SHRIMP U-Pb zircon ages for the UHP metamorphosed granitoid gneiss in Altyn Tagh and their geological significance

摘要

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/238U ages of 5 spots from the mantles vary from 484+11 to 491+12 Ma with Th/U ratios from 0.01 to 0.03, yielding a weighted average age of 487 + - 10 Ma (2cr) as the age of the UHP metamorphosed granitoid gneiss. The 206Pb/238U ages from the oscillatory zoned cores range from 809 + - 19 to 885 + - 21 Ma with Th/U 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 + - 10 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.