Shoshonitic- and adakitic magmatism of the Early Paleozoic age in the Western Kunlun orogenic belt, NW China: Implications for the early evolution of the northwestern Tibetan plateau

The Western Kunlun orogenic belt in the northwestern margin of the Tibetan plateau contains two magmatic belts; early Paleozoic belt in the northern part of Western Kunlun Terrane (WKT), and early Mesozoic belt in the southern part of WIC. Both formed from northward subduction of the Paleo-Tethys. The early Paleozoic belt contains large Datong and Qiukesu igneous complexes and many smaller plutons. The Datong complex is mainly composed of dark-colored porphyritic syenite and monzonite with minor light-colored dykes of granite and monzonite. The dark-colored rocks are characterized by moderate SiO2 (58.2-69.3 wt.%), and high Al2O3 (153-17.1 wt.%), total alkali (Na2O + K2O = 8.07-10.2 wt.%) and ratios of K2O/Na2O (0.77-1.83). They plot in shoshonite field, and show high abundances of LILE including LREE ((La/Yb)(n) = 15.4-26.2; mean 20.2) with pronounced negative anomalies of Nb-Ta-P-Ti in normalized trace elemental patterns and weak negative anomalies of Eu (delta(Eu) = 2Eu(n)/(Sm-n + Gd-n) = 0.68-0.80). The light-colored rocks contain slightly higher concentrations of SiO2 (60.3-72.0 wt.%), similar Al2O3 (14.7-17.6 wt.%), and slightly lower total alkalis (6.57-9.14 wt.%) than dark-colored rocks. They show adakitic geochemical signatures with low Y (5.80-17.2 ppm) and Yb (0.63-1.59 ppm), and high Sr/Y (>40). U-Pb zircon dating indicates that shoshonitic rocks and adakitic dykes formed at similar to 444 Ma to similar to 443 Ma, and a separate small adakitic plug at similar to 462 Ma. The mean epsilon(Hf)(t) values of zircon range from -1.6 to -0.94 (n = 14) with T-DM2 of similar to 1.5 Ga for shoshonitic rocks and SHS(t) values from -1.8 to + 0.72 (n = 12) with T-DM2 of 1.4 to 1.5 Ga for adakitic rocks. Shoshonitic rocks show initial Sr-87/Sr-86 and epsilon(Nd)(t) of 0.7092-0.7100 and -3.9 to -3.2, respectively, and adakitic rocks yield initial Sr-87/Sr-86 and epsilon(Nd)(t) of 0.7099-0.7134 and -3.6 to -3.1, respectively. Similar Sr, Nd, and Hf isotope compositions for the shoshonitic and adakitic rocks suggest similar ancient rocks as their sources. Combined with the geological development of the area, we propose that a local extension in the WKT over the subducting Paleo-Tethys has induced the upwelling of asthenospheric mantle and partial melting in the lower crust during mid-Ordovician to early Silurian. The primary shoshonitic melt evolved into the parental magmas for the shoshonitic rocks and adakitic rocks through fractional crystallization of hornblende. (C) 2017 Elsevier B.V. All rights reserved.