Granitoid magmatism and tectonic evolution in the eastern part of the West Qinling: Constraints from geochemistry, zircon U-Pb chronology and Hf isotopic

In the eastern part of the West Qinling Orogenic Belt, a large number of Indosinian granites are developed, which run through the entire West Qinling from north to south. In this paper, a detailed study of the petrology, geochemistry, zircon U-Pb chronology and Lu-Hf isotopic composition were made for the Chaijiazhuang, Taibai, Zhoujiashan, and Miba plutons in the eastern part of the West Qinling. The lithology of these plutons is quartz diorite, granodiorite and monzogranite. Geochemical data suggest that these granitic rocks are belonging to high-K calc-alkalic series. Zircon LA-ICP-MS data indicate that these plutons formed from 237Ma to 219Ma, and shows two stages of magmatic activity. The Chaijiazhuang monzogranite has the geochemical characteristics of adakite, such as high Sr (578 x 10(-6) similar to 661 x 10(-6)) (La/Yb)(N) ratios (37.44 similar to 41.73), and low Y (3.48 x 10(-6) similar to 5.50 x 10(-6)) and Yb (0.3 x 10(-6) similar to 0.5 x 10(-6)), and weakly negative Eu anomalies (delta Eu = 0. 85 similar to 1.00). The Miba and Zhoujiashan plutons also show the characteristics of high Sr (Sr > 400 x 10(-6)) and low Yb (Yb < 2 x 10(-6)), but Taibai monzogranite is similar to Himalaya -type granite (Sr < 400 x 10(-6), Yb < 2 x 10(-6)). The zircon in situ Lu-Hf isotopic analysis shows that the epsilon(Hf)(t) values of the Chaijiazhuang, Taibai, Zhoujiashan, and Miba plutons are -5.1 similar to 2.2, -4.8 similar to -0.9, -6.0 similar to -2.7, and -6.3 similar to -3.9, respectively, and the corresponding two -stage model ages (t(DM2)) are 2153 similar to 1495Ma, 2120 similar to 1767Ma, 2227 similar to 1932Ma, and 2249 similar to 2033Ma. The Hf isotopic compositions of zircons suggest that the granitoids in the eastern part of the West Qinling were mainly derived from Paleoproterozoic to Mesoproterozoic crustal materials, and a little of juvenile crust material might be added in the source area of Chaijiazhuang pluton. At the same, the composition of crystalline basement tends to age gradually from north to south. The granitoids that show the characteristics of adakite may be formed from partial melting of the thickened lower crust, while the Himalaya -type granitoids are the products of partial melting of the normal lower crust. The continental collision in the Triassic period resulted in the thickening of the crust, and the partial melting of the thickened lower crust at different depths resulted in the formation of the granitoids with adakite or Himalaya -type geochemical characteristics in this region.