that match all age provinces of the Amazonian craton. By contrast, Nd isotopic data for Cadomian basement (ca. 2.0 Ga Icart Gneiss) closely resemble those for 2.1-2.0 Ga (Eburnian) granitoids in West Africa. Basement of similar isotopic characteristics to that of Cadomia also provided the source for Cambrian sedimentary rocks in the Meguma terrane that contain detrital zircons that match each of the age provinces of the West African craton. East Avalonia shows Nd isotopic affinities with both West Avalonia and Cadomia and may straddle a suture between their respective basements. Zircon ages and Nd isotopic data for Carolina and the Florida subsurface (Suwannee terrane) also show similarities with those for West Avalonia but suggest a more evolved crustal component of possible Grenville age, supporting their derivation from more westerly present-day peri-Amazonian positions. Such variations in basement isotopic signatures provide important constraints for Neoproterozoic paleogeographic reconstruction of the now-dismembered Avalonian-Cadomian orogenic belt and may be of more general application to the palinspastic restoration of dispersed Precambrian terranes.

Contrasting basement isotopic signatures and the palinspastic restoration of peripheral orogens: Example from the Neoproterozoic Avalonian-Cadomian belt

Article

Full-text available

Jan 1994
GEOLOGY

The Avalonian-Cadomian orogenic belt developed at the periphery of a Neoproterozoic supercontinent oblique to interior collisional orogenic sutures associated with supercontinent amalgamation. As a result, parts of the Avalonian-Cadomian orogenic belt are likely to have evolved upon different cratonic basements. Available Nd and U-Pb (detrital zircon) isotopic data support this contention and suggest that West Avalonia lay adjacent to the Amazonian craton during the Neoproterozoic, whereas Cadomia and, possibly, East Avalonia occupied positions adjacent to the West African craton. Such variations in basement isotopic signatures provide important information for Neoproterozoic paleogeographic reconstruction of the now-dismembered orogenic belt and may be of more general application to the palinspastic restoration of dispersed Precambrian terranes.

U--Pb SHRIMP Dating of Detrital Zircons from Paleozoic and Mesozoic Sandstone in Israel and Jordan

Article

Full-text available

Jan 2006

Origin of northern Gondwanaland Cambrian Sandstone revealed by detrital zircon SHRIMP dating

Article

Full-text available

Mar 2003
GEOLOGY

Voluminous Paleozoic sandstone sequences were deposited in northern Africa and Arabia following an extended Neoproterozoic orogenic cycle that culminated in the assembly of Gondwana. We measured sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of detrital zircons separated from several Cambrian units in the Elat area of southern Israel in order to unravel their provenance. This sandstone forms the base of the widespread siliciclastic section now exposed on the periphery of the Arabian-Nubian shield in northeastern Africa and Arabia. Most of the detrital zircons we analyzed yielded Neoproterozoic concordant ages with a marked concentration at 0.55 0.65 Ga. The most likely provenance of the Neoproterozoic detritus is the Arabian-Nubian shield; 0.55 0.65 Ga was a time of posttectonic igneous activity, rift-related volcanism, and strike-slip faulting there. Of the zircons, 30% yielded pre-Neoproterozoic ages grouped at 0.9 1.1 Ga (Kibaran), 1.65 1.85 Ga, and 2.45 2.7 Ga. The majority of the pre-Neoproterozoic zircons underwent Pb loss, possibly as a consequence of the Pan-African orogeny resetting their provenance. Rocks of the Saharan metacraton and the southern Afif terrane in Saudi Arabia (˜1000 km south of Elat) are plausible sources of these zircons. Kibaran basement rocks are currently exposed more than 3000 km south of Elat (flanking the Mozambique belt), but the shape of the detrital zircons of that age and the presence of feldspar in the host sandstone are not fully consistent with such a long-distance transport. Reworking of Neoproteorozoic glacial detritus may explain the presence of Kibaran detrital zircons in the Cambrian of Elat, but the possibility that the Arabian-Nubian shield contains Kibaran rocks (hitherto not recognized) should also be explored.

Terrane accretion and dispersal in the northern Gondwana margin. An Early Paleozoic analogue of a long-lived active margin

Article

Full-text available

Apr 2003
TECTONOPHYSICS

Cited By (since 1996): 52, Export Date: 11 October 2012, Source: Scopus, doi: 10.1016/S0040-1951(03)00023-4, Language of Original Document: English, Correspondence Address: Gutiérrez-Alonso, G.; Departamento de Geología, Universidad de Salamanca, Salamanca 33708, Spain; email: gabi@gugu.usal.es, References: Bonjour, J.L., Peucat, J.J., Chauvel, J.J., Paris, F., Cornichet, J., U-Pb zircon dating of the Early Paleozoic (Arenigian) transgression in western Brittany (France): A new constraint for the Lower Paleozoic time-scale (1988) Chemical Geology, 72, pp. 329-336;

Origin of the Rheic Ocean: Rifting along a Neoproterozoic suture?

Article

Full-text available

May 2006
GEOLOGY

The Rheic Ocean is widely believed to have formed in the Late Cambrian-Early Ordovician as a result of the drift of peri-Gondwanan terranes, such as Avalonia and Carolina, from the northern margin of Gondwana, and to have been consumed in the Devonian Carboniferous by continent-continent collision during the formation of Pangea. Other peri-Gondwanan terranes (e.g., Armorica, Ossa-Morena, northwest Iberia, Saxo-Thuringia, Moldanubia) remained along the Gondwanan margin at the time of Rheic Ocean formation. Differences in the Neoproterozoic histories of these peri-Gondwanan terranes suggest the location of the Rheic Ocean rift may have been inherited from Neoproterozoic lithospheric structures formed by the accretion and dispersal of peri-Gondwanan terranes along the northern Gondwanan margin prior to Rheic Ocean opening. Avalonia and Carolina have Sm-Nd isotopic characteristics indicative of recycling of a juvenile ca. 1 Ga source, and they were accreted to the northern Gondwanan margin prior to voluminous late Neoproterozoic arc magmatism. In contrast, Sm-Nd isotopic characteristics of most other peri-Gondwanan terranes closely match those of Eburnian basement, suggesting they reflect recycling of ancient (2 Ga) West African crust. The basements of terranes initially rifted from Gondwana to form the Rheic Ocean were those that had previously accreted during Neoproterozoic orogenesis, suggesting the rift was located near the suture between the accreted terranes and cratonic northern Gondwana. Opening of the Rheic Ocean coincided with the onset of subduction beneath the Laurentian margin in its predecessor, the Iapetus Ocean, suggesting geodynamic linkages between the destruction of the Iapetus Ocean and the creation of the Rheic Ocean.

Geodynamic setting and geochemical signatures of Cambrian–Ordovician rift-related igneous rocks (Ossa-Morena Zone, SW Iberia)

Article

Full-text available

Apr 2003
TECTONOPHYSICS

An important rifting event, accompanied by massive igneous activity, is recorded in the Ossa-Morena Zone of the SW Iberian Massif (European Variscan Orogen). It likely culminated in the formation of a new oceanic basin (Rheic ocean?), remnants of which appear presently accreted at the southern margin of the Ossa-Morena Zone. Rifting propagated diachronously across the zone from the Early Cambrian to the Late Ordovician, but by Early Ordovician time, the existence of a significant tract of new ocean is evidenced by a breakup unconformity. Although early stages of rifting were not accompanied by mantle-derived igneous activity, a pronounced increase of the geothermal gradient is indicated by partial melting of metasedimentary protoliths in the upper and middle crust, and by coeval core-complex formation. Geochemistry of the main volume of igneous rocks, emplaced some million years later during more mature stages of rifting, suggests an origin in a variably enriched asthenospheric source, similar to that of many OIB, from which subsequent petrogenetic processes produced a wide range of compositions, from basalt to rhyolite. A tectonic model involving collision with, and subsequent overriding of, a MOR is proposed to account for the overall evolution, a present-day analogue for which lies in the overriding of the East Pacific Rise by North America and the rifting of Baja California.

Avalonian and related peri-Gondwanan terranes of the Circum-North Atlantic

Book

Jan 1996

The "Urra Formation": age and Precambrian inherited record

Article

Jan 2006

Mapa Tectónico de la Península Ibérica

Article

Jan 1974

El magmatismo calcoalcalino del Precámbrico terminal en la zona de Ossa Morena (Macizo Ibérico). Revista de la Sociedad Geológica de España 2: 7-21

Article

Jan 1989

Esquema geológico do Maciço de Évora

Article

Jan 1983

A. Carvalhosa

Contribuição para o conhecimento geológico da região entre Portel e Ficalho (Alentejo)

Article

Jan 1965

A. Carvalhosa

The Ediacara Biota: A terminal neoproterozoic experiment in the evolution of life

Article

Jan 1998

G.M. Narbonne

The Ediacara biota is a distinctive assemblage of large, soft-bodied organisms that characterizes terminal Neoproterozoic (latest Precambrian) strata worldwide. Some Ediacaran organisms apparently were the root-stock for the Phanerozoic evolution of animals; other bizarre forms may represent a failed experiment in Precambrian evolution. The Ediacara biota and its nonactualistic preservation and ecosystem characterized the final 20 m.y. of the Proterozoic, and disappeared near the beginning of the Cambrian "explosion" of shelly and burrowing animals.

Large scale sediment dispersal associated with the late Neoproterozoic assembly of Gondwana

Article

Jan 2002

Internal structure of the Évora High-grade Terrains and the Montemor-o-Novo Shear Zone (Ossa-Morena Zone, Portugal)

Article

Jan 2003

Cape Town's Table Mountain reveals rapid Pan-African uplift of its basement rocks

Article

Jan 1998
J AFR EARTH SCI

Noticia explicativa. Carta Geologica de Portugal na 1/250 000. Folha 7 : Portimão

Book

Jan 1984

Alternative approach to the Variscan Belt in southwestern Europe: Preorogenic paleobiogeographical constraints

Chapter

Jan 2002

M. Robardet

Most of the models proposed for the Variscan Belt of southwestern Europe have been based on structural, metamorphic, and magmatic data. An alternative approach is proposed here, based on sedimentological and faunal data that allows the definition and delimitation of different domains and evaluation of their relative positions and relations within the preorogenic paleogeography. Several previous models, which have not taken account of pre-Variscan paleogeographical data, can no longer be sustained. In particular, models that propose two distinct oceanic sutures within the French part of the belt should be abandoned. A model with a single Variscan ocean would accord much more satisfactorily with paleobiogeographical limitations. If it is accepted that, during pre-Variscan time, the Mid-North Armorican regions were not in their present positions, but formed an extension of the Central Iberian region, a hypothesis based on faunal and sedimentary evidence, then the single-ocean model proves more appropriate for the French section of the Variscan Belt.

The Moroccan Hercynides

Chapter

Jan 1991

Geological exploration of Morocco began at the end of the last century and developed rapidly. The first reliable synthesis of much of the country was made by Gentil and illustrated by a map at a scale of 1/500000 (Gentil 1920). Several regional descriptions were carried out by the Geological Survey, progressing roughly from north to south. Choubert summarized the geology with a synoptic map at 1/500000 (Choubert 1952), followed in 1985 by the current 1/1000000 map, which includes the southern provinces. More recent work has often involved collaboration between the Geological Survey and other organizations such as the rapidly growing Moroccan universities. Present research is devoted to thematic studies that are largely structural, and detailed mapping at scales of 1/100000 and 1/50000.

The structure and evolution of the Hercynian foldbelt in the Iberian Peninsula

Chapter

Dec 1987

Steiger, R. H. & Jaeger, E. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359-362

Article

Oct 1977
EARTH PLANET SC LETT

Assembling West Gondwana in the Neoproterozoic: Clues from the S??o Francisco craton region, Brazil

Article

Apr 2001
GEOLOGY

The São Francisco craton, which consists of Archean and Paleoproterozoic basement, now forms eastern Brazil; it once was at the center of West Gondwana. Distinct Neoproterozoic Brasiliano (Pan-African) orogenic belts border the craton's margins. Crosscutting relationships among these belts, along with stratigraphic features of the cover in the craton's interior, provide constraints on the sequence of collisions leading to the assembly of West Gondwana. The earliest collision involved the São Francisco Congo and Rio de la Plata cratons and resulted in the formation of the Southern Brasília belt. Next came the closure of the Brazilide ocean by collision between the São Francisco Congo and Amazonia cratons. This event produced the Northern Brasília belt, which now defines the northwest side of the São Francisco craton. An intracontinental rift, which merged southward with a narrow sea, once was between the São Francisco and Congo cratons; closure of this rift and sea led to the formation of the Araçuaí and West Congo orogen, and to the southwestward extrusion of the Ribeira belt. Continued convergence of Amazonia against the São Francisco Congo craton caused the eastward extrusion of the Borborema province. The final stage of West Gondwana assembly closed a basin between the Rio de la Plata and Amazonia cratons and created the Paraguai belt.

Birth of a volcanic passive margin in Cambrian time: Rift paleogeography of the Ossa-Morena Zone, SW Spain

Article

Jul 2006
PRECAMBRIAN RES

Mapping at 1:10000 scale of both north (N) and south (S) flanks of the Variscan Monesterio antiform in the Ossa-Morena Zone (southern Spain), in association with detailed petrographic and major element analyses of magmatic rocks has revealed the building of a passive, but volcanically active, oceanic margin in Cambrian time. This margin was located in northern Gondwana and represents the beginning of an extensional tectonic regime that concluded with the opening of the Rheic Ocean. At time of eruption (Middle Cambrian), the N flank was located on the northern coastline of a marine basin in the process of passive rifting. The vents, mainly located in its western part, were subaerial continental and/or shallow marine. On the S flank, eruptions and deposits were shallow marine, occurring as lava flows and pillows interstratified within shallow marine sediments. On both flanks, intrusives were emplaced after the extrusives. Lateral thickness variations of the sedimentary deposits testify to the building, during four distinct periods, of a rifted passive margin with a shifting locus of volcanic activity over time. The composition of magmas was bimodal. The mafic magmas are silica-saturated, alkaline to sub-alkaline and somewhat Ti-rich, perhaps originating from partial melting of a subcontinental mantle that was less metasomatized with time. The youngest magmatism, with an affinity close to that of MORB, likely had an asthenospheric source. The alkaline felsic products are diverse and may have originated from a very heterogeneous continental crust. In each set, partial melting relationships prevail, suggesting that the melts rose along open fractures as soon as they formed. In the context of lithospheric thinning, it is possible that mantle and crustal melting was triggered by decoupling between rheologically different materials.

Early Cambrian palaeogeography and the probable Iberia–Siberia connection

Article

Jul 2002
TECTONOPHYSICS

Alexander P. Gubanov

The end of the Proterozoic–beginning of the Cambrian is marked by some of the most dramatic events in the history of Earth. The fall of the Ediacaran biota, followed by the Cambrian Explosion of skeletonised bilaterians, a pronounced shift in oceanic and atmospheric chemistry and rapid climatic change from ‘snowball earth’ to ‘greenhouse’ conditions all happened within a rather geologically short period of time. These events took place against a background of the rearrangement of the prevailing supercontinent; some authors view this as a sequence of individual supercontinents such as Mesoproterozoic Midgardia, Neoproterozoic Rodinia and Early Cambrian Pannotia. Assembled in the Mesoproterozoic, this supercontinent appears to have existed through the Neoproterozoic into the Early Cambrian with periodic changes in configuration. The final rearrangement took place during the Precambrian–Cambrian transition with the Cadomian and related phases of the Pan-African orogeny. The distribution of Early Cambrian molluscs and other small shelly fossils (SSF) across all continents indicates a close geographic proximity of all major cratonic basins that is consistent with the continued existence of the supercontinent at that time. Subsequently, Rodinia experienced breakup that led to the amalgamation of Gondwana, separation of Laurentia, Baltica, Siberia and some small terranes and the emergence of oceanic basins between them. Spreading oceanic basins caused a gradual geographic isolation of the faunal assemblages that were united during the Vendian–Early Cambrian.

Cadomian Orogens, peri-Gondwanan correlatives and Laurentia–Baltica connections

Article

Jul 2002
TECTONOPHYSICS

A search for ancient detrital zircons in Zimbabwean sediments

Article

Dec 1988

86 detrital zircon grains from Archaean quartzites from the Mweza and Shurugwi greenstone belts have been dated by the ion probe SHRIMP. Nine of these give ages of 3.75-3.80 Ga, older than any Zimbabwean rocks so far satisfactorily dated, and the Tokwe-Zvishavane gneisses are a possible source for these old grains. However, none are comparable in age with the 4.2 Ga zircons discovered in Western Australia. If such old grains exist in the sample it is unlikely (P <0.05) that their abundance could exceed 3%. Ages of the younger grains are broadly consistent with the presumed age of the quartzites sampled, though some high Th/U grains in the Shurugwi sample may by anomalously young. -Authors

A Sequence of Pan-African and Hercynian Events Recorded in Zircons from an Orthogneiss from the Hercynian Belt of Western Central Iberia--an Ion Microprobe UPb Study

Article

Aug 2004

A study of inherited zircons in granitoid rocks from the South Portuguese and Ossa-Morena Zones, Iberian Massif: Support for the exotic origin of the South Portuguese Zone

Article

Jul 2002
TECTONOPHYSICS

Trace element and U–Pb isotopic analyses of inherited zircon cores from a sample of Gil Márquez granodiorite (South Portuguese Zone, SPZ) and Almonaster nebulite (Ossa-Morena Zone, OMZ, in the Aracena Metamorphic Belt) have been obtained using laser ablation-inductively coupled plasma-mass spectrometry. These data reveal differences in the age of deep continental crust in these two zones. Inherited zircon cores from the Ossa-Morena Zone range at 600±100 Ma, 1.7–2 Ga and 2.65–2.95 Ga, while those from the South Portuguese Zone range at 400–500 and 700–800 Ma. These data support the “exotic” origin of the South Portuguese Zone basement relative to the rest of Iberian Massif. The young ages of inherited zircon cores and Nd model ages of magmatic rocks of the South Portuguese Zone are comparable to results from granulite facies xenoliths and granitic rocks from the Meguma Terrane and Avalonia and support a correlation between the basement of the southernmost part of the Iberian Massif and the northern Appalachians.

Precambrian—Cambrian boundary and correlation from southwestern and central part of Spain

Article

May 1984
GEOL MAG

This paper comprises the first palaeontological correlation between the Upper Proterozoic–Lower Cambrian Series of Ossa-Morena, Luso-Oriental-Alcúdica and Galaico-Castellana Zones of the Iberian Peninsula. The authors show the palaeontological events on acritarchs, stromatolites, cyanophyta, soft-bodied metazoa, trilobites, archaeocyathans and skeletal microfossils from three representative sequences of the above mentioned zones and discuss the position of the Precambrian–Cambrian boundary in the sequences. The Sierra de Córdoba general Series (Ossa-Morena Zone) has an erosive discontinuity between the rocks with the first record of Cambrian metazoan activity ( Skolithos sp., Monomorphichnus sp., Phycodes pedum and Rusophycus sp.) and the rocks with a palynological association ( Bavlinella faveolata, Protosphaeridium flexosum, Trachisphaeridium sp., aff. Octoedryxium truncatum, Phycomicetes ? sp. and cf. Ooidium sp.) which suggests a Lower–Middle Vendian age. For this reason we suggest that the Precambrian–Cambrian boundary be placed at this unconformity. The upper part of the Sierra de Guadalupe Series (Luso-Oriental-Alcúdica Zone) includes detrital beds with Phycodes pedum, Planolites sp. and Treptichnus sp. which are also the first record of Cambrian metazoan activity. In the middle part of the sequences, acritarchs attributed to the genus Micrhystridium are found at the top of the Calcareous Beds. This is taken to indicate an early Cambrian age. Vendotaenids and Bavlinella faveolata are found in abundance, thus indicating a Late Vendian age for the middle part of this unit. Thus, we propose the Precambrian–Cambrian boundary be located in the upper part of the Calcareous Beds. In the Rio Uso Series (Galaico-Castellana Zone), the Azorejo Sandstones contain Rusophycus gr. radwanskii, Planolites sp. and Gordia sp. Trace fossils produced by trilobite-like arthropoda ( Monomorphichnus ) have been cited in the upper part of the underlying Pusa Shales. Moreover, Octoedryxium truncatum and Bavlinella faveolata are found in the lower part of this unit which suggest that the Precambrian–Cambrian boundary could be situated in the Pusa Shales. Calcareous microfossils related to annelid polychaetes are found associated with Upper Vendian acritarchs in the Calcareous Beds of Sierra de Guadalupe. They are the oldest record of skeletal metazoa in the Spanish Series. The medusoids found are associated with a Vendian flora and they are also the oldest non-skeletal metazoa record known in the Iberian Peninsula.

Neoproterozoic-early Cambrian geology and palaeontology of Iberia

Article

Nov 1994
GEOL MAG

Neoproterozoic-early Cambrian deposition in Central Iberia can be accommodated in a model that implies a generalized stretching of the crust during an extensional event which closely followed the Cadomian phase of the Pan-African Orogeny and which eventually could have included transcurrent components. An extensional phase with transcurrent components during the deposition of the Ibor and lower Rio Huso groups is regarded as a probable cause of widespread ponding resulting in the juxtaposition of platform and basinal successions, eventually leading to anoxic conditions in Pusa shale deposition times. A possible cause for repeated collapse events developing olistostromes and intra-sequential folding could be sought in this tectonic context. The fossil contents of these units and their change with time are described. -from Authors

Proterozoic and Cambrian successions in Upper Silesia: An Avalonian terrane in southern Poland

Article

Sep 1997
GEOL MAG

Malgorzata Moczydlowska

All Cambrian series and several Cambrian biozones have been recognized using acritarch biochronology within the siliciclastic successions underlying Upper Silesia in southern Poland. The entire Cambrian succession is around 580 m thick and contains rare Lower Cambrian trilobites of the Acado-Baltic faunal province. Acritarch associations are taxonomically comparable to those recorded in Baltica, Laurentia and Gondwana, but their closest taxonomic affinity is with Iberia. The Cambrian succession accumulated in a shallow shelf environment and is almost flat-lying, unmetamorphosed, uncleaved and in normal stratigraphic order. It underlies paraconformably Lower Devonian deposits and overlies unconformably steeply dipping metasediments of undetermined Precambrian age. Tectonic deformation and metamorphism to greenschist grade in these Precambrian strata must have occurred in the Proterozoic, and are attributed to the Cadomian orogeny because similar Cadomian basement complexes occur in the adjoining Brno Massif and in the East Avalonian and Armorican terranes. Upper Silesia appears to be a stable crustal block bordered by deep faults whose sedimentary cover has not been affected by tectonic deformation other than faulting. Based on the recognition of Cadomian age basement, the distribution of trilobites and acritarchs and the tectonostratigraphic relationships to adjacent areas, the Upper Silesia terrane is interpreted to be a distal segment of East Avalonia that in Cambrian times faced Iberia. An extension of the Tornquist Suture from the Intra-Sudetic Fault is seen in the Kraków-Myszków Fault Zone at the margin of Upper Silesia. The Intra-Sudetic Fault zone and the Kraków-Myszków Fault Zone contain Early Palaeozoic rocks deformed during the Caledonian orogeny, and mark the boundary between the Caledonian accretionary belt and areas unaffected by this orogeny.

Inherited arc signature in Ediacaran and Early Cambrian basins of the Ossa-Morena Zone (Iberian Massif, Portugal): Paleogeographic link with European and North African Cadomian correlatives

Article

Feb 2006
PRECAMBRIAN RES

Geochemical data from clastic rocks of the Ossa-Morena Zone (Iberian Massif) show that the main source for the Ediacaran and the Early Cambrian sediments was a recycled Cadomian magmatic arc along the northern Gondwana margin. The geodynamic scenario for this segment of the Avalonian-Cadomian active margin is considered in terms of three main stages: (1) The 570–540 Ma evolution of an active continental margin evolving oblique collision with accretion of oceanic crust, a continental magmatic arc and the development of related marginal basins; (2) the Ediacaran–Early Cambrian transition (540–520 Ma) coeval with important orogenic magmatism and the formation of transtensional basins with detritus derived from remnants of the magmatic arc; and (3) Gondwana fragmentation with the formation of Early Cambrian (520–510 Ma) shallow-water platforms in transtensional grabens accompanied by rift-related magmatism. These processes are comparable to similar Cadomian successions in other regions of Gondwanan Europe and Northwest Africa. Ediacaran and Early Cambrian basins preserved in the Ossa-Morena Zone (Portugal and Spain), the North Armorican Cadomian Belt (France), the Saxo-Thuringian Zone (Germany), the Western Meseta and the Western High-Atlas (Morocco) share a similar geotectonic evolution, probably situated in the same paleogeographic West African peri-Gondwanan region of the Avalonian-Cadomian active margin.

Tectonic setting of Cadomian low-pressure metamorphism in the central Ossa-Morena zone (Iberian massif, SW spain)

Article

Apr 1992
PRECAMBRIAN RES

The upper Proterozoic Serie Negra (a monotonous, pre- and syntectonic, slate-greywacke series including volcanic episodes) crops out widely in the central area of the Ossa Morena Zone (SW Iberian Massif, Iberian Peninsula). This unit was affected by complex late Precambrian tectonothermal events (the Cadomian Orogeny) during which two phases of deformation (D1 and D2) are recognized in the 610-585 Ma time span. The second deformational episode (D2) was the most prominent and was related to progressive, low pressure/high temperature metamorphism (LP-HT) reaching the highest grade in the lower structural levels, where thermal evolution led to extensive migmatization and gave rise to the generation of different kinds of anatectic granitoids.

The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: A review

Article

Apr 2001
TERRA NOVA

Ph. Matte

The Variscan belt of western Europe is part of a large Palaeozoic mountain system, 1000 km broad and 8000 km long, which extended from the Caucasus to the Appalachian and Ouachita mountains of northern America at the end of the Carboniferous. This system, built between 480 and 250 Ma, resulted from the diachronic collision of two continents: Laurentia–Baltica to the NW and Gondwana to the SE. Between these two continents, small, intermediate continental plates separated by oceanic sutures mainly have been defined (based on palaeomagnetism) as Avalonia and Armorica. They are generally assumed to have been detached from Gondwana during the early Ordovician and docked to Laurentia and Baltica before the Carboniferous collision between Gondwana and Laurentia–Baltica. Palaeomagnetic and palaeobiostratigraphic methods allow two main oceanic basins to be distinguished: the Iapetus ocean between Avalonia and Laurentia and between Laurentia and Baltica, with a lateral branch (Tornquist ocean) between Avalonia and Baltica, and the Rheic ocean between Avalonia and the so-called Armorica microplate. Closure of the Iapetus ocean led to the Caledonian orogeny: a belt resulting from collision between Laurentia and Baltica, and from softer collisions between Avalonia and Laurentia and between Avalonia and Baltica. Closure of the Rheic ocean led to the Variscan orogeny by collision of Avalonia plus Armorica with Gondwana. A tectonic approach allows this scenario to be further refined. Another important oceanic suture is defined: the Galicia–Southern Brittany suture, running through France and Iberia and separating the Armorica microplate into North Armorica and South Armorica. Its closure by northward (or/and westward?) oceanic and then continental subduction led to early Variscan (430–370 Ma) tectonism and metamorphism in the internal parts of the Variscan belt. As no Palaeozoic suture can be detected south of South Armorica, this latter microplate should be considered as part of Gondwana since early Palaeozoic times and during its Palaeozoic north-westward drift. Thus, the name Armorica should be restricted to the microplate included between the Rheic and the Galicia–Southern Brittany sutures.

Response of detrital and zircon and monazite, and their U-Pb isotopic systems, to regional metamorphism and host-rock partial melting, Cooma Complex, Southeastern Australia

Article

Aug 2001
AUST J EARTH SCI

I. S. Williams

Progressive Early Silurian low-pressure greenschist to granulite facies regional metamorphism of Ordovician flysch at Cooma, southeastern Australia, had different effects on detrital zircon and monazite and their U–Pb isotopic systems. Monazite began to dissolve at lower amphibolite facies, virtually disappearing by upper amphibolite facies, above which it began to regrow, becoming most coarsely grained in migmatite leucosome and the anatectic Cooma Granodiorite. Detrital monazite U–Pb ages survived through mid-amphibolite facies, but not to higher grade. Monazite in the migmatite and granodiorite records only metamorphism and granite genesis at 432.8 ± 3.5 Ma. Detrital zircon was unaffected by metamorphism until the inception of partial melting, when platelets of new zircon precipitated in preferred orientations on the surface of the grains. These amalgamated to wholly enclose the grains in new growth, characterised by the development of {211} crystal faces, in the migmatite and granodiorite. New growth, although maximum in the leucosome, was best dated in the granodiorite at 435.2 ± 6.3 Ma. The combined best estimate for the age of metamorphism and granite genesis is 433.4 ± 3.1 Ma. Detrital zircon U–Pb ages were preserved unmodified throughout metamorphism and magma genesis and indicate derivation of the Cooma Granodiorite from Lower Palaeozoic source rocks with the same protolith as the Ordovician sediments, not Precambrian basement. Cooling of the metamorphic complex was relatively slow (average ~12°C/106 y from ~730 to ~170°C), more consistent with the unroofing of a regional thermal high than cooling of an igneous intrusion. The ages of detrital zircon and monazite from the Ordovician flysch (dominantly composite populations 600–500 Ma and 1.2–0.9 Ga old) indicate its derivation from a source remote from the Australian craton.

Conventional and ion-microprobe U-Pb dating of detrital zircons of the Tentud�a Group (Serie Negra, SW Spain): implications for zircon systematics, stratigraphy, tectonics and the Precambrian/Cambrian boundary

Article

Mar 1993

Conventional multi-grain and ion-microprobe dating of detrital zircons from a very low grade metagraywacke of the Tentuda Group (upper part of the Serie Negra, Ossa-Morena Zone, SW Spain) reveals an uppermost Vendian age for the deposition of the metagraywacke. The youngest detrital zircon grain provides a maximum depositional age of about 565 Ma. Thus, these data contradict earlier Middle to Upper Riphean (ca. 1350-850 Ma) estimates on the age of the Tentuda Group and favour a Precambrian/Cambrian boundary falling into the range of 540 to 530 Ma. The presence of about 20% of Pan-African detrital zircons ranging from about 700 to 550 Ma indicates the derivation from Gondwana. From the upper intercept ages of the fan-shaped data field defined by conventionally determined zircon fractions, it can be deduced that 2.1 Ga old zircons as well as Archean zircons existed in the provenance(s) of the Serie Negra sediments. This mixing of crustal components of different ages is in line with the Nd crustal residence age of 1.9 Ga. The latter value, as well as other model ages of the Iberian Massif, indicates unusually high amounts of ancient crust to be present in the strata. This is different to other (meta)sediments of the European Hercynides and suggests that the Iberian strata of uppermost Precambrian age may contain the detritus of more internal, older parts of Gondwana than other European strata of comparable ages. Geochemical data on the analysed sample and further metagraywackes of the Tentuda Group argue for a deposition in an arc environment. Such a scenario would conform with the syn- to post-orogenic shallow marine deposition of the studied sediments. Furthermore, an upper time limit for the pre-Lower Cambrian deformational history, including two phases of regional deformation, is given by the maximum age of deposition, implying a very short time interval for deposition and deformation of the Tentuda Group. Concerning the U-Pb systematics of detrital zircon fractions, it is probable that numerous, previously published conventional multi-grain zircon data on (meta)sedimentary rocks of the European Hercynides readily can be explained by the presence of up to 20% of Pan-African detrital zircons and later Phanerozoic lead loss during metamorphic transformation of the sedimentary protoliths. Moreover, this implies that such metasediments originated from post-Pan-African sedimentary precursors.

Isotopic evidence for the Precambrian provenance and Caledonian metamorphism of high grade paragneisses from the Seve Nappes, Scandinavian Caledonides - II. Ion microprobe zircon U-Th-Pb

Article

Oct 1987

Ion microprobe U-Th-Pb analyses of residual cores and metamorphic mantles of zircons from three high grade paragneisses from the Seve Nappe Complex, north-western Sweden, show that: 1) The sediments comprising the protolith of the Seve Nappes gneisses over a distance of 250 km were probably derived from similarly-aged source terranes. 2) Those source terranes were dominated by rocks with ages in the range 1400 to 1730 Ma, with minor components at least as young as 1000 Ma. The oldest component identified is 173012 Ma old. 3) Those rocks had U and Th contents normal for felsic igneous rocks. 4) The gneiss protolith was metamorphosed to granulite grade during the Caledonian. There is evidence to suggest that the peak of metamorphism may not have been synchronous throughout the Nappes. 5) The metamorphism probably included a reduction of about a factor of ten in the gneisses' Th/U.

West African provenance for Saxo-Thuringia (Bohemian Massif): Did Armorica ever leave pre-Pangean Gondwana? – U/Pb-SHRIMP zircon evidence and the Nd-isotopic record

Article

Nov 2004

Neoproterozoic rocks in the Saxo-Thuringian part of Armorica formed in an active margin setting and were overprinted during Cadomian orogenic processes at the northern margin of Gondwana. The Early Palaeozoic overstep sequence in Saxo-Thuringia was deposited in a Cambro-Ordovician rift setting that reflects the separation of Avalonia and other terranes from the Gondwana mainland. Upper Ordovician and Silurian to Early Carboniferous shelf sediments of Saxo-Thuringia were deposited at the southern passive margin of the Rheic Ocean. SHRIMP U/Pb geochronology on detrital and inherited zircon grains from pre-Variscan basement rocks of the northern part of the Bohemian Massif (Saxo-Thuringia, Germany) demonstrates a distinct West African provenance for sediments and magmatic rocks in this part of peri-Gondwana. Nd-isotope data of Late Neoproterozoic to Early Carboniferous sedimentary rocks show no change in sediment provenance from the Neoproterozoic to the Lower Carboniferous, which implies that Saxo-Thuringia did not leave its West African source before the Variscan Orogeny leading to the Lower Carboniferous configuration of Pangea. Hence, large parts of the pre-Variscan basement of Western and Central Europe often referred to as Armorica or Armorican Terrane Assemblage may have remained with Africa in pre-Pangean time, which makes Armorica a remnant of a Greater Africa in Gondwanan Europe. The separation of Armorica from the Gondwana mainland and a long drift during the Palaeozoic is not supported by the presented data.

Zircon Zonation Patterns as Revealed by Cathodoluminescence and Backscattered Electron Images - Implications for Interpretation of Complex Crustal Histories

Article

Nov 1993
CHEM GEOL

Zircon exhibits an extraordinary memory. Its stability, durability, low solubility and low elemental diffusivities combine to preserve in it a record of most of the important events that have affected it, its host rocks, and the crust of which it is a part. Zonation in zircon grains delineates the boundaries of discrete geochemical packages formed at different times, each effectively a closed system. The elemental and isotopic compositions of these packages reflect the timing and conditions of growth events, and the morphology of the zonation indicates qualitatively the nature of both growth and intervening degradation events.Cathodoluminescence (CL) and backscattered electron (BSE) imaging reveals detailed zonation patterns that are commonly invisible or barely visible with conventional transmitted and reflected light microscopy. Characteristic patterns are visible in almost all zircons that serve to distinguish igneous from metamorphic growth, to distinguish truncation surfaces of different types (e.g., sedimentary fracturing vs. resorption), and possibly to identify ancient metamictization. Zircons from many rocks record multistage histories that reflect two or more events; those from rocks such as peraluminous granites and high-grade paragneisses are especially likely to reveal long and complex histories.Studies of zonation patterns in zircons provide a clear, though qualitative, history of a rock and its heritage. Furthermore, they provide the basis for a quantification of that history. Elemental and isotopic compositions can reveal the environment in which a zone grew. U-Pb analysis of a zone provides an age for its growth. shrimp analyses that are not guided by detailed knowledge of zonation can straddle two (or more) zones and a discordant U-Pb result from such an analysis may falsely suggest Pb loss, and important growth zones may be missed entirely. Thus, the combined use of CL, BSE, electron microprobe and ion probe methods can elucidate complex crustal histories.

Ore lead ratios in a continuously changing Earth

Article

Dec 1975
EARTH PLANET SC LETT

A critical re-assessment of the construction of simple ore lead isotopic development curves is followed by three fresh approximations, all designed to involve the minimum possible number of assumptions. All are based on the Russell-Reynolds algorithm which in its simplest form involves knowledge only of ratios, not of ages. We apply the calculations to a restricted class of ore leads, and employ the latest constant values for the U and Th isotopes.Model I treats all data as being of equal weight, and shows that the deletion or inclusion on the Canyon Diablo meteorite data makes no differences to the derived parameters.Model II demonstrates that essentially the same parameters result if the simple curve is forced through the meteorite point; i.e. questions about homogeneity or otherwise of “initial terrestral” Pb a unimportant to the regression.Model III makes allowance for the known discrepancy in young “model ages” by providing for a steady linear change in U/Pb and Th/Pb. The additional assumption of one fixed time point proves necessary. An age close to 430 m.y. for Captains Flat, N.S.W., yields acceptable age estimates for most other deposits investigated. No claim is made for the uniqueness of this solution, but the derived evidence for steady growth in U/Pb accompanied by a slight decline in Th/U, seems compatible with a crustal source for the lead ores concerned.

Zircon U-Pb geochronology of paragneisses and biotite granites from the SW Iberian Massif (Portugal): Evidence for apalaeogeographical link between the Ossa-Morena Ediacaran basins and the West African craton

Abstract

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