Cathodoluminescence (CL) and mineral inclusions investigations show that zircons from Yinggelisayi UHP metamorphosed granitoid gneiss in Altyn Tagh exhibit core-mantle-rim internal texture with magmatic residual core, the metamorphic overgrowth mantle and the late stage rim. SHRIMP 206Pb/ 238 U ages of 5 spots from the mantles vary from 484±11 to 491±12 Ma with ThlU ratios from 0.01 to 0.03, yielding a weighted average age of 487±10 Ma (20") as the age of the UHP metamorphosed granitoid gneiss. The 206Pb/ 238 U ages from the oscillatory zoned cores range from 809±19 to 885±21 Ma with ThIU ratios ranging from 0.42 to 0.83, which are interpreted as the crystallization age of the protolith of the granitoid gneiss. The metamorphic age (487±1O Ma) of the UHP metamorphosed granitoid gneiss, well consistent with the peak metamorphic age (about 500 Ma) of the UHP eclogite with crystallization age of the protolith between 809±19 and 885±21 Ma at Jianggelisayi in western Altyn Tagh, indicates the existence of a UHPM belt along the south margin of the Altyn Tagh as a result of deep subduction during Early Paleozoic age of previously existing continental crust. Different types of UHP metamorphic rocks have been recently discovered in the Altyn Tagh[I-4], the northern margin of Qadam Basin[S-7], the southwestern Tianshan Mountains [8,9] and the northern Qinling Mountains [IO,ll] in Central and Western China. And these areas have attracted focus attention of geologists at home and abroad to the studying of UHP metamorphism and continental deep subduction. However, as newly discovered UHP metamorphic terranes, some questions have been arisen about the space and time extent of these UHP metamorphic rocks, peak metamorphic ages, the correlation to the deep subduction of continental crust as well as the tectonic relation between these UHP metamorphic Chinese Science Bulletin Vol. 49 No. 23 December 2004 ARTICLES terranes. The recently discovered garnet lherzerlites[l] and their direct country rocks-garnet-bearing granitoid gneisses[2,4] and K-feldspar garnet clinopyroxenites [2,12] from Yingelisayi in the Altyn Tagh have all experienced UHP metamorphism and are the products of deep subduction of crustal rocks. But it is still unclear whether or not these UHP metamorphic rocks construct the same UHP metamorphic belt with UHP eclogites in the western part of the south margin of the Altyn Tagh and the tectonic relationship of these rocks with other UHP rocks discovered in Western China because of the lack of accurate isotopic data of these rocks. Based on the CL images, mineral inclusions and SHRIMP U-Pb dating of zircons, the authors report in this paper the metamorphic age of 487±10 Ma for the UHP metamorphosed granitoid gneisses from Yinggelisayi in the Altyn Tagh. This age is consistent well with that of the UHP eclogites (about 500 Ma) in the western Altyn Tagh, and provides further evidence that different kinds of UHP rocks discovered in this area construct a UHP metamorphic belt along the south margin of Altyn Tagh, and also ...
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