The Paleozoic-Mesozoic magmatic evolution of the Eastern Tianshan, NW China: Constraints from geochronology and geochemistry of the Sanchakou intrusive complex

The Sanchakou intrusive complex (containing quartz diorite, porphyritic granodiorite, granodiorite, fine-grained granodiorite, diorite porphyry and hornblende gabbro) is located at the eastern part of the Dananhu-Tousuquan island arc belt, Eastern Tianshan. Zircon U-Pb ages suggest that the above six units were emplaced at 446–238 Ma. The Sanchakou intrusive complex therefore provides a unique opportunity to gain a better understanding of the Paleozoic-Mesozoic magmatism and its evolution in the Eastern Tianshan.

The granitoids form a subduction related calc-alkaline to low K (tholeiitic) magmatic series, characterized by enrichment in LILE and depletion in HFSE. They have low initial ⁸⁷Sr/⁸⁶Sr, high ε_Nd(t) and ε_Hf(t) values. As well as the above geochemical features, the quartz diorite, porphyritic granodiorite and fine-grained granodiorite have low Nb/U and Ta/U values, relatively high Mg^#, Ti/Zr and Ti/Y values, indicating a potential role of the subducted slab in their petrogenesis. The granodiorite units have low Nb/U, Ta/U and Mg^# values, and high (La/Yb)_N values, suggesting an origin from the melting of juvenile lower crust. The Sanchakou diorite porphyry has positive ε_Nd(t) and ε_Hf(t), low SiO₂ and (⁸⁷Sr/⁸⁶Sr)_i, and high MgO, Mg^#, Ti/Y and Ti/Zr values, indicating it was likely sourced from depleted mantle. The Mesozoic hornblende gabbro has low SiO₂, relatively high TiO₂, MgO, Al₂O₃ and (La/Yb)_N, and minor depletion in Nb-Ta, implying an E-MORB-like mantle source.

Integrating available tectonic, stratigraphic and geochemical data, we propose that the Sanchakou quartz diorite, porphyritic granodiorite, granodiorite and fine-grained granodiorite were formed in a double-side subduction system. The younger Permian diorite porphyry was produced in a post-collisional extensional setting and the Triassic hornblende gabbro was likely formed in an intraplate environment.