Journal of Metamorphic Geology最新文献

High-pressure rocks from the island of Ios in the Greek Cyclades were examined to resolve the P–T conditions reached during subduction of the two distinct lithotectonic units that are separated by the South Cycladic Shear Zone (SCSZ)—the footwall complex composed of Hercynian basement gneisses, schists and amphibolites, and the hangingwall complex composed of blueschists and eclogites. A combination of elastic tensor quartz inclusion in garnet (QuiG) barometry and Zr-in-rutile (ZiR) trace element thermometry was used to constrain minimum garnet growth conditions. Garnet from the hangingwall (blueschist) unit record formation pressures that range from 1.5 to 1.9 GPa and garnet from the footwall basement complex record garnet formation pressures of 1.65–2.05 GPa. ZiR thermometry on rutile inclusions within garnet establishes the minimum temperature for garnet formation to be ~480–500°C. That is, there is no evidence in the QuiG and ZiR results that the rocks of the blueschist hangingwall and basement experienced different metamorphic histories during subduction. This is the first reported observation of blueschist facies metamorphism in the Hercynian basement complex. A model is proposed in which initial subduction occurred along a relatively shallow P–T trajectory of ~11°C/km and then transitioned to a steeper, nearly isothermal trajectory at a depth of ~45 km reaching similar peak metamorphic conditions of ~500–525°C at 2.0 GPa for all samples. Such a change in the subduction path could be accomplished by either an increase in the rate of subduction or an increase in the angle of the subduction zone. The present juxtaposition of samples with contrasting mineral assemblages and garnet growth histories is interpreted to have arisen from differences in bulk compositions and variations in the preservation of high-pressure prograde mineral assemblages during exhumation. The existence of similar P–T conditions and prograde paths in the two units does not require that the rocks were all metamorphosed at the same time and that the SCSZ experienced little movement. Rather, it is suggested that the two units experienced prograde and peak metamorphism at different times and were subsequently juxtaposed along the SCSZ.

High-grade metamorphic rocks are widely exposed along the SE–NW- to E–W-trending shear zones in the Truong Son Belt, Central Vietnam, but few petrological studies have been conducted in this area. Herein, we report the occurrence of mylonitized granulites that crop out along the Dai Loc shear zone in the southernmost Truong Son Belt. Detailed petrographic analysis, geochemistry and P–T–t estimates of the evolution of two granulite samples are presented to elucidate the formation processes of these high-grade metamorphic rocks. The results indicate that the rocks underwent two distinct metamorphic cycles. The first cycle (M1) is characterized by coarse-grained granulite mineral assemblages, defining a tight clockwise P–T path with near-isobaric heating to a near ultrahigh-temperature peak at low pressure, followed by cooling. The prograde mineral assemblage (M1a) is indicated by inclusions of cordierite + sillimanite + biotite + quartz + spinel ± plagioclase in coarse-grained garnet, orthopyroxene and cordierite. The mineral assemblage of garnet + orthopyroxene + cordierite + plagioclase + K-feldspar + ilmenite + melt ± biotite (M1b) defines the peak P–T conditions of 5.3–6.3 kbar and 850–920°C. Post-peak cooling (M1c) is marked by the formation of quartz + biotite symplectites around garnet and orthopyroxene. The second cycle involved medium-pressure amphibolite facies metamorphism (M2), characterized by domainal development of fine-grained kyanite-bearing mineral associations. Petrographic observations indicate that these fine-grained associations were formed during mylonitization. Zircon U–Pb dating reveals that the timing of granulite facies metamorphism appears to be coeval with the intrusion of a post-collisional granitoid at 430–410 Ma. Granulite facies metamorphism and crustal melting were probably driven by asthenospheric mantle upwelling triggered by slab breakoff during the Early Palaeozoic. Considering previous structural and geochronological studies, the second metamorphic event likely occurred during the Triassic Indosinian orogeny.

Chloritoid and kyanite coexist in metapelites from the high-pressure/low-temperature Massa Unit in the Alpi Apuane metamorphic complex (Northern Apennines, Italy). The composition of chloritoid is extremely variable throughout the Massa Unit. Fe-chloritoid occurs in association with hematite-free, graphite-bearing schists, whereas strongly zoned Fe-Mg chloritoid is found with hematite and kyanite. We investigated the effect of different bulk Fe₂O₃ contents in controlling chloritoid composition through phase equilibria modelling of four selected samples, representative of the different chloritoid-bearing parageneses found in the Massa Unit. The ferric iron content, measured through wet chemical titration, ranges from 0 (graphite-chloritoid schist) to 73% of the total iron (hematite-chloritoid schist). We show that Mg-rich chloritoid compositions and stability of kyanite at greenschist to blueschist facies conditions can be reproduced in the MnO–Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (MnNKFMASHTO) chemical system only considering the presence of significant amounts of ferric iron as part of the bulk composition. The stabilization of kyanite at lower grade is directly linked to the presence of Fe₂O₃, which renders the reactive bulk rock composition effectively enriched in Al₂O₃ with respect to Fe and Mg. We also document that high Fe₂O₃ contents exacerbate the effect of chloritoid fractionation, producing strongly zoned Fe-Mg-chloritoid grains. Finally, the P–T modelling of the Massa Units performed in this study allows, for the first time, the recognition of a two-stage evolution at peak conditions, with an earlier pressure peak (1.2–1.3 GPa at 350–400°C), and a later thermal peak (0.7–1.1 GPa at 440–480°C), compatible with subduction, underthrusting and exhumation of the Adria continental margin during growth of the Northern Apennine orogenic wedge.

The integration of garnet-based petrologic constraints with multimineral geochronologic data in eclogites and blueschists allows the timing and rate of subduction zone metamorphism to be constrained. We present a combined garnet Lu–Hf/Sm–Nd and zircon/rutile U–Pb geochronology study on three eclogites, a garnet-bearing blueschist, and a micaschist from the Changning–Menglian orogenic belt, a newly discovered ultrahigh-pressure metamorphic belt in southeast Tibet, in order to characterize tectono-metamorphic events and determine the duration of Paleo-Tethys oceanic subduction. Integration of phase equilibrium modelling and conventional thermobarometry for the eclogites defines a clockwise P–T path evolving from blueschist facies conditions at ~1.4 GPa and ~505–530°C to peak eclogite facies conditions at ~2.8 GPa and ~630–640°C, followed by isothermal decompression to amphibolite facies at ~1.0 GPa and ~630–650°C. The Lu–Hf ages of c. 239–236 Ma obtained for the eclogites and the blueschist are indistinguishable from the rutile U–Pb age of c. 239 Ma obtained for the eclogites and, combined with the observation of well-preserved Rayleigh-fractionation-style Mn and Lu zoning profiles in garnet, reflect the timing of early prograde garnet growth. The Sm–Nd ages of c. 242–236 Ma reflect a later period of garnet growth, evidenced by flat and/or M-shaped Sm zoning profiles. Each of the Sm–Nd ages overlaps, within uncertainty, with its corresponding Lu–Hf age (i.e., from the same garnet fraction). The consistency of the Lu–Hf and Sm–Nd ages indicates a short overall duration of garnet growth from blueschist to eclogite facies metamorphism, reflecting rapid subduction of the oceanic slab. The magmatic zircon U–Pb dates of c. 247 Ma constrain the protolith age of these metabasaltic rocks. The close protolith and the high-pressure metamorphic ages, together with the consistent garnet Lu–Hf and Sm–Nd ages and the overlapping youngest and oldest metamorphic ages of the oceanic-type and continental-type eclogites, respectively, suggest a fast tectonic transition from divergence to convergence highlighted by rapid oceanic subduction, continuous transition from oceanic to continental subduction, and a rapid cooling of the subduction interface.

Two eclogite samples from the Haut-Allier record a prograde evolution from ~20 kbar, 650°C to 750°C, 22–23 kbar followed by heating up to 850–875°C and partial melting. Incipient decompression in high-pressure granulite facies conditions (19.5 kbar, 875°C) was followed by exhumation to high-temperature amphibolite facies conditions (<9 kbar, 750–850°C). Following a detailed geochemical, petrological, and geochronological investigation using trace-element data and laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircon, apatite, and rutile, the eclogites reveal an Ordovician (c. 490 Ma) rifting event followed by Devonian (c. 370–360 Ma) subduction and Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central. The previously proposed Silurian age for the subduction, which strongly influenced many tectonic models, is definitively rejected. In the light of other geological data from the French Massif Central, including the lithological and geochemical zoning of calc-alkaline Devonian volcanism, we propose a southward polarity of the subduction and question the very existence of the so-called Massif Central Ocean. Furthermore, we infer that following subduction, the eclogites were relaminated to the upper plate and exhumed at the rear of the magmatic arc pointing to similarities with the geodynamics of the Bohemian Massif.

The petrochronological record of zircon is particularly complex. Metamorphic zircon with clear eclogitic rare-earth elements patterns (no Eu anomaly and flat heavy rare-earth elements) and inclusions (garnet, rutile, and omphacite) shows concordant apparent ages that spread from c. 380 down to c. 310 Ma. This apparent age pattern strongly contrasts with the well-defined age of apatite and rutile of c. 350 Ma. Apparent zircon ages younger than 350 Ma unequivocally testify that zircon can recrystallize outside the conditions of the eclogite facies, which resets the U–Pb while preserving an apparent eclogitic signature. Local fractures filled by analcite, thomsonite, plagioclase, and biotite testify to late interaction of the eclogites with alkaline fluids at relatively low temperatures. This interaction, possibly at c. 310 Ma or later, could lead to the recrystallization of zircon while leaving apatite unaffected.

The Dom Feliciano Belt of southern Brazil and Uruguay represents part of a larger Neoproterozoic orogenic system formed during the amalgamation of Western Gondwana. The hinterland and foreland domains in parts of the belt preserve deformation structures and metamorphic assemblages that developed during early crustal thickening from c. 650 Ma. However, the metamorphic history of the southern foreland, in Uruguay, and its relationship with the hinterland, is not so well understood. We show that metamorphism in the southern hinterland is characterized by near-isothermal decompression from ~10 kbar (~770°C) down to ~6 kbar, reflecting exhumation from depths of ~40 km during convergent thrusting and crustal thickening. This metamorphic event and associated magmatism is constrained by garnet Lu–Hf and zircon U–Pb dating to c. 655–640 Ma, supporting age and P–T constraints from previous studies. In contrast, prograde metamorphism in the foreland supracrustal rocks reached maximum lower-amphibolite facies conditions (~6–7 kbar and ~550–570°C) and is constrained by garnet Lu–Hf dating to 582 ± 23 Ma. An exposed sheet of imbricated foreland basement rocks reached partial melting at upper-amphibolite facies conditions, and metamorphism is similarly constrained to c. 585–570 Ma by monazite U–Pb dating. The data indicate that metamorphism in the foreland occurred during a sinistral transpressional event c. 55–85 Ma after the start of crustal thickening recorded in the hinterland, whereby strain partitioning during sinistral transpression led to imbrication in the foreland and oblique thrusting of the basement over more distal supracrustal rocks. This event is coeval with transpressional deformation in the Kaoko and Gariep belts, indicating a distinct two-stage tectonic history driven by the three-way convergence between the Congo, Kalahari, and South American cratons.

One of the most striking geological features of the Pamir is the south-dipping lithospheric slab beneath the orogen characterized by an intracontinental Wadati-Benioff zone. A widely accepted hypothesis over the past 40 years interprets the slab to represent southward subducted cratonic Asian continental lithosphere, which predicts significant cratonic Asia-sourced crustal materials (e.g., Tarim Basin) beneath the Pamir. Alternatively, recent studies have interpreted the slab to be lithosphere delaminated from the base of the Pamir. To test these hypotheses, depth–tectonic affinity relations of crustal xenoliths carried by Miocene volcanic rocks in the eastern Pamir, interpreted to be sourced from the Pamir deep lithosphere, are used to determine whether they represented Asian affinity cratonic crust. Thermodynamic calculations, zircon U–Pb geochronology combined with rare earth element analysis, and whole-rock major-trace element and Sr–Nd isotopic analyses document that (1) eclogite and pyroxenite xenoliths (~31–43 kbar/~960–1170°C) are the deepest sourced portions of the lithosphere from ~100 to 140 km depth, the protoliths of which represent the mid-lower crustal rocks of the Cretaceous Pamir magmatic arc, rather than material from cratonic Asia, and (2) granulite xenoliths (~20 kbar/~900°C) represent the Cenozoic lower crustal rocks of Pamir terranes from ~70 km depth. These results indicate the south-dipping slab represents delaminated Pamir lower crust and mantle lithosphere, rather than intracontinental subduction of Asian lithosphere, and further support the hypothesis of minimal Cenozoic northward translation of the Pamir.

The Pikwitonei Granulite Domain (PGD), located in the northwestern Superior Province, is one of the largest Neoarchean high-grade metamorphic domains in the world, and is a key to understanding the Neoarchean crustal evolution of the Superior Province. Here we report results of a study on ultrahigh temperature (UHT) granulites with a sapphirine + quartz-bearing peak assemblage from the Sipiwesk Lake area in the PGD. Three stages of metamorphic assemblage development are recognized based on petrographic observations: Pre-peak stage is marked by garnet, sillimanite, K-feldspar, and biotite inclusions within sapphirine and orthopyroxene. Peak stage is charactered by the typical UHT associations of sapphirine + quartz and sapphirine + orthopyroxene. The retrograde stage is represented by the retrograde formation of cordierite, biotite and sillimanite in the matrix. Phase equilibrium modelling based on the bulk compositions of sapphirine-bearing granulites suggest that the rocks have experienced extensional UHT metamorphism in excess of 1035°C at pressures of 7.9–9.0 kbar, followed by isobaric cooling process. SHRIMP U–Pb dating of metamorphic zircons in sapphirine-bearing granulite record the timing of the UHT event in the PGD with a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2681 ± 13 Ma. This study in combination with other metamorphic P–T paths and age information reveals that this UHT metamorphism at 2.68 Ga in PGD was generated by upwelling asthenosphere due to slab break-off in collision during the amalgamation of the Superior Province.

Melt inclusions (MIs) in high-temperature metamorphic rocks provide a unique window into crustal anatexis in collisional orogenic belts and have been widely used to characterize compositions of anatectic melts as well as melting mechanisms. In this study, MIs hosted by peritectic garnet were for the first time identified in an Al₂SiO₅-free graywacke-type paragneiss from the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet. These MIs occur as nanogranites in the rims of porphyroblastic garnet, exhibit negative crystal shapes with an average diameter of ~12 μm and consist of a mineral assemblage of biotite + quartz + plagioclase + K-feldspar ± muscovite. Re-homogenization experiments of these nanogranites were conducted at a pressure of 1.5 GPa and temperatures of 800°C, 850°C and 900°C and produced homogeneous glasses at 850°C. The homogenized glasses are strongly peraluminous and calc-alkalic in composition, with 66.43–71.31 wt.% SiO₂, 12.64–15.06 wt.% Al₂O₃, high alkaline (5.41–7.22 wt.%) and low ferromagnesian (2.72–4.46 wt.%) contents. They are lower in silica and CaO but higher in K₂O compared with MI produced by fluid-present melting of metasedimentary rocks, thus indicating fluid-absent melting. These glasses are also characterized by enrichment of large ion lithophile elements (particularly Cs and Rb), depletion of Ba and Sr, low contents of light rare earth elements (3.6 to 33.7 ppm), high Rb/Sr ratios (6.19–37.3) and low Nb/Ta ratios (2.55–18.7). In combination with phase equilibrium modelling, these compositional features suggest that a sequential dehydration melting of muscovite and biotite was responsible for the production of MI during prograde metamorphism of the studied paragneiss. By compiling MI data published in the literature, we show that dehydration melting of metasedimentary rocks from the Himalayan orogen can produce initial melts with various peraluminous and granitic compositions.

The Central Qilian terrane (CQT) of the northern Tibetan Plateau played a key role in the tectonic evolution of the Proto-Tethyan Ocean in the Tethysides, but its formation and tectonic attribution have been hotly debated. Here, we report the discovery of eclogites and HP mafic granulites in the Beidahe Complex of the western CQT. These occur as blocks of various sizes within a sequence of metavolcanic–sedimentary rocks, exhibiting typical a ‘block-in-matrix’ and thrust imbrication structure. The eclogite facies metamorphic rocks preserve distinct mineral assemblages and textures corresponding to prograde, peak, and retrograde metamorphism. By combining phase equilibrium modelling with SHRIMP and LA-ICPMS U–Pb dating of metamorphic zircon, sphene, and rutile, the Late Neoproterozoic–Cambrian (c. 553–516 Ma) eclogite facies peak with low thermal gradients of 10–14°C/km, Cambrian (c. 515–506 Ma) post-peak decompression and Ordovician (c. 495–455 Ma) cooling histories for these metabasic rocks have been restored. These constitute hairpin-type clockwise pressure–temperature–time (P–T–t) paths depicting in detail the sequence of deep subduction and subsequent exhumation in Central Qilian during the Late Neoproterozoic to Early Palaeozoic. Our new findings suggest that the CQT represents a Japan-type arc-accretionary system that formed as a result of the North Qilian oceanic plate, one of the major branches of the Proto-Tethys Ocean, being subducted southward. Eclogites in the Beidahe Complex in western CQT offer the earliest (c. 553 Ma) metamorphic record of subduction in the Qilian orogen, indicating that the North Qilian Ocean commenced subducting southward prior to the Late Neoproterozoic.