U-Pb ages and Hf isotopes of detrital zircons from siltstone, Bianmagou, North Qilian Mountains
Wang Yueming1, Qian Qing2,3, Zhang Jianxin4
1. Gansu Geology-Mineral Science & Technology Information Center, Lanzhou 730000;
2. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029;
3. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101;
4. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037
Multiple rock units, including the Neo-Proterozoic basement, Early Paleozoic ophiolites, arc granites and volcanics, and Late Paleozoic sediments, are preserved in the Bianmagou-Dachadaban area of Zoulang Nanshan, North Qilian, providing important records of the Neo-Proterozoic to Late Paleozoic geological evolution of the orogen. In Bianmagou a suite of gray-green turbiditic rocks composed of silicious mudstone, siltstone, greywacke, and conglomerate are covered unconformably on the Chaidanuo granite batholith, while the age, sedimentary sources and tectonic significance of the tuibidites are still unclear. Most detrital zircons from the siltstones are magmatic and lack of abrading to roundness by transport, and were formed during the time periods of 529~484 Ma and 466~442 Ma, respectively. The youngest zircon grain was formed at 425 Ma or in Meso-Proterozoic. It is likely that the detrital zircons were derived mainly from the Chaidanuo granitoids(516~505 Ma), subordinately from the Ordovician-Silurian intermediate-felsic magmatic rocks, and rarely from the Paleozoic ophiolites and Meso-Proterozoic basement rocks. Based on rock association of the turbidites, age distribution of detrital zircons from the siltstone, as well as evidences from former studies, it is proposed that the Bianmagou turbidites were mostly likely formed in a relic forearc basin of Silurian.