Age And Tectonic Setting Of Neoproterozoic Gneissic Granites In The Southern Yili Block (Nw China) And Implications For The Origins Of The Continental Blocks In Sw Central Asian Orogenic Belt

Several ribbon-like continental fragments with Precambrian basement were wrapped within Palaeozoic accretionary complexes in the southwest Central Asian Orogenic Belt (CAOB). Their origins and tectonic evolution are essential for understanding the accretionary orogeny of the CAOB and crustal growth versus continental reworking in Phanerozoic. The southern Yili Block in NW of China is one of the major continental fragments in SW CAOB. In order to better constrain the tectonic settings of the Precambrian basement of the southern Yili Block, we present new data of field structures, whole-rock geochemical compositions, zircon U-Pb ages and Lu-Hf isotopic ratios of the Neoproterozoic gneissic granites. Ductile deformation characterized by mylonitic foliations and stretching/mineral lineation is commonly recognized in the gneissic granites, which were cross-cut by undeformed granitoids and mafic dykes. The gneissic granites yielded zircon U-Pb ages of 968 +/- 5 Ma, 948 +/- 8 Ma, and 873 +/- 9 Ma. These gneissic granites show geochemical features of high potassium, peraluminous, calc-alkaline series, enrichment of Th, K, Rb and depletion of Ba, Nd, Sr. They show zircon epsilon(Hf)(t) values from -5.5 to 2.0 and two-stage zircon Hf model ages between 1.7 and 2.1 Ga. These new results indicate that the Neoproterozoic gneissic granites in the southern Yili Block derived from partial melting of meta-greywackes that most likely had formed in a continental arc setting. A comparison of stratigraphy, magmatism and detrital zircon ages suggests that the southern Yili Block is similar with the northern Yili Block and Chinese Central Tianshan Block in term of Precambrian basement, and these continental fragments might have originated from Neoproterozoic orogenic belts.