The Tianhu granite pluton consists of six intrusive rocks, which can be divided into three lithofacies units but belong to one superunit. The roundshaped plutonic body was the product of three intrusive actitites of the same magma. Zircon LAICPMS UPb dating result shows that the Tianhu pluton intruded in early Triassic between 241Ma and 243Ma and the product of early Indosinian period. Zircon insitu Hf isotopic composition analysis yields the εHf(t) and tDM2 values of -2.6～5.5 and twostage mode age of 941～1435Ma for the Tianhu pluton, suggesting that the magma originated from depleted mantlederived juvenile crustal rock. Geochemically, the pluton is characterized by the high content of SiO2（69.14%～76.62%）, and high K2O+Na2O value（7.39%～8.83%), low contents of Ti, Ca, Fe and Mg, with the ratios of （K2O+Na2O）/Al2O3 varying between 0.69 and 0.82, and the ratios of A/CNK between 0.95 and 1.04. All these features indicate that the pluton belongs to calcalkaline granite. The pluton is also rich in LILE（K, Rb）, HFSE（Ba, Th）and depleted in Ta, Nb, Ce, Hf, Zr, Sm, Y and Yb. Chondritenormalized REE distribution pattern of the Tianhu pluton displays right skewed shapes, distinct fractionation between LREE and HREE, with slightly negative Eu anomalies （δEu=0.24～0.78）. Petrological and geochemical studies of the Tianhu pluton indicate that the pluton resulted from mantle magma underpalting and intraplating induced partial melting of the Mesoneoproterozoic newborn lower crust. Magma intrusion was driven by northward subduction of the Tethys Ocean from late Hercynian to early Indosinian and subsequently intense intracontinental extension in the eastern Tianshan Mountains.