Early Paleozoic high-Mg adakite froin Yindongliang in the eastern section of the North Qilian: Implications for geodynamics and Cu-Au mineralization

Early Paleozoic tectono-magmatic-mineralization in the eastern section of the North Qilian Mountains was important responses to the plate tectonic evolution of the North Qilian. From west to east, the granitoids of Mijiashan, Bianqianggou, Huangjiawashan, Quwushan ( Yindongliang rock bodies) and Nanhuashan in the Alax paleo-continental margin to the north of the early Paleozoic Laohushan back-are basin, stretching in NW-NWW and intruding into originally called as the Yinggou Group and Precambrian metamorphic rocks, are an important tectono-magmatic belt. Our study results show the geochemical characteristics of the Yindongliang granodiorites from Quwushan are very consistent with those of the typical circum-Pacific adakites. They have high Al2O3 ( > 15%, Na2O ( > 4.0% (4.51% similar to 4.92%), Na2O/K2O = 1.63 similar to 2.40, and SiO2 > 56%), but low K2O contents ( 1.96 similar to 2.77%) belonging to calc-alkaline series; and high Sr (599 similar to 691 x 10(-6)) low Y (6.3 similar to 8.1 x 10(-6))and Yb (Yb 40(81 similar to 94), Y/Yb > 10, La/Yb > 20 (22 similar to 45), and higher Mg-# (58 similar to 64), Ni (31 similar to 38 x 10(-6)) and Cr(35 similar to 43 x 10(-6)) contents compared with the typical adakites, so called as high-Mg adakite. LREE in the samples are obviously enriched and HREE and Y depleted in the Chondrite-normalized REE patterns with slightly negative Eu anomalies ( Eu/Eu* similar to 0.94), ( La/Yb)(N) > 10( 16 similar to 32). The primitive-mantle-normalized trace element spider diagrams for the rocks show enrichment in LILEs( Rb, Ba,Sr and Th), negative Nb, Ta anomalies and remarkably positive Sr anomalies, different from island arc calc-alkaline rocks. That means that the high-Mg adakite found in Quwushan was derived from the partial melting of the young subducted oceanic crust of the Laohushan back-arc basin, northwards subducting beneath the Alax paleo-continental margin in Early Paleozoic. Meanwhile, Cu-Au mineralization in Yindongliang ore deposits had very close relationship with the high-Mg adakitic magmatism, indicating the Cu-Au ore deposits related to adakites can be new ore-prospecting strategy in the eastern section of the North Qilian Mountains.