Fig 3 - uploaded by Samuel Angiboust
Content may be subject to copyright.
Backscattered electron (BSE) images for Polar Urals samples. a. White jadeitite specimen (PU2) showing highly disrupted clasts with evidence for fracturing, dissolution, indentation and re-precipitation. Several clinopyroxene generations can be identified with increasing diopside component towards clast rim. b. Example of a cryptic jadeitite breccia showing extensive fracturing, comminution and dissolution of dark white jadeitite clasts (locally exhibiting oscillatory zoning pattern). Location of sample UR25 given in Fig. 1a. c. Dark, amphibole-rich granofels that is heavily brecciated, with similar dissolution-reprecipitation features as well as extensive healed microfractures (sample PU2). d. Fractures associated with the growth of several amphibole compositions such as richterite (Rct) and eckermannite (Eck) overgrowing magnesio-katophorite (Mkt). Note that omphacite grows lately within a crack in textural equilibrium with Rct and Eck (sample UR11b).
Source publication
An increasing number of seismological studies report transient seismicity clusters in the mantle wedge several kilometers above the subduction interface. Their physical significance with respect to subduction zone seismo-tectonics remains poorly understood. Jadeitites are known to form and/or be associated with mantle wedge serpentinites in the c....
Contexts in source publication
Context 1
... outer rims are commonly lined by Ca-rich, omphacitic compositions as well as interstitial phlogopitic micas (Angiboust et al., 2021; Fig.S1; Table 1 and Table S1). Despite an apparently homogeneous macroscopic texture, detailed petrographic investigations reveal that jadeite crystals exhibit widespread fracturing, dissolution and replacement textures (Fig.3a,b). Vein systems that range in color from white (jadeite composition) to green (Cr-rich jadeite or Cr-rich omphacite; Fig.2a) are ubiquitously found crosscutting the host jadeitite matrix (see also Franz et al, 2014). ...
Context 2
... grains. Orientation maps (Fig.4b) show that [001] axes are dominant along the fine-grained shear zone, whereas [100] and [010] axes become important approaching the margins of the shear band. Intracrystalline misorientation EBSD maps, depicting misorientation angles between each data point and the mean orientation of the parent crystal ( Fig. S3), show that grains at the shear zone margins are highly misoriented. In addition, the finer grains within the shear band are consistently devoid of intragranular misorientation except for strongly misoriented larger ...
Context 3
... comparative and analytical investigation reveals that most of the studied samples from Russia, Guatemala and Myanmar display widespread and similar markers of fracturing that have received little attention in previous studies (e.g., Dobretsov & Ponomareva, 1968;Shi et al., 2009b). Brittle deformation markers span a broad range, from Type I breccias typical of damage zones (e.g. , Figs.2, 6, 10a, 13a), to Type II variably foliated cataclasites that are more diagnostic of strongly localized fault systems (e.g., Rowe et al., 2011;Angiboust et al., 2015;Oncken et al., in revision; Figs.5, 9, 13b), to Type III hydraulic breccias thought to develop at high fluid pressure conditions as extensional veining (e.g. Woodcock et al., 2007). ...
Context 4
... pattern, sometimes hardly visible using optical microscopy, requires the use of specific analytical approaches such as cathodoluminescence imaging or electronic microscopy to be documented (e.g., Figs.3, 12; Shi et al., 2003;Sorensen et al., 2006;Takahashi et al., 2017). Pure extensional fracturing (e.g., hydrofracturing; Type III; Fig.13c) has been identified in several samples, with fragments of the host that appear to have been snatched into clinopyroxene or amphibole-rich veins (Fig.10b). ...
Context 5
... 2021a). Interestingly, the main brecciation/cataclasis event in sample KAS04 is followed by an episode of hydraulic brecciation where Naamphibole precipitates Type III; Fig. S2). Similar deformation patterns have been observed in dark granofels that also comprise breccias (Fig.6), foliated cataclasites (Fig.5) and multiple fracturing events ( Fig.3c; Fig. S5; Table ...
Context 6
... common feature reported in jadeitites worldwide is the enrichment in Ca, Fe and Mg of jadeitic clinopyroxenes due to the infiltration of Ca-Fe-Mg-rich fluids associated with dissolution-precipitation processes during late fluid-rock interaction events (e.g., Sorensen et al., 2006;Garcia-Casco et al., 2009;Harlow et al., 2011;Cardenas-Parraga et al., 2012;Angiboust et al., 2021; this study). The formation of dissolution features (Fig.3c), re-equilibration of clast rims (Fig.8a) and the metasomatic overprint left in the rock record (Fig.9b) are witnesses to the apparently corrosive nature of the infiltrating fluids, which were at thermodynamic disequilibrium with respect to the infiltrated material. ...
Context 7
... discussed earlier, the release of Cr may be related to serpentinization of the host peridotite during fluid-rock interaction. It is important to note that (i) jadeite/omphacite is found growing within brecciated white jadeitites (Figs.3b, 8a and 11c) and amphibole-rich dark granofels (Fig.6d); and (ii) amphibole growth is observed in brecciated dark granofels (Fig.5b) and between white jadeitite breccia fragments (Figs.7f,11a,11b). From these observations, it appears that the incoming fluid composition was not buffered by the wall-rock composition and that brittle events ...
Context 8
... lithologies. Furthermore, the widespread formation of texturally late micas (e.g., phlogopite in the Polar Urals: Angiboust et al., 2021, see also Fig. S1; phengite or phlogopite in Guatemala: Harlow et al., 2011;Flores et al., 2013; white mica in W. Japan: Shigeno et al., 2012) associated with enrichments in LILEs and other fluid mobile elements (Figs. S3 and S4) confirm that fluid sources were variable in time, switching from a mafic oceanic crust signature to a more hybridized composition with transient highs in sedimentary input (e.g., Sorensen et al., 2006Sorensen et al., , 2010Morishita et al., 2007;Meng et al., 2011Meng et al., , 2016Harlow et al., 2015;Chen et al., 2018;Cardenas-Parraga ...
Context 9
... seems clear that nearly lithostatic pore fluid pressure was sustained throughout the entire jadeitite drain activity as demonstrated by the ubiquitous evidence for fluidrock interaction in the three types of brittle features reported here ( Table 1; see also Kuznetsov et al., 1986 andAngiboust et al., 2021). Yet, transient overpressures likely triggered the formation of some of the (explosive) breccia features (Type III) within already-tectonized Type I and Type II domains (Figs.11c,13a,b). These findings are consistent with the episodic opening of vein systems and filling of voids by omphacitic compositions documented by Garcia-Casco et al. (2009) and Cardenas-Parraga et al. (2012) in the Sierra del Convento serpentinite mélange (E. ...
Context 10
... rocks have long been recognized as potential markers of seismic deformation (e.g., Sibson, 1986;Angiboust et al., 2012;Melosh et al., 2014). Evidence of fault-zone rocks with diagnostic elements such as damage zones and fault cores (Fig.13a, b) highlights for the first time that jadeitites (and associated dark amphibole-rich granofels) host abundant brittle deformation events, most likely along fast-slipping seismogenic fault planes. Estimating slip or strain rates along paleo-fault surfaces (or shear zones) is a challenging task, subject to great uncertainties. While Oncken ...
Context 11
... earlier stated by Magloughlin (1992) andSwanson (1992), cataclasites and pseudotachylytes may be closely intricated in fault zone rocks, both exhibiting evidence for open-system modification of the pristine host composition. Thus, the structures herein observed may have formed through (i) metasomatic fault-zone alteration associated with fluid influx within a finely-crushed cataclastic domain (likely a structure as shown in Fig.13b), followed by (ii) fast-slip along narrow zones where local melting of the previously-formed cataclasites occurred (thus explaining the enrichment in omphacitic component of "shards" and devitrified spherulites, Fig.9d-h). ...
Context 12
... rims are commonly lined by Ca-rich, omphacitic compositions as well as interstitial phlogopitic micas (Angiboust et al., 2021; Fig. S1; Table 1 and Table S1). Despite an apparently homogeneous macroscopic texture, detailed petrographic investigations reveal that jadeite crystals exhibit widespread fracturing, dissolution and replacement textures (Fig. 3a, b). Vein systems that range in colour from white (jadeite composition) to green (Cr-rich jadeite or Cr-rich omphacite; Fig. 2a) are ubiquitously found crosscutting the host jadeitite matrix (see also Franz et al., 2014). These veins are filled by idiomorphic to fibrous clinopyroxene crystals with locally oscillatory and/or dendritic ...
Context 13
... grains. Orientation maps (Fig. 4b) show that [001] axes are dominant along the fine-grained shear zone, whereas [100] and [010] axes become important approaching the margins of the shear band. Intracrystalline misorientation EBSD maps, depicting misorientation angles between each data point and the mean orientation of the parent crystal (Fig. S3), show that grains at the shear zone margins are highly misoriented. In addition, the finer grains within the shear band are consistently devoid of intragranular misorientation except for strongly misoriented larger ...
Context 14
... This pattern, sometimes hardly visible using optical microscopy, requires the use of specific analytical approaches such as cathodoluminescence imaging or electronic microscopy to be documented (e.g., Figs. 3, 12; Shi et al., 2003;Sorensen et al., 2006;Takahashi et al., 2017). Pure extensional fracturing (e.g., hydrofracturing; Type III; Fig. 13c) has been identified in several samples, with fragments of the host that appear to have been snatched into clinopyroxene or amphibole-rich veins (Fig. 10b). When several brittle events can be distinguished, Type III events always occur late in the sequence (Table 1). In all localities, it is always the original white jadeitite material ...
Context 15
... 2021a). Interestingly, the main brecciation/cataclasis event in sample KAS04 is followed by an episode of hydraulic brecciation where Na-amphibole precipitates Type III; Fig. S2). Similar deformation patterns have been observed in dark granofels that also comprise breccias (Fig. 6), foliated cataclasites (Fig. 5) and multiple fracturing events ( Fig. 3c; Fig. S5; Table ...
Context 16
... breccias and cataclasites are classically reported for rather shallow -and hence colder -environments (<15 km depth, T < 300 • C; Sibson, 1986), a number of field examples have recently shown that such Fig. 13. a, b and c. Sketches showing the key features of the three main categories of tectonized jadeitites identified in this comparative study. d. Sketch depicting the white jadeitite structure (derived from observations from the Pus'yerka body) at the time of brecciation, indicating the various elements identified in the field and their ...
Context 17
... of jadeitic clinopyroxenes due to the infiltration of CaFe-Mg-rich fluids associated with dissolution-precipitation processes during late fluid-rock interaction events (e.g., Sorensen et al., 2006;García-Casco et al., 2009;Harlow et al., 2011;Cárdenas-Párraga et al., 2012;Angiboust et al., 2021; this study). The formation of dissolution features (Fig. 3c), re-equilibration of clast rims (Fig. 8a) and the metasomatic overprint left in the rock record (Fig. 9b) are witnesses to the apparently corrosive nature of the infiltrating fluids, which were at thermodynamic disequilibrium with respect to the infiltrated material. The absence of carbonates in most jadeitite occurrences suggests that ...
Context 18
... discussed earlier, the release of Cr may be related to serpentinization of the host peridotite during fluid-rock interaction. It is important to note that (i) jadeite/omphacite is found growing within brecciated white jadeitites (Figs. 3b, 8a and 11c) and amphibole-rich dark granofels (Fig. 6d); and (ii) amphibole growth is observed in brecciated dark granofels (Fig. 5b) and between white jadeitite breccia fragments (Figs. 7f, 11a, 11b). From these observations, it appears that the incoming fluid composition was not buffered by the wall-rock composition and that brittle events ...
Context 19
... by the ubiquitous evidence for fluid-rock interaction in the three types of brittle features reported here (Table 1; see also Kuznetsov et al., 1986 andAngiboust et al., 2021). Yet, transient overpressures likely triggered the formation of some of the (explosive) breccia features (Type III) within already-tectonized Type I and Type II domains (Figs. 11c, 13a, b). These findings are consistent with the episodic opening of vein systems and filling of voids by omphacitic compositions documented by García- Casco et al. (2009) andCárdenas-Párraga et al. (2012) in the Sierra del Convento serpentinite mélange (E. Cuba), and also in other subduction HP-LT mélanges environments (e.g., Muñoz-Montecinos ...
Context 20
... rocks have long been recognized as potential markers of seismic deformation (e.g., Sibson, 1986;Angiboust et al., 2012;Melosh et al., 2014). Evidence of fault-zone rocks with diagnostic elements such as damage zones and fault cores (Fig. 13a, b) highlights for the first time that jadeitites (and associated dark amphibole-rich granofels) host abundant brittle deformation events, most likely along fast-slipping seismogenic fault planes. Estimating slip or strain rates along paleofault surfaces (or shear zones) is a challenging task, subject to great uncertainties. While Oncken ...
Context 21
... may be closely intricated in fault zone rocks, both exhibiting evidence for open-system modification of the pristine host composition. Thus, the structures herein observed may have formed through (i) metasomatic fault-zone alteration associated with fluid influx within a finely-crushed cataclastic domain (likely a structure as shown in Fig. 13b), followed by (ii) fast-slip along narrow zones where local melting of the previously-formed cataclasites occurred (thus explaining the enrichment in omphacitic component of "shards" and devitrified spherulites, Fig. 9d-h). Subsequent shear deformation contributed to the apparent banding systematically observed in Type II fault rocks ...
Similar publications
![(Color online) ADF-STEM images of SrTiO3 in the [001] projection with...](publication/364066972/figure/fig2/AS:11431281207897841@1701353855039/Color-online-ADF-STEM-images-of-SrTiO3-in-the-001-projection-with-different-focus_Q320.jpg)
Extension of the focal depth of a sub-nanometer-sized electron probe in a scanning transmission electron microscope was demonstrated using an electron Bessel beam. We observed atomically-resolved annular dark-field scanning transmission electron microscope images of SrTiO3 in the [001] direction with defocus using an electron quasi-Bessel beam gene...
В статье представлены анализ и результаты научных исследований техногенных шлаковых отходов свинцового производства. После закрытия и ликвидации завода, оставшиеся отходы, в виде шлаков, являются источником экологического загрязнения почвы, грунтовых вод и воздуха. Шлаки свинцового производства содержат большое количество токсичных соединений, таки...
We report the first observation of the coupling of strong optical near fields to wavepackets of free, 100 eV electrons with <50 fs temporal resolution in an ultrafast point-projection microscope. Optical near fields are created by excitation of a thin, nanometer-sized Yagi-Uda antenna, with 20 fs near-infrared laser pulses. Phase matching between e...
Precise monitoring of fluid flow rates constitutes an integral problem in various lab-on-a-chip applications. While off-chip flow sensors are commonly used, new sensing mechanisms are being investigated to address the needs of increasingly complex lab-on-a-chip platforms which require local and non-intrusive flow rate sensing. In this regard, the d...
![Figure 1: (a) Simplified tectonic map of China [51]. (b) Geological map...](publication/373744794/figure/fig1/AS:11431281187162239@1694135903839/a-Simplified-tectonic-map-of-China-51-b-Geological-map-of-Longhua-chalcedony_Q320.jpg)
A low-crystallinity index chalcedony was found in the rhyolitic ignimbrite of the Late Jurassic Zhangjiakou Formation, located in Longhua County, Hebei Province, China. This chalcedony occurs as fillings along the fragile fractures of the host rock and is distinct from any other chalcedony deposits, such as the known basalt and carbonate-related ty...
Citations
... Nonetheless, several authors related the cauliflower structure of garnet to fast growth and seismic activity 54,55 . Moreover, the brecciation of omphacite-rich, strong rock types at subduction zone high-pressure conditions has been in some cases associated with deep seismic ruptures and transiently high pore pressure 56,68,69 . Additionally, dilation breccias similar to the ones described in this study are also found at shallower (crustal) depths in carbonate rocks and have been related to seismic failure 20,57 . ...
Metamorphic fluids, faults, and shear zones are carriers of carbon from the deep Earth to shallower reservoirs. Some of these fluids are reduced and transport energy sources, like H2 and light hydrocarbons. Mechanisms and pathways capable of transporting these deep energy sources towards shallower reservoirs remain unidentified. Here we present geological evidence of failure of mechanically strong rocks due to the accumulation of CH4-H2-rich fluids at deep forearc depths, which ultimately reached supralithostatic pore fluid pressure. These fluids originated from adjacent reduction of carbonates by H2-rich fluids during serpentinization at eclogite-to-blueschist-facies conditions. Thermodynamic modeling predicts that the production and accumulation of CH4-H2-rich aqueous fluids can produce fluid overpressure more easily than carbon-poor and CO2-rich aqueous fluids. This study provides evidence for the migration of deep Earth energy sources along tectonic discontinuities, and suggests causal relationships with brittle failure of hard rock types that may trigger seismic activity at forearc depths.
... At least three of these seismicity patterns originate from the subducted slab at 75, 105 and, 135 km depths, ascending until they become indistinguishable from crustal seismicity. These structures, sometimes referred to as mantle-wedge earthquakes (Halpaap et al., 2019;Angiboust et al., 2021) have been reported in this region, featuring a finger-shaped zone of seismicity above the subducting slab (Chang et al., 2017(Chang et al., , 2019. This type of seismicity may be associated with the upward migration of melt (Špičák et al., 2004) or fluid (Špičák et al., 2009) products of dehydration in the subducting plate (Halpaap et al., 2019). ...
... Exhumed subduction channel mélanges are natural laboratories for the study of fluid-rock interaction and mass-transfer at the subduction interface and the subduction channel. Fluid evolved from subducting slabs is able to flow and transport elements over large distances in the subduction environment, triggering chemical-mechanical processes that are responsible for hydrofracturing, seismicity and varied metasomatic transformations and mass recycling in the subduction-suprasubduction system (Ague, 2017;Angiboust et al., 2014Angiboust et al., , 2017Angiboust et al., , 2021aAngiboust et al., , 2021bBebout, 1991Bebout, , 2007Bebout, , 2014Bebout and Penniston-Dorland, 2016;Beinlich et al., 2010;Blanco-Quintero et al., 2011c;Bouilhol et al., 2012;Cannaò et al., 2015Cannaò et al., , 2016Cannaò et al., , 2020Connolly and Galvez, 2018;Gilio et al., 2019;Hermann et al., 2006;Konrad-Schmolke et al., 2011;Lázaro et al., 2011;Muñoz-Montecinos et al., 2020Penniston-Dorland et al., 2012;Scambelluri et al., 2004Scambelluri et al., , 2019Scambelluri and Tonarini, 2012;Sorensen et al., 2010;Spandler et al., 2011;Taetz et al., 2016;Vitale-Brovarone et al., 2014;Zack and John, 2007). ...
Exotic blocks of massive antigorite-serpentinite (antigoritite) document a deep-seated subduction channel in the Villa Clara serpentinite-matrix mélange, central Cuba. The petrological and geochemical characteristics of antigoritite allow distinguishing two types of rock: i) antigoritite and ii) dolomite-bearing antigoritite. Both types are intimately related in field exposures and represent deep peridotite infiltrated by H2O-CO2 fluid mixtures that triggered antigoritization and local carbonation. Fluid infiltration continued after antigoritization forming a vein network as a potential response to hydrofracturing that precipitated tremolitite in the veins and triggered fluid-antigoritite reaction forming blackwalls. The mineralogical and chemical zoning in the blackwalls (Atg + Chl + Tr adjacent to antigoritite and Chl + Tr adjacent to the tremolitite vein) attest for multi-step metasomatic processes during fluid-rock interaction characterized by advection of infiltrating fluid towards the blackwall and, possibly, by diffusion out of the blackwall towards the fluid-filled vein. Tentative thermodynamic modeling of the blackwall domain Atg + Chl + Tr points vein network formation at 400–500 °C and 5–10 kbar during exhumation in the subduction channel, suggesting the infiltration of deep-seated pressurized fluid that triggered hydrofracturing. The chemical compositions antigoritites, veins and blackwalls indicate a LILE- and LREE-enriched fluid evolved from the subducting plate, while SrNd isotope systematics are compatible with an external fluid composed of a mixture of fluids evolved from sediments and, probably to a lesser extent, altered oceanic crust.
... The scale of the transition between the situations of Norsi and Cavo, although possibly diachronous and documenting different stages of the history of the Ligurian subduction, is on the order of tens of kilometers, consistent with the transitions between accretionary and erosional margins observed in modern subduction zones when trenches experience subduction of topographic highs or incipient collision (e.g., New Britain Trench; Galewsky & Silver, 1997;Honza et al., 1989;Riker-Coleman et al., 2006). Similar lateral changes in deformation style and composition of mélange blocks and its fabrics have also been observed in Arosa by and are supported by numerical modeling, showing transient changes between underplating and erosion (Angiboust et al., 2021). ...
We document an exhumed plate boundary shear zone—a subduction channel—developed at the contact between a fossil Cretaceous‐Eocene accretionary prism (Ligurian Units) and the underlying continent‐derived nappes of the Northern Apennines. The subduction channel, referred to as the Norsi‐Cavo Complex (NCC), is continuously exposed for ∼10 km along strike on the island of Elba. The NCC consists of oceanic sediments and serpentinites, coupled at the base of the prism. In its northernmost exposures the NCC crops out as a serpentinite mélange with blocks of sedimentary rocks. There, evidence of pervasive fluid‐rock interaction is present. We interpret this mélange as the result of material transfer from the upper plate to the subduction channel through tectonic erosion at the base of the prism. The southern part of the NCC preserves a series of tectonic slices of shales and limestones (Palombini shales) coupled with ultramafic rocks through serpentinite shear zones containing lenses of massive serpentinites, mafic rocks, and sediments. Since sedimentary slices preserve the same structures of the overlying Ligurian Units, produced by accretionary prism deformation, we interpret this complex as the result of material transfer from the prism to the subduction channel and its subsequent underplating at the base of the prism. Based on our interpretation, the NCC fossil subduction channel formed during E‐verging deformation over the W‐directed subduction of the Ligurian Ocean during the early development stages of the Apennines. This deformation terminated when the continental margin entered the subduction.
... However, only hot fluids deliver Al in appreciable quantity to the slab surface, that is, at different depths depending on slabs thermal structure. This not only supports a metasomatic origin for jadeites (Angiboust, Muñoz-Montecinos, et al., 2021;Harlow & Sorensen, 2005), but it suggests that the alumina content (e.g., their aluminum saturation index) of subduction zone fluids may be used as chemo/geothermometer. The experimental evidence for the high incompatibility of K in high-P/T metasediments (Schmidt, 2015;Schmidt et al., 2004) suggests that the mobility of this element may be elevated at depth. ...
We investigate the chemical budget of subduction zones at sub‐solidus conditions using a thermodynamic‐numerical simulation in which all major rock components are treated as soluble and potentially mobile in aqueous fluids. This new strategy significantly improves the accuracy of predicted fluid‐rock equilibrium compositions in open petrological systems. We show that all slabs release volatiles and nonvolatiles to the mantle wedge, contributing to its refertilization. But some mobile constituents, such as alkali and alumina, may be trapped along layer boundaries or traverse without interaction depending on chemical contrasts between adjacent lithologies. The accumulation of igneous alumina and silica in the limestones of the central‐eastern Pacific slabs drives their decarbonation and is marked by metasomatic garnet growth. Those slabs are also predicted to lose much of their alkalis before sub‐arc depth. Even when they are produced in the altered mafic and ultramafic layers, fluids reach the slab/mantle wedge interface with distinct compositional signatures that are typical of the sedimentary cover. We distinguished supply and transport limited regimes of element subduction by testing the sensitivity of our mass balance to changes in slab hydration state (HS). Transport limited slabs sensitive to HS include notably a hotspot of carbon release to the mantle wedge (e.g., Costa Rica). Finally, we show that the quantitative budgets do depend on the geometry of fluid flows, and on assuming that slabs are mechanically continuous structures, which is questionable. Taken together, these insights will help better constrain the long‐term chemical evolution of the shallow planetary interior, and the thermomechanical behavior of the subduction interface.
This study illustrates the field relationships of jadeitite-bearing block-in-matrix sequences on Syros and Tinos, Cycladic Blueschist Unit, and adds additional U–Pb zircon ages for jadeitites to the geochronological database. The results confirm the importance of Cretaceous ( c. 80 Ma) and Eocene ( c. 50 Ma) processes in their geological evolution. Interpretations suggesting that the jadeitites were formed by complete metasomatic replacement of a pre-existing rock are not fully supported by field observations. In at least some cases, the formation of jadeitite is likely due to precipitation from Na-Al-Si-rich aqueous fluids, which also caused variable metasomatic alteration of the host rock. Unambiguous age constraints for formation of the Syros and Tinos jadeitites are not available. A relationship to Eocene blueschist facies metamorphism recorded in the associated metamafic rocks seems plausible. However, since high-pressure overprinting of pre-Eocene jadeitite is also conceivable, there is a much larger time window for jadeitite formation, framed by Cretaceous ( c. 80–76 Ma) protolith ages of various mélange blocks and the waning stages of blueschist facies metamorphism ( c. 40 Ma). Field observations are consistent with the interpretation that the mélange-like occurrences on Syros and Tinos record, to varying extent, multi-stage processes that include detachment of mafic rocks from the subducting plate, local infiltration of Na-Al-Si-rich aqueous fluids, exhumation via a serpentinitic matrix in a subduction channel and reworking of the primary block-in-matrix fabric by sedimentary or tectonic processes during accretionary wedge formation.
Ophiolitic peridotites in Burma (Myanmar) occur along three major tectonic zones, the Kaleymyo–Nagaland suture, Indo-Burman ranges, the Jade Mines belt, and the Tagaung–Mytkyina belt. These belts all show harzburgite–lherzolite–dunite peridotites, but the Hpakan-Taw Maw region (Jade Mines belt) hosts jadeitites including pure jadeite, mawsitsit (chromium-rich jadeite) kosmochlore (chromium-rich clinopyroxene), and albitite. High Na and Al contents of jadeitites require either very unusual Al-rich, Si-poor protoliths, or extensive fluid metasomatism, or both. The Hpakan jadeitites formed by Na-, Al-, (and Si) metasomatic alteration of pyroxenite–wehrlite intrusions into harzburgite–dunite, from widespread fluid alteration. Fluids could have been derived from a mid-Jurassic intermediate pressure subduction event during ophiolite formation and emplacement. In the Indawgyi Lake area, normal ophiolitic peridotites, including harzburgite and dunite with pyroxenite veins, have not been jadeitised. Gabbros related to the Jade Mines ophiolite gave a U-Pb zircon age of 169.71±1.3 Ma (MSWD 2.2), similar timing to the Myitkyina ophiolite (173 Ma) to the east, suggesting that the ophiolite belts were originally continuous. The jade ‘boulders’ in the Uru conglomerate beds at Hpakan have also resulted from normal in-situ serpentinisation weathering processes, followed by limited fluvial mass transport processes along the Uru river.
Supplementary material: https://doi.org/10.6084/m9.figshare.c.6655269
Deep Slow Slip and Tremors (SSTs) are a combination of transient clusters of tectonic tremors and slow slip associated with extremely elevated fluid pressures. SSTs are thought to reflect a transition from viscous to brittle plate interface rheology and likely exert a first‐order control on megathrust seismicity. Nevertheless, the deformation mechanisms governing the source of SSTs remain elusive. We herein document the occurrence of vein networks precipitated and brecciated within the deep SST region under blueschist‐facies conditions. These lawsonite‐rich vein sets exhibit extensive evidence of brittle deformation and are spatially related to localized, finely milled (cataclastic) shear bands. Petro‐geochemical data reveal that brittle deformation was accompanied by the injection of several ultramafic‐, mafic‐ and metasedimentary‐derived fluid pulses, imprinting characteristic Cr, high field strength elements, and light over heavy rare earth elements positive anomalies in the vein breccias while leaching light rare earth elements from the cataclastic blueschist host. Our results suggest that metamorphic veins represent zones of mechanical anisotropy within the rock volume prone to localized shearing, brittle deformation and episodic injection of externally derived fluids. These networks demonstrate the importance of former vein sets as structural heterogeneities in triggering fluid‐controlled brittle creep events. The combined effects of high pore fluid pressures and rheological heterogeneities in the form of metamorphic veins could trigger the nucleation and propagation of SSTs at the margins of this mechanically anisotropic environment, and thus determine where slip will take place along deep subduction interfaces.
The generation, transport, and recrystallization of slab-derived melts/fluids play a critical role in the deep recycling of elements in subduction zones. While boron (B) isotope systematics have been invoked as an important tracer of these processes, its behavior during metamorphic dehydration and partial melting of deeply subducted continental slabs, and the partitioning of B isotopes between minerals and melts/fluids is not fully understood. Here, we investigate these processes through an in situ study of the major, trace-element, and B-Sr isotope variations in different occurrences of tourmaline in migmatite from the Yuka terrane, North Qaidam orogen (China), which resulted from partial melting of a continental slab at different stages during subduction and exhumation. Based on textural and detailed high-resolution X-ray mapping studies, tourmaline was classified into four paragenetic generations (Tur-I, Tur-II, Tur-III, and Tur-IV). Dravitic Tur-I occurs in melanosomes and shows increasing Fe, Ca, and Ti contents from the core to the outer rim. In addition, it has relatively homogeneous Sr isotope values (0.7407–0.7416) and decreasing δ11B values (-3.8 to -8.6‰) and XMg (Mg/(Mg + Fe)) ratios, indicating formation in a rock buffered by an aqueous fluid during the prograde to peak metamorphism. Schorlitic Tur-II occurs within selvage zones between melanosomes and leucosomes, and yields high-Fe values and low δ11B (-13.5 to -10.9‰) and more variable 87Sr/86Sr (0.7343–0.7418) values, indicating crystallization in the presence of a hydrous melt external derived from breakdown of Fe-rich mineral(s) during partial melting of the subducted slab. Dravitic Tur-III formed in the matrix and also enveloped Tur-II. It shows homogeneous 87Sr/86Sr values (0.7411-0.7420) and decreasing δ11B values (-6.8 to -9.9‰) and XMg ratios as well as increasing Fe and Ti contents from core to outer rim. Formation of Tur-III reflects a transitional stage from hydrous melt to aqueous fluid during exhumation. Tur-IV in the leucosomes is essentially a schorl-dravite solid solution with small amounts of Ca. Its 87Sr/86Sr values (0.7402–0.7416) and δ11B values (-11.4 to -8.5‰) are intermediate between the respective values of Tur-II and Tur-III. The formation of Tur-IV likely results from interaction between melt and fluid and, based on its chronological sequence, and is interpreted to have formed during the exhumation stage of the Yuka terrane.
Overall, the variable XMg ratios and δ11B values in the different generations of tourmalines are a consequence of the evolution of the melt/fluid at different depths within the deeply subducted slab. Decreasing δ11B values from Tur-I to Tur-II and Tur-III are controlled by the breakdown of different minerals during partial melting or metamorphic dehydration of the subducted slab, while the co-variations of the elemental geochemistry and B isotopic compositions of tourmaline reflect different depths of formation during subduction and exhumation of the lithosphere. These observations suggest tourmaline may serve as a useful tracer of multiple melt/fluid–rock interactions and of boron cycling in continental subduction zones. The heterogeneity of δ11B in melts/fluids at different depth levels of the continental subducted slab may also result in locally variable B isotope values in syn- and post-collisional magmas.
