Geochronological, geochemical, and Sr-Nd-Hf isotopic studies of the Aketas adakitic granites in Eastern Junggar: Petrogenesis and tectonic implications

The Aketas adakitic granites are located in the Dulate Arc, in the northern part of the eastern Junggar region, Xinjiang (NW China). In this study, we determined the zircon U-Pb isotopic compositions, bulk-rock Sr-Nd-Hf isotopic compositions, and major and trace elements of Aketas granite samples. These samples show a narrow range in SiO2 content (65.05-67.19wt.%) as well as high Al2O3 (14.95-18.09wt.%; A/CNK=0.96-1.08), Na2O+K2O (7.94-8.42wt.%), and CaO (2.05-3.48wt.%) content. Similar to modern adakites, the samples have high Na2O (4.55-5.61wt.%) and Sr (554.00-915.00ppm) content, high Sr/Y ratios (85.10-127.38), and low Y (6.51-7.12ppm) and Yb (0.42-0.79ppm) content. The low initial Sr-87/Sr-86 ratios (0.70298-0.70350) and positive epsilon Nd(t) (+5.1 to +7.0) and epsilon Hf(t) values (+10.4 to +14.9) suggest that the Aketas adakitic granites were derived from the partial melting of lower crustal material in a subduction setting. This is further evidenced by high Mg# values (42.00-65.67), high SiO2 and K2O content, and low Cr (<17.30ppm), Co (<9.62ppm), and Ni (<12.60ppm) content. Bidirectional subduction occurred between eastern Junggar and the Altai Orogen during the Late Carboniferous, and in combination with other exposed Palaeozoic granites in eastern Junggar, the Aketas adakitic granites may represent the latest record of this subduction event. The emplacement of the Aketas adakitic granites implies that eastern Junggar was involved in southward subduction at 308 +/- 2.3Ma; therefore, the final collision between the Siberian and Kazakhstan blocks likely occurred between 310.3 and 307Ma. Copyright (c) 2017 John Wiley & Sons, Ltd.