The Geochemistry Characteristics And Geological Significance Of The Early Paleozoic Clastic Rocks In Rongma, North Tibet

Rongma, of Nagqu in Tibet, is in the north of south Qiangtang terrane. And a typical unconformity is located in Wenquan, about 3km from west of Rongma. The Middle and Late Ordovician is parallel unconformity on thick-bedded low grade metamorphic quartz-sandstones which has been interbedded by thin layer of marl.More than 200 detrital zircons dating results indicate that the maximum sedimentary age of the clastic rocks at the bottom of Tashishan Formation is 471±6Ma. And nearly 600 detrital zircons dating results indicate that the maximum sedimentary age of the low-grade metamorphic quartz-sandstone is 527±7Ma. The maximum sedimentary age gap between the clastic rocks above the unconformity and those underlying it is up to 56 Myr, which indicates that thers is an obvious depositional hiatus between these two strata. And the time when the parallel unconformity formed was Early Ordovician. Recently it was reported that the low grade quartz-sandstones were intruded by Early Ordovician granitoids(471-477Ma). So the low grade quartz-sandstones can be classified in Late Cambrianera. The researches of the REE distribution pattern and oxygen fugacity calculation for the detrital zircon don’t show a quite ideal result.The detrital zircons of the clastic rocks above the unconformity were mainly from crystalline rocks which formed in the late period of the Pan-African event and close to the irprovenance. This implies that the Late Pan-African crystalline rocks had exhumed in Early Ordovician, providing material sources to the Middle and Late Ordovician sedimentary rocks. The discovery of the parallel unconformity of Ordovician indicates that the south Qiangtang terrane used to be one part of the Gondwana supercontinent, like Himalayan terrane and Lhasa terrane and so on. The distinctive difference in Paleozoic strata between the south Qiangtang and north Qiangtang terranes indicates that there is an ancient ocean basin between the two terranes in the Early Ordovician at least, and the two terranes evolved independently.