Zircon U-Pb age and petrogenetic discussion on Jianshetun adakite in Balinyouqi, Inner Mongolia

In this paper petrology, geochemistry, zircon U-Pb dating and in-situ Hf isotopic analyzing for Jianshetun granodiorite in SE Inner Mongolia are reported. LA-ICP-MS zircon dating gives the emplacement age of Early Triassic (248.5 +/- 2.3Ma). for the pluton. Petrological and geochemical study indicate that the aluminum saturation index (A/CNK) of the rock is 1.02, belonging to calk-alkaline I-type granite. The rock has high SiO2 (>= 56%) and Al2O3 (>= 15%) contents, and the Na2O and K2O contents range between 4.34% similar to 4.41% and 2.03% similar to 2.07% respectively, with Na2O/K2O ratio varying from 2.10 to 2.17; the rock has high content of Sr (>400 x 10(-6)) low content of Y (<= 18 x 10(-6)) and Yb (<= 1.9 x 10(-6)), and has notable fractionation between LREE and HREE, with (La/Yb)(N) varying between 16.92 +/- 17.38. Besides, there is not negative Eu anomaly in the chondrite-normalized REE pattern. All of the geochemical characteristics above indicate that the Jianshetun pluton belongs to adakite. The epsilon(Hf) (t) of in-situ zircon Hf isotopic analysis is between 11.2 similar to 14.4, showing the magma originated form depleted source. Considering regional magmatism, stratigraphy and palaeontology data, it is thought that the pluton is the product of partial melting of the thickened newborn lower crust after the Late Permian closure of the Paleo-Asian Ocean along Xar Moron River suture belt.