Geochemistry, Geochronology And Its Tectonic Implications For The Kuanping Complex In The West Part Of The North Qinling Orogen

The North Qinling Orogen (NQO) is mainly composed of four lithostratigraphic units including the Kuanping Group, Erlangping Group, Qinling Group and Danfeng Group. Accurate chronological and tectonic constraints for each unit have great implications for exploring and reconstructing the formation and evolution process of the NQO. Based on extensive literature reading and field study, we selected the Kuanping Group from the Tianshui-Baoji area, Gansu Province in the west part of the NQO as the research object, and conduct systematic field geological investigation, petrology, geochemistry and in situ zircon U-Pb dating on the low-grade metasedimentary, meta-mafic rock and gneissic granite from the Dongcha, Guchuangou, Xieyuguan and Dazhengou profiles. According to the studies above, several conclusions are given as follows:1. The Kuanping complex in the west part of the NQO consists of low-grade metased imentary, meta-mafic rock and gneissic granite. The clastic rock is dominated by high maturity feldspar-quartz sandstone which formed in the passive continental margin. The protolith of the meta-mafic rock should be intermediate-basic igneous rock which formed within the continental rift environment. The Dongcha granite shows characteristics of A-type granite and the Guchuangou granite is peraluminous S-type granite.2. The zircon chronology studies show that the low-grade metasedimentary of the Kuanp ing Group has a formation age less than 530Ma, the meta-mafic rock formed in 1771 ± 15Ma, and the Dongcha and Guchuangou granite yield zircon U-Pb ages of 1775±14Ma and 2197 ±19Ma, respectively. The crystallizing ages for the protolith of the mate-mafic rock and granitic rock are much older than the sedimentary age of the Kuanping Group clastic rock, suggesting that they do not share a common lithostratigraphic unit, and the ancient igneous rocks which could be an orogenic involvement from the southern margin of the North China Block (NCB) should be eliminated from the Kuanping Group.3. Detrital zircon of the clasitic rock from the Kuanping Group yield peak ages of ～600-800 Ma、～950-1100Ma、～2500Ma and ～3400Ma. The ages of ～2500Ma and ～3400Ma are statistically indistinguishable with age spectrum of the NCB. The age of ～950-1100Ma is identical with the magmatic event in the NQO. The age of ～600-800 Ma is identical with the South Qinling/Yangtze Block. These above indicate that the clastics of the Kuanping Group contemporaneously derived from the NCB, NQO and northern margin of the Yangtze Block, in addition, the Kuanping Group clasitic rock should deposit in a rift basin between the NCB and NQO.