Ordovician parallel unconformity in Qiangtang terrane, northern Tibet: Implications to Early Paleozoic evolution of northern Tibetan regions

Several issues have been disputed for a long time in Qiangtang terrane, northern Tibet, such as whether it has the metamorphic basement, as well as the pre-Cenozoic tectonic history. Here we report the Tashishan Formation, which is located in the northern portion of southern Qiangtang terrane, of Middle and Late Ordovician is parallel unconformity on thick-bedded fossil-free low-grade metamorphic quartz sandstones which has been interbedded by thin layer of marl. Nearly 600 detrital zircons dating results indicate that the maximum sedimentary age of the low-grade metamorphic quartz-sandstone is 527 +/- 7Ma. And more than 300 detrital zircons dating results indicate that the maximum sedimentary age of quartz sandstone at the bottom of Tashishan Formation is 471 +/- 6Ma. The sedimentary age gap between the quartz sandstones above the unconformity and the underlying low-grade metamorphic quartz sandstones is up to 56Myr, indicative of an obvious depositional hiatus between the two quartz sandstones. This further confirms that the unconformity between the two quartz sandstones is Early Ordovician. An independent piece of evidence is that the fossil-free low-grade quartz sandstones were intruded by Early Ordovician granitoids (471 similar to 477Ma). The sedimentary rock underlying the unconformity is, therefore, named as Rongma Formation and classified in Late Cambrian era in this study. Cathodoluminescence images and geocluonological studies.indicate that detrital zircons of the quartz sandstones above the unconformity were mainly from crystalline rocks which formed in the late period of the Pan-African event and close to their provenance. This implies that the Late PanAfrican crystalline rocks had exhumed in Early Ordovician, and then, underwent denudation, providing material sources for the Middle and Late Ordovician sedimentary rocks. The discovery of the parallel unconformity of Ordovician reflects that the south Qiangtang terrane is one portion of the Gondwana supercontinent, as Himalayan and Lhasa terranes. The distinctive difference in Paleozoic strata between South Qiangtang and North Qiang-tang terranes shows that there is an ancient ocean basin between the two terranes in the Early Ordovician at least, and the two terranes evolved independently.