Petrology Of Tight Sandstones Within Northwestern Tibetan Regions:Implications To The Tectonic Evolution Of The Northwestern Tibet

Northwest Tibet as the most controversial area of Gondwana evolution has been attracting a large number of scholars.In the Qiangtang central upheaval,due to fossil scarcity the age of large area of metamorphic quartz sandstone has not been unified,which directly led to tectonic evolution is not clear in the northwest Tibet before the Mesozoic.Through the field investigation,we found a large area of tight sandstone outcrop in chaduogangri,and basal conglomerate in caimaer cuo.Petrographic characteristics show that the low-grade metamorphic quartz sandstones occurring in the Chaduogangri and caimaer cuo,respectively,are exactly similar.They are mainly composed of quartz,experiencing greenschist facies metamorphism to yield a new assemblage of albite + muscovite + chlorite among early quartz grains,and both parallel unconformable cover by quartz sandstone..The maximum sedimentary ages of the tight sandstones beneath the parallel unconformity are 520? 8 Ma,and are intruded by the granitoids with ages of 480 Ma.However the maximum sedimentary ages of the tight sandstones above the parallel unconformity are 460 ? 8 Ma,showing that a significant depositional hiatus between the two sandstones and thus confirming that the parallel unconformity was formed during the Ordovician times.In addition to,compareing Yang Yao(2014)reported Rong Ma Xiang parallel unconformity with the two unconformities,we can find that three unconformities were formed in the same period.It can be seen that the chaduogangri,cemar cuo and Rong maxiang all belong to South Qiangtang block.In the northwestern Tibet,the northern border of Gondwanaland is Hongji Mountains belt instead of the Buerga fault zone.