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We report here for the first time long prism/needle-shaped polycrystalline quartz aggregates and square/parallelogram-shaped single-phase quartz inclusions in omphacite and garnet of ultrahigh pressure (UHP) eclogite from the Yinggelisayi area in the South Altyn UHP belt. Based on their morphology and mineral composition, these quartz inclusions in omphacite and garnet are quartz paramorphs after stishovite. The minimum peak pressure of the eclogite is estimated to be >8–9 GPa at 800–1000 °C based on the stability field of stishovite. This new evidence, together with previous discovery of kyanite + spineal exsolutions in former stishovite of the pelitic gneiss from the same region, suggests a ultra-deep subduction and exhumation of the South Altyn continental rocks to/from mantle depths in the stability field of stishovite. Evidence of ultra-deep subduction of continental materials might be more common and diverse than previous thought. [ABSTRACT FROM AUTHOR]

Ultrahigh pressure (UHP) rocks are often overprinted by high pressure (HP) granulite-facies metamorphism during exhumation. Here we investigate retrograde eclogite from the Lvliangshan area in North Qaidam which is associated with abundant felsic veins. The eclogite adjacent to the vein shows conversion to garnet amphibolite. Petrographic, mineralogical, geochemical, geochronological data suggest that the eclogite has been experienced peak eclogite facies metamorphism at c. 440 Ma, followed by granulite facies retrogression at c. 420 Ma, and amphibolite facies overprinting at <420 Ma. The retrograde eclogite has microstructural records for partial melting with elongated plagioclase grains occurring as veinlets along the garnet-ilmenite grain boundaries and interspersed in clinopyroxene. Moreover, highly cuspate, wedge–shaped and triangular plagioclase grains are distributed along the garnet boundaries, and banded felsic veins (Pl + Qz) occur along the boundaries of amphibole in garnet amphibolite. Whole-rock geochemistry suggests that the felsic veins have 438.1–1106 ppm Sr and 2.87–12.18 ppm Y with Sr/Y ratios from 81.98–166.2, exhibit pronounced positive Eu anomaly, and depletion of HREE, resembling those of adakitic rocks. Felsic veins contain zircon grains with an U-Pb age of ∼422 Ma, similar to the age of HP granulite–facies metamorphism (420 Ma) of retrograde eclogite. Combined with the identical Hf isotope compositions between the granulite–facies zircon grains from the host retrograde eclogite and those from the felsic vein, we infer closed system partial melting during HP granulite facies retrogression of the retrograde eclogite. The tonalitic (high Sr/Y ratio and enrichment of LREE) chemical feature, low Rb/Sr and high Ba/Rb ratios of the felsic veins suggest that zoisite and sodic amphibolite dehydration-melting together account for the formation of felsic veins during exhumation from the eclogite–facies to granulite–facies stages. [ABSTRACT FROM AUTHOR]

Abstract: Petrography, mineral chemistry and pressure–temperature (P–T) estimates were carried out for the eclogite from the South Altyn in NW China. The results suggest three stages of metamorphism: an ultra-high pressure (UHP) eclogite-facies metamorphism at 717–871°C and ≥2.8GPa, a high pressure (HP) granulite-facies metamorphism at 624–789°C and 1.42–1.52GPa, and an amphibolite-facies metamorphism at 597–728°C and 0.99–1.17GPa. Cathodoluminescence investigation revealed that zircons from the retrograde eclogite display a distinct core-rim structure. Cores are grey-white luminescent and contain mineral inclusions of Garnet+Omphacite+Rutile+Quartz, suggesting eclogite-facies metamorphic origin. The rims are dark grey luminescent and contain Garnet+Clinopyroxene+Pagioclase inclusions, forming at HP granulite-facies conditions. A few residual zircon grains with mottled internal structure also occur as the metamorphic cores. LA-ICPMS zircon U–Pb dating yielded three discrete age groups: (1) a Neoproterozoic protolith age of 752±7Ma for the residual grains, (2) an eclogite-facies metamorphic age of 500±7Ma for the metamorphic cores, and (3) a HP granulite-facies retrograde age of 455±2Ma for the rims. These ages indicate that the protolith of the Altyn eclogite probably formed in response to breakup of the Rodinia supercontinent during the Neoproterozoic; it was subjected to continental deep subduction and UHP metamorphism during early Paleozoic (~500Ma) and subsequently underwent two stages of retrograde metamorphism during exhumation. The petrological and geochronological data suggest a clockwise P–T–t path for the UHP eclogite. According to pressures and ages for the peak UHP eclogite-facies and the retrograde HP granulite-facies metamorphism, an exhumation rate of 1.2mm/yr was estimated for the eclogite, which is considerably slower than that of some UHP rocks from other UHP terranes (>5mm/yr). While the peak metamorphic age of 500Ma is consistent with previous dates of 480–504Ma, it is 40–60Myr older than the HP/UHP metamorphic ages of 420–461Ma for UHP eclogites in North Qaidam. The retrograde metamorphic age is 455±2Ma for the Altyn eclogite, which is 30–55Myr older than ~400–425Ma for the North Qaidam eclogites. These age differences suggest that the South Altyn and North Qaidam eclogites do not belong to the same HP/UHP metamorphic zone. [Copyright &y& Elsevier]