UHP metamorphic evolution and SHRIMP geochronology of a coesite-bearing meta-ophiolitic gabbro in the North Qaidam, NW China

We have for the first-time observed the well-preserved coesite inclusions in a kyanite-eclogite from the Shaliuhe terrane of the North Qaidam ultra-high pressure (UHP) metamorphic belt, NW China. This provides convincing evidence for UHP metamorphism for the North Qaidam eclogite. Detailed petrography indicates that the kyanite-eclogite records complex metamorphic histories that can be broadly described in terms of three stages: (1) a pre-peak metamorphic stage characterized by the mineral assemblage of Grt + Amp + Pl + Qtz at P = 0.49-0.67 GPa and T = 410-490 degrees C; (2) a peak eclogite-facies stage with a mineral assemblage of Grt + Omp + Ky + Phn + Rt +/- Qtz/Coe at P = 2.7-3.4 GPa and T = 610-700 degrees C; (3) a retrograde "stage" represented by reaction texture which includes the garnet reaction rims, the breakdown of omphacite, the symplectitic breakdown of kyanite, the retrograded garnet amphibolite assemblage and the latest greenschist-facies overprint. Zircon U-Pb SHRIMP dating shows the peak UHP metamorphic age of the kyanite-eclogite to be similar to 440 Ma. These data indicate that the kyanite-eclogite had ever been subducted to mantle depth of similar to 100 km in the end of the Ordovician. All these observations allow the construction of a P-T path for the kyanite-eclogite, suggesting a cold geothermal gradient and rapid subduction, ultimately followed by rapid exhumation in response to continental collision in Shaliuhe terrane and throughout the North Qaidam UHP metamorphic belt. (C) 2008 Elsevier Ltd. All rights reserved.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000266895800009&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701Geosciences, MultidisciplinarySCI(E)CPCI-S(ISTP)4