DEPOSITION-FILLING PROCESS AND PROVENANCE OF LUSHI BASIN,EAST QINLING MOUNTAINS(CENTRAL CHINA)DURING THE MIDDLE EOCENE AND THE FORCING MECHANISM
The thick Cenozoic deposits in the Qinling intermountain basins are of great significance for understanding geomorphic evolution of the Qinling Mountains(Center China)and palaeoenvironment changes of East Asia.However,the deposition-filling process and the geomorphic and climatic evolution of these Meso-Cenozoic intermountain basins have not been fully understood.In view of the above problems,this paper analyzes sedimentology and provenance of a Middle Eocene fluvial-lacustrine deposit sequence(34°04′03″N,111°09′ 17″E),which is located at Lushi Basin,East Qinling Mountains,Central China.The Middle Eocene strata of Lushi Basin can be divided into Zhangjiacun Formation,Lushi Formation and Dayu Formation.Zhangjiacun Formation is alluvial fan-fluvial facies,Lushi Formation is shallow to middle-deep lake deposit,and Dayu Formation is fluvial-alluvial fan sediments.We measure U-Pb ages of detrital zircons from 27 layers of the Lushi Basin depositional sequence using LA-ICP-MS(Laser Ablation Inductively Coupled Plasma Mass Spectrometry).Zircon U-Pb ages of Zhangjiacun Formation and Lushi Formation are characterized by 200～250 Ma(T-J)and 400～500 Ma(D-S-O-(€))as their main age peaks,700～1000 Ma(Pt3)as the secondary peak.Some samples have 1800 Ma(Pt1)and 2500 Ma(Ar3)age peaks.By comparing with ages of rocks of the potential sources,we suggest that sediments of Zhangjiacun and Lushi formations are mainly derived from the relatively distant North Qinling Orogenic Belt and South Qinling Orogenic Belt.Zircon U-Pb ages of lower Dayu Formation are characterized by 1000～1100 Ma(Pt2),1800 Ma(Pt1)and 2300 Ma(Pt1)as their characteristic peaks.The provenance changed to the relatively proximal North Qinling Orogenic Belt and South-North China Craton in lower Dayu Formation.The contribution of proximal source increased at first and then decreased,which may be related to river headward erosion by regional tectonic uplift in the basin filling during the Middle Eocene.Sedimentary environment of upper Dayu Formation changed into alluvial fan.The zircon proportion of 130Ma rapidly increases,which may be related to uplift and exhumation of Late Mesozoic granite in the southeast Luanchuan area,close to the Lushi Basin.The Middle Eocene fluvial-lacustrine deposit sequence in Lushi Basin reveals the process of formation to vanishing of the Lushi palaeo-lake and history of surface erosion,transport and deposition in this mountain-basin system,and it records two regional uplift events of East Qinling Mountains.
万方数据
维普
