Petrogenesis of the Qinghe post-orogenic pluton and its implications for the Late Paleozoic tectonic evolution of the Altai orogenic belt

Abstract

The Altai Orogenic Belt, situated between Siberia plate and Kazakhstan-Junggar plate, is part of the Central Asian Orogenic Belt. Controversy has long surrounded the timing of the orogeny termination, which hampers us from better understanding the Paleozoic tectonic evolution of the Altai orogenic belt. The Permian granitoids are widely distributed in the southern Chinese Altai and show typical alkaline or A-type affinities, probably reflecting an extensional tectonic setting after collision. This forms the justification of this study in which we focus on the geochronology and geochemistry of the Qinghe pluton in order to evaluate the late-stage tectonic evolution of the Altai orogenic belt. The Qinghe pluton at the southeastern margin of the Altai orogenic belt is composed mainly of monzogranite with minor diorite. New geochronological data reveal Early Permian emplacement ages, including 283 +/- 3Ma for the porphyritic monzogranite, 280 +/- 2Ma for the monzogranite, 286 +/- 2Ma for the mylonitic monzogranite, and 269 +/- 1Ma for the pyroxene diorite porphyrite. The Qinghe monzogranites are characterized by high silicon (SiO2 = 61. 98% similar to 73.35%), alkali (K2O + Na2O = 5.8% similar to 8.72%) and alkalinity ratio (AR = 2.12 similar to 3.65), and low calcium (CaO = 1.2% similar to 3.76%), belonging to high kalium calc-alkaline series with Rittmann Index = 2.38 similar to 2.54 and K2O/Na2O = 0.78 similar to 1.06. They are also depleted in Ba, Sr, P, Ti, Nb and Ta, with negative Eu abnormalies (delta Eu = 0.46 similar to 0.78) and the ratio of 10000 x Ga/Al (2.85 similar to 2.47), characterized by A-type granitoids that were emplaced at the end of the collision orogeny. In addition, there are the positive epsilon(Hf) (t) values (ranging from +4.04 to + 11.78) and two stage model ages (t(DM2)) of 880 similar to 694Ma, 923 similar to 633Ma, 875 similar to 555Ma and 1030 similar to 635Ma for these intrusions, suggesting Neoproterozoic mantle material or juvenile crust as the source material. Together with the previous studies about contemporary enriched alkali (A-type) granite along the southeast margin of the Altai orogenic belt, this study suggests that the Qinghe pluton was generated by remelting of Neoproterozoic juvenile lower crust, probably caused by the underplating of mantle-derived magma with subsequent crystal fractionation. Therefore, the collision orogeny of the Altai orogenic belt must have ended in the Early Permian, followed by a post-collisional extensional event.