Petrogenesis Of Late Triassic Yanzhiba Granite Pluton In South Qinling, Central China

Early Triassic, the Mianlue Ocean crust began subducted and closed. Finally, in late Triassic, South China block and Qinling micro-block converged and collided, and thus induced great amount of Indosinian magmas all around the South Qinling orogeny. According to the study of these Indosinian granitoids, we can understand more about the forming and evolution of South Qinling Orogen or even the whole Qinling Orogen. Located in the Ningshan area of South Qinling, the Wulong granite complex is the biggest granite complex among these Indosinian granitoids which exposure in the South Qinling area. The Yanzhiba granite pluton, located in the easternmost part of the Wulong granite complex, is one of the major components of Early Mesozoic magmatism in the Ningshan area, South Qinling. The study of the Yanzhiba granite pluton helps to understand the formation and evolution of the Wulong granite complex. However, previous research on this rock is relatively weak, only sporadic age and radio isotopic data reported. Therefore in this paper, we make a lot of samples more systematically across the Yanzhiba granite pluton. Then, we make more systematical Geochemical Study to these samples, including zircon U-Pb geochronology, petrology, petro-geochemistry and radioactive Sr-Nd-Pb isotope Geochemistry study. We hope that through these studies, we can more comprehensive understand the forming age, geochemistry feature and petrogenesis type of Yanzhiba granite pluton. Further, we focus on the discussion of its magma source and forming Geodynamic background in order to explore its petrogenesis.The main results of this study are as follows:1. Zircon U-Pb dating results of five granite samples bearing biotite from boundary to centre of the Yanzhiba pluton give 206Pb/238U ages of 202.9 ± 3.5 Ma,201.0 ±3.0 Ma,202.1 ± 2.6 Ma,200.4 ± 5.4 Ma and 205.5 ± 3.3 Ma, respectively. A weighted mean 206Pb/238U age of 201.9 ± 1.5 Ma suggests that the main body of Yanzhiba pluton formed in a same stage magma evolution. It’s main body formed in the middle or advanced stage of the evolution of Wulong granitic magmas (233-190Ma).2. The Yanzhiba granite pluton is mainly consist of biotite granite; the rock have main characteristic of I type granite. Specifically express as follow:high silica(the mean SiO2 content is 72.33%), alkali-rich(Na2O+K2O=7.67%-8.94%), sodium-rich(Na2O= 3.40%-4.54%> 3.2%), high potassium(K2O=3.62%～5.54%). The mean A/CNK(ASI) is 1.06< 1.1, belonging to weakly peraluminous and high-potassic calc-alkaline series. It has feature with enrichment of large-ion lithosphile and high field-strength elements (such as Rb, Th, U, K and Pb) and depletion in elements Such as Sr, P, Nb, Ta and Ti. Some part has features transition to A-type granite:high FeO/MgO and Ga/Al ratio.3. Initial 87Sr/86Sr ratios range from 0.70072 to 0.70678, initial 8εNd values from-5.3 to -2.0, and have relatively high initial Pb isotopic ratios:(206Pb/204Pb)t: 17.737-18.940, (207Pb/204Pb)t:15.511-15.600, (208Pb/204Pb)t:37.439-38.076. These Pb isotopic features imply a Yangtze affinity of the Yanzhiba granite. Linear variations of initial 87Sr/86Sr ratios and initial εNd values with varying SiCh contents suggest assimilation of the surrounding rocks during the emplacement of the magma. Initial Sr-Nd isotopic compositions are similar to those of Neoproterozoic volcanic-sedimentary sequences in South Qinling (such as Yaolinghe, Wudang and Bikou Groups). Combined Sr-Nd-Pb radiogenic isotopic compositions indicate that the magma source of the Yanzhiba granite originated from Neoproterozoic crustal materials with an affinity of the Yangtze block.4. To synthesize the research results of Qinling Orogen, we proposed a model of formation mechanism of the main body of the Yanzhiba granite pluton as follow:The early Mesozoic, as the Mianlue Ocean crust subduction and reduction, finally, the closure of Mianlue Ocean led the Yangtze craton collided with South Qinling terrane. At about 202Ma, in an extension tectonic environment, Neoproterozoic crustal materials with an affinity of the Yangtze block induced decompression partial melting. During the process of the magmas rising and emplacement, the magmas assimilate some wall, rock; finally emplaced and consolidated as a granite pluton with characteristic of I type granite.