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

• An alkaline complex emplaced along faults in the Tianshan, Southern Altaids
• Representative gabbros and rhyolites formed at ca. 286 Ma.
• Derivation from depleted mantle, but not from an old continental crustal source
• Transtension under oblique subduction setting generating mantle derived magma
• Fractionation of mantle-derived magmas for A-type granitic mechanism

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 ± 2.1 Ma and 286.7 ± 2.1 Ma, 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.

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