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Inclusions of polycrystalline K-feldspar aggregates after K-cymrite (KAl[Si.sub.3][O.sub.8] x n[H.sub.2]O) were discovered in garnet from a Dulan eclogite in the Qaidam ultrahigh-pressure (UHP) terrane, western China. The eclogite consists of garnet ([Alm.sub.56][Grs.sub.23][Prp.sub.20][Sps.sub.01]), omphacite ([Jd.sub.35][Aeg.sub.6][Aug.sub.59]), and minor ruffle and apatite. The 20 to 200 [micro]m inclusions vary in shape from prismatic, hexagonal to rounded, and exhibit palisade and mosaic textures. Host garnets show radial fractures, similar to those surrounding quartz pseudomorphs after coesite. Some inclusions consist of almost end-member K-feldspar ([Or.sub.99-100][Ab.sub.0-1]) polycrystalline aggregates, whereas others are composed of >90 vol% K-feldspar ([Or.sub.96-99][Ab.sub.1-4]) with minor secondary albite occurring along the margins of the inclusions. Raman spectra of K-feldspar crystalline aggregates vary slightly reflecting various degrees of Si-Al ordering, and show a Raman peak at ~390-395 [cm.sup.-1], typical for cymrite structure. These characteristics of the K-feldspar polycrystalline inclusions imply the presence of former K-cymrite in the Dulan eclogite formed at >3 GPa at ~720 [degrees]C. The occurrence of K-cymrite in UHP eclogite is significant because of its potential as an important carrier of crustal K and [H.sub.2]O to the upper mantle. Keywords: K-cymrite, K-feldspar, inclusion, eclogite, Raman spectrum, North Qaidam UHP terrane

The Yangkou garnet-peridotite complex in the Sulu UHP terrane, eastern China, consists of several blocks, tens of meters across, enclosed within gneiss. The blocks consist mainly of garnet lherzolite and garnet clinopyroxenite with small amounts of eclogite. The complex displays a finely layered stRucture from outcrop to thin-section scales. Lherzolitic garnet is generally homogenous in composition ([Prp.sub.64-67][Alm.sub.23-24][Grs.sub.10-12][Sps.sub.1]). Garnet (PrpsI-63) from clinopyroxenite contains more pyrope component than eclogitic garnets ([Prp.sub.29-35]). Porphyroblastic garnet from clinopyroxenite and eclogite contains abundant exsolved rods of futile and apatite ([+ or -]Cpx). Lherzolitic garnet exhibits an unusual sinusoidal REE pattern characterized by a decrease in abundance from La to Pr and an increase from Nd to the HREE, and shows Ba and Pb enrichment and negative Nb and Ni anomalies in primitive mantle-normalized trace-element plots. In contrast, diopside shows a normal REE pattern with decreasing abundances from La to Lu, a peak at Ba and Pb, and depletion in Co and Ni. Garnet from eclogite has a REE pattern with increasing abundances from La to Lu, or a maximum at Eu. Eclogitic clinopyroxene shows humped REE patterns with a peak at Nd. Except for slightly lower [[delta].sup.18][O.sub.VSMOW] of +4.40% for olivine, O-isotopic compositions (+5.03 to +5.53%) of analyzed garnets and clinopyroxenes from all three rock types lie in the range of mantle [[delta].sup.18]O values of +5.1 to +6.6%0. Small O-isotope fractionations ([DELTA]Grt-Cpx: -0.16 to -0.35) in eclogite suggest isotopic equilibrium. On the basis of petrochemical and isotopic data, this garnet-peridotite complex was initially a mantle peridotite cut by mafic veins. These rocks experienced metasomatism and subduction-zone UHP metamorphism at 750 [+ or -] 50 [degrees]C and 40 to 45 kbar with a low thermal gradient. The fine-scale layering probably formed as a result of strong shear deformation during exhumation.

Eclogites interlayered with gneiss, and minor quartzite in the Qinglongshan near Donghai are characterized by unusuall abundant (15-40 vol%) hydrous phases including talc. phengite, and epidote; many also contain kyanite. Garnet hosts, both prograde (paragonite, amphibole, epidote) and peak stage (omphacite, epidote, phengite, kyanite) mineral inclusions. Several eclogites contain talc rimmed by barroisite: optically and compositionally similar coarse-grained amphibole in other samples indicates that the reaction Omp + Tlc = Amp has completely consumed tale. Estimated peak conditions of 30-35 kbar, 600-700 [degrees]C. are consistent with polycrystalline quartz pseudomorphs after coesite included in garnet, omphacite, epidote, and kyanite, and up to 3.6 Si pfu (11 O atom basis) in phengite. Garnet-epidote oxygen barometry on the peak metamorphic assemblage indicates oxygen fugacities above the Hem-Mag buffer, consistent with the epidote + talc assemblage and 5-20 mol% aegerine component in omphacite. The high oxygen fugacity calculated in this study as well as previously documented negative oxygen isotope values recorded by these rocks may both reflect alteration by oxidizing, meteoric water in a hydrothermal system. Oxidized conditions during peak metamorphism may explain the extreme scarcity of microdiamond in this area. The Ep + Tlc assemblage is stabilized by high oxygen fugacity, and demonstrates that talc-bearing eclogites are not restricted only to unusually Mg-rich bulk composition.

The Rizhao garnet clinopyroxenite occurs as small mantle slices faulted into subducted continental crust in the Sulu ultrahigh-pressure (UHP) terrane, China. Two representative garnet clinopyroxenites with exsolation microstructures were studied: (1) a massive ilmenite-rich specimen, composed of coarse-grained clinopyroxene containing abundant exsolution rods of grossular-rich garnet (>20 vol%) + ilmenite (~3-4 vol%) in a fine-grained matrix of clinopyroxene + garnet + ilmenite; and (2) megacrystic garnet-bearing sample, characterized by garnet containing inclusions of clinopyroxene and ilmenite; the clinopyroxene inclusions show exsolution rods and blebs with variable amounts of garnet (1-15 vol%) and minor ilmenite. Both exsolved and matrix garnets have similar grossular-rich compositions. The aggregate composition of clinopyroxene + garnet + ilmenite intergrowth is similar to the whole-rock composition. We propose that the parental phase of exsolved garnet + ilmenite lamellae and clinopyroxene host was either a homogeneous clinopyroxene (hypothesis A) or a majoritic garnet (hypothesis B) that experienced three discrete evolution stages. If the parental phase was clinopyroxene (AB[O.sub.3]), with an Si deficiency in the B site, the Rizhao garnet clinopyroxenite could have an initial assemblage clinopyroxene [+ or -] Grt [+ or -] Ilm formed at near-solidus conditions ([less than or equal to]1400[degrees]C, at 5 GPa) in the upper mantle. The second stage is defined by garnet exsolution from the primary clinopyroxene involving decreasing temperature and/or increasing pressure, related to continental subduction. Coexisting clinopyroxene host and garnet exsolution ([Grt.sub.exs]) recrystallized at temperatures of ~900[degrees]C at an assumed minimum P of 5 GPa. The third stage is represented by recrystallization of exsolution phases to form the fine-grained matrix of Cpx + Grt + Ilm at 700[degrees]C and [is greater than or equal to] 3 GPa during the early exhumation of the UHP terrane. The speculative hypothesis B implies that the proposed majoritic garnet was from the mantle transition zone (

Garnet peridotites from the Kokchetav ultrahigh-pressure (UHP) massif contain abundant volatile and high field strength element (HFSE) bearing minerals, such as Ti-clinohumite and zircon. These characteristics are interpreted to be related to fluid-infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area. The zircons from the peridotites were dated by using sensitive high-resolution ion microprobe (SHRIMP) and yielded apparent U-Pb ages of 554-494 Ma (weighted mean age, 528 Ma) that are mostly consistent with the timing of UHP metamorphism deduced from diamond-bearing country rocks in this massif. These zircons have an almost flat rare earth element (REE) pattern and very low REE concentrations; these characteristics are similar to those observed in kimberlitic zircons. Inherited zircon cores, although only rarely preserved, yielded apparently Proterozoic ages and have different trace element characteristics compared to the overgrowth rims. These features indicate that the mantle metasomatic events and the recrystallization of Ti-clinohumite and zircon were due to HFSE-enriched fluid infiltration during the UHP metamorphism at great depths. The metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts, because these rocks have characteristically high volatile and HFSE concentrations such as those of the Ti-clinohumite-bearing garnet peridotites. Keywords: mantle metasomatism, Ti-clinohumite, zircon, U-Pb geochronology, ultrahigh-pressure metamorphism.

Clinoenstatite is an uncommon mineral, and has been documented in Sulu ultrahigh-pressure (UHP) garnet pyroxenite. However, clinoenstatite (CLEN) was rarely reported in the previous studies of ophiolitic mantle peridotites. The studied ophiolitic peridotites are recovered from a drilling hole of the Luobusa ophiolitic massif, Tibet that is unique in occurrences of super reducing UHP minerals in both chromitite and peridotite. Our petrographic and TEM studies reveal abundant CLEN as lamellae associated with diopside exsolution in orthoenstatite (OREN) from the Luobusa mantle peridotite. In addition, termination of CLEN lamellae and Guinier-Preston zones (nanometer-scale precipitates with compositions between enstatite and diopside) are also observed in the OREN. Both CLEN lamellae and host OREN have nearly identical (Mg 0.9 Fe 0.1 ) 2 Si 2 O 6 compositions. The CLEN lamellae in host OREN from two core samples have even numbers of 9 Å (1 0 0) fringes without twins, and are oriented parallel to (1 0 0) of the host OREN. The crystallographic data of the CLEN are d 100 = 9.06 Å (R1), d 020 = 4.34 Å (R2), d 120 = 4.03 Å (R3), R1 ∠ R2 = 90°, R1 ∠ R3 = 64° and R2 ∠ R3 = 26°, whereas the host OREN have d 100 = 18.1 Å (R1), d 020 = 4.48 Å (R2), d 120 = 4.34 Å (R3), R1 ∠ R2 = 90°, R1 ∠ R3 = 77° and R2 ∠ R3 = 13°. No antiphase domain boundaries are observed in TEM images; it probably indicates that the clinoenstatite is the low- P polymorph of clinoenstatite (LCLEN) with P 2 1 / c symmetry. The exsolution of diopside and the presence of Guinier-Preston zones are attributed by cooling of primary high- T Ca-bearing (very minor) OREN. The transformation of LCLEN from OREN may have occurred mainly by cooling along the P-T boundaries between the OREN and LCLEN stability fields; shear stress only enhances the transition. The occurrence of CLEN lamellae in OREN and the existence of super reducing UHP minerals in peridotite provide an additional constraint for a very deep mantle origin of the Luobusa peridotite. [ABSTRACT FROM AUTHOR]

Polycrystalline microdiamonds are rare in ultrahigh-pressure (UHP) rocks worldwide. Among samples collected at Erzgebirge, Germany, we found abundant polycrystalline microdiamonds as inclusions in zircons from a quartzofeldspathic rock. To illuminate their origin and forming age, we investigated morphologies and Raman spectra of 52 microdiamond inclusions, and dated the zircon host. The zircons have low Th/U values (0.03–0.07) and a concordia U/Pb age of 335.8 ± 1.9 Ma. Polycrystalline diamond (10–40 µm) consists of many fine-grained crystals (1.5–3 µm) with different orientations; discrete single diamonds (2–20 µm) are rare. All measured Raman spectra show an intense diamond band at 1332–1328 cm−1and have a negative correlation with full width at half maximum (FWHM) of 5.8–11.3 cm−1. These data combined with previously reported diamond band data (1331–1337 cm−1) are compatible with those of diamond inclusions in various host minerals from other UHP terranes, but are different from those of ureilite diamonds. The Erzgebirge microdiamonds in zircon do not display visible disorderedsp3-carbon, but show downshifting of the Raman band from the ideal value (1332 cm−1), and have a broader diamond band (FWHM >3 cm−1) than those of well-ordered diamonds. These features may reflect imperfect ordering due to rapid nucleation/crystallization during UHP metamorphism and rapid exhumation of the UHP terrane. Graphite inclusions in zircon show a typical G-band at 1587 cm−1. Our study together with previously reported C-isotopic compositions (δ13C, −17 to −27‰) of diamond and occurrences of fluid/melt inclusions in diamond and garnet indicates that Erzgebirge microdiamonds are metamorphic, have an organic carbon source, and crystallized from aqueous fluids. Limited long-range ordering suggested by the Raman spectra is a function of theP–Ttime of crystallization and subsequent thermal annealing on decompression. Combined with regional geology, our work further constrains the tectonic evolution of the Erzgebirge terrane. [ABSTRACT FROM AUTHOR]

Previous research on super-reducing ultrahigh-pressure (SuR UHP) phases from the Tibetan ophiolitic chromitites were mainly conducted on isolated grains extracted from extremely large samples. This approach has been questioned because of possible contamination. To elucidate the occurrence and origin of these SuR UHP minerals, we studied 33 thin sections and rock chips of three ophiolitic chromitites from the Yarlung Zangbo suture zone. Here we report and analyze unambiguously in situ SuR UHP assemblages from the ophiolitic chromitites by electron probe micro-analyzer, scanning microscope and Laser Raman spectroscope. The SuR UHP and associated phases include: (1) blue moissanite as inclusions in olivine (Fo96-98), and in olivine domains between disseminated chromite grains; (2) multiple inclusions of moissanite + wüstite + native Fe in olivine; (3) FeNi and FeCr alloys in olivine and chromite; and (4) native Fe and Si in chromite. Crustal asphaltum and h-BN also occur as inclusions in chromite. Our documented in situ SuR UHP phases, combined with the previously inferred existence of ringwoodite + stishovite, all indicate that these assemblages formed under a highly reducing environment (oxygen fugacities several orders of magnitude lower than that of the iron-wüstite buffer) in the mantle transition zone (MTZ) and in the deep upper mantle. Diamond + moissanite with distinct 13C-depleted compositions from chromitites have a metasedimentary carbon source. Associations with existing crustal minerals in chromitites demonstrate that carbon-bearing metasedimentary rocks were recycled into the mantle through subduction, and locally modified its composition. Finally we propose a three-stage model to explain the formation of SuR UHP phase-bearing chromitite. Discoveries of SuR UHP phases in Luobusa and other ophiolitic podiform chromitites from the polar Ural Mountains and from Myanmar imply existence of a new type of ophiolitic chromitite. Such occurrences provide an additional window to explore the physical-chemical conditions of the MTZ, mantle dynamics, and the profound recycling of crustal materials. [ABSTRACT FROM AUTHOR]

Eclogites interlayered with gneiss and minor quartzite in the Qinglongshan near Donghai are characterized by unusually abundant (15-40 vol%) hydrous phases including talc, phengite, and epidote: many also contain kyanite. Garnet hosts both prograde (paragonite, amphibole, epidote) and peak stage (omphacite, epidote, phengite, kyanite) mineral inclusions. Several eclogites contain talc rimmed by barroisite: optically and compositionally similar coarse-grained amphibole in other samples indicates that the reaction Omp + Tlc = Amp has completely consumed talc. Estimated peak conditions of 30-35 kbar, 600-700 °C, are consistent with polycrystalline quartz pseudomorphs after coesite included in garnet, omphacite, epidote, and kyanite, and up to 3.6 Si pfu (11 O atom basis) in phengite. Garnet-epidote oxygen barometry on the peak metamorphic assemblage indicates oxygen fugacities above the Hem-Mag buffer, consistent with the epidote + talc assemblage and 5-20 mol% aegerine component in omphacite. The high oxygen fugacity calculated in this study as well as previously documented negative oxygen isotope values recorded by these rocks may both reflect alteration by oxidizing, meteoric water in a hydrothermal system. Oxidized conditions during peak metamorphism may explain the extreme scarcity of microdiamond in this area. The Ep + Tlc assemblage is stabilized by high oxygen fugacity, and demonstrates that talc-bearing eclogites are not restricted only to unusually Mg-rich bulk compositions. [ABSTRACT FROM AUTHOR]

The Shuanghe ultrahigh-pressure (UHP) slab in the Dabie Mountains consists of layered coesite-bearing eclogite, jadeite quartzite, marble and biotite gneiss, and is fault bounded against hosting orthogneiss. Representative assemblages of eclogite are Grt+Omp+Coe+Rt±Ky±Phn±Mgs; it formed at P>27 kbar and 680–720±50 °C. During exhumation, these UHP rocks experienced multistage retrograde metamorphism. Coesite was overprinted by quartz aggregates, phengite by biotite±muscovite and rutile by titanite. Garnet was successively replaced by a thin rim of Amp, Amp+Pl, and Amp+Ep±Bt+Pl (minor). Omphacite and kyanite were replaced by Amp+Pl±Cpx (or ±Bt) and by Zo+Pl+Ms±Mrg±Bt, respectively. Secondary calcite occurs as irregular pockets in some layers. An outcrop near the UHP slab border is composed of ∼20 thin, concordant layers of foliated eclogites, amphibolite and gneissic rocks of variable bulk composition. These layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low-amphibolite facies through hydration, metasomatism and recrystallization. Retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions. Retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of CO2-rich and low-salinity aqueous, intensity of deformation and mineral resistance to alteration. The fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites, and/or as isolated pockets in extensive or less altered eclogite layers. [Copyright &y& Elsevier]

Focuses on a transmission electron microscopy (TEM) study of intergrowths of clinoenstatite and orthoenstatite in ultrahigh-pressure garnet pyroxenites from the Dabie-Sulu terrane in China. Analytical techniques and results of TEM; Description of Donghai garnet pyroxenite; Clinoenstatite formed by transformation of orthoenstatite at a retrograde stage due to gentle shearing or slow cooling; Mineral compositions of garnet pyroxenites.

Reports findings of hydroxyl-rich topaz in the Sulu kyanite quartzite from eastern China. Discussion of the petrochemical characteristics and stabilities of hydroxyl-rich topaz; Role of fluid in metamorphic processes related to continental subduction and collision; Origin of woodhouseite.

Studies the exsolution textures exhibited by ultrahigh-pressure (UHP) minerals of the Maowu mafic-ultramafic complex in the Dabieshan, east-central China. Sample description; Crystal structure; Discussion.

The Zedang ophiolite, containing pods of massive chromitite, crops out in the eastern segment of the Yarlung-Zangbo suture zone, Southern Tibet. Chromite grains in the Zedang chromitite bodies have high Cr# [Cr/(Cr + Al)] (0.73–0.82) and Mg# [Mg/(Mg + Fe)] (0.73–0.79), and low TiO 2 content (0.13–0.25 wt%); they contain diopside (Di) exsolution lamellae and abundant inclusions of diopside, enstatite, hydrous silicates (e.q., Cr-bearing amphibole), and serpentine. The inclusions occur as isolated single-phase or multiphase (clinopyroxene + amphibole ± platinum group mineral) grains. Diopside lamellae-bearing chromite was probably a precursor CaFe 2 O 4 -phase (CF phase) forming at >12.5 GPa. Individual octahedral serpentine inclusions are likely pseudomorphs after isometric ringwoodite. Both the CF phase and ringwoodite crystallized within the mantle transition zone (MTZ). Equilibrium pressure-temperature (P-T) conditions of coexisting clinopyroxene-orthopyroxene (Cpx-Opx) inclusions are 8.5–21.5 ± 1.5 kbar, and 996–1097 ± 30 °C. These P-T conditions suggest that the inferred parental melts of the Zedang chromitites were boninitic. The boninitic liquid in turn was generated from depleted, metasomatized mantle in a supra-subduction zone (SSZ) environment. Linear alignment of hydrous silicate inclusions in chromitites imply fluid infiltration along fracture zones at shallow depths during consolidation of the ophiolite. Based on our new data, we propose a new three-stage model to explain the formation and evolution of the Zedang chromitites. It experienced a long journey from (i) the MTZ forming high-P chromite (containing ultrahigh-pressure minerals), (ii) to the upper mantle attending to the formation of the major part of the chromite, whereas it's host chromitites formed at melt-rock reaction process, and (iii) where high-P chromite was incorporated in low-P neoblastic chromite during subducting slab rollback-induced channel return flow to shallow depths (involving fluid input and alteration). [Display omitted] • The Zedang chromitites are mixtures produced in subduction channels. • Mineral inclusions provide information constraining the origin of the chromitite. • A new 3-stage model explains the formation and evolution of the Zedang chromitites. [ABSTRACT FROM AUTHOR]