UHP Metamorphism Research Articles

Exhumation of an Ultrahigh‐Pressure Slice From the Upper Plate of the Caledonian Orogen—A Record From Titanite in North‐East Greenland

AbstractUltrahigh‐pressure (UHP) rocks in North‐East Greenland lie within a larger region of high‐pressure Laurentian crust formed in the overthickened upper plate of the collision with Baltica. Coesite‐bearing zircon dates UHP metamorphism to 365–350 Ma, which formed at the end of the Caledonian collision as a result of intracontinental subduction facilitated by strike‐slip faults that broke the lithosphere. Rutile is the stable Ti‐bearing phase at UHP, while titanite forms on the retrograde path. Trace elements and U‐Pb in titanite were analyzed for six UHP gneisses. Zr‐in‐titanite temperatures range from 764 to 803°C and lie on the isobaric part of the pressure‐temperature path at 1.2 GPa, which fits Ti‐phase stability determined by thermodynamic modeling. Large (>600 μm), zoned titanite preserves three distinct trace element patterns that are due to metamorphism, melting and garnet breakdown. Weighted mean 206Pb/238U ages range from 347 ± 5 Ma to 320 ± 11 Ma, but age variation as a function of trace element domain for individual samples is not resolvable within uncertainty. Titanite records a prolonged period of exhumation that is also seen in the zircon record, where phengite decompression melting started at ca. 347 Ma, leucosome emplacement accompanied retrograde metamorphism from 350 to 330 Ma; and titanite grew during isobaric cooling from 345 to 320 Ma when the UHP rocks stalled at lower crustal levels. The same transforms that originally break the lithosphere play a significant role in channeling the UHP rocks back to the lower crust via buoyancy driven exhumation, after which time titanite formed.

Lattice preferred orientation of quartz in granitic gneisses from Tso Morari Crystalline Complex, Eastern Ladakh, trans-Himalaya: evaluating effect of Dauphiné twin in dynamic recrystallization during exhumation

Abstract The Tso Morari Crystalline complex (TMCC) of eastern Ladakh, India, is part of the north Indian continental margin and is characterized by eclogitic enclaves embedded within ortho- and paragneisses known as the Puga Gneiss. Two fault zones bound the TMCC: the Karzok fault to the southwest and the Zildat fault to the northeast. In the present study, we carried out Electron Backscatter Diffraction study of quartz of 10 samples collected from the Puga Gneiss. The relict and recrystallized quartz grains were treated separately to understand the deformation conditions of the Puga Gneiss during early and late deformation stages related to UHP metamorphism and final stage of exhumation during retrogression, respectively. Microstructural observations suggest dynamic recrystallization in quartz and plagioclase at different temperature ranges. Misorientation analysis of both relict and recrystallized quartz grains reveals presence of Dauphiné Twins. Lattice preferred Orientation (LPO) of <c> axis of relict quartz grains generally shows more than one point maxima indicating that the relict grains preserve LPO developed during different stages of metamorphism/deformation. On the other hand, LPO of <c> axis of recrystallized grains from Karzok and Zildat fault zones shows asymmetric single girdle either normal or at an angle to the foliation plane, which suggests simple shear. We conclude that grain size reduction and recrystallization of the Puga Gneiss was greatly influenced by Dauphiné Twin and the final exhumation of the TMCC took place in a simple shear environment aided by activity along its two binding fault zones.

Fluid effect on zircon O and U-Pb isotopes during ultrahigh-pressure metamorphism: Insights from the Dora-Maira Massif of the Western Alps

Zircon geochemistry such as U-Pb and O isotopes have been widely used in dating and tracing complex geological processes. However, it still remains unclear how fluid action affects zircon geochemistry during metamorphic and metasomatic processes in subduction zones. Here a systematic study on zircon U-Pb dating, O isotopes and trace elements as well as whole-rock O isotopes was carried out for the coesite-bearing whiteschists, jadeite quartzites and granitic gneisses from the Dora-Maira Massif, Western Alps. Whole-rock and zircon geochemistry supports a common protolith, i.e., Permian S-type granites, for the above three types of rocks and an intense fluid metasomatism during the Alpine orogeny to form whiteschists and jadeite quartzites. Zircon cores in all samples have nearly identical δ18O values (9‰–11‰), whereas their apparent 206Pb/238U ages show a greater variability due to Pb loss during metamorphism. Zircon rims formed in the late Eocene to early Oligocene can be categorized into two types. Type-I rims occur in granitic gneisses and jadeite quartzites. They have high δ18O values consistent with zircon cores, but much lower contents of P and Y as well as lower Th/U ratios than the cores. Their growth can be attributed to internal metamorphic fluid action at the UHP metamorphic stage. Type-II rims occur in whiteschists and jadeite quartzites. They have remarkably lower δ18O values (5‰–8‰) and Th/U ratios (<0.01), compared with zircon cores and Type-I rims. Their growth can be ascribed to external fluids during the metasomatic process. Some zircon domains in whiteschists and jadeite quartzites show a positive correlation between δ18O values and apparent 206Pb/238U ages, which suggest the simultaneous impacts on U-Pb-O isotopes during external fluid metasomatism. This process can be attributed to the fluid-assisted dissolution and recrystallization of protolith zircons. Especially, coesite inclusions that would have been expected to occur only in young zircon rims formed during UHP metamorphism are also observed in the relict magmatic zircon cores, indicating that the fluid-related metasomatism at the UHP metamorphic conditions also affected these pre-existing protolith-related cores. Therefore, fluid action in subduction zones reveals significant impacts on both the U-Pb and O isotope systems of zircon, especially when external metasomatic fluids are involved. Therefore, a detailed study on zircon, including microstructure, mineral inclusion and geochemical data of different growth and recrystallization domains, is needed in order to unravel continental crustal evolution based on zircon U-Pb ages and O isotope compositions.

Petrology of UHP eclogite-facies felsic schist in the Western Tianshan subduction zone, China

Although quartzo-feldspathic metasedimentary rocks are widespread in high pressure-ultrahigh pressure (HP-UHP) metamorphic belts worldwide, their petrogenesis and metamorphic evolution is poorly understood. We discovered an UHP eclogite-facies felsic schist in the Western Tianshan metamorphic belt, China. Petrological observations and phase equilibria modeling both indicate the felsic schist experienced UHP metamorphism in the coesite stability field. In particular, it experienced prograde metamorphism at 21–24 kbar, 445°C–470°C, a pressure peak at 25–28 kbar and 490°C–525°C, and eventually heating with decompression to 20 kbar and 560°C. The obtained clockwise P-T path was consistent with those of other lithologies (eclogite and pelitic schist) from the same belt, which provides new evidence for the coherent exhumation of the UHP unit of the Western Tianshan metamorphic belt. The final uplift of the Western Tianshan oceanic crust to the surface is attributed to fluid activity and late tectonic deformation.

Non-cratonic Diamonds from UHP Metamorphic Terranes, Ophiolites and Volcanic Sources

Non-cratonic Diamonds from UHP Metamorphic Terranes, Ophiolites and Volcanic Sources

Metamorphism of retroeclogites from the Passos Nappe, Southern Brasília Orogen

High-pressure metamafic rocks from the Passos Nappe, Southern Brasília Orogen, have been traditionally interpreted as highly retrogressed eclogites, even though no reliable P-T conditions were available thus far. In this work, samples from the least retrogressed sample were studied using petrography, mineral and whole-rock chemical analysis, thermodynamic modelling, and rutile U–Pb geochronology, with the objective of unravelling its metamorphic history. The studied rocks are, descriptively, hornblende-garnet-clinopyroxene schists. Garnet and hornblende occur in a matrix composed of clinopyroxene-plagioclase symplectite, with garnet being surrounded by plagioclase-hornblende coronas. A combination of thermodynamic modelling, Zr-in-rutile and Ti-in-quartz thermobarometry suggests peak temperature to be 725 ± 30 °C, attained at a minimum pressure of ≈16 kbar, and data from rutile and quartz crystals included in garnet qualitatively indicate an earlier stage at higher P and lower T, constraining a clockwise P-T path. No evidences of UHP metamorphism were found thus far. A lower intercept age of 579 ± 23 Ma was calculated from rutile data, which is identical, within uncertainty, to rutile ages obtained previously in units immediately above the rocks studied here. The data suggests that these rocks were submitted to a different P-T path compared to the adjacent units, and that they were subducted to depths of at least ≈60 km and tectonically emplaced amid the surrounding metapelitic rocks before 579 Ma, which is compatible with current models for the evolution of the Southern Brasilia Orogen.

Metamorphic evolution of mafic granulites in the Kuruksayi area of the South Altyn Orogen, West China: Insights from petrography, phase equilibria modeling and geochronology

The South Altyn high-pressure–ultrahigh-pressure (HP–UHP) metamorphic belt in West China records the processes of deep continental subduction (>150–300 km). The HP mafic granulite pods hosted by massive granitic gneiss were for the first time discovered in the Kuruksayi area of the South Altyn Orogen, and they consist mainly of garnet, clinopyroxene, plagioclase, hornblende, biotite, and quartz, with minor rutile, ilmenite, and K-feldspar. The garnets vary in grain size and chemical zoning, and can be divided into three types: subhedral porphyroblastic (g1), anhedral porphyroblastic (g2), and neoblastic fine (g3) grains. Four generations of mineral assemblage (M1–M4) are recognized, and are used to constrain the P–T evolution of the mafic granulites using phase equilibria modeling calculated by THERMOCALC. The first generation (M1) is represented by the core domains of g1 (g1C) that have inclusions of rutile and quartz and conspicuous chemical zoning. A pre-peak prograde stage of metamorphism from ~ 15.8 kbar/600 °C to ~ 19.2 kbar/655 °C is recorded by g1C. The second generation (M2) is inferred to consist of garnet + omphacite + phengite + rutile + quartz and is recorded in the mantle domains of g1 (g1M). Modeling suggests that this is representative of conditions that reached a maximum P–T of ~ 36.2 kbar/920 °C. The third generation (M3) is characterized by intergrowths of clinopyroxene + plagioclase and biotite + plagioclase, and this stage is recorded by the compositions of g2 with modeling suggesting an evolution from ~ 17.8 kbar/820 °C to ~ 11.5 kbar/826 °C, consistent with near-isothermal decompression. The last generation (M4) is characterized by kelyphitic rims of plagioclase + hornblende around garnet and the hornblende coronas that encompass symplectitic clinopyroxene, and is recorded by the compositions of g3. Modeling suggests evolution from ~ 11.5 kbar/826 °C to ~ 8.7 kbar/735 °C, compatible with near-isobaric cooling. These four stages of metamorphism together establish a clockwise P–T path, which indicates a series of tectonic processes that took the continental slab from subduction to exhumation. Zircon U–Pb dating by LA–ICP–MS yielded two distinct clusters of ages: c. 897 Ma magmatic protolith ages and c. 494 Ma metamorphic ages. The metamorphic ages are interpreted to represent the timing of the eclogite-facies metamorphism, and are similar to the metamorphic ages of other HP/UHP rocks elsewhere in the South Altyn Orogen. Accordingly, we propose that the Kuruksayi mafic granulites underwent UHP metamorphism similar to that recorded in other South Altyn eclogites exposed in Jianggalesayi and Danshuiquan to the west and Yinggelisayi to the east.

Cathodoluminescence Microscopy of Zircon in HP- and UHP-Metamorphic Rocks: A Fundamental Technique for Assessing the Problem of Inclusions versus Pseudo-Inclusions

This paper shows how a faulty approach to the study of mineral inclusions in zircon can lead to misleading interpretations of the geological context. We present and discuss two well-documented examples. Zircon grains separated from HP metamorphic jadeitite of the Rio San Juan Complex, Dominican Republic, and from UHP pyrope quartzite of the Dora Maira Massif, northern Italy, were studied using cathodoluminescence (CL) techniques, in combination with mineral inclusion and age data. In general, zircon from both localities shows inherited magmatic core domains with oscillatory zoning and metamorphic rims. The magmatic cores of zircon from the jadeitite yield ages of 115–117 Ma and host jadeite and omphacite which are of metamorphic origin and formed at about 78 Ma. Zircon from lawsonite blueschist, representing the country rock of the jadeitite, contains domains with oscillatory zoning that are nearly identical in age to the zircon cores from the adjacent jadeitite, and also contains younger metamorphic minerals such as lawsonite, albite, phengite (Si3.68), chlorite, and omphacite. Similar observations were made on the magmatic cores of zircon from the pyrope quartzite. These are about 275 Ma in age and host pyrope, phengite (Si3.55), talc, and kyanite, all of which formed during UHP metamorphism at about 35 Ma. Zircon from the biotite-phengite-gneiss country rock (metagranite) shows oscillatory zoning and yields ages that are identical to those of the magmatic cores of zircon from pyrope quartzite, which thus reflect granitic intrusion ages. The country-rock zircon also encloses metamorphic minerals with ages of about 35 Ma. Such minerals are, for example, garnet and phengite, as well as a polymineralic assemblage of clinopyroxene+garnet+phengite+quartz, that point to formation at UHP metamorphic conditions around 40 kbar/750 °C. Based on these examples we suggest an effective approach centered on key evidence from CL studies to show that magmatic domains of zircon may actually contain pseudo-inclusions which were not entrapped during an early stage of formation, but were instead introduced during later metamorphic or metasomatic events along microcracks representing pathways for fluid influx. Cathodoluminescence microscopy is thus an excellent tool for avoiding such pitfalls by allowing distinction between true inclusions and pseudo-inclusions in zircon.

Two-stage garnet growth in coesite eclogite from the southeastern Papua New Guinea (U)HP terrane and its geodynamic significance

Mineral compositions and textures of Late Miocene coesite eclogite from Tomagabuna Island were investigated to constrain P–T conditions during UHP metamorphism and subsequent exhumation. Two stages of garnet growth (core and rim), at eclogite-facies conditions, were documented. In addition to core garnet (I), peak assemblages include omphacite, coesite, phengite, and rutile. Rim garnet (II), amphibole, paragonite, plagioclase, quartz, and accessory biotite and spinel were formed after peak pressure conditions. Although a compositional gradient is present at the core–rim garnet interface, the garnet core has a relatively flat compositional profile, suggesting its crystallization in the coesite stability field. Together with minerals formed subsequent to peak pressure (UHP) conditions, the composition of the garnet rim indicates heating of the eclogite during its decompression into the amphibolite facies mineral stability field. Based on compositional zoning in garnet and application of diffusion modelling, we propose that eclogite-facies metamorphism of the mafic protolith and its host lithologies occurred at, or near, UHP conditions. The core garnet (I) formed at 650 °C at 8 Myr in the coesite stability field and was partially resorbed during the onset of exhumation. We infer that garnet rim growth at < 7.1 Ma occurred at P–T conditions corresponding to the boundary between the eclogite and amphibolite facies fields. Diffusion modelling for the garnet core–rim boundary compositions suggests a transient heating event (0.3 Myr) occurred at 1.5 GPa that we infer resulted from heat transport within the Australian—Woodlark plate boundary zone as the coesite eclogite was exhumed. Results caution that apparent P–T paths documented for many UHP terranes may not result from isothermal decompression corresponding to peak temperatures. Instead, paths may link P–T points’ set during transient mineral growth events when the UHP rocks form within the subduction channel, and during subsequent heating events, when UHP rocks are exhumed from mantle depths.

UHP metamorphism recorded by coesite-bearing metapelite in the East Kunlun Orogen (NW China)

AbstractThe East Kunlun Orogen (EKO) is the NW part of the Central China Orogenic Belt, which records the evolutionary history of the Proto- and Palaeo-Tethys Oceans from the Cambrian to the Triassic. An Early Palaeozoic eclogite belt has been recognized in recent years, which extends discontinuously for ∼500 km as three eclogite-bearing terranes. In this study, we report an integrated study of zircon grains from mica-schists accompanying the eclogites, in terms of mineral inclusions, U–Pb age systematics andP–Tconditions. The presence of coesite is identified, as inclusions within the metamorphic domain of zircons, which provides unambiguous evidence for subducted terrigenous clastic rocks of the Proto-Tethys Ocean exhumed from coesite-forming depths. U–Pb dating of the metamorphic zircons yields a concordia age of 426.5 ± 0.88 Ma, which is likely to be the time of ultrahigh-pressure metamorphism in the Kehete terrane.P–Tcalculations suggest that metapelite may have experienced a clockwiseP–Tpath with peakP/Tconditions of 685 ± 41 °C and &gt;28 kbar, and equilibrated at 482–566 °C and 5.6–8.9 kbar during subsequent exhumation. The high-pressure – ultrahigh-pressure (HP-UHP) metamorphic belt within the EKO may have formed by collision between the Qaidam Block and the South Kunlun Block, as a consequence of the closure of the Proto-Tethys Ocean.

Multi-Stage Metamorphism of the UHP Pelitic Gneiss from the Southern Altyn Tagh HP/UHP Belt, Western China: Petrological and Geochronological Evidence

The kyanite-bearing garnet pelitic gneiss from the Jianggalesayi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of kyanite and spinel exsolution microstructure in quartz (precursor stishovite). In this study, three stages of retrograded metamorphism (M2–M4) after the UHP metamorphism (M1) were identified for the UHP pelitic gneiss. The HP granulite-facies stage (M2) was characterized by the mineral assemblage of garnet+kyanite+K-feldspar+rutile+quartz±ilmenite, recording the P-T condition of >1.12 GPa and ≈850–930 oC. The granulite-facies stage (M3) was represented by the mineral assemblage of garnet rim+K-feldspar+sillimanite (Sill1)+biotite (Bt1)+plagioclase (Pl1)+ilmenite+quartz, and confined under P-T conditions of 0.5–0.8 GPa and ≈770–795 oC. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl2, Sill2 and Bt2 in the matrix, and the P-T conditions were 0.4–0.6 GPa and 350 km) to the lower crust depth (≈40–20 km), resulting in an average exhumation rate of 9.11–9.70 mm/yr. In the southern Altyn Tagh region, the HP and UHP rocks from different areas had identical peak metamorphic ages. Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. The continental crustal were subducted to different depths along the direction of the subduction channels at ≈500 Ma, suffered different grade metamorphism, and then returned to the surface along the subduction channel.

Rare peak and ubiquitous post-peak zircon in eclogite: Constraints for the timing of UHP and HP metamorphism in Erzgebirge, Germany

Erzgebirge eclogite occurs in three high-pressure (HP) units, including one UHP unit (Unit 1). Each sample investigated contains either magmatic or metamorphic zircon but never both. This is in contrast to many literature examples of eclogite with more than one generation of zircon. We infer that metamorphic zircon formed at the expense of a magmatic precursor, and that the process either went to completion (consuming all magmatic zircon) or did not occur at all. Metamorphic zircon is only present in UHP eclogite and one HP eclogite from Unit 2. Another sample from Unit 2 and one from Unit 3, which experienced the lowest metamorphic PT conditions of all samples, contain only magmatic zircon. The U-Pb SIMS age of c. 540 Ma of igneous zircon, being somewhat younger than average Hf model ages (i.e., 567 and 591 Ma), is attributed to crystallization in the basaltic or gabbroic precursor that formed from a juvenile mantle source.The time of peak metamorphism is recorded by only two zircons from UHP eclogite giving ages of 360.5 ± 5.4 Ma and 359.7 ± 5.3 Ma that well agree with a previous three-point Sm-Nd isochron of 360 ± 7 Ma. All other grains gave younger ages, mainly between 340 and 330 Ma but covering a range of c. 350–315 Ma, which is attributed to retrogression at amphibolite-facies conditions. We infer (i) that this post-eclogitic zircon replaced peak-metamorphic grains (only in UHP eclogite) as well as magmatic zircon and (ii) that peak-metamorphic zircon did not form in non-UHP eclogite. Growth of peak and post-peak zircon was governed by fluid and/or melt. The latter is related to decompressional melting in felsic host rocks (and possibly also to late-orogenic granitic magmatism). Fluid is either of external origin (melt-derived or from dehydration reactions in subducting footwall units) or due to decompressional release of internal fluid (molecular water from fluid inclusions or structural water in omphacite and garnet). The new and former age data can best be reconciled with a two-stage exhumation process. Initial, buoyancy-driven exhumation was fast and terminated as the HP/UHP slices reached lower crustal levels where the rocks presumably rested for 10–20 million years, followed by slow exhumation associated with gravitational collapse of the Variscan orogen. The majority of metamorphic zircon (with ages of 340–330 Ma) grew at lower crustal levels, where the rocks presumably rested for 10–20 million years, and other grains (younger than 330 Ma) during the final, slow exhumation that partly overlapped with the time of granitic magmatism.

The coherent ultrahigh-pressure terrane of the Tianshan meta - ophiolite belt, NW China

Blocks of basic rocks metamorphosed at high- and ultrahigh-pressures (HP-UHP) are often embedded in metasedimentary or granitic rocks that do not show evidence for HP metamorphism. The contrasting maximum depth reached by these isolated rock fragments, normally eclogites, and the enveloping rocks is commonly seen as evidence for a form of tectonic mélange. In the Tianshan meta-ophiolite belt of NW China, some reported eclogites and pelitic-felsic schists adjacent to coesite-bearing rocks apparently lack evidence of UHP conditions. Their deduced maximum pressures are 1.5–2.5 GPa. These HP blocks have been regarded as dismembered ophiolite fragments each with its own metamorphic history generated by a forced return flow in a subduction channel. Here we reappraise the P-T estimates, formerly derived by various P-T tools, of these reported HP rocks, including pelitic schists, mafic eclogites and eclogitic rocks of volcaniclastic origin, from the Akeyazi area. P-T pseudosections in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O contoured for garnet core or mantle composition (thermocalc version 3.33) have been used for deducing peak P-T conditions. It was found that all investigated samples were subject to UHP metamorphism (2.8–3.5 GPa at 520–580 °C). The used thermodynamic dataset for these P-T estimates has also been testified valid for typical UHP-LT rocks worldwide. We suggest that garnet isopleth thermobarometry-derived P-T estimates are potentially robust in accordance with UHP mineralogical evidence such as coesite, but are sensitive to garnet solution models and effective bulk compositions. Inconsistent P-T results and related tools involving sodic clinopyroxene and phengite should be dismissed in many cases, if not all. The consequences of the reevaluated P-T data are that eclogites, both interlayered and blocky, and the associated host rocks comprise a coherent Tianshan UHP terrane, not a highly disrupted mélange.

Low-δ18O mantle-derived magma in Panjal Traps overprinted by hydrothermal alteration and Himalayan UHP metamorphism: Revealed by SIMS zircon analysis

We report two generations of low-δ18O zircons from the Himalayan eclogites and their host gneisses. In situ SIMS δ18O analyses on single zircon crystals (with known age and Hf isotope ratios) from two populations of chemically distinct zircons demonstrate a complex history: (1) an early low-δ18O mantle-derived magma, (2) followed by post-emplacement high-temperature meteoric-water alteration and finally (3) crystallization of new, low-δ18O minerals during the ultrahigh-pressure metamorphism. Magmatic zircon (269Ma) in Group I eclogites yielded δ18O values from 1.9 to 4.6‰ VSMOW with an average value of 4.0±0.2 (n=35, the error is 2SD analytical precision and “n” represents number of analyzed spots), which is lower than the typical mantle values (5.3±0.6, 2SD). In contrast, metamorphic zircons (45Ma) in Group II eclogites preserve unusually low, negative δ18O values from −3.9 to −2.7‰ (average: −3.4±0.4, n=35, 2SD). Zircons in felsic gneiss that surround Group II eclogites have inherited magmatic cores (ca. 260Ma) with δ18O values of ca. 2.9‰, which decrease to −0.1‰ in metamorphic (ca. 45Ma) rims. These zircons preserve lower δ18O values than would be equilibrated with typical mantle. The low-δ18O values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative δ18O values in metamorphic zircons indicate hydrothermal alteration after crystallization of the mafic magmas but before growth of metamorphic zircons. This study reports evidence for melting of subducted low-δ18O ocean crust to form low-δ18O mantle-derived mafic magmas as previously proposed by Cartwright and Valley (1991, Geology 19, 578–581) for Proterozoic Scourie Dikes.

Quartz and orthopyroxene exsolution lamellae in clinopyroxene and the metamorphic P–T path of Belomorian eclogites

ABSTRACTThe Shirokaya Salma eclogite‐bearing complex is located in the Archean–Palaeoproterozoic Belomorian Province (Russia). Its eclogites and eclogitic rocks show multiple clinopyroxene breakdown textures, characterized by quartz–amphibole, orthopyroxene and plagioclase lamellae. Representative samples, a fresh eclogite, two partly retrograded eclogites, and a strongly retrograded eclogitic rock, were collected for this study. Two distinct mineral assemblages—(1) omphacite+garnet+quartz+rutile±amphibole and (2) clinopyroxene+garnet+amphibole+plagioclase+quartz+rutile+ilmenite±orthopyroxene—are described. Based on phase equilibria modelling, these assemblages correspond to the eclogite and granulite facies metamorphism that occurred at 16–18 kbar, 750–800°C and 11–15 kbar, 820–850°C, respectively. The quartz–amphibole lamellae in clinopyroxene formed during retrogression with water ingress, but do not imply UHP metamorphism. The superfine orthopyroxene lamellae developed due to breakdown of an antecedent clinopyroxene (omphacite) during retrogression that was triggered by decompression from the peak of metamorphism, while the coarser orthopyroxene grains and rods formed afterwards. The P–T path reconstructed for the Shirokaya Salma eclogites is comparable to that of the adjacent 1.9 Ga Uzkaya Salma eclogite (Belomorian Province), and those of several other Palaeoproterozoic high‐grade metamorphic terranes worldwide, facts allowing us to debate the exact timing of eclogite facies metamorphism in the Belomorian Province.

O-190 Volatile transfer and recycling at UHP metamorphism; constraint from CCSD (Chinese Continental Scientific Drilling) eclogites

O-190 Volatile transfer and recycling at UHP metamorphism; constraint from CCSD (Chinese Continental Scientific Drilling) eclogites

Age of UHP metamorphism in the Western Mediterranean: Insight from rutile and minute zircon inclusions in a diamond-bearing garnet megacryst (Edough Massif, NE Algeria)

Diamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U–Pb and trace elements analyses of zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 μm in size) analyzed in situ provide a U–Pb age of 32.4±3.3 Ma interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (≤30 μm) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740–810 °C, most likely during HP retrograde metamorphism. U–Pb analyses provide a significantly younger age of 20.7±2.3 Ma attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica–Sardinia block and most likely resulted from subduction rollback that was driven by slab pull.

CO2‐bearing fluid inclusions associated with diamonds in zircon from the UHP Kokchetav gneisses

CO2‐bearing fluid inclusions coexisting with diamonds were identified in zircons from diamondiferous gneiss in the Kokchetav Massif. This discovery provides evidence for the presence of CO2 in UHP fluids and diamond formation in moderately oxidized conditions in the Kokchetav gneiss. Fluid and multiphase solid inclusions coexisting in zircons represent immiscible melt and fluid captured close to the peak metamorphic conditions for the Kokchetav UHP gneiss. Most of CO2‐bearing inclusions are CO2+H2O mixtures except for some cases when they also contain daughter phases (e.g. muscovite, calcite and quartz) tracing the presence of aqueous and solute‐rich fluids at different phases of UHP metamorphism. Decrease of pressure and temperature may have been responsible for the reduction of solutes in the CO2‐bearing fluid. The lack of CO2‐bearing inclusions in garnet porphyroblasts from diamond‐bearing gneiss, as well as the common coexistence of aqueous CO2‐bearing inclusions with calcite, testify that most likely all CO2 in fluid was consumed by the calcite‐forming reaction and hydrous melt was the only remaining growth medium during retrograde metamorphism of the Kokchetav UHPM gneisses. Neither K‐cymrite nor kokchetavite was identified among daughter phases in the hydrous melt inclusions in garnet, which indicates that they hardly could originate in a metapelitic system. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Dating of ultramafic rocks from the Western Alps ophiolites discloses Late Cretaceous subduction ages in the Zermatt-Saas Zone

AbstractThe Zermatt-Saas Zone was part of the Middle to Late Jurassic Tethyan lithosphere that underwent oceanic metamorphism during Mesozoic time and subduction during Eocene time (HP to UHP metamorphism). In upper Valtournanche, serpentinite, metarodingite and eclogite record a dominant S2 foliation that developed under 2.5±0.3 GPa and 600±20°C during Alpine subduction. Serpentinites contain clinopyroxene and rare zircon porphyroclasts. Clinopyroxene porphyroclasts show fringes within S2 with similar compositions to that of grains defining S2. Zircon cores show zoning typical of magmatic growth and thin fringes parallel to the S2 foliation. These features indicate crystallization of clinopyroxene and zircon fringes during HP syn-D2 metamorphism, related to the Alpine subduction. The U–Pb zircon dates for cores and fringes reveal crystallization at 165±3.2 Ma and 65.5±5.6 Ma, respectively. The Middle Jurassic dates are in agreement with the known ages for the oceanic accretion of the Tethyan lithosphere. The Late Cretaceaous - Paleocene dates suggest that the Zermatt-Saas Zone experienced high-pressure to ultra-high-pressure (HP–UHP) metamorphism at c. 16 Ma earlier than previously reported. This result is in agreement with the evidence that in the Western Alps the continental Sesia-Lanzo Zone reached the subduction climax at least from 70 Ma and was exhumed during ongoing oceanic subduction. Our results are further evidence that the Zermatt-Saas ophiolites diachronically recorded heterogeneous HP–UHP metamorphism.

Further observations related to a possible occurrence of terrestrial ahrensite

Clusters of aligned, highly elongate, prismatic quartz (Qtz) rods occur in a few fayalite (Fa) crystals in an eulysite from a recently identified ~1.8 Gy UHP site in central West Greenland (Glassley et al. 2014). Additional detailed analyses of the crystallography and phase compositions of these olivines were conducted to evaluate the postulate that the Qtz rods formed during inversion of super-silicic ahrensite to Fa+Qtz during decompression. These new observations show the Qtz rods consistently occur in crystallographically coherent clusters with the Qtz grains aligned parallel to [100] of Fa. The contrasting compositions of coexisting primary UHP Fa and Fa postulated to have formed by inversion of ahrensite are consistent with the inversion scenario. We thus conclude that all available data are consistent with the postulate that ahrensite was part of the equilibrium phase assemblage formed during UHP metamorphism and that it inverted to Fa+Qtz upon decompression. If true, this would represent the first occurrence of terrestrial ahrensite formed through natural tectonic processes.