Geochemical Features And Geological Significance Of Biotite Monzonitic Granite Of Sanshilipu In Qinling Orogenic Belt

Qinling orogenic belt experienced a complex tectonic evolution, accompanied by a lot of activities of granitic magma. Especially the Mesozoic granites outcropped in the wild of south Qinling. According to the studies of granite, the geological evolution mechanism of the south Qinling will be recognized. The granite of Sanshiiliipu outcropped from about 10 square kilomerters of Shangdan tectonic belt. Based on the filed survey result of geology, mineralogy, petrology, geochemistry, high-precision LA-ICP-MS zircon U-Pb geochronology, this paper studies about magmatic source, the formation age of granite, tectonic evolution and geodynamic setting. With the main achievements as followed:The granite of sanshilipu with biotite monzonite granite for porphyritic structure, it has massive structure, mainly composed of plagioclase, K-feldspar, quartz, biotite, amphibole,etc. Apatite, epidote, magnetite, zircon can been found in accessory minerals.Geochemical studies of rock show that ω(SiO2) = 61.44%~68.49%,ω(Na2O) = 3.56% ~4.58%, A/CNK=0.85~1.19, δ=2.28~3.23, which belongs to high-Kcalc-alkaline series. The granite of Sanshilipu is I-type granite in the SiO2-K2 O diagram. Chondrite-normalized enrichment of LREE flat HREE with slightly negative Eu anomalies. The trace element shows obvious enrichment in LILE(Th, Sr, Pb) and relative depletion in HFSE(Nb, Ta, Ti).The granite of Sanshilipu contains of a lot of zircon. CL image shows that zircon has obvious magmatic volatile band and narrower band width. Zircon chondrite-normalized rare earth element, which is leftist with high HREE. Th/U is greater than 0.4, indicating that they are magmatic zircon. By LA-ICP-MS zircon U-Pb dating and getting the weighted average of 205±0.96 Ma, representing the age of magmatic crystallization, which shows that the granite rock mass formed in the late Triassic.The rock of Shahewan is close to the rock of Sanshilipu. The previous research shows that a crust-mantle mixed with melting characteristics about the rock of Shahewan. As for the rock of Sanshilipu, Mg# between 38.9 and 61.3, most of the samples ranged between 45 and 60. It indates the occurrence of crust and mantle miscible. In addition, The similar Nb/Ta(8.34~8.75) between the granite of sanshilipu and lower crust(8.3), indates that the source material is mainly for the lower crust. Previous studies have shown that a collision has occurred between the Yangtze plate and the Southern Qinling Mountains, making the crust thickening and delamination occurs. A large number of granite was produced in this period. The granite of Sanshilipu mainly originated from the lower crust and contained of a lot of mantle material. So the rock formation is the result of delamination in the late Triassic.Based on comprehensive research work, it shows that the rock of sanshilipu belongs to I-type granite. It was formed in the Late Triassic(about 205Ma). After the Mianlue ocean closed, according to Yangtze Plate happened to get larger scale unterthrusts northwards, making the crust thickening in the South Qinling Mountains area, and forming the I-type granite. It can be seen that the granite of Sanshilipu was formed in the late Triassic post-orogenic extensional encironment.