Ultrahigh-pressure Metamorphism Research Articles

A Raman spectroscopic study of diamond and disordered sp3‐carbon in the coesite‐bearing Straumen Eclogite Pod, Norway

AbstractRaman spectral point analysis and Raman mapping have been applied to study the nature of carbon‐bearing microinclusions C3‐1 and C5‐1 within zircon in phengite–kyanite–coesite eclogite sample 62a from the Straumen Eclogite Pod in the Western Gneiss Region (WGR), S.W. Norway. Graphitic carbon is always present with a degree of structural disorder comparable to that found in other ultra high pressure metamorphic (UHPM) terranes; it is compatible with an origin by a partial retrogression of the coexisting diamond due to depressurization by exhumation. Hematite coexists with diamond in C3‐1. In both microinclusions, the Raman wavenumber position of the diamond peak varies from point to point, from the ideal value of 1332 cm−1, frequently down to ∼1328 cm−1 and sometimes down to ∼1322 cm−1 in C3‐1. The bandwidth also increases proportionally. Some spectra reveal doublets with peaks at ∼1328 and ∼1322 cm−1 and even a triplet. These Raman data are discussed in terms of one or other kind of structural disorder of sp3‐carbon, for example metamictization, overtemperature or overpressure, nitrogen impurities and especially stacking disorder, that is, with the possible existence or coexistence of 3C, 2nH and/or 3mR polytypes in a mixture in the same monocrystal. The question of a metamorphic or a contamination origin of the diamond and the disordered sp3‐carbon in slide 62a is evaluated; numerous arguments favour the UHPM origin. These new data contribute to the ongoing debate on the various petrological and geodynamical models (including possible deviatoric stress) for the creation of diamond in UHPM rocks in general. This new locality of diamond at Straumen, the fourth in the WGR but the first in the southwestern part of the UHPM area of the WGR, provides a minimum pressure estimate for UHP metamorphism of ∼3.5 GPa. Taken together with other published P–T estimates, 3.75 ± 0.75 GPa covers all of the estimates of the minimum pressure for coesite‐, diamond‐ or orthopyroxene‐bearing eclogite types in the WGR.

Exhumation mechanisms of melt‐bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates

AbstractA series of 2D petrological–thermomechanical numerical experiments was conducted to: (i) characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic (UHPM) rocks during collision of spontaneously moving plates and (ii) study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high‐temperature–ultrahigh pressure metamorphic (HT–UHPM) rocks. To this end, the models include fluid‐ and melt‐induced weakening of rocks. Five distinct modes of exhumation of (U)HPM rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows: vertical crustal extrusion, large‐scale crustal stacking, shallow crustal delamination, trans‐lithospheric diapirism, and channel flow. The variation in exhumation mechanisms for (U)HPM rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and/or did not include fluid‐ and melt‐induced weakening of rocks. This difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid‐ and melt‐related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments, such as slab breakoff, vertical crustal extrusion, large‐scale stacking, shallow crustal delamination and relamination, and eduction of the continental plate. Consequently, the significance of channel flow for the exhumation of UHPM rocks may have been overstated based on the results of the earlier numerical experiments. In addition, the results from this study extend over a larger proportion of the high‐temperature range of P–T conditions documented from UHPM rocks, including those retrieved from HT–UHPM rocks, than the results of experiments from previous numerical models. In particular, the highest peak metamorphic temperatures (up to 1000 °C) are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab. Nonetheless, some extreme temperature conditions which have been suggested for the Kokchetav and Bohemian massifs, perhaps up to 1100–1200 °C, are still to be achieved in experiments using numerical models.

Decoupling of U–Pb and Lu–Hf isotopes and trace elements in zircon from the UHP North Qaidam orogen, NE Tibet (China): Tracing the deep subduction of continental blocks

Eclogites and gneisses from the Yuka and Xitieshan areas in the North Qaidam orogen, NE Tibet, experienced ultrahigh-pressure (UHP) metamorphism in the early Paleozoic, when the Qaidam block subducted northwards beneath the Qilian block. U–Pb ages, trace-element patterns and Hf-isotope compositions of zircon in the two rock types were used to trace the isotopic and elemental changes within zircon during continental subduction and exhumation, and to examine the links between the igneous protolith and tectonic behavior of the subducting block. Zircon data suggest that the eclogites had igneous protoliths with ages of ca. 785±20Ma (2σ, MSWD=3.4, n=5), and most zircons metamorphically recrystallized during eclogitization at ca. 441±2Ma (2σ, MSWD=1.15, n=41); minor changes were caused by retrogression during ~417–409Ma. Trace elements of these zircons were incompletely reset in the prograde and retrograde processes; however, their Lu–Hf isotopic compositions retained the primary igneous 176Hf/177Hf ratios, reflecting a large extent of initial heterogeneity (εHf(t)=+14.0 to −19.5), even within single grains. Lu/Hf ratios decreased as a result of the eclogitization, and then increased again during retrogression. The behaviors of the U–Pb and Lu–Hf isotopic systems as well as the trace elements of the eclogitic zircons were decoupled during the early Paleozoic orogeny.Zircons from the gneisses mostly preserved their primary diverse signatures (εHf(t)=+15.8 to −17.9; crustal model ages=2.54–0.80Ga), but some ~440Ma metamorphic grains show lower trace-element concentrations than the Proterozoic zircons. U–Pb and Lu–Hf isotopic data in this study and from the literature reveal the existence of Neoarchean (ca. 2.8–2.5Ga) components in the Qaidam block as the initial protoliths of the gneisses, which were intruded by depleted-mantle (DM) magmas at ~2.1–2.0Ga and 1.1–0.8Ga. The igneous protoliths of the eclogites were emplaced into the protoliths of gneisses at ~850–750Ma, and are interpreted here as having formed by magmatic mixing between juvenile components from the DM and evolved material from the Archean Qaidam crust. This 1.1–0.8Ga magmatism has largely “rejuvenated” the Qaidam bock, at least its northern margin, and added more mafic material into its deep crust and upper mantle, as also indicated by geophysical observations. This lithosphere, and perhaps lower crust, of this continental block would become denser during subduction because of the extensive generation of garnet in the deep lithosphere, causing it to act as a “sinker.” The intrinsically basic nature of the deep levels of the Qaidam block pre-determined its deep subduction into the mantle depths at ~440Ma, and then the less dense upper crust was detached from the deeper levels and exhumed later.

Multi-stage exhumation of young UHP–HP rocks: Timescales of melt crystallization in the D’Entrecasteaux Islands, southeastern Papua New Guinea

On the D’Entrecasteaux Islands, southeastern Papua New Guinea (PNG), a series of domes expose the youngest known ultrahigh-pressure (UHP) to high-pressure eclogites and associated gneisses. Abundant evidence of partial melting is preserved throughout the D’Entrecasteaux domes in the form of variably deformed leucosomes, dikes, and plutonic bodies. In order to understand (1) the timing of melt emplacement relative to the UHP metamorphism and (2) the complete exhumation and deformational history of the eclogites and surrounding gneisses, a series of leucosomes, dikes, and eclogites from the Mailolo dome in western Fergusson Island were analyzed by U–Pb ID-TIMS zircon geochronology. Zircons extracted from the eclogite at the UHP locality yield 206Pb/238U TIMS dates from 5.60±0.22 to 4.61±0.18Ma. Zircons from three foliation-parallel leucosomes are interpreted to record melt crystallization from 3.49±0.01Ma to 2.98±0.02Ma. In comparison, weakly deformed granitic–granodioritic dikes that cut the leucosomes yield zircon 206Pb/238U dates of 2.48±0.01Ma–2.420±0.002Ma. Based on these new and previously published results, the eclogites may have undergone UHP metamorphism for several million years, from 7.1±0.7Ma to 4.61±0.18Ma; based on TIMS dates alone, a more restricted interval of 5.6–4.6Ma is proposed. Following UHP metamorphism, the eclogite and gneisses were exhumed to lower crustal depths, intruded by foliation-parallel leucosomes by ca. 3.5–3.0Ma and by ca. 2.45Ma late discordant dikes. Multiple generations of leucosomes and dikes allow a timeline for the exhumation of the PNG UHP terrane to be constructed: mantle (∼100km depths) to the base of the crust in ∼1my, storage in the lower crust for ∼1my, and final emplacement into the shallow crust ∼1my later.

Grenville-age orogenesis in the Qaidam-Qilian block: The link between South China and Tarim

The link between the South China and Tarim blocks is a key issue in defining the tectonic framework of China. We report here that rocks from the Paleozoic (Caledonian) ultrahigh-pressure metamorphic (UHPM) belt also record a Grenville-age orogenic event in the Qaidam-Qilian block that lies between the South China and Tarim blocks. This event is marked by the presence of a juvenile Grenville-age continental margin that had been subducted to and exhumed from mantle depths of 100–200km at the present-day location of northern Tibetan Plateau. The magmatic and metamorphic ages of 900–1000Ma recorded in the gneisses provide direct evidence for this Grenville-age orogeny that extends northwestward from the Yangtze block, to the Qaidam-Qilian block, and to the Tarim block. This Grenville-age orogen along the North Qaidam UHPM belt (1) represents the link for the once South China–Qaidam-Qilian–Tarim continent, which, we name here as the “South-West China United Continent (SWCUC)”, had existed before the India–Asia collision, (2) sets a framework for precise reconstruction of Supercontinent Rodinia, and (3) presents an example of multi-epoch tectonic recycles, represented by the Neoproterozoic Grenvillian orogenesis and the Early Paleozoic Caledonian orogenesis.

UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland, Swedish Caledonides

The first evidence for ultrahigh-pressure (UHP) metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides is recorded by kyanite-bearing eclogite, found in a basic dyke within a garnet peridotite body exposed close to the lake Friningen in northern Jämtland (central Sweden). UHP metamorphic conditions of ~3GPa and 800°C, within the stability field of coesite, are constrained from geothermobarometry and calculated phase equilibria for the peak-pressure assemblage garnet+omphacite+kyanite+phengite. A prograde metamorphic evolution from a lower P–T (1.5–1.7GPa and 700–750°C) stage during subduction is inferred from inclusions of pargasitic amphibole, zoisite and kyanite in garnet cores. The post-UHP evolution is constrained from breakdown textures, such as exsolutions of kyanite and silica from the Ca-Eskola clinopyroxene. Near isothermal decompression of eclogite to lower crustal levels (~0.8–1.0GPa ) led to formation of sapphirine, spinel, orthopyroxene and diopside at granulite facies conditions. Published age data suggest a Late Ordovician (460–445Ma) age of the UHP metamorphism, interpreted to be related to subduction of Baltoscandian continental margin underneath an outboard terrane, possibly outermost Laurentia, during the final stages of closure of the Iapetus Ocean. The UHP rocks were emplaced from the hinterland collision zone during Scandian thrusting of the nappes onto the Baltoscandian foreland basin and platform. The record of P–T conditions and geochonological data from UHP rocks occurring within the allochthonous units of the Scandinavian Caledonides indicate that Ordovician UHP events may have affected much wider parts of the orogen than previously thought, involving deep subduction of the continental crust prior to final Scandian collision between Baltica and Laurentia.

원생대 이후 트라이아스기까지의 남한과 동북아시아의 지구조 진화

최근 송림 조산운동시기인 트라이아스기에 홍성 지역의 에클로자이트와 경기육괴 내에 광범위하게 나타나는 충돌 후 화성암이 형성되었음이 확인되었다. 이러한 새로운 발견에 의해 1980년대 말에 확인된 남중국판과 북중국판의 충돌대인 트라이아스기 친링-다비-수루 충돌대가 한반도의 홍성 지역으로 연결된 후 양평 지역을 거쳐 오대산 지역으로 연결되는 모델이 제시되었다. 그 이후 충돌대는 일본 서남부의 고생대 섭입 복합체와 북한의 고생대 섭입 복합체인 연변대로 연결될 가능성이 높다. 이럴 경우 경기육괴는 홍성-양평-오대산 충돌대를 경계로 나뉘어져야하며 충돌대 북부는 북중국판에 남부는 남중국판에 대비될 것이다. 남중국판과 북중국판의 충돌은 페름기에 한반도로부터 시작하여 중국 쪽으로 진행되었으며 트라이아스기 말에 충돌이 완료되었다. 다비-수루-홍성-양평-오대산 충돌대를 따라 동측에서 서측으로 변성조건이 체계적으로 변화한다. 가장 동측에 해당하는 오대산 지역에서는 245-230 Ma 초고온 변성작용이 한반도의 서측에 해당하는 홍성지역에서는 230Ma 고압변성작용이 그리고 중국의 수루, 다비 지역에서는 220Ma의 초고압 변성작용이 일어났다. 충돌대를 따라 나타나는 체계적인 변성조건의 변화는 충돌 전 섭입된 해양지각의 양이 동측에서 서측으로 증가하여 섭입하던 대륙판과 해양판의 분리되는 깊이가 동측에서 서측으로 점차 증가하였기 때문인 것으로 생각된다. 한반도 남부 영남육괴와 남중국판 남부에 광범위하게 나타나는 페름기-트라이아스기 섭입관련 화성암은 북중국판과 남중국판의 페름기-트라이아스기 충돌이 남중국판 남쪽 경계부에서의 섭입작용을 시작시켰을 가능성이 있음을 지시한다. 즉 한반도의 송림 조산운동 시기에는 경기육괴지역에서 대륙충돌이 일어나 한반도가 형성되었으며 동시에 대륙충돌에 의해 영남육괴 남측에 섭입대를 형성되었을 것으로 예상된다. 홍성-양평-오대산 충돌대를 경계로 하여 경기육괴 북부와 남부는 모두 콜롬비아 초대륙 형성 후기인 1870-1840 Ma에 대륙충돌이나 섭입 작용에 의해 화성 및 변성작용을 강하게 받았다. 옥천 변성대는 중국 남중국판 내에 나타나는 760Ma 경에 형성된 난후아 열곡대에 대비될 수 있으며 그럴 경우 경기육괴 남부와 영남육괴는 각각 남중국판의 북부 양쯔판과 남부 커테시아판에 대비된다. 최근 경기육괴 변성퇴적암, 옥천변성대 등에서 광역적으로 발견되는 실루리아기-데본기의 쇄설성 저어콘은 경기육괴 내에 데본기 혹은 그 이후 고생대 시기의 퇴적암이 존재할 가능성을 제시하고 있으며 홍성과 광천 지역에서 확인된 데본기의 변성작용과 함께 페름기-트라이아스기의 대륙충돌 이전인 고생대 중기에도 경기육괴에 활발한 지구조 운동이 일어났음을 지시한다. Recent studies reveal that eclogite formed in the Hongseong area and post collision igneous rocks occurred throughout the Gyeonggi Massif during the Triassic Songrim Orogeny. These new findings derive the tectonic model in which the Triassic Qinling-Dabie-Sulu collision belt between the North and South China blocks extends into the Hongseong-Yangpyeong-Odesan collision belt in Korea. The belt may be further extended into the late Paleozoic subduction complex in the Yanji belt in North Korea through the Paleozoic subduction complex in the inner part of SW Japan. The collision belt divides the Gyeonggi Massif into two parts; the northern and southern parts can be correlated to the North and South China blocks, respectively. The collision had started from Korea at ca. 250 Ma and propagated to China. The collision completed during late Triassic. The metamorphic conditions systematically change along the collision belt:. ultrahigh temperature metamorphism occurred in the Odesan area at 245-230Ma, high-pressure metamorphism in the Hongseong area at 230 Ma and ultra high-pressure metamorphism in the Dabie and Sulu belts. This systematic change may be due to the increase in the depth of slab break-off towards west, which might be related to the increase of the amounts of subducted ocecnic slab towards west. The wide distribution of Permo-Triassic arc-related granitoids in the Yeongnam Massif and in the southern part of the South China block indicate the Permo-Triassic subduction along the southern boundary of the South China block which may be caused by the Permo-Triassic collision between the North and South China blocks. These studies suggest that the Songrim orogeny constructed the Korean Peninsula by continent collision and caused the subduction along the southern margin of the Yeongnam Massif. Both the northern and southern Gyeonggi Massifs had undergone 1870-1840 Ma igneous and metamorphic activities due to continent collision and subduction related to the amalgamation of Colombia Supercontinent. The Okcheon metamorphic belt can be correlated to the Nanhua rift formed at 760 Ma within the South China blocks. In that case, the southern Gyeonggi Massif and Yeongnam Massif can be correlated to the Yangtz and Cathaysia blocks in the South China block, respectively. Recently possible Devonian or late Paleozoic sediments are recognized within the Gyeonggi Massif by finding of Silurian and Devonian detrital zircons. Together with the Devonian metamorphism in the Hongseong and Kwangcheon areas, the possible middle Paleozoic sediments indicate an active tectonic activity within the Gyeonggi Massif during middle Paleozoic before the Permo-Triassic collision.

Velocity structure heterogeneity and tectonic motion in and around the Tan–Lu fault of China

The depth distribution of the Moho discontinuity and the P-wave velocity structure in and around the Tan–Lu fault were analyzed employing data from seismic reflection and refraction sounding from greater than 40 profiles. The results suggest that high P-wave velocity zones have emerged in the upper crust of the Dabie and Sulu ultrahigh pressure metamorphic (UHPM) belts. Anomalous zones of low velocity in the lower crust and the deep Moho discontinuities imply that the low crustal velocity structure zones exit in the Dabie and Sulu region. The high-velocity anomalous zones in the upper crust and low-velocity anomalous zones in the lower crust beneath the Sulu region are all located northeast of the northern segment of the Tan–Lu fault. The high- and low-velocity anomalous zones beneath the Dabie region are located southwest of the southern segment of the Tan–Lu fault. Such a distribution pattern of the velocity-anomalous zones may show some evidence of the left strike-slip motion regime of the Tan–Lu fault. The low velocity structure in the crust emerged beneath the joining region between the Dabie and Sulu orogens near the Tan–Lu (T–L) fault.

Formation of ferrian chromite in podiform chromitites from the Golyamo Kamenyane serpentinite, Eastern Rhodopes, SE Bulgaria: a two-stage process

The Golyamo Kamenyane serpentinite is a portion of a metaophiolite, located in the Upper High-Grade Unit of the metamorphic basement of the Eastern Rhodope Metamorphic Complex, SE Bulgaria. It consists of metaharzburgite and metadunite hosting layers of metagabbro and some chromitite bodies. All these lithologies were affected by ultrahigh-pressure (UHP) metamorphism and subsequent retrograde evolution during exhumation. Chromite from chromitites can be classified into four textural groups: (1) partly altered chromite, (2) porous chromite, (3) homogeneous chromite and (4) zoned chromite. Partly altered chromite shows unaltered, Al-rich cores with unit cell size of 8.255 A and Cr# [Cr/(Cr + Al) atomic ratio] = 0.52–0.60, Mg# [Mg/(Mg + Fe2+) atomic ratio] = 0.65–0.70 and Fe3+/(Fe3+ + Fe2+) = 0.20–0.30, surrounded by porous chromite, with a cell size of 8.325 A, Fe3+/(Fe3+ + Fe2+) < 0.20 and values of Cr# and Mg# evolving from 0.60 to 0.91 and 0.65–0.44, respectively, from core to rim. The chemical composition of porous chromite varies within the following ranges: Cr# = 0.93–0.96, Mg# = 0.48–0.35 and Fe3+/(Fe3+ + Fe2+) = 0.22–0.53. Its unit cell size is very constant (8.350 A). Most pores in porous and partly altered chromite are filled with chlorite, which also occurs between chromite grains. Homogeneous chromite has Fe3+/(Fe3+ + Fe2+) = 0.55–0.66, Cr# = 0.96–0.99, Mg# = 0.32–0.19 and a cell size of 8.385 A. The cores of zoned chromite are similar to those of partially altered chromite, but the rims are identical to homogeneous chromite. Although chlorite predominates in the silicate matrix of homogeneous and zoned chromite, it coexists with some antigorite, talc and magnesiohornblende. Mineral data and thermodynamic modeling allow interpretation of the alteration patterns of chromite as the consequence of a two-stage process developed during retrograde metamorphic evolution coeval with fluid infiltration. During the first stage, chromite reacts in the presence of fluid with olivine to produce chlorite and Cr- and Fe2+-rich residual chromite (ferrous chromite) at ~700 to ~450 °C. This dissolution–precipitation reaction involves continuous chromite mass loss resulting in the development of a porous texture. This stage takes place progressively on cooling under water-saturated and reducing conditions. The second stage mainly consists of the formation of homogeneous chromite with ferrian chromite composition by the addition of magnetite to the porous ferrous chromite during a late oxidizing hydrothermal event.

The geochemical characteristics of metabasites with a pseudo-pillow structure from Ganghe, Dabie Orogen, China

Several metabasite lenses in Ganghe, Central Dabie, that were previously described as pillow lavas are studied by elemental, Sr–Nd–Pb isotopic, and mineral oxygen isotopic analysis as well as zircon SHRIMP U–Pb dating. Zircon U–Pb geochronology results indicate that the protolith emplacement age of these metabasites is approximately 717±38Ma, consistent with the age of the volcanoclastic rocks in the same unit, and that they experienced the Triassic HP eclogite-facies retrograde metamorphism at 221±2Ma during exhumation after subduction to mantle depth and peak ultra-high pressure metamorphism. The low δ18O values of −5.5‰ to −2.0‰ indicate that the protoliths underwent high temperature meteoric-hydrothermal alteration before subduction but had no seawater interaction. These metabasites had similar formation processes, water–rock interactions and metamorphisms as other eclogite-facies rocks cropped out in the Central Dabie terrain. They showed negative abnormalities in Nb, Sr, and Ti content and positive abnormalities in Ba, Th, and Pb content; they also showed LREE enrichment. The insusceptible Sm–Nd isotopes during metamorphism yielded εNd (t)=−12 to −10 and TDM=2.2–2.8Ga for samples from lenses #1 to #3 and −7 to −6 and 2.1–2.2Ga for lens #4; the samples also showed low radiogenic Pb isotope compositions of (206Pb/204Pb)i=15.34–16.50, (207Pb/204Pb)i=15.23–15.32, and (208Pb/204Pb)i=35.93–37.04. The data suggest that the protolith sources of the metabasites were contaminated to variable degrees by old crustal materials during formation. Unlike the Maowu layered intrusions, which were contaminated by upper crust, the magmas of the metabasites were contaminated by lower crust in the magma chamber and during eruption. It can be concluded that the protoliths of these metabasites were derived from old crustal-contaminated mantle sources and initially emplaced in the crust at the Neoproterozoic and that they were altered by meteoric water at high temperatures. In this respect, they might be similar to the Neoproterozoic mafic intrusions in the North Huaiyang terrain. However, the studied metabasites experienced the Permo-Triassic subduction and metamorphism, whereas the North Huaiyang Neoproterozoic mafic intrusions did not.

Exsolution of Garnet and Clinopyroxene from High-Al Pyroxenes in Xugou Peridotite, Eastern China

Serpentinized massif peridotite in the Xugou, Su-Lu ultrahighpressure (UHP) metamorphic belt, eastern China, preserves texturally old (porphyroclastic) orthoand clinopyroxene with up to two generations of lamellae of garnet, clinopyroxene and Mg-chromite. Their crystallographic orientation with respect to the host pyroxene is consistent with an origin by solid-state exsolution. Comparison of integrated mineral chemistry with simplified and natural chemical datasets suggests that both aluminous precursor pyroxenes were in equilibrium at a minimum pressure of 4 GPa and within a temperature range of about 1300^15008C. Steep isopleths of Ca in orthopyroxene imply that exsolution occurred during cooling. Al diffusion modelling suggests growth of widely spaced lamellae in orthopyroxene down to about 9008C. Integrated Al contents between wide lamellae record a minimum of 4 GPa pressure during cooling. Compositionally uniform exsolved minerals were formed at 4·3 0·3 GPa and 730 308C and reflect a cratonic geotherm with about 33 mWm 2 surface heat flow. The peridotite matrix mineral assemblage of olivineþ orthopyroxene garnet Mg-chromite clinopyroxene phlogopite records strain-induced recrystallization that partially to completely replaced precursor porphyroclasts. The recrystallized minerals lack lamellar exsolution. Recrystallized orthopyroxene, with Al2O3 at 0·13 wt %, indicates conditions of 5·5 0·3 GPa and 760 308C, which are higher-grade metamorphic conditions than those preserved in the chemically equilibrated exsolution microstructures. Both estimates overlap with the range reported for the Early Mesozoic UHP metamorphism in the region (4·0^ 6·7 GPa and 760^9708C).Major element melt models applied to previously published Xugou peridotite data suggest high degrees of melt extraction (30^35 %) in the garnet peridotite stability field (3^ 4·5 GPa) until garnet and clinopyroxene exhaustion. Coincidence in pressure and in the order of temperature of equilibration of precursor pyroxenes and peridotite melting implies that peridotite formation occurred at 135 km depth in the subcontinental lithospheric mantle (SCLM) beneath the Archaean North China Craton. Subsequent refertilization, mineral exsolution and chemical re-equilibration during long-term cooling in the SCLMoccurred prior to deformation and incorporation of the mantle fragment into the continental crust during UHP metamorphism at a minimum depth of 170 km. Because the Xugou precursor pyroxenes and peridotite formed at depths greater than the regional SCLM (c. 90 km), we infer that the orogenic peridotite massif formed part of the former hanging wall of the Archaean SCLM, which delaminated after the Late Mesozoic.

Phase relations during peak metamorphism and decompression of the UHP kyanite eclogites, Pohorje Mountains (Eastern Alps, Slovenia)

Phase relations among the mineral assemblages of UHP kyanite eclogite were investigated in the Pohorje Mountains of the Eastern Alps. Ultrahigh-pressure metamorphism resulted from intracontinental subduction during the Cretaceous (ca. 92Ma). Kyanite-bearing eclogites are associated with meta-ultramafic rocks including UHP garnet peridotites and are embedded in metapelitic gneisses and micaschists. The kyanite eclogites contain a peak metamorphic assemblage of garnet, omphacite, kyanite and phengite. Pyrope-rich garnet is unzoned and almost free of inclusions. The non-stoichiometric supersilicic omphacite contains up to 5mol% of Ca-Eskola molecule. Breakdown of omphacite during decompression resulted in exsolution of oriented rods of silica. Phengite contains up to 3.5 Si a.p.f.u. Polycrystalline quartz inclusions in peak-pressure minerals – garnet, omphacite and kyanite – are surrounded by radial fractures diagnostic of the former presence of coesite. Peak-pressure minerals are replaced by symplectites of diopside+plagioclase+amphibole after omphacite, plagioclase+biotite after phengite and sapphirine+corundum+spinel+anorthite after kyanite. Sapphirine has composition close to (Mg, Fe)12.4 Al38.9 Si4.5 O80 in average, which is amongst the most aluminous yet reported. Peak metamorphic conditions were constrained from calculated phase equilibria in the NKCFMASH system with the fixed bulk-rock composition, and conventional geothermobarometry. This approach led to consistent results, the calculated peak P–T conditions of 3.0–3.7GPa and 710–940°C, in the stability field of coesite and the same range as metamorphic conditions recorded by the associated garnet peridotites. This implies that eclogites and their host rocks were subducted to depths of about 100km. The relatively high temperature at peak pressure, compared to UHP rocks of Tertiary age in the Western Alps where mostly oceanic crust was subducted, probably resulted from radiogenic heat production by subducting continental crust, in the intra-continental setting of the Cretaceous subduction zone in the Eastern Alps.

Trace-element transport during subduction-zone ultrahigh-pressure metamorphism: Evidence from western Tianshan, China

Subduction-zone metamorphism is considered to be a major chemical filter for both the arc magmatism and mantle compositional heterogeneity. To understand the element transport during this process, we conducted a petrographic and geochemical study of the bulk-rock blueschists and eclogites from the western Tianshan ultrahigh-pressure metamorphic belt, northwest China. By examining correlations among incompatible elements, we show that high field strength elements (HFSEs), rare earth elements (REEs), Th, and U are relatively immobile, whereas Pb and Sr are mobile in both basaltic and sedimentary protoliths during subduction-zone metamorphism. K, Rb, Cs, and Ba are mobile in rocks of basaltic protolith but immobile in rocks of sedimentary protolith because of the presence and persistent stability of white mica throughout their petrologic history. The commonly observed enrichment of some immobile elements (e.g., U, light [L] REEs) in arc magmas may thus not be caused by subduction-zone aqueous fluids. The lack of Rb/Sr-Sm/Nd correlation in these metamorphosed rocks is inconsistent with the observed first-order Sr-Nd isotope correlation in oceanic basalts, suggesting that the residual ocean crust that has undergone subduction-zone metamorphism cannot be the major source material for oceanic basalts, as widely believed, although it can contribute to mantle compositional heterogeneity in general.

Ultrahigh-pressure metamorphism in the archean-proterozoic mobile belt (Gridino complex, Karelia, Russia)

412 Ultrahighpressure metamorphism is considered as an indicator of the modern style of plate tectonics, generally, and of subduction, in particular (1). Accord� ing to some data, the formation of this style started on the Earth in the Neoproterozoic (1); according to other data, it began 2 b. y. before, at the Meso- Neoarchean boundary (2). The absence of ultrahigh� pressure complexes with an older age than the Neoproterozoic is an important argument in favor of the first hypothesis. In this paper we describe the results of petrological investigation of a metagabbro dyke in the Gridino eclogite complex (Karelia) dem� onstrating that ultrahighpressure events were realized in the subducting lithospheric slab already in the Pale� oproterozoic. The Gridino complex is part of the Belomorian mobile belt of Karelia (3-5). It represents a tectonic plate of northwestern strike occurring in the coastal area and on the islands of the White Sea up to 60 km from Sukhaya Bay in the northwest to the Suprotivnye Islands in the southeast at a width of 6-7 km. The main volume of the complex is represented by migma� tized gneiss containing rock boudines of various sizes and compositions (3, 4). Rocks of the basic composi� tion (metabasites), namely eclogite and amphibolites, strongly prevail among boudines in gneiss; amphiboli� tized metaultramafites, marbles, zoisite, and scapolite rocks are rarely observed. The complex is cut by dykes and small massifs of metagabbronorites metamor� phosed under the conditions of the eclogite and upper amphibolite facies, as well as intrusions and veins of plagiogranites (3). Dating of eclogites from this com� plex demonstrates a wide range of ages from the Neoarchean (2.74 Ga) to the Paleoproterozoic (1.9 Ga) (3-5). This paper contains the results of study of metagab� bro dyke cutting migmatized amphibole-biotite gneiss on Bezymyannyi Island. Metagabbro has massive structure, with fineto mediumgrained, heterogranoblastic texture. The main rockforming minerals are garnet, clinopyrox� ene, plagioclase, orthopyroxene, biotite, amphibole, and quartz, which are related to different stages of metamorphic evolution of the rock, according to the

Metamorphic chemical geodynamics in continental subduction zones

Chemical geodynamics is an integrated discipline that studies the geochemical structure and tectonic evolution of geospheres with the aim of linking tectonic processes to geochemical products in the Earth system. It was primarily focused on mantle geochemistry, with an emphasis on geochemical recycling in oceanic subduction zones. It has been extended to geochemical reworking and recycling under high-pressure (HP) to ultrahigh-pressure (UHP) conditions in all convergent plate margins. In particular, UHP terranes, along with UHP metamorphic minerals and rocks in continental subduction zones, represent natural laboratories for investigating geochemical transport and fluid action during subduction and exhumation of continental crust. As a result of this extension, the study of UHP terranes has significantly advanced our understanding of tectonic processes in collisional orogens. This understanding has principally benefited from the deciphering of petrological and geochemical records in deeply subducted crustal rocks that occur in different petrotectonic settings. This review focuses on the following issues in continental subduction zones: the time and duration of UHP metamorphism, the origin and action of metamorphic fluid/melt inside UHP slices, the element and isotope mobilities under HP to UHP conditions during continental collision, the origin of premetamorphic protoliths and its bearing on continental collision types, and the crustal detachment and crust–mantle interaction in subduction channels. The synthesis presented herein suggests that the nature of premetamorphic protoliths is a key to the type of collisional orogens and the size of UHP terranes. The source mixing in subduction channels is a basic mechanism responsible for the geochemical diversity of continental and oceanic basaltic rocks. Therefore, the geochemical study of HP to UHP metamorphic rocks and their derivatives has greatly facilitated our understanding of the geodynamic processes that drive the tectonic evolution of convergent plate margins from oceanic subduction to continental collision. Consequently, the study of chemical geodynamics has been developed from oceanic subduction zones to continental collision zones, and it has enabled important contributions to development of plate tectonic theory.

Constraining peak P–T conditions in UHP eclogites: calculated phase equilibria in kyanite‐ and phengite‐bearing eclogite of the Tromsø Nappe, Norway

AbstractKyanite‐ and phengite‐bearing eclogites have better potential to constrain the peak metamorphic P–T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromsø Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak‐pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca‐Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular (XGrs = 0.25–0.29), but lower pyrope‐content (XPrp = 0. 37–0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P–T conditions to 3.2–3.5 GPa and 720–800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2–3.5 GPa &amp; 730–780 °C). Peak‐pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh‐pressure (UHP) conditions to 1.3–1.5 GPa at 740–770 °C is constrained by the garnet rim (XCaGrt = 0.18–0.21) and symplectite clinopyroxene (XNaCpx = 0.13–0.21), and to 0.5–0.7 GPa at 700–800 °C by sapphirine (XMg = 0.86–0.87) and spinel (XMg = 0.60–0.62) compositional isopleths. UHP metamorphism in the Tromsø Nappe is more widespread than previously known. Available data suggest that UHP eclogites were uplifted to lower crustal levels rapidly, within a short time interval (452–449 Ma) prior to the Scandian collision between Laurentia and Baltica. The Tromsø Nappe as the highest tectonic unit of the North Norwegian Caledonides is considered to be of Laurentian origin and UHP metamorphism could have resulted from subduction along the Laurentian continental margin. An alternative is that the Tromsø Nappe belonged to a continental margin of Baltica, which had already been subducted before the terminal Scandian collision, and was emplaced as an out‐of‐sequence thrust during the Scandian lateral transport of nappes.

Metamorphic P-T evolution of high-pressure eclogites from garnet growth and reaction textures: insights from the Kaghan Valley transect, northern Pakistan

The Himalayan Metamorphic Belt of the Kaghan Valley transect in northern Pakistan is a key area where rocks of the Indian Plate have been subducted to greater depths >100 km beneath the Asian Plate and experienced high- to ultrahigh-pressure metamorphism. At least three distinct metamorphic events can be recognized in the metamorphosed mafic rock units (eclogites and amphibolitized eclogites) of the Kaghan Valley: (i) prograde; (ii) peak; and (iii) retrograde stages. Chemical composition, zoning pattern and inclusions in garnet, and metamorphic reactions among major mineral assemblage (garnet + clinopyroxene + quartz + amphibole + epidote) show that core portions of garnet grew under the prograde stage, whereas most of the middle portions were developed or recrystallized under the high-pressure stage with P–T conditions at 2.3 ± 0.4 GPa and 766 ± 107°C. The rim portions of garnet in association with amphibole and presence of symplectites indicate amphibole-eclogite to amphibolite-facies (retrograde) stage with average P–T conditions at 1.5 ± 0.2 GPa and 710 ± 75°C. Zoning in garnet shows Ca-poor wider cores, Ca-rich thin middle portions, and chlorite and amphibole development along the outermost rims, suggesting prograde growth at cores and homogenous middle portions and retrogressive formation at rims during the amphibolite-facies stage. These features suggest that most of the garnet grew during subduction of the Indian Plate lithosphere before the India–Asia collision, whereas rim portions developed during rapid exhumation in the post-collision stage.

Eclogites from the Belomorian Mobile Belt ( Kola Peninsula): geology and petrology

The paper focuses on the metamorphic geology of the oldest crustal eclogites discovered in the Late Archean tonalite-trondhjemite-granodiorite (TTG) complex of the Belomorian Mobile Belt on the Kola Peninsula. Eclogite bodies are, most likely, widespread. We studied one of the key objects, the Kuru-Vaara quarry, where several tens of retrogressed eclogite blocks randomly embedded in the TTG gneisses were stripped at the benches. Based on the field observations, two visually different types of eclogites have been recognized: “southern”, strongly retrogressed coarse-grained, and “northern”, well-preserved fine-grained. The southern eclogite blocks bear evidence of their partial melting with the formation of veins and melt percolation channels. The northern eclogite blocks show no evidence of melting. Despite the significant mineralogic difference, both types of eclogites can be assigned to amphibole eclogite facies. The applied jadeite solubility geobarometers yielded the minimum pressures of ∼12 kbar for the northern eclogites and ∼14–14.5 kbar for the southern ones. The used geothermometers yielded ∼700°C and ∼750°C, respectively. But the presence of quartz lamellae in Na-clinopyroxenes in both types of eclogites and their bulk compositions corresponding to high-Mg basalts suggest that the Kuru-Vaara eclogites might have reached the field of ultrahigh-pressure metamorphism. Analysis of the tentative P- T paths of metamorphic evolution for both types of eclogites showed that their burial–exhumation cycle might have taken as short as a few million years. The set of presented data suggests that the formation of the Kuru-Vaara eclogites was related to the subduction of the Archean oceanic crust, which should have differed in composition and structure from the modern oceanic crust.

Eocene high grade metamorphism and crustal anatexis in the North Himalaya Gneiss Domes, Southern Tibet

Determination of the timing and geochemical nature of early metamorphic and anatectic events in the Himalayan orogen may provide key insights into the physical and chemical behavior of lower crustal materials during the early stage of tectonic evolution in large-scale collisional belts. The Yardoi gneiss dome is the easternmost dome of the North Himalayan Gneiss Domes (NHGD), and contains three types of amphibolites with distinct mineral assemblage, elemental and radiogenic isotope geochemistry, as well as various types of gneisses. SHRIMP zircon U/Pb analyses on the garnet amphibolite and garnet-bearing biotite granitic gneiss yield ages of nearly peak metamorphism at 45.0±1.0 Ma and 47.±1.8 Ma, respectively, which are 2 to 4 Ma older than the age for partial melting in migmatitic garnet amphibolite (43.5±1.3 Ma). Available data have demonstrated that ultra-high pressure metamorphism in the Tethyan Himalaya occurred at ∼55 Ma, and high amphibolite facies to granulite facies metamorphism at 45 to 47 Ma. In addition, partial melting at thickened crustal conditions occurred at 43.5±1.3 Ma, which led to the formation of high Sr/Y ratios two-mica granites. The high-grade metamorphic rocks in the NHGD may represent the subducted front of the Indian continental lithosphere. In large collisional belts, fertile components in crustal materials could melt and form granitic melts with relatively high Na/K and Sr/Y ratios under thickened crustal conditions, significantly different from those formed by decompressional melting during rapid exhumation.

A granulite record of multistage metamorphism and REE behavior in the Dabie orogen: Constraints from zircon and rock-forming minerals

A combined study of mineral inclusions, U–Pb ages and trace elements was carried for zircon and coexisting minerals from granulite in the North Dabie Terrane (NDT) of the Dabie–Sulu ultrahigh-pressure metamorphic (UHP) zone, east-central China. The results provide insights into the exhumation history of NDT and into rare earth element (REE) behavior during retrogression. Besides inherited cores and one magmatic rim, zircons separated from the granulite record three episodes of metamorphism under different P–T conditions: (1) 223.8±2.3Ma for domains that contain Grt+Cpx±Rt±F−Ap±Aln inclusions without plagioclase and show flat HREE patterns without negative Eu anomalies, representing peak eclogite-facies event; (2) 213.3±2.1Ma for domains that contain Pl±Cpx±Grt±Qtz±Ap inclusions and show rather flat HREE patterns with negative Eu anomalies, corresponding to granulite-facies retrogression; (3) 199.9±3.3Ma for domains that contain Amp±Pl±Qtz±Ap inclusions and show high REE contents with steep HREE patterns and remarkable negative Eu anomalies, representing amphibolite-facies overprinting. Therefore, the UHP eclogite in NDT experienced decompression heating during the initial exhumation, with local hydration in the late stage of the Triassic continental collision.Garnet in the granulite is composed of a corroded core with embayed outline and spongy texture and an overgrowth rim. There is equilibrium distribution of HREE between garnet rim and granulite-facies zircon domain, confirming the geological interpretation of 213.3±2.1Ma for the granulite-facies metamorphism. There is the prograde HREE depletion in porphyroblastic garnet from core to rim and the continuous decrease of HREE from the eclogitic to granulitic zircons, suggesting that the metamorphic transformation from eclogite-facies to granulite-facies took place in a closed system. On the other hand, the amphibolitic zircons show steep HREE patterns and significantly elevated REE contents, suggesting that the amphibolite-facies metamorphism took place in an open system with introduction of external fluids containing more REE to the granulite-facies rocks. Residual clinopyroxene partially replaced by amphibole has remarkably elevated REE contents relative to euhedral primary clinopyroxene. There is equilibrium distribution of REE between the residual clinopyroxene and the amphibole, marking fluid action during the amphibolite-facies overprinting.