Genisis Of The Neoproterozoic Granite Within Tianquan County,Sichuan Province And Its Geological Implications

The Neoproterozoic granite at the Pendaikou is mainly distributed in the Pendaikou-Huangshaping area where is about 9 km northwest of Tianquan County,Sichuan Province.The rock mass distributes in NW direction with irregular shape and around 67 Km² areas.The northern edge of the rock mass intrude on the early-formed diorite and the eastern and southern parts were covered by the Sinian stratum.The rock mass of the granite can be divided into the southeastern marginal facies and the central facies of the Midwest.The rock type of the marginal facies is biotite monzogranite,which is composed of alkali feldspar?34%-40%?,plagioclase?26%-32%?,quartz?20%-25%?and biotite?5%-8%?.The accessory minerals are magnetite,zircon,apatite,and titanite.The rock type of the central facies is monzogranite,which is composed of alkali feldspar?35%-40%?,quartz?25%-35%?,plagioclase?20%-25%?and biotite?1%-2%?.The accessory minerals are zircon,apatite,titanite,magnetite,and leucosphenite,with monazite and rutile locally.The major elements of the granite have the characteristics of having a high content of SiO₂?67.96%⁷3.95%,71.40%average?,alkali-rich?content of Na₂O+K₂O is 8.34%-9.06%,8.5%average?and sodium-rich?The ratio of Na₂O/K₂O is0.80-2.03,1.44 average?.The Ritman index?=2.35-3.06 belongs to calc-alkaline??=1.8-3.3?.A/CNK is 0.98-1.08,an average of 1.01,is quasi-aluminum-weak over aluminum,the corundum content of the standard mineral is less than 1%.The DI values of the rock samples ranged from 84.82-95.77,with an average value of 91.8,reflecting the strong crystallization differentiation of the magma.The increase of SiO₂ shows a decreasing trend of the main elements such as TiO₂,FeO,MgO,CaO,P2O5,which suggests that biotite monzonitic granite and monzonitic granite evolved from comagmatic differentiation.P,Nb,Ti,Sr,Ba,and other elements are obviously depleted in the granite at the Pendaikou,and Th,U,and K are enriched,showing the characteristics of type I granite.At the same time,in the identified diagrams of granite's genetic types with 10000*Ga/Al as abscissa,all points fall into the area of type I granite and the ratio of Rb/Sr is 0.35-1.08?the mean value is 0.62?,which is very close to the mean value of type I granite?0.61?and further indicates that the granite at the Pendailou is type I granite.The ratio of Nb/Ta and Sm/Nd of the rock mass reflects the characteristics of its crust source,and the ratio of Eu/Sm indicates that it may have been contaminated by mantle materials.And the Sm/Nd ratios of granite at the Pendaikou?between 0.152and 0.183,0.171 average?are less than 0.3,which further indicates that the granite in this area originated from the lower crust.On the other hand,the enrichment of Zr and the loss of Nb in rocks also indicate that the source rocks may be mainly continental crust components,while enrichment of K and enrichment of other incompatible elements U and Th also indicate that the source rocks may be continental crust components.Therefore,the above characteristics reflect that the granite in this area is originated from the partial melting of basic igneous rocks in the lower crust and may be contaminated by mantle materials.The geochronological study of the LA-ICP-MS zircon U-Pb of the Pendaikou's granite shows that its zircon U-Pb age is730.5±5Ma,indicating that the Pendaikou's granite was formed in the middle Neoproterozoic.Located in the western margin of the Yangtze Block,the tectonic events in the middle Neoproterozoic are generally considered to be related to the evolution of the Rodinia supercontinent.Combining with regional tectonic evolution and geochemical characteristics,the tectonic environment of the Pendaikou's granite is identified.It is shown that the Pendaikou's granite has the geochemical property of island-arc granite.It can be inferred that the western margin of the Yangtze Block was in the island-arc tectonic environment in the middle Neoproterozoic,and the granite at the Pendaikou is the important evidence of the island-arc magmatic activity in this period.Therefore,the whole South China block,including the Yangtze Block,should be on the edge of the Rodinia supercontinent during this period.