Geochronology, geochemistry and petrogenesis of Early Permian alkaline magmatism in the Eastern Tianshan: Implications for tectonics of the Southern Altaids

Abstract

In the early Permian large volumes of volcanic rocks developed in the Eastern Tianshan of the southern Altaids. The Shaerhu alkaline complex, which occurs along a NW-trending transcurrent fault in the Dananhu arc, is composed of alkaline gabbro intrusions, granites and rhyolites; the gabbros and rhyolites have similar zircon crystallization ages of 286.5±2.1Ma and 286.7±2.1Ma, respectively. The granitic and rhyolitic rocks have typical A-type granite geochemical signatures, i.e. high oxide ratios (in wt.%): K2O+Na2O, (K2O+Na2O)/CaO, K2O/MgO, and SiO2, high trace element values: Zr, Nb, Ga, Ce, Y, and REE, and high Zr+Ce+Y, and 10,000∗Ga/Al ratios. However, spidergrams and REE patterns indicate major depletions in Ba, Sr, P, Ti and Eu. The presence of positive εNd(t) values of +7.0 to +11.2 and low (87Sr/86Sr)i (0.70148–0.70416) indicates that these alkaline rocks were derived from a depleted mantle, and not from old continental crust. The geochemical characters indicate that the rocks of the complex have the same source and that fractionation was important in their generation. A-type granitic rocks are the most highly fractionated of alkaline basic rocks. In summary, the Shaerhu complex is a product of mantle-derived alkaline magma fractionation, which is one of the main mechanisms of A-type granite genesis. The felsic rocks of the Shaerhu complex have the geochemical signature of an A2-type granite, and thus were not derived from a rift or mantle plume. In the early Permian, oblique subduction in the southern Altaids gave rise to strike-slip extensional faults, which controlled the emplacement of large volumes of mantle-derived melts.