Petrogenesis of Triassic granites in Xiaohongshan area, Beishan orogenic belt: Constraints from zircon U-Pb ages and Hf isotopes

In this paper, zircon U-Pb ages, Hf isotopes and whole rock geochemical compositions of Triassic granite-porphyry in Xiaohongshan area of Beishan orogenic belt were systematically studied.Zircon LA-MC-ICP-MS U-Pb dating results show that the two ages of granite-porphyry are 211.8±1.6 Ma(the confidence value is 95%, n=24, MSDW=2.4)and 205.9±1.7 Ma(the confidence value is 95%, n=23, MSDW=2.9)respectively, which intruded at the late stage of Late Triassic.The granite-porphyry in the study area is characterized by high silicon, rich alkali, quasi-aluminum, and poor calcium, magnesium, and iron.It belongs to the series of high potassium calc-alkaline to potassic basanite with high degree of differentiation.It belongs to highly differentiated Ⅰ-type granite.It is enriched in Rb, Th, U, La, Ce and other large ion lithophile elements and depleted in Nb, P, Ti, Ba and Sr. It is characterized by low Sr, high Yb and Y with obvious negative europium anomaly.The ε_Hf(t)values range from -1.43 to 9.93, which are relatively high, and the Hf isotopic crustal model ages range from 610 Ma to 1335 Ma, indicating that the granite-porphyry was derived from the juvenile crust with mantle imprint mixed with remelting old crust. According to the data obtained in this paper and other regional geological data acquired recently, it is proposed that, under the post-orogenic extension system, the heat brought by the underplating of basic magma caused the partial melting of the juvenile crust from Neoproterozoic to Paleozoic, and suffered magma contamination of the remelting of the old crust of Lower Proterozoic.In addition, the magma was finally emplaced in the middle and upper crust after fractional crystallization, forming the the Late Triassic granite-porphyry in the study area.