Journal of Metamorphic Geology最新文献

Subduction infancy corresponds to the first few million years of the start of subduction following heat transfer from the incipient mantle wedge towards the slab-top, as witnessed by metamorphic soles which represent slivers of oceanic crust metamorphosed up to granulite facies conditions welded beneath obducted ophiolites. In this study, integrated petrological, geochemical, mineralogical, geochronological, and thermodynamic studies were carried out on samples from the Yushugou high-temperature metamorphic ophiolitic complex (YHTM) in the South Tianshan Accretionary Complex (STAC), where a massive exposure of coherent granulite accompanied by a thick peridotite body is preserved. Bulk-rock compositions and Sr–Nd–Hf isotopes demonstrate the petrogenesis of meta-basalts with oceanic island basalt (OIB)-like and mid-ocean ridge basalt (MORB)-like affinities, with little infiltration by subduction-derived melts and/or fluids (e.g., no negative Nb–Ta anomalies). Thermodynamic modelling and U–Pb chronology reveal that the YHTM meta-basalts experienced granulite facies metamorphism of ~840–940°C and ~0.92–1.02 GPa at c. 392 Ma and then possibly reheating and zircon alteration in the Carboniferous. In addition, detrital zircons in sedimentary host rocks of the YHTM show limited Precambrian records and offer maximum depositional ages of c. 410–400 Ma together with the oldest Palaeozoic cluster around c. 470–450 Ma. It is suggested that the YHTM granulites could be of Ordovician–Silurian protolith and such an age pattern significantly deviates from those of adjacent terranes (the Central Tianshan, STAC, and North Tarim Craton). Combined with the compilation of pressure–temperature–time estimates of the YHTM and ages of regional ophiolites, arc intermediate-mafic rocks, A-type granites, and deformation, a model of induced, temporarily northward, intra-oceanic subduction initiation is proposed, which probably occurred along the previously existing weak zone close to a seamount or oceanic plateau in the earliest middle Devonian during the northward subduction of the South Tianshan Ocean (STO). Anomalously high geothermal gradients could be triggered by asthenosphere upwellings, further facilitating the formation of OIB-type metamorphic soles. The YHTM, which represents the remnant of metamorphic soles and associated ophiolites, was finally emplaced to the north margin of the STAC during the relatively long-term (c. 160 million years) accretion and continuous subduction of the STO before its closure. This finding also presents a new natural example of OIB-type metamorphic soles as a snapshot of fossil intra-oceanic subduction infancy during the complex evolutionary history of the STO.

The formation of most jadeitites and other jadeite-rich rocks (jadeitoids) during subduction is thought to occur by precipitation (P-type) or metasomatism (R-type) by infiltration of Na-Al-Si-rich aqueous fluids because of the compositional similarity of the rocks to inferred subduction fluids. Whether these rocks can form by isochemical metamorphism (I-type) during subduction is still hotly debated. A characteristic of I-type jadeitoid is that it exhibits a similar prograde metamorphic record as associated eclogite, in contrast to P- and R-type jadeitite and jadeitoids that are typically enclosed in serpentinite derived from the mantle wedge and either lack a prograde metamorphic history (R-type and P-type) or probably experience a prograde history (R-type) that is difficult to discern owing to the high variance of the jadeite-dominated assemblages and alteration by subduction fluids. The recently discovered Baqing (eastern-central Tibet) jadeitoid is enclosed by quartzo-feldspathic schist and has a peak metamorphic assemblage of almandine + jadeite/omphacite + phengite/paragonite + rutile + quartz, similar to eclogite. Abundant mineral inclusions in almandine, especially rutile inclusions with increasing Zr contents from the core to rim of almandine, provide an opportunity to further decode the jadeitoid-forming processes. In this study, pseudosections and Zr-in-rutile thermometry, together with conventional geothermobarometers, were employed to decipher the metamorphic history of Baqing jadeitoids. Two analysed Baqing jadeitoids exhibit a similar clockwise P–T path, starting from early metamorphic conditions of 5–7 kbar, 350–440°C, to different peak conditions (27–29 kbar, 730–760°C, or 20–23 kbar, 670–710°C), followed by relatively consistent retrograde metamorphic conditions of 6–7 kbar, 530–600°C. This result indicates a similar subduction history to the Baqing eclogite. In addition, the Baqing jadeitoids show similar geochemical characteristics to some Na-rich, K-depleted and Ca-depleted sedimentary rocks or plagiogranite. Therefore, we propose an isochemical genesis for the Baqing jadeitoid, rather than a metasomatic origin.

In the Altai Accretionary Wedge, several periods of Barrovian- and Buchan-type metamorphic cycles were dated from Ordovician to Permian. However, the timing and link between these cycles are not clear, and their causes are debated. In order to contribute to the understanding of Barrovian- to Buchan-type evolution of the accretionary wedges, we studied an area composed of three parallel belts in the easternmost extremity of the Hovd domain located in Mongolian Altai Zone: garnet gneiss in the north, garnet–staurolite–kyanite schist overprinted by ±sillimanite±cordierite±andalusite-bearing assemblages in the centre and garnet–sillimanite gneiss in the south. Petrography, garnet zoning and thermodynamic modelling indicate that the garnet gneiss from the northern belt records burial from ~510°C and ~3–4 kbar to ~600°C and ~5 kbar, followed by heating to ~660°C and decompression to ~4.5 kbar. The garnet–staurolite–kyanite schist from the central belt records burial from ~550°C and ~3–4.5 kbar to ~640–680°C and ~7 kbar, followed by decompression to the sillimanite stability field at ~650°C and ~6 kbar. Crystallization of cordierite, andalusite, late muscovite and chlorite in some samples indicates cooling on decompression to ~540°C and ~3.5 kbar. In the southern gneiss belt, the garnet–sillimanite gneiss with almost unzoned garnet suggests re-equilibration at ~6 kbar and ~710°C. In situ U–Pb monazite and xenotime dating carried out inclusions in porphyroblasts and matrix grains revealed Carboniferous and Permian ages. The monazite and xenotime from gneisses of the northern and southern belts record Carboniferous and Permian ages, which are interpreted as Carboniferous crystallization at c. 347 Ma associated with metamorphic peak, followed by Permian (re)crystallization at c. 300 and 283 Ma. In the central belt, rare Carboniferous xenotime grains in a garnet–staurolite–kyanite–andalusite–muscovite schist indicate a possible Carboniferous age of the prograde metamorphism. Predominant ages between c. 280 and 260 Ma recorded by monazite are interpreted as a result of complete recrystallization during an LP metamorphic overprint. The Carboniferous ages from the gneisses can be interpreted as constraining the timing of the exhumation of deep crustal rocks to shallow crustal levels. This event corresponds to the formation of crustal-scale migmatite-magmatite domes in the Mongolian Altai Zone. The prograde Barrovian assemblages in the central schist belt are interpreted as having formed contemporaneously during burial in a synform between the migmatite-magmatite domes. The Permian ages reflect LP–HT metamorphism, best recorded by the Buchan-type assemblages in the central schist belt, and are related to massive heat flux from tectonically mobile deep partially molten crust. Correlation with similar Barrovian- and Buchan-type episodes from the Chinese Altai Zone indicates multiple compressional and extensional events in the upper plate a

In situ age and trace element determinations of monazite, rutile and zircon grains from an ultrahigh temperature (UHT) metapelite-hosted leucosome from the Napier Complex using laser split-stream analysis reveal highly variable behaviour in both the U–Pb and trace element systematics that can be directly linked to the microstructural setting of individual grains. Monazite grains armoured by garnet and quartz retain two concordant ages 2.48 and 2.43 Ga that are consistent with the previously determined ages for peak UHT metamorphism in the Napier Complex. Yttrium in the armoured grains is unzoned with contents of ~700 ppm for the garnet-hosted monazite and in the range 400–1,600 ppm for the monazite enclosed within quartz. A monazite grain hosted within mesoperthite records a spread of ages from 2.43 to 2.20 Ga and Y contents ranging between 400 and 1,700 ppm. This grain exhibits core to rim zoning in both Y and age, with the cores enriched in Y relative to the rim and younger ages in the core relative to the rim. A monazite grain that sits on a grain boundary between mesoperthite and garnet records the largest spread in ages—from 2.42 to 2.05 Ga. The youngest ages in this grain are within a linear feature that reaches the core and is connected to the grain boundary between the garnet and mesoperthite; the oldest ages are observed where monazite is in contact with garnet. Yttrium in the grain is enriched in the core and depleted at the rim with the strongest depletions where monazite is adjacent to grain boundaries between the silicate minerals or in contact with garnet. The unarmoured monazite grains have lower intercept ages of 1.85 Ga, which overlaps with the bulk of ages determined from the rutile and is coincident with a previously reported zircon age obtained through depth profiling from the Napier Complex. The age and chemical relationships outlined above illustrate decoupling between the geochemical and geochronological systems in monazite. Individual grains are suggestive of a range of processes that modify these systems, including volume diffusion, flux-limited diffusion and fluid-enhanced recrystallization, all operating at the scale of a single thin section and primarily controlled by host mineral microstructural setting. These findings illustrate how the development of simple partitioning coefficients (cf. garnet/zircon) and geospeedometry based on experimentally determined diffusion coefficients on grain separates may not be achievable. However, it highlights the utility of combining age and trace element concentrations from multiple accessory minerals with microstructural information when trying to build a complete history of tectonothermal events experienced by an ancient rock system that has undergone a prolonged history of thermal, deformational and fluid flow events.

Recent studies of the Cretaceous lower arc crust exposed in Fiordland, New Zealand, conclude that shear zones are sites of melt migration and mass transfer through the deep crust. Here, we investigate the 4–10 km-wide George Sound Shear Zone, which cuts the Western Fiordland Orthogneiss, comprising two main rock types: two-pyroxene gneiss and hornblende gneiss. Previous studies infer a predominantly igneous origin for the two types of gneiss. However, this study finds that melt-rock interaction within the George Sound Shear Zone formed the hornblende gneiss from the precursor two-pyroxene gneiss. Petrographic analyses of samples collected in transects across the shear zone show hydration reaction textures ranging from rims of hornblende + quartz around pyroxene grains to complete replacement of pyroxene grains. Plagioclase is recrystallized and partially replaced by clinozoisite. Additionally, biotite mode increases towards the higher strain rocks in the shear zone. Backscatter images and polarized light microscopy show microstructures indicative of former melt-present deformation, including (a) interconnected mineral films of quartz and K-feldspar along grain boundaries, (b) grains that terminate with low apparent dihedral angles, (c) interstitial grains, with some (d) undulose extinction in plagioclase and (e) serrated grain boundaries. In addition, zircon microstructures are consistent with Zr mobility, further supporting the former presence of melt; geochemical data show enrichment of Zr in the hornblende gneiss as compared to the two-pyroxene gneiss. From the above observations, it is inferred that a felsic to intermediate hydrous melt migrated through the George Sound Shear Zone reacting with the host two-pyroxene gneiss of the Western Fiordland Orthogneiss. Melt migration along grain boundaries was deformation assisted, (i) causing hydration of pyroxene to hornblende + quartz, and plagioclase to clinozoisite, (ii) increasing proportions of biotite within the shear zone and (iii) causing depletion of Cr + Ni and Zr enrichment in the hydrated rock. Our interpretation is supported by published observations of pegmatite dyke swarms that intruded into the George Sound Shear Zone, the P-T conditions of deformation and characterization of microstructures that contrast sharply with those typically found in mylonitic rocks formed under solid-state metamorphic conditions. Our results confirm that hydrous shear zones within otherwise anhydrous country rock are retrogressive and may represent evidence of melt migration through zones of deformation.

Granulite facies metapelitic gneisses collected over a $�200�\times �120$ km exposed area of the Kakamas Domain of the Namaqua–Natal Metamorphic Province in southern Namibia all contain similar garnet–sillimanite–cordierite–biotite–quartz–K-feldspar–ilmenite $�\pm$ plagioclase $�\pm$ magnetite mineral assemblages. These assemblages are interpreted to have equilibrated at suprasolidus retrograde conditions, and most samples contain distinct biotite- or sillimanite-free peak assemblages. Pseudosection modelling constrains extremely uniform residuum solidus conditions of $�5�.�5�\pm �1$ kbar and $�790�\pm �30$°C for the entire Kakamas Domain. Estimated peak metamorphic conditions overlap with these but are more smeared out at between 4 and 7 kbar at 760°C to potentially more than 900°C. The uniformity of residuum solidus conditions is not coincidental, but is a consequence of retrograde re-equilibration due to minor melt retention after peak metamorphism. Re-equilibration could only stop once all retained melt had crystallized, which required the concomitant growth of a hydrous mineral to account for its H₂O component. Biotite is the most stable hydrous mineral in these rocks, such that the residuum $�P$–$�T$ conditions in the Kakamas Domain reflect the upper-$�T$ stability of biotite, and also corresponds to the intersection of the well-known biotite–sillimanite melting reaction that consumed all biotite during prograde metamorphism. The calculated melt fertility of the sample suite indicates that the variable amounts of heat consumed to overcome the latent heat of fusion could have caused a 25°C spread in the peak temperature achieved by the most and least fertile samples. Peak temperature in the Kakamas Domain may have been as much as 100°C higher than residuum solidus conditions for specific samples but cannot be confidently constrained as it is obscured by the effects of both thermal buffering during prograde metamorphism and melt retention during retrograde metamorphism. Both processes are an inescapable part of the evolution of all granulite facies rocks, but their effects are most pronounced in fertile rocks like metapelites that are traditionally the preferred lithology for quantifying the $�P$–$�T$ history of exhumed terranes.