P-T path of Early Paleozoic pelitic high-pressure granulite from Danshiiiquan area in Altyn Tagh.

The peak metamorphic mineral assemblage ( g + sill + ksp + ru + q) in garnet-kyanite-bearing biotite gneiss from Altyn Group of Qingshuiquan area, South Altyn, indicates that the gneiss is a suit of high-pressure pelitic granulite, and the metamorphic condition is T850℃ and P11kbar according to the THERMOCALC 3.23. Four stages metamorphic evolution are described by means of P-T pseudosections for selected metapelite specimens, showing a clockwise retrograde metamorphic P-T path with rapid exhumation and fast cooling. Zircon CL images show the uniform internal structure of core-rim microstructure, the cores preserve magmatic zone and the rims display sector and planar structure. LA-ICP-MS in situ trace element analyses indicate that spots from the core have high HREE concentration, and their Th/U ratio are mostly close to 0.4, showing feature of magmatic zircon; and spots from the rim show slightly depleted HREE pattern, the corresponding Th/U ratios are all lower than 0.1 , indicating their metamorphic growth coexisting with garnet. LA-ICP-MS in situ dating yields the metamorphic age of 486±5Ma from the rims, and the detrital protolith ages varying from 719Ma to 1780Ma from the cores. The metamorphic age of 486±5Ma obtained in this paper is nearly the same as the existing metamorphic ages of Jianggalesayi and Yinggelasayi UHP terrane, it provides us further evidence that there is an Early Paleozoic HP-UHP metamorphic belt along the south of Altyn Tagh. In addition, the youngest protolith age of 719Ma undoubtly indicates that the Altyn Group formed during Neoproterozioc instead of Paleoproterozoic.