Geochemical and Sr-Nd-Pb isotopic characteristics of the granitoids of Xiaoshan Mountain area on the southern margin of North China Block and its geological significance

Systematic and comprehensive petrological and geochemical study investigates the formation of eight northeastern Yanshanian granites in the southern margin of North China Block in Xiaoshan Mountain area, China Petrologically, adamellite is domain. Geochemically,, according to major elements, eight granites are silicate and belong to high K-shoshonite series; all granites have similar trace element patterns (including REE), obvious partition of LREE from HREE (La/Yb)(N) = 9.52 similar to 41.21 (avg. 26.16), and weakly abnormal Eu (delta Eu =0.82 similar to 1.35), and enrich in Rb, Ba, Th, K, Pb, Hf and Y, and deplete in Ta, Nb, Zr, P and Ti. High Sr (392.8 x 10(-6) similar to 916.9 x 10(-6), avg. 678.8 x 10(-6)), low Y (8.12 x 10(-6) similar to 21.34 x 10(-6), avg. 14.86 x 10(-6)) and Yb (0.503 x 10(-6) similar to 1.756 x 10(-6), avg. 1.26 x 10(-6)) characterize a garnet-bearing magma source in a thick lower crust under North China Block. Initial isotopic Sr ratio (I-Sr = 0.70645 similar to 0.71022, avg. 0.70828) and initial epsilon Nd (epsilon(Nd)(t) = similar to 19.7 similar to -3.4, avg. -14.6) disclose a crustal magma source. The proterozoic Nd depleted mantle model ages (t(2DM)) of the granites are concentrated in 1827 similar to 2372Ma, and present high radiogenic Pb and initial Pb ratios (Pb-206/Pb-204 = 17.728 similar to 18.720 (avg. 17.905); Pb-257/Pb-204 = 15.4-44 similar to 15.656 (avg. 15.544); Pb-208/Pb-204 = 37.519 similar to 38.707 (avg. 38.187). All isotopic data suggest that the batholith was problebly formed by partial melting of the South Qinling crystalline basement with the participatioin of Taihua Group, Xiong' er Group and mantle materials. When extension environment substitute compression conditions caused by subduction of Yangtze Block with Qinling micro-block under North China Block, delamination of thick garnet-bearing crust and asthenosphere upwelling provide heat to melt the thick bottom crust, and forward, to form magma. This suggests that the crystalline basement under South Qinlin Orogen through detachment in north direction into the crystalline basement of Xiaoshan Mountain area, and the north of Xiaoshan Mountain area probably is the northern margin of East Qinling Orogen.