Determination and geological implication of the Middle Jurassic post-collisional granitoids in Taerqi area, central Great Xing' an Range.

The Great Xing' an Range is characterized by voluminous Phanerozoic granites, and the studies of the stages, characteristics and genesis of the granitic magmatism play important roles in the exploration of tectonic-magmatic evolution in the Northeast China. In this study, we undertook zircon U-Pb dating, whole rock major elements and trace elements, as well as zircon Hf isotopic analyses of the Taerqi complex in the central Great Xing' an Range, with an aim to determine their formation timing, petrogenesis and tectonic settings. The Taerqi complex is the product of Early Carboniferous, Middle Jurassic and Early Cretaceous magmatisms, and the Middle Jurassic granitoids are the major part of the Taerqi complex, which are composed of granodiorite, monzogranite and syenogranite. Zircons from the dated granitoids are euhedral to subhedral in shape, and are characterized by typical oscillatory growth zoning and high Th/U ratios (0. 35 similar to 2. 02), indicating a magmatic origin. LA-ICP-MS zircon U-Pb data indicates that the Middle Jurassic granitoids were emplaced in 173 similar to 166Ma These granitoid samples have SiO2 contents of 70. 54% similar to 77. 96%, Na2O/K2O ratios of 0. 86 similar to 1. 28, Al1O3 contents of 11.67% similar to 14. 83%, and TiO2 contents of 0.07% similar to 0. 60%, with low MgO contents (0. 02% similar to 0. 74%), and A/CNK values of 0. 9 similar to 1. 1, indicating they belong to high-K calc-alkaline I-type granite, with metaluminous to weakly peraluminous affinities. These granitoid samples are enriched in Rb, Th, U, Zr and light rare earth elements, and depleted in Ba, Nb, Ta, Sr, Ti and heavy rare earth elements, with medium to strong negative Eu anomalies. In addition, zircons from the Middle Jurassic granitoids have initial Hf-176/Hf-177 ratios of 0. 282874 to 0. 282985, and their epsilon(Hf)(t) values range in + 7. 01 similar to +10. 9, with corresponding two-stage model age of 768 similar to 516Ma. The above geochemical characteristics suggest that the primary magma was derived from partial melting of juvenile crustal material accreted during the Neoproterozoic to Phanerozoic with plagioclase and hornblende in the residue, of which, the magma then experienced fractional crystallization of plagioclase and K-feldspar during magmatic evolution. Combined with previous studies, we conclude that the studied Middle Jurassic granitoids were emplaced in a post-collisional extensional setting, which was likely related to the lithospheric delamination caused by the closure of the Mongo-Okhotsk Ocean.