关键词: 西秦岭, 印支期, 花岗岩, 俯冲沉积物, 古特提斯洋

Abstract: This paper reports systematically petrological and SHRIMP zircon U-Pb dating, whole-rock geochemical and Sr-Nd-Hf isotopic research for the Fengxian Pluton. We discussed the magmatic evolution, source characteristics, tectonic and geodynamic settings of these plutons. Our study shows that: 1. The Fengxian pluton is composed of granodiorite and quartz monzonite, invaded in the Middle Triassic (~237.5 Ma). The SiO₂ content of the rock is 61.7~66.7 wt %, belonging to the high potassium calc-alkaline series, with relatively high MgO content (1.49-2.43 wt%) and Mg# value (0.53-0.58), belonging to the high-K calc-alkaline series. They are enriched in light rare earth elements (LREE) and lack marked negative Eu anomalies. They are also enriched in LILE and depleted in HFSE. Their high Sr (474×10-6-629×10-6) low Y (6.85×10-6-7.60×10-6) and Yb (0.57×10-6- 0.72×10-6) value are similar to those of adakites. They show εNd(t) values of -4.2 to -5.8, initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7072-0.7077 and ε_Hf(t) values of -1.1 to -4.3. We believe that the Fengxian pluton is mainly derived from subducted sediments in the continental margin environment of the northward subduction of the Paleo-Tethys oceanic slab. The origin of the Fengxian pluton suggests that the Animaqing-Mianlue oceanic crust had subducted under the West Qinling before 237.5 Ma, and the eastern segment of the West Qinling remains a continental arc till the Middle Triassic.

Key words: West Qinling, Indosinian, Granitiods, subducted sediments, Paleo-Tethys