DETRITAL ZIRCON PROVENANCE OF THE CENTRAL SONGPAN-GANZI TURBIDITES, NORTHERN TIBETAN PLATEAU: IMPLICATIONS FOR THE FILLING HISTORY AND TECTONIC EVOLUTION

The Songpan-Ganzi Complex (SGC) in the northern Tibetan Plateau is regarded as the largest exposure of Triassic strata on earth. It contains exceptionally thick (averaging ~8 km) and widespread (> 300,000 km²) deep-water oceanic clastic deposits. The source and filling history of this sedimentary record is a key to better understanding of the interaction between tectonic settings and basin sedimentation and subsequent evolution of the Eastern Paleo-Tethys Ocean. However, the source of those deposits remains ambiguous or controversial, as the SGC given complicated structural overprint by subsequent tectonism.

This study presents 1788 single detrital zircon U-Pb ages by LA-ICP-MS analysis from 18 central SGC Triassic sandstone samples and 2 East Kunlun basement metasedimentary rocks. The results show that the detrital zircon ages of the turbidites mainly consists of 5 populations: 240-310 Ma, 400-480 Ma, 750-1000 Ma, 1700-2000 Ma, and 2300-2600 Ma. Early Paleozoic and Permian ages indicate a dominant source from the East Kunlun arc, while the bimodal distribution of Precambrian zircon populations is consistent with sediment contribution from the North China block. The Qinling-Dabie belt and South China block likely served as the subordinate sources due to the presence of Late Devonian–Carboniferous and Neoproterozoic zircon found in some samples. The detrital zircon results from the metasedimentary rocks collected from east part of the East Kunlun reveal a Neoproterozoic-Mesoproterozoic age with a peak at ~920 Ma, which may be the source of Neoproterozoic zircons in the central Songpan-Ganzi area. The turbidites from central SGC do not have obvious signature of the Qiangtang block, which usually contain Pan-African (550-650 Ma) zircons. This conclusion can be reinforced by the paleocurrent measurement results, which demonstrate the dominant south and southwest directed paleoflow. The vertical distributions of 9 samples from the Dari section show that the lower turbidites were fed by the East Kunlun and North China block, while the upper ones were mainly fed by the East Kunlun and Qinling-Dabie belt. This probably implies the uplift and enhanced erosion in the north margin collision belts of the Eastern Paleo-Tethys Ocean occurred during the Late Triassic.