Age,geochemistry, Magma Mixing Of Zhongchuan Granitoid Pluton In The Western Qinling

The West Qinling belt is an important part of the Central Orogenic belt, located in the western part of the Qinling belt. The Zhongchuan granitoid pluton, located between the Lixian-Luoba-Suolongkou Fault and Lixian-Taoping Fault in the West Qinling belt, mainly intruded into the Devonian System Sujiaba Group and Xihanshui Group. This pluton consists of mid-coarse-grained porphyritic biotite monzogranites, mid grained porphyritic biotite monzogranites and mid-fine-grained monzogranites, these monzogranites are concentric rings from the outside to the inside. The rock mass develop magmatic enclaves, and also the fine-grained granite dikes.Zircon LA-ICPMS dating for the porphyritic monzogranite from the Zhongchuan pluton yielded age of 221±1Ma(MSWD=0.26), fine-grained monzogranite of 217±1Ma(MSWD=0.107), magmatic enclave bearing phenocryst of 220±1Ma(MSWD=0.107), fine-grained magmatic enclave of 216±1Ma(MSWD=0.26) and fine-grained granite vein of 207±1Ma(MSDW=0.29). These indicat that the rocks of the pluton become younger from its margin to center, the host rocks and the enclaves have the similar ages and granite dikes are youngest.Different rock types of Zhongchuan granitoid pluton are similar in mineral compositions, including feldspar, quartz, biotite, hornblende without garnet, cordierite and other aluminum rich minerals. Fine-grained monzogranites contain small amount of muscovite without amphibole. Enclaves are fine-grained quartz diorite mainly distributed in the margin of the pluton with mineral composition of biotite, hornblende, quartz and feldspar, some magmatic enclaves bearing K-feldspar megacryst.The host rocks are rich in silica, aluminum, alkali with A/CNK=0.99~1.15, indicating that they are respectively metaluminous or weakly peraluminous and high K cal-alkaline series and I type granites. Fine-grained monzogranites is highly fractionated I type granite with A/CNK>1.1 and poor in magnesium and iron with a very small amount of muscovite. The magmatic enclaves are rich in aluminum, alkali, but poor in silica, indicating that they are high-K calc-alkaline or shoshonite and metaluminous series. The host rocks and magmatic enclaves are enriched in large ion lithophile elements(LREE, Rb, Ba, K), and depleted in high-field strength elements(HREE, Zr, Hf, Ta, Nb, P, Ti).Nd isotopic compositions of the host rocks and magmatic enclaves are mainly characterized by εNd(t) values of-7.31~-8.73 and-5.32~-5.69, TDM2 of 1.59~1.71 Ga and 1.43~1.46 Ga, respectively. Zircon Hf isotopic compositions of the host rocks and magmatic enclaves are mainly characterized by εHf(t) values of-7.02~-0.31 and-3.0~0, TDMC of 1.27~1.70 Ga and 1.2~1.5Ga, respectively. These characteristics suggest that the host rocks are from partial melting of ancient crustal material and magmatic enclaves from the lithospheric mantle. Both of them have mixed features.The gold deposits located neighbouring the Zhongchuan pluton(171~210Ma) are temporal and spatial with this pluton. Mineralization data indicate that ore-forming material are similar to that of the rocks. These suggest that the granitoid pluton and these gold deposits may have homology and inheritance evolution characteristics.