Depositional age and sedimentary provenance of the Luchuan group in the southern margin of the North China Craton and its significance for regional tectonic evolution: Constraints from zircon U-Pb geochronology and Hf isotope
Abstract:There is a long-standing controversy about depositional age and tectonic significance of Luanchuan Group, which locates joint part of two tectonic units of Qinling orogenic belt and North China Craton (NCC). In this paper, the provenance and regional comparative analysis of Luanchuan Group from the perspective of zircon U-Pb chronology and Hf isotope could provide important evidence of late Precambrian tectonic evolution in the southern margin of NCC. The crystallization age of epi-metamorphic trachyte from Dahongkou Formation in the upper part of the Luanchuan Group is 854 ± 8 Ma, and its genesis is mainly due to the recycling of Late Paleoproterozoic continental crust. The depositional age of Luanchuan Group is limited in 917 to 844 Ma by the youngest clastic zircon U-Pb age of metaclastic rocks of Luanchuan Group in Luanchuan area and crystallization age of epi-metamorphic trachyte of Dahongkou Formation and related intrusive rocks, which belongs to Neoproterozoic Qingbaikou Period. There are similar U-Pb age and two-stage Hf model age (TDM2) of detrital zircons among the clastic rocks from Luanchuan Group in Luanchuan area, which consist of upper and middle parts of Baishugou Formation to Yuku Formation, the former are mostly between 1.85 and 1.10 Ga, whose most probable provenance is North Qinling terrane (NQT), and the latter are between 1.33 and 2.92 Ga, which indicates that the provenance area has undergone 4 times of Proterozoic crustal accretion of ~1.45 Ga, ~1.8 Ga, ~2.1 Ga and ~2.5 Ga. The conversion of the provenance of Luanchuan Group from NCC to NQT and its sedimentary tectonic setting from passive continental margin to collisional orogeny and post-collisional extension environment during sedimentation of baishugou Formation are suggested by marked difference of detrital zircon age spectrum between lower parts of Baishugou Formation and other strata of Luchuan Group. NQT would belong to a part of NCC, and a Meso-Neoproterozoic tectonic belt similar to NQT developed at least in eastern and northern margin of NCC. The tectonic belt was strongly involved in process of Columbia supercontinent breakup and Rodinia supercontinent convergence, and was briefly combined with NCC during early Neoproterozoic era to transform into the main provenance supply area of peripheral sedimentary basin of NCC.