Zircon Dating And Geochemistry Of Clastic Sedimentary Rocks In The Gonghe-Huashixia Area, Qinghai Province And Their Geological Implications

The Gonghe-Huashixia area is a key node of conjunction and transformation of the Central Orogenic Belt. It connects with Qaidam block and eastern Kunlun orogenic belt by Wahongshan fault in the west, with western Qinling by Duomaohe fault in the east, with Baryan Har-Songpan orogenic belt by Mianlue-A'nyemaqen suture in the south ,and with Qilian orogenic belt by Nanshan fault in Qinghai lake in the north.The clastic sedimentary rocks in this area are classified as graywacke and arkose, and their material source should be a mixture of mafic or its partial melting product, metamorphosed basement and/or its anatexis magma, as well as granites.The total REE contents of the rocks,the negative Eu anomaly and the fractional degree of HREE and LREE decrease from Proterozoic to Late Paleozoic,which may imply accretion of provenance crust in Late Paleozoic; the total REE contents of the rocks and the fractional degree of HREE and LREE increase and the negative Eu anomaly become more evident, which may imply recycled crustal substance increasing in the process of anatexis of the provenance.Nd model ages of the clastic sedimentary rocks in Gonghe-Huashixia area range between 1.1-1.3Ga and 1.4-2.0Ga, peak at 1.2Ga and 1.7 Ga. The histogram of Nd model ages is similar to that of Qaidam block , eastern Kunlun orogenic belt and Qilian orogenic belt,but it is obvious different from that of western Qinling, which implys the source materials of the clastic sedimentary rocks in Gonghe-Huashixia area are consist of eastern Kunlun orogenic belt , Qilian orogenic belt and Qaidam block, and western Qinling is impossible as a main source.The U-Pb age distribution feature of the detrital zircon implys the Middle Carboniferous to Permian clastic sedimentary rocks of 750-1000Ma in this area are consist of substances from crustal accretion of Qilian orogenic belt and north margin of the Qaidam basin in Neoproterozoic, and the rocks of >1000Ma are consist of basement rocks of Qilian orogenic bel, Qaidam block and western Qinling. The U-Pb age distribution feature of Permian detrital zircon which shows a significant peak of 250-300Ma is obvious different from that of Middle Carboniferous to Permian.There was no or almost no 250-300Ma ages reported in Qilian orogenic belt and Qaidam block, yet the 200-300 granites are widely spread in eastern Kunlun orogenic belt. It illuminates that the 200-300Ma sediments are mainly from eastern Kunlun orogenic belt which is a main resource of the clastic sedimentary rocks in Gonghe-Huashixia area together with Qilian orogenic belt and Qaidam block. The U-Pb age distribution feature of Triassic is similar to that of Permian, but the peak of 250-300Ma becomes more evident. It illuminates that provenance of the Triassic clastic sedimentary rocks is the same with Permian, but the eastern Kunlun orogenic belt contributes more.Hf isotopic compositions of zircons show four dominant age ranges of 350Ma,500-1000Ma,1.7-2.5Ga and 3.8-4.1Ga for accretion of provenance crust...