Tectonic Framework of North Qinling Mountain and North Qilian Mountain Conjunction Area in Early Paleozoic： A Study of the Evidences from Strata and Tectonic-Magmatic Events

Based on comparative analysis of Ordovician strata located in Wuwie-Helan-Liquan area which is in the conjunction area of North Qinling and North Qilian,it is preliminarily deduced that it was the same ocean covering North Qilian,North Qinling and North China area in Ordovician.In Early to Middle Ordovician,a continued sedimentary basin existed,changing from continental margin sedimentary series to typical basin sedimentary series.It was developed from interior Ordos carbonate plat form to southwest marg in of Ordos,then to margin of North Qilian Lobby plat form,and finally to North Qilian volcanic rocks basin.Moreover,study on tectonic magmatic event indicated that volcanic rocks developed in Guanzizhen ophoilite have the same feat ures as N-MORB and those developed in Gaojiahe ophoilite are the same as E-MORB.This suggested that they are derived from mid-ocean ridge setting and represented the west extending of Shangdan Paleozoic ocean basin.The east area of North Qilian is preliminarily confirmed to have N-MORB type basalts which can be found in Chisha,BA-type basic volcanic rocks found in Hongtubu and IAB-type acid volcanic rocks found in Chenjiahe.This provided clear clues to the existence of Paleozoic ocean in east section of North Qilian.Detailed data of petrology,geochemistry and isotope dating show both west section of North Qinling and east section of North Qinlian devel oped 450～391Ma and 408～415 Ma granitic magmatic events.The former are related with subduction or collisional events and the later related with collisional events.All above indicated that the same ocean covering North Qinling and North Qilian had undergone the same tectonic evolution.In other words,North Qilian and North Qiling have the same framework and the same dynamical system in Early Paleozoic.Comparative analysis indicated that,Shangdan fault in East Qinling corresponds with Wushan-Tang zangfault; the Erlangping Formation in North Qinling corresponds with Huluhe Formation in North Qilian; and Qinling Formation in North Qinling corresponds with Longshan Formation in North Qilian.