PETROFACIES AND PROVENANCE OF UPPER PERMIAN-LOWERMOST TRIASSIC SANDSTONES, WUTONGGOU LOW-ORDER CYCLE, BOGDA MOUNTAINS, GREATER TURPAN-JUNGGAR RIFT BASIN, NW CHINA: IMPLICATIONS ON UNROOFING HISTORY OF NORTH TIANSHAN

The upper Permian-lowermost Triassic sandstones in Wutonggou low-order cycle (WTG) exposed in Bogda Mountains provide great opportunity to reconstruct the missing parts and to understand the forming processes of the North Tianshan suture (NTS), a key element in the southern part of Central Asian Orogenic Belt. The WTG sandstones were deposited in the greater Turpan-Junggar intracontinental rift basin, bounded by NTS to the south in NW China. Point counting data of 60 sandstones, gravel lithology from 17 beds, and paleocurrent directions from 13 beds were acquired from Zhaobishan (ZBS), Taodonggou (TDG), north Tarlong (NTRL) sections in the southern, and Dalongkou (DLK) section in the northern foothill of Bogda Mountains. Two petrofacies have been identified. Petrofacies 1 (P1) is quartz and feldspar enriched with a mean composition of Q₄₇F₃₁L₂₃ and occurs throughout ZBS and upper part of TDG and NTRL sections. P1 suggests dominant volcanic and plutonic and subordinate metamorphic and sedimentary sources. Petrofacies 2 (P2) is lithic enriched with a mean composition of Q₈F₁₃L₇₉ and occurs in DLK and lower parts of TDG and NTRL sections. P2 suggests volcanic and sedimentary sources. The occurrence of P1 and plutonic and metamorphic gravel clasts in ZBS section indicates that the volcanic, plutonic, and metamorphic rocks from NTS were the main sources. The occurrence of P2, and variable paleocurrent directions in DLK section, suggest the sedimentary and volcanic rocks from rift shoulders and NTS were the sources. The upsection change from P2 to P1 in TDG and NTRL sections suggests the sources altered from mixed sedimentary and volcanic rocks from rift shoulders and NTS to volcanic, plutonic, and metamorphic rocks from NTS only. The compositional variations within the greater Turpan-Junggar basin suggest the compositions of NTS are diverse spatially. The western NTS might contain a large amount of volcanic and sedimentary rocks; the central NTS abundant volcanic rocks with plutons. Additionally, the upsection increase of quartz and granitic gravel content occurs throughout the middle to upper ZBS and upper TDG and NTRL sections suggest the plutons had been exposed at least during Changhsingian. This study suggests the NTS is a collage that plutons were juxtaposed against the volcanic arcs and accretionary complex.