APPLICATION OF TWO DETRITAL ZIRCON GEOCHRONOLOGY TECHNIQUES TO THE DEEP-WATER DEPOSITS OF THE SONGPAN-GANZI COMPLEX, CENTRAL CHINA

Detrital zircon provenance investigations have proven to be indispensable in reconstructing sediment dispersal patterns, paleogeographic configurations and unroofing histories, and are beginning to be applied to predicting petroleum reservoir distribution. Its wide application is due primarily to the availability of two single-grain U-Pb isotopic measurement techniques: the sensitive high-resolution ion microprobe (SHRIMP) and laser-ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS). Both techniques were used to determine U-Pb ages for 870 detrital zircon grains from eleven Middle Triassic-Upper Triassic turbidite sandstone samples of the Songpan-Ganzi complex, a massive turbidite basin of central China. In order to calibrate the two techniques used in this study, 70 detrital zircon grains from two different samples composed primarily (~90%) of Paleozoic zircons were first analyzed using the SHRIMP-RG at Stanford University; the same spots were subsequently analyzed using the LA-ICP-MS at the University of Arizona. Histograms and probability curves plotted using 20 Ma bins produce similar detrital zircon age spectra from SHRIMP and LA-ICP-MS data sets; however, SHRIMP analyses use a smaller sample volume than LA-ICP-MS analyses, which affected age determinations for some complex grains. The detrital zircon age data from the Songpan-Ganzi complex, together with ~250 paleocurrent measurements, discriminate two separate turbidite deposystems in what had appeared to be one vast turbidite complex. The turbidite system of the northeastern Songpan-Ganzi complex contain zircon grains derived from the North China block and the Qinling-Dabie orogen, whereas turbidite system of the central Songpan-Ganzi complex initially was sourced solely by erosion of the Qinling-Dabie orogen during early Late Triassic time, then by an integrated drainage tapping Qinling-Dabie, North China Block and South China block sources during middle to late Late Triassic time. These separate deep-water deposystems were later tectonically shortened, amalgamated, and metamorphosed due to continental collision during the early Jurassic.