Paper No. 28-5
Presentation Time: 9:00 AM-1:00 PM
CONSTRAINTS ON AMALGAMATION OF THE SOUTHERN CENTRAL ASIAN OROGENIC BELT FROM DETRITAL ZIRCON FROM THE UPPERMOST CARBONIFEROUS TO LOWER TRIASSIC GREATER TURPAN-JUNGGAR RIFT BASIN, BOGDA MOUNTAINS, NW CHINA
The Permian-Triassic time is an important transition between Paleozoic continental amalgamation and Cenozoic orogenic reactivation in the southern Central Asian Orogenic Belt (CAOB). Sections in Tarlong-Taodonggou (TT) and Zhaobishan (ZBS) in the south Bogda Mountains, NW China and Dalongkou (DLK) in the north, are 100 km apart and ~7,000 m in total thickness. U-Pb dates from 1,860 detrital zircons from 19 uppermost Carboniferous-Lower Triassic sandstones constrain the timing of accretion. Dates from 3 Gzhelian-Asselian samples in TT peak at ~300 Ma ranging from 280-340 Ma, and minor peaks are at 360, 400-500 and 1500 Ma. The dates, along with petrographic and paleocurrent data, suggest North Tianshan (NTS) arc was the source. Sakmarian-Kungurian samples from TT and ZBS have a main peak at 290-310 Ma and Kungurian samples have minor peaks at 330, 360, 400-500, and 900 Ma. The main peak indicates the same NTS provenance. Precambrian zircons are from texturally immature sandstones, suggesting non-recycled zircons from an old continent. Guadalupian samples in all three areas have a main peak from 310-330 Ma, indicating a common NTS provenance. However, TT samples show a 265 Ma peak, indicating contribution from contemporaneous loessial sediments. ZBS samples have unique peaks at 390-500 Ma and 1.5-2.0 and 2.6-3.0 Ga that suggest the East Junggar arc to the east was a source, having been exposed during the tectonic reorganization indicated by the major unconformity and difference in age spectra between Cisuralian and Guadalupian. Dates from Wuchiapingian-mid Olenekian samples from three areas indicate the NTS source persisted. The contribution from the East Junggar source persisted in ZBS and appeared in the late Permian in DLK and Early Triassic in TT, suggesting its increasing contribution extended farther into the basin. Zircon dates from a mid-Triassic sandstone in DLK suggest the pattern persisted across a major regional early-mid Triassic unconformity. This study indicates that sediments in Bogda Mountains are from the NTS, East Junggar and an old continent. It provides constraints on the timing of amalgamation of the southern CAOB; the NTS accreted to Junggar block before the Gzhelian and closure of the Paleo-Asian Ocean (?) and accretion of East Junggar started in the Guadalupian.