Ultrahigh-pressure Metamorphic Rocks Research Articles

Excess 40Ar-free phengite in ultrahigh-pressure metamorphic rocks from the Lago di Cignana area, Western Alps

Ar/Ar analyses of phengites and paragonites from the ultrahigh-pressure metamorphic rocks (zoisite–clinozoisite schist, garnet–phengite schist and piemontite schist) in the Lago di Cignana area, Western Alps were carried out with a laser probe step-heating method using single crystals and a spot dating method on thin sections. Eight phengite and two paragonite crystals give the plateau ages of 37–42 Ma with 96–100% of 39Ar released. Each rock type also contains mica crystals showing discordant age spectra with age fractions (20–35 Ma) significantly younger than the plateau ages. Phengite inclusions in garnet give ages of 43.2 ± 1.1 Ma and 44.4 ± 1.5 Ma, which are significantly older than the spot age (36.4 ± 1.4 Ma) from the matrix phengites, and the plateau ages from the step-heating analyses. Inclusion ages (43 and 44 Ma) are consistent with a zircon SHRIMP age (44 ± 1 Ma) in this area. These results suggest that the oceanic materials that underwent a simple subduction related UHPM, form excess 40Ar-free phengite and that the peak metamorphism is ca. 44 Ma or little older. We suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic 40Ar from the crystals. This has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age.

Ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: II Oxygen isotope and fluid inclusion distributions through vertical sections

In order to reconstruct the formation and exhumation mechanisms of UHP metamorphic terrains, the Chinese Continental Scientific Drilling Program (CCSD) has been carried out in Donghai of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, East China. Eclogite, gneiss, amphibolite (retrograded from eclogite), ultramafic rocks, and minor schist and quartzite have been drilled. Aiming to reveal the fluid behaviour in a vertical sequence of an UHP slab, we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot-holes PP2 and ZK 703 of the CCSD. More than 540 laser-ablation oxygen isotope analyses on garnet, omphacite, quartz, kyanite, amphibole, phengite, rutile, epidote, amphibole, plagioclase, and biotite from various rocks in the depth range of 0–3,000 m (mainly eclogite and gneiss) show that the investigated rocks can be divided into two groups: 18O-depleted rocks (as low as δ18O = −7.4‰ for garnet) indicate interaction with cold climate meteoric waters, whereas 18O-normal rocks (with bulk δ18O > +5.6‰) have preserved the O-isotopic compositions of their protoliths. Meteoric water/rock interaction has reached depths of at least 2,700 m. Oxygen isotope equilibrium has generally been achieved. Isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology, but heterogeneous on the scale of a few metres. Oxygen isotope distributions in the vertical sections favour an “in situ” origin of the UHP metamorphic rocks. The very negative δ18O eclogites usually have higher hydroxyl-mineral contents than the normal δ18O rocks, indicating higher water content during UHP metamorphism. Fluid inclusion data suggest that rocks with depleted 18O compositions have had different fluid histories compared to those with normal δ18O values. Rocks with depleted 18O mainly have primary medium-to-high salinity inclusions in omphacite, kyanite and quartz, and abundant secondary low-salinity or pure water inclusions in quartz, indicating a high-salinity-brine-dominated fluid system during peak UHP metamorphism; no carbonic inclusions have been identified in these rocks. By contrast, primary very high-density CO2 inclusions are commonly found in the rocks with normal δ18O values. These observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the Dabie-Sulu UHP metamorphic rocks.

First report of garnet–corundum rocks from southern India: Implications for prograde high-pressure (eclogite-facies?) metamorphism

Abstract We report here for the first time the occurrence of garnet and corundum in Mg–Al-rich rocks at Sevitturangampatti (Namakkal district) in the Palghat-Cauvery Shear Zone System (PCSS), southern India. The rocks contain several rare mineral assemblages such as garnet–corundum–sillimanite–cordierite–sapphirine–spinel–Mg-rich staurolite, garnet–corundum–sodic gedrite–cordierite–sillimanite/kyanite, garnet–Mg-rich staurolite–sillimanite/kyanite, sodic gedrite–Mg-rich staurolite–corundum–sapphirine, biotite–corundum–sapphirine and sodic gedrite–sapphirine–spinel–cordierite. Both garnet and corundum in these rocks occur as coarse-grained (1 mm to 10 cm) porphyroblasts in the matrix of sillimanite, cordierite and gedrite. Kyanite is common as inclusions in garnet, but matrix aluminosilicates are mainly sillimanite. The presence of rare garnet + corundum, which has so far been reported from kimberlite xenoliths, aluminous eclogites and ultrahigh-pressure metamorphic rocks as well as in high-pressure experiments, suggests that the assemblage is an indicator of an unusually high-pressure event, which has not been recorded in previous studies from southern India. Phase analysis of quartz-absent MAS system also suggests high-pressure stability of the assemblage. The inference of high pressure metamorphism is also supported by the presence of Mg-rich [Mg/(Fe + Mg) = 0.51] staurolite, which has been reported from high-pressure rocks, included from cores of coarse-grained garnet and gedrite. Porphyroblastic occurrence of garnet + corundum as well as staurolite and kyanite inclusions suggests that the area underwent prograde high-pressure metamorphism, probably in the eclogite field. The rocks subsequently underwent continuous heating at 940 to 990 °C, suggesting ultrahigh-temperature (UHT) metamorphism along a clockwise trajectory. Sapphirine + cordierite and spinel + cordierite symplectites between garnet and sillimanite suggest near isothermal decompression after the peak event. Evidence for such high-pressure metamorphism followed by UHT conditions along a single clockwise exhumation path is the first report from southern India. The Namakkal area occurs within the central domain of the PCSS, which defines the probable collisional suture between the Archean craton in the north and the Proterozoic Madurai Block in the south. We propose that the rocks reported in this study testify to metamorphism at mantle depths (eclogite-facies) and their subsequent heating at extreme thermal conditions during collisional orogenic event and subsequent rapid exhumation associated with the final suturing of the Gondwana assembly in the late Pan-African. The high-pressure metamorphism and subsequent UHT event reported in this study have important bearing on the tectonic evolution of the continental deep crust in southern India.

Pargasite and ilmenite exsolution texture in clinopyroxenes from Hujialing garnet-pyroxenite, Su-Lu UHP terrane, Central China: a geodynamic implication

We report here the discovery of amphibole exsolution texture in the Hujialing garnet-pyroxenite, Su-Lu ultrahigh pressure (UHP) metamorphic terrane. These rocks together with coesite-bearing eclogites were exhumed from mantle depths. The garnet-pyroxenite consists mainly of garnet and diopside with minor pargasitic amphibole, ilmenite, Mg-Al spinel, magnetite, tremolite, orthopyroxene, and olivine. Diopside contains abundant garnet inclusions as well as pargasite and ilmenite lamellae. The present mineral assemblages observed in the Hujialing garnet-pyroxenite indicate that the parental mineral of the reported exsolution texture should have a composition of mixture of pargasite, ilmenite and diopside. Primary clinopyroxene in the Hujialing garnet-pyroxenite contains ∼2000 ppm H 2 O, suggesting a peak metamorphic pressure probably at 8.0 GPa. Prevailing exsolution of pargasite (stage I) and ilmenite (stage II) within clinopyroxenes suggests an at least two-stage exsolution process, which implies a complex exhumation history for the Hujialing garnet-pyroxenite. Texture relationships apparently show that the pargasite lamellae formed earlier than the ilmenite lamellae in diopsides. These observational data implies that the exhumation of these UHP metamorphic rocks were accomplished in two steps, one at depths ∼100 km, another at much shallower levels.

Sm-Nd and zircon SHRIMP U-Pb dating of Huilanshan mafic granulite in the Dabie Mountains and its zircon trace element geochemistry

The mafic granulites from Huilanshan are outcropped on the center of the Luotian dome in the northern Dabie Mountains. The Sm-Nd isochron defined by granulite-facies metamorphic minerals (garnet+clinopyroxene+hypersthene) yields an age of 136 ± 18 Ma indicating the early Cretaceous granulite-facies metamorphism. The cathodoluminescence (CL) images of zircons from the granulite show clearly core-mantle-rim structures. The zircon cores are characterized by typical oscillatory zoning and highly HREE enriched patterns, which suggests their magma origin. Some zircon cores among them with little Pb loss give SHRIMP U-Pb ages ranging from 753 to 780 Ma, which suggests that the protolith of Huilanshan granulite is Neoproterozoic mafic rocks. The zircon mantles usually cut across the oscillatory zone of the zircon cores have 3-10 times lower REE, Th, U, Y, Nb and Ta contents than the igneous zircon cores but have high common Pb contents. These characteristics suggest that they were formed by hydrothermal alteration of the igneous zircons. The part of zircon mantles with little Pb loss give a similar SHRIMP U-Pb age (716-780 Ma) to the igneous zircon cores, which implies that the hydrothermal events occurred closely to the magmatic emplacement. In view of the strong early Cretaceous magmatism in the Luotian dome, consequently, the Huilanshan mafic granulite was formed by heating of the Neoproterozoic mafic rocks in mid-low crust, which caused the granulite-facies metamorphism underneath the Dabie Mountains. The similarity between the granulite metamorphic age (136±18 Ma) defined by Sm-Nd isochron and K-Ar age of 123-127 Ma given by amphible from the gneiss in Luotian dome suggests a rapid uplifting of the Luotian dome, which may result in further exhumation of the ultrahigh pressure metamorphic rocks in the Dabie Mountains.

Deformation‐enhanced metamorphic reactions and the rheology of high‐pressure shear zones, Western Gneiss Region, Norway

AbstractMicrostructural and petrological analysis of samples with increasing strain in high‐pressure (HP) shear zones from the Haram garnet corona gabbro give insights into the deformation mechanisms of minerals, rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions. Scanning electron microscopy with electron backscattering diffraction (SEM–EBSD), compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization. Plagioclase with a shape‐preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the (0 0 1)[1 0 0] slip system. In omphacite, crystallographic preferred orientations indicate slip on (1 0 0)[0 0 1] and (1 1 0)[0 0 1] during deformation. Fine‐grained garnet deformed by diffusion creep and grain‐boundary sliding. Ilmenite deformed by dislocation glide on the basal and, at higher strains, prism planes in the a direction. Relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500–650 °C and 0.5–1.4 GPa and continued during prograde metamorphism to ultra‐high‐pressure (UHP) conditions. Both products and reactants show evidence of syn‐ and post‐kinematic growth indicating that prograde reactions continued after strain was partitioned away. The restriction of post‐kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn‐kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away, demonstrating that deformation played an important role in enhancing metamorphic reactions. Two important consequences of deformation observed in these shear zones are: (i) the homogenization of chemical composition gradients occurred by mixing and grain‐boundary migration and (ii) composition changes in zoned metamorphic garnet by lengthening diffusion distances. The application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100–150 MPa and lower limits for strain rates of 10−12 to 10−13 s−1 as deformation conditions for the shear zones in the Haram gabbro that were produced during subduction of the Baltica craton and resulted in the production of HP and UHP metamorphic rocks.

Petrogenesis of Early Cretaceous intrusions in the Sulu ultrahigh-pressure orogenic belt, east China and their relationship to lithospheric thinning

Geochronological, geochemical, whole-rock Sr–Nd and zircon Hf isotopic analyses have been carried out on two suites of Late Mesozoic mafic to felsic magmatic rocks in the Sulu orogenic belt (east-central China) with the aim of characterizing their petrogenesis and tectonic implications. The Shijiusuo monzogranite has a SHRIMP zircon 206Pb / 238U age of 127 ± 2 Ma and an 40Ar / 39Ar age on hornblende of 123.5 ± 0.4 Ma. A mafic enclave from this pluton has a SHRIMP zircon 206Pb / 238U age of 124 ± 3 Ma and a hornblende 40Ar / 39Ar age of 124.2 ± 0.4 Ma, indicating coeval crystallization of the mafic enclaves and host monzogranite. Whole rock 40Ar / 39Ar dating gives an emplacement age of 111.2 ± 0.1 Ma for the mafic dikes. The monzogranites have low MgO and Cr, high Na 2O, and Sr–Nd–Hf isotopic data ( 87Sr / 86Sr > 0.7085, ɛ Nd( t) = − 20.5 and ɛ Hf( t) = − 22.5 to − 56.6) consistent with derivation from Late Archean to Paleoproterozoic lower crust with involvement of mantle materials. The presence of coeval mafic enclaves with high ɛ Nd( t) and ɛ Hf( t) values indicates magma mixing and involvement of mantle-derived materials in the generation of the Shijiusuo pluton. The mafic dikes that intrude the monzogranite have characteristics of ultrapotassic rocks. Their geochemical features, such as high MgO (Mg# up to 75) and Cr (up to 1233 ppm), low TiO 2 (1.11–1.24 wt.%) and total Fe 2O 3 (8.33–9.09 wt.%), enrichment in LILEs (e.g., Rb, Ba, Sr) and LREEs, and depletion in HFSE (e.g., Nb, Ta and Ti), together with initial 87Sr / 86Sr ratios of 0.7076–0.7078 and negative ɛ Nd( t) values (− 17.6 to − 18.2), indicate they were derived from an amphibole-bearing, refractory lithospheric mantle. The Shichang–Fangzi monzodioritic to monzonitic rocks have zircon SHRIMP U–Pb ages of ∼122 Ma. These rocks have Sr, Nd and Hf isotopic compositions (initial 87Sr / 86Sr = 0.7083–0.7088, ɛ Nd( t) = − 16.5 to − 17.7 and ɛ Hf( t) = − 20.4) similar to the mafic dikes in the nearby Shijiusuo pluton, indicating they were derived from a common source. High Rb and HREE, low Sr and Ba and strongly negative Eu anomalies indicate that the monzodioritic magmas resulted from pyroxene- and plagioclase-dominated fractionation of magma derived from an enriched mantle source. Taken together, these features indicate that Early Cretaceous magmatism in the Sulu orogenic belt was not related to Late Triassic subduction or exhumation of the ultrahigh-pressure metamorphic rocks that characterize the Sulu region; instead they resulted from mantle–crust interaction in an extensional setting, most likely induced by widespread removal of lithospheric mantle in the eastern North China Craton during the Early Cretaceous.

Low-Grade Metamorphic Rocks in the Dabie-Sulu Orogenic Belt: A Passive-Margin Accretionary Wedge Deformed during Continent Subduction

Greenschist-facies metasedimentary and meta-igneous rocks occur continuously along the northern margin, and sporadically in the interior, of the Dabie-Sulu orogenic belt in east-central China. An integrated study of geochronological, petrological, and paleontological observations demonstrates that precursors of flysch-facies metasedimentary rocks were deposited along the northern, passive continental margin of the Yangtze plate prior to the Triassic, and that protoliths of the meta-igneous rocks are a product of Middle Neoproterozoic bimodal magmatism along the northern margin of this plate. Except for the striking contrast in metamorphic grade, these low-grade rocks generally can be correlated in protolith origin and age with ultrahigh-pressure metamorphic rocks within the orogenic belt. Relationships in time and space between these rocks of contrasting grades can be reasonably interpreted through an accretionary wedge model that links their evolution with continent subduction. The low-grade metamorphic rocks of the subducting accretionary wedge consist of two parts: (1) large masses of metasedimentary rocks (including slates, schists, phyllites, metasandstones, and marble) along the northern margin of the Dabie-Sulu orogenic belt and deformed igneous rocks of Middle Neoproterozoic age; (2) sporadic outcrops in the interior of the belt of metavolcanics, metaclastics, phyllite, and marbles. During Triassic subduction of the Yangtze plate, the sedimentary cover and its underlying basement were partly scraped off by the overthrusted North China plate. The scraped-off materials accumulated in front of the overriding plate, forming an accretionary wedge that underwent deformation and metamorphism under greenschist-facies conditions. The present study also provides a constraint on the location of the Triassic suture zone between the North China and Yangtze plates. It is located below, or north of, the accretionary wedge (i.e., the Beihuaiyang zone in the Dabie region and the Wulian-Penglai zone in the Sulu region) rather than along the northern margin of the ultrahigh-pressure metamorphic zones.

40Ar-39Ar Thermochronological Constraints on the Exhumation of Ultrahigh-Pressure Metamorphic Rocks in the Sulu Terrane of Eastern China

In order to better understand the uplift history of the Sulu ultrahigh-pressure metamorphic terrane, 40Ar-39Ar thermochronological analysis was conducted on minerals extracted from both metamorphic (gneiss and retrograded eclogite) and igneous (pegmatite, granite, and mafic intrusion) rocks from this terrane. Samples of the major lithotectonic units were collected from key localities, including Rongcheng, Laoshan, Yangkou, Taohang, and Rizhao. The analyses of metamorphic rocks yielded ages for hornblende ranging from 222 to 186 Ma, biotite from 130 to 119 Ma, and K-feldspar from 112 to 108 Ma. Hornblende, biotite, and K-feldspar from igneous rocks yielded exclusively late Mesozoic ages from 137 to 97 Ma. A two-stage cooling path for the metamorphic rocks were revealed. A slow cooling (1.9 to 3.8°C/m.y.) during the period of ∼220 to ∼130 Ma was followed by fast cooling (6.8 to 16.7°C/m.y.) during ∼130 to ∼100 Ma. The time span and cooling rate of the second stage coincide with those of the analyzed post-collisional igneous rocks as well as with the formation of rift basins in the region. The study strongly indicates that extensional tectonics played an important role in the final stage of rapid exhumation of the ultrahigh-pressure rocks in the Sulu terrane.

On the transformation pathways of alpha-PbO2-type TiO2 at the twin boundary of rutile bicrystals and the origin of rutile bicrystals

Extraction and electron irradiation (under transmission electron microscopy) of an epitaxial nanometer-thick α-PbO 2 -type TiO 2 slab between twinned rutile bicrystals in ultra-high pressure metamorphic rock caused phase changes into a modified fluorite-type and then an amorphous phase. This martensitic-type transition process accounts for the dislocations and stacking faults of the slab and disordering of Ti in the adjoined rutile bicrystals. Additional hydrothermal experiments of sol-gel TiO 2 -Al 2 O 3 performed at 8.5–9 kbar and 675–800°C in the piston-cylinder apparatus indicated that twinned rutile bicrystals were shaped in mirror image without the formation of α-PbO 2 -type TiO 2 slab at the twin boundary and with no other planar defects for the bicrystals. The twinned bicrystals can be rationalized by growth and/or coalescence processes. Accordingly, it is not justified to assume a precursor phase of α-PbO 2 -type structure for twinned rutile bicrystals when there is no such relic. Rutile, unless exsolved epitaxially from a host mineral such as garnet, does not constitute evidence for unusually deep burial for ultra-high pressure terranes.

Plastic flow of coesite eclogite in a deep continent subduction regime: Microstructures, deformation mechanisms and rheologic implications

Ultrahigh pressure (UHP) metamorphic rocks are continental crust that once has been subducted to mantle depth and subsequently exhumed to the surface in collisional orogenic belts. Despite the long distance transport of the UHP rocks in a deep subduction channel and the fast strain rate required for rapid exhumation, little evidence for UHP deformation is found in the exhumed UHP terrains so far identified. Some UHP rocks have even preserved primary igneous and volcanic structures. These observations have led to the postulation that the stress level is low (a few MPa) in the deep subduction zones. Deformation microstructures of some eclogite mylonites from a ductile shear complex in the Sulu UHP metamorphic belt, eastern China, provide for the first time convincing evidence that dislocation creep is the predominant deformation mechanism for the major constituent minerals of the UHP rocks in a highly strain-localized shear zone at the UHP conditions. Omphacite, phengite and kyanite are the main strain-accommodating phases and are dynamically recrystallized through progressive subgrain rotation and grain boundary migration. Coesite is either fractured, with irregular extinction in the moderately deformed eclogite or pseudomorphed by polycrystalline quartz aggregates that have a foam structure. Garnet does not show optically discernable plastic deformation. P–T conditions for the deformation are estimated to be P≈3.3 GPa and T≈624 °C using garnet–omphacite–kyanite–phengite–coesite/quartz thermobarometry, suggesting that deeply subducted continental crust remains “cold” at the upper mantle depths. Large part of the deeply subducted crust remains relatively undeformed. Strain is strongly localized into narrow shear zones. Stress level in the deep subduction channel is extrapolated on the order of tens of MPa, much higher than it was previously thought.

Two Ultrahigh-Pressure Metamorphic Events Recognized in the Central Orogenic Belt of China: Evidence from the U-Pb Dating of Coesite-Bearing Zircons

A ~4000 km long ultrahigh-pressure metamorphic (UHPM) belt in northern China has been documented on the basis of the discovery of coesite-bearing rocks in the Altun-North Qaidam terrane in the western Central Orogenic Belt (COB), and diamond-bearing rocks in Qinling in the central and Dabie-Sulu terrane in the east. New SIMS and SHRIMP U-Pb dates of zircons from coesitebearing UHPM rocks indicate two UHPM events: one in the early Paleozoic and the other in the Triassic. Coesite-bearing zircons from a North Qaidam gneiss yielded UHP metamorphic ages of 452 ± 13.8 Ma and retrograde ages of 419 ± 6.7 Ma. A diamond-bearing gneiss from Qinling gave a lower intercept age of 502 ± 45 Ma, and an upper intercept age of 1545 ± 100 Ma, whereas a Qinling eclogite sample gave a lower intercept age of 493 ± 170 Ma and an upper intercept age of 1381 ± 82 Ma. The lower and upper intercept ages of the Qinling samples are interpreted as UHPM and protolith ages of the rocks, respectively. Coesite-bearing zircons from a Qinglongshan eclogite in the south Sulu belt yielded early Paleozoic UHPM ages of 441 ± 9 Ma, 449 ± 9 Ma, and 442 ± 9 Ma, whereas the core of a zircon containing plagioclase and apatite inclusions gave a protolith age of 761 ± 13 Ma. These age data suggest that the early Paleozoic UHPM rocks extend from west to east for about 4000 km across the COB, whereas the Triassic UHPM belt extends across the Dabie-Sulu region for about 1000 km. Based on available geochronological and geochemical data, we suggest the following tectonic model for evolution of the COB. At about 1000 Ma, the area was amalgamated to form the Rodinian continent, which contained ophiolitic fragments of oceanic affinity. This part of Rodinia was then rifted at about 800-750 Ma to form an oceanic basin with a variety of MORB and intruded by granitic plutons. Closure of this ocean basin produced Neoproterozoic ophiolites and granitic gneisses. The UHPM rocks, along with subduction-related island-arc volcanics and granites of early Paleozoic age suggest a second cycle of rifting and subduction along the COB, whereas the Triassic UHPM rocks record a final subduction and collision event between the North China and South China blocks.

Microdiamonds in Ultrahigh-Pressure Metamorphic Rocks

Since the first report of microdiamonds of metamorphic origin in crustal rocks of the Kokchetav Massif, northern Kazakhstan, diamonds have been described from several other ultrahigh-pressure (UHP) metamorphic terranes. In situ diamond is the best indicator of ultrahigh-pressure conditions (>4 GPa), and testifies to subduction of continental crust to depths within the diamond stability field followed by relatively rapid exhumation. In contrast to other UHP terranes, the Kokchetav Massif contains rocks with unusually abundant diamonds, particularly in the Kumdy-Kol region. Kumdy-Kol diamonds exhibit diverse morphologies, dependent upon the host rock. Raman and cathodoluminescence spectra and carbon isotope compositions differ between core and rim, indicating two distinct growth stages.

Pressure dependence and anisotropy of P-wave velocities in ultrahigh-pressure metamorphic rocks from the Dabie–Sulu orogenic belt (China): Implications for seismic properties of subducted slabs and origin of mantle reflections

The compressional wave velocities (Vp), pressure derivatives (Vp′) and anisotropy ( A) of three types of eclogites and country rocks from the Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt, China, have been measured under confining pressures up to 800 MPa. Type-1 eclogites, which are coarse-grained and subjected to almost no retrograde metamorphism, experienced recovery-accommodated dislocation creep at peak metamorphic conditions (in the diamond stability field). Type-2 eclogites are fine-grained reworked Type-1 materials that experienced recrystallization-accommodated dislocation creep under quartz/coesite boundary conditions during the early stage of exhumation. Type-3 eclogites are retrogressed samples that were overprinted by significant amphibolite facies metamorphism during a late stage of exhumation within the crust. Type-1 eclogites are richer in Al 2O 3 and MgO but poorer in SiO 2 and Na 2O than Type-2 and Type-3 eclogites. Anisotropy of Type-1 and Type-2 eclogites is generally low (<4%) because volumetrically important garnet is elastically quasi-isotropic, while Type-3 eclogites can exhibit high anisotropy (>10%) due to the presence of strongly anisotropic retrograde minerals such as amphibole, plagioclase and mica. The transition of the pressure dependence of velocity from the poroelastic to elastic regimes occurs at a critical pressure ( P c), which depends mainly on the density and distribution of microcracks and in turn on the exhumation history of rocks. The Vp–pressure relationship can be expressed by Vp= a(ln P) 2+ bln P+ c ( P≤ P c) and Vp= V 0+ DP ( P≥ P c), where P is the confining pressure, a and b are constants describing the closure of microcracks below P c, c is the velocity when P is equal to one (MPa), V 0 is the projected velocity of a crack-free sample at room pressure, and D is the intrinsic pressure derivative above P c. When data are curve-fit, pressure derivatives and anisotropy as functions of pressure are determined. The average Vp of the eclogites in the linear regime is 8.42+1.41×10 −4 P for Type-1, 7.80+1.58×10 −4 P for Type-2, and 7.33+2.04×10 −4 P for Type-3, where Vp is in km/s and P in MPa. The decrease in V 0 and increase in D from Type-1 to Type-3 eclogites are attributed to a decrease in garnet content and an increase in retrograde minerals. The NE–SW trending, NW-dipping, slab-like high Vp anomaly (8.72 km/s at a depth of 71 km) which extends from the Moho to at least 110 km beneath the Dabie–Sulu region, can be interpreted as the remnant of a subducted slab which is dominated by Type-1 eclogites and has frozen in the upper mantle since about 200–220 Ma. Such relic crustal materials, subducted and preserved as eclogite layers intercalated with felsic gneiss, garnet–jadeite quartzite, marble and serpentinized peridotite, could be responsible for regionally observed seismic reflectors in the upper mantle.

Provenance of Jurassic sediments in the Hefei Basin, east-central China and the contribution of high-pressure and ultrahigh-pressure metamorphic rocks from the Dabie Shan

The provenance of the Jurassic sediments in the Hefei Basin is constrained by compositions of the detrital K-white micas and garnets, and SHRIMP dating of the detrital zircons, which can help to understand the evolution and to reconstruct the paleogeographic distribution of HP–UHP rocks in the Jurassic Dabie Shan. (1) For the oldest Mesozoic sediments at the bottom of the Fanghushan Formation ( J 1), the predominance of the early Paleozoic and Luliang (1700–1900 Ma) zircons indicates a major source from the North China Block. However, Neoproterozoic zircons as the major component in other Jurassic sediments indicate that the source rocks were mainly derived from the exhumed Yangtze Block in the Dabie Shan. (2) The co-occurrence of high-Si phengites and Triassic zircons provides stratigraphic evidence that the first exposure of the UHP rocks at the Earth's surface in the Dabie Shan occurred in the Early Jurassic during deposition of the Fanghushan Formation. (3) From the east to the west of the Hefei Basin, there is a spatial variation in the compositions for detrital micas and garnets, and in the U–Pb ages of detrital zircons. Evidently, HP–UHP rocks were widely distributed at outcrop in the eastern Dabie Shan. In contrast, they were less important in the western Dabie Shan during the Jurassic.

Ultrahigh pressure (&gt;7 GPa) gneissic K-feldspar (-bearing) garnet clinopyroxenite in the Altyn Tagh, NW China: Evi-dence from clinopyroxene exsolution in garnet

The exsolution of clinopyroxene and rutile in coarse-grain garnet is found in the gneissic K-feldspar(-bearing) garnet clinopyroxenite from Yinggelisayi in the Altyn Tagh, NW China. The maximum content of the exsolved clinopyroxene in the garnet is up to >5% by volume. The reconstructed precursor garnet (Grt1) before exsolution has a maximum Si content of 3.061 per formula uint, being of supersilicic or majoritic garnet. The peak-stage metamorphic pressure of >7 GPa is estimated using the geobarometer for volume percentage of exsolved pyroxene in garnet and the Si-(Al+Cr) geobarometer for majoritic garnet, and the temperature of about 1000℃ using the ternary alkali-feldspar geothermometer and the experimental data of ilmen-ite-magnetite solid solution. The protoliths of the rocks are intra-plate basic and intermediate ig-neous rocks, of which the geochemical features indicate that they are probably the products of the evolution of basic magma deriving from the continental lithosphere mantle. The rocks are in outcrops associated with ultrahigh pressure garnet-bearing lherzolite and ultrahigh pressure garnet granitoid gneiss. All of these data suggest that the ultrahigh pressure metamorphic rocks in the Altyn Tagh are the products of deep-subduction of the continental crust, and such deep- subduction probably reaches to >200 km in depth. This may provide new evidence for further discussion of the dynamic mechanism of the formation and evolvement of the Altyn Tagh and the other collision orogenic belts in western China.

Fluid inclusions evidence for differential exhumation of ultrahigh pressure metamorphic rocks in the Sulu terrane

Differential exhumation was petrologically recognized in ultrahigh pressure metamorphic rocks from the southern and northern parts of the Sulu terrane. While a normal exhumation occurred for eclogites and gneisses in south Sulu, granulite-facies overprinting of ultrahigh pressure metamorphic rocks took place with high retrograde temperatures in north Sulu. A study of fluid inclusions reveals trapping of five type fluid inclusions in high and ultrahigh pressure eclogite minerals and vein quartz in the Sulu terrane. These are A-type N2±CO2 inclusion trapped at high and ultra-high pressure eclogite-facies metamorphic condition, B-type pure-CO2 liquid phase inclusion with higher density trapped during granulite-facies overprinting metamorphism of eclogites, C-type CO2−H2O inclusion and D-type hypersaline inclusion trapped in high pressure eclogite-facies re-crystallization stage, and E-type low salinity H2O inclusion trapped in the latest stage of ultrahigh pressure exhumation (amphibolite-facies retrogression). Identification of crowded-distributing pure-CO2 liquid inclusions with higher density trapped in garnet of eclogites provides an evidence for granulite-facies overprinting metamorphism in the north Sulu terrane.

中国東部，スールー・ダービー帯に産出する超高圧変成岩（藍晶石-石英エクロジャイト）の上昇時における部分融解

中国東部，スールー・ダービー帯に産出する超高圧変成岩（藍晶石-石英エクロジャイト）の上昇時における部分融解

Domain structures in rutile in ultrahigh-pressure metamorphic rocks from Dabie Mountains, China

According to the HRTEM study, the UHP jadeite–quartzite mineral (Rutile, TiO 2) in Anhui Province, Dabie Mountains, China, has ultrastructures such as 〈011〉 two-dimensional commensurable modulated structures or superstructures, {011} twin domain structures, dislocations and crystal deformations. The SAED patterns and HRTEM images indicate the existence of the deformations and stacking faults on the interface of {011} twin crystal of rutile and its two-dimensional commensurate modulated structures with repetition period 0.753 nm (3 d 011) has tetragonal symmetry, cell parameters a=3a 0=1.377 nm ( a 0=0.459 nm ), c=c 0=0.3 nm. The modulated structures of rutile were probably caused by the isomorphic replacement of Ti 4+ and position modulation or occupation modulation of oxygen atoms in different degree; the deformation structures reveal that during the process of crystallization and mineralization, this mineral may be affected by the geological environment (such as temperature, pressure and stress), metamorphism and deformation.

UHP-metamorphic rocks from Dora Maira/Western Alps and Kokchetav/Kazakhstan: New insights using cathodoluminescence petrography

Thin sections of ultrahigh pressure (UHP) metamorphic rocks from the Dora Maira Massif (Italy) and the Kokchetav Massif (Kazakhstan) were investigated using the hot cathode cathodoluminescence (CL) technique. Coloured images of important, but otherwise invisible growth features could be easily identified with this tool within seconds. These features are in excellent correlation with chemical variations of minerals revealed by electron microprobe (EMP). Generally, CL is induced by activator-elements (e.g. Mn and REE) and lattice defects whereas so-called quencher-elements like Fe may reduce or even extinct luminescence. Since X-ray-intensity mapping images (MAPS) of minerals can take up to 50 hours, the CL-method represents an ideal and rapid approach prior to chemical characterization. In addition to typical carbonates such as calcite, Mg-bearing calcite and dolomite, a number of rock forming and accessory minerals including Mg- and Mg-Ca-garnets, diopsidic and jadeitic pyroxenes, kyanite, K-feldspar, quartz, coesite, diamond, zircon, apatite, and bearthite were examined. Features observed in garnets include small-scale oscillatory zoning patterns, changes in morphology during growth as well as different crack generations which were partly annealed. SiO2 phases (coesite, quartz, chalcedony) as well as exsolution textures of dolomite and Mg-bearing calcite are easy to distinguish due to their different CL-colours. Pyroxene displays complex zonation patterns and -to some extent- exsolution-textures of K-feldspar. Kyanite reveals distinct growth zones; in combination with mineral inclusion studies it is possible to discriminate between different kyanite-forming reactions. The different crystallographical orientation of twinned kyanite crystals leads to various luminescence colours, thus, the suture of the twin plane is well defined. Prior to SHRIMP analyses, knowledge of the internal structures of zircon is indispensable. Even very tiny coesite crystals are easy to distinguish from quartz or chalcedony by their disparate luminescence colours. Accessory luminescent minerals like diamond, apatite, bearthite are easy to identify in thin section even if they occur in very small abundance within the matrix or as inclusions. The CL method presented here for UHP-metamorphic rocks is recommended as a pathfinder for the discovery of internal structures of minerals prior to their chemical characterization using EMP.