Petrology Of The Piaoac Granites Related W-Sn Mineralization,Cao Bang,Northern Vietnam

Cretaceous magmatic rocks are widespread in North Vietnam-South China Block(NV-SCB),which are close spatial and genetic relationship to tin polymetallic mineralization.Studies of origin and genesis of these magmatic rocks are not only significant for understanding the evolution of NV-SCB,but also for a better exploration strategy for tin polymetallic deposits.At the present time,a lack of deep research constraints the genesis and tectonic setting of these magmatic related mineralization.The focus on the data of magmatic rock related mineralization in the NV-SCB have been published and comparing with tectonic model in NV and SCB that can give us relative clear idea mechanism and processes during Late Cretaceous periodIn this study,we present new geological,geochemical,mineral chemistry and Hf isotopic data of Piaoac granites massif,Cao Bang province,North Vietnam and comparing with the literature data of Dulong tin polymetallic district in South China.The objectives of this research shed new light on the origin of granites related the tungsten-tin mineralization in the Cao Bang area.In addition,we intend to provide further constraining on the petrogenesis and tectonic setting of the Late Cretaceous rock in NV-SCB.The Piaoac massif includes the medium-to fine-grained two mica granites,with texture porphyry of feldspars.These granites show high silica(Sio2 from 72.37 to 73.07 wt.%),strong peraluminous affinity(ASI=1.61-1.65),enrichment in Rb,U,Ta,and P,and depletion in Ba,Nb,La,Eu and Ti.Zircon form the intrusions are characterized by high Th/U ratios(>0.1),clear oscillatory zone indicating the zircons are magmatic origin.The crystallization age of Piaoac granitic rocks is around 82 Ma,which represent Late Cretaceous.The low CaO/Na2O ratios(<0.3)and high SiO2 contents on the igneous rocks suggest that the granitic melts forming by partial melting of metapelitic rock.These features coupled with negative ?Hf(t)value(-9.69 to-0.9)and the crustal model age(TDM2)of these granites suggest that the parental magma was produced from the Proterozoic metasedimentary(1.2 Ga to 1.7 Ga)which is consistent with basement rocks in SCB.Tungsten-tin mineralization in these granites were effect by two major factor,first,through the high fractional crystallization of plagioclase,biotite,and feldspar,tungsten-tin accumulate in granitic melts,second,low oxygen fugacity(below MH),would efficiently removal of W-Sn into a hydrothermal fluid,and lead to deposition of W-Sn mineral phases.In combining with literature data of granite related tin-tungsten in Southwest China,which form lithospheric thinning and asthenospheric upwelling,Piaoac granites share similar characteristic and close temporal,suggesting same tectonic setting.Piaoac granites formed in an extensional setting related with the Late Cretaceous magmatism occurring large-scale lithospheric extensional in South China Block.