Geochronology and geochemistry of the Qinling Group in the eastern Qinling Orogen.

Petrology and geochemistry of the basement metamorphic rocks from the Qinling Group in the Luonan, Ningshan and Changan County from Shanxi Province and Xichuan County from Henan Province of the Eastern Qinling Orogen indicate that the samples are mainly composed of metavolcanic rocks and metasedimentary rocks. LA ICP-MS zircon U-Pb dating results suggest that three orthometamorphites were all formed in early Neoproterozoic (971～843Ma) and metasedimentary rock contains a number of Neoproterozoic detrital zircons. The depositional time of the sedimentary rock should be middle-late Neoproterozoic on the basis of the youngest age (859Ma) of concordant detrital zircons and early Paleozoic metamorphic age. Therefore, the Qinling Group probably is a complex consisting mainly of early Neoproterozoic igneous rocks and middle-late sedimentary rocks. The metamorphism of the Qinling Group occurred mainly in Caledonian period and locally in Yanshanian period, suggesting that the major orogeny in the northern Qinling area took place in early Paleozoic time. Geochemical characteristics also indicate that Neoproterozoic volcanic rocks in the Qinling Group formed in a volcanic arc tectonic setting and the sedimentary rock deposited on continental arc to active continental margin environment, suggesting the Qinling orogenic belt was a volcanic arc during early Neoproterozoic. The volcanic and sedimentary rocks of the Qinling Group are very similar to those in the western Yangtze craton in the formation time and tectonic setting. Therefore, the Qinling Group distributing along southern margin of northern Qinling Orogen should belong to the Yangtze craton and it was an island arc in the northern margin of Yangtze craton.