SHRIMP zircon U-Pb dating and geochemical characteristics of Shihama granite pluton in northeastern Hunan province

Distributed in northeastern Hunan province and intruded in Neoproterozoic strata, Shihama granite pluton is mainly composed of micro-fine-grained porphyritic biotite granodiorites of early stage and fine-grained porphyritic biotite monzonitic granites of late stage.SHRIMP zircon dating of the granites gives an age of 157±2 Ma which belongs to Late Jurassic.The granites belong to magnesian,metaluminous-weakly peraluminous,high-K calc-alkaline-shoshonite series,with SiO2=68.26% ～68.53% and K2O/Na2O=1.37～1.59.The rocks show enriched large-ion lithophile elements,depleted high field-strength elements,low values of Rb/Sr(0.40～0.56),high values of ΣREE(171.48～183.81),with δEu=0.86～0.93,(La/Yb)N=27.11～45.87,eNd=-5.11,T2DM=1.63 Ga. Studies indicate that the Shihama granites belong to high-K calc-alkaline granites(KCG) originated form the crust-mantled mixed source ivolving large amount of mantle source materials.It is believed that the granite pluton is the result of tectonic extension during the late period of orogenetic movement and is formed in the geodynamic background related to the transition of tectonic regime.