Highly Fractionated Granite:Mineral,Geochemical Features And Their Implications

As a major component of continental crust,granites have long been served as the most important subject in geology.The classification of granite has been an important issue.Approximately 20 different schemes have been proposed during the last 30 years.In this study,we systematically summarize the most commomly 4 classification of granite: Alphabetic classification;Barbarin's classification;Magnetite-and Ilmenite-series granitic rocks;and Frost petro-chemistry classification,and briefly review the advantages and disadvantages of the above 4 classification.Highly fractionated granite is a special type of granite,and it's the late stage product of magma evolution.It is difficult to determine the genetic type in some rock masses.In addition,highly fractionated granite has a closely relation to W-Sn mineralization.Therefore,the identification of the distinguishing marks of highly differentiated granites is of great significance for the classification of granite and prospecting W-Sn deposits.This paper collects geochemical data of from 38 typical highly fractionated granites in the world,and classifies them into 8 groups according to their petrographic characteristics: alaskite;aplite;Li-F granite;tourmaline-bearing granite;muscovite granite;two mica granite;biotite granite;alkaline melano-minerals granite,and lithofacies and geochemical characteristics are summarized.We used Modan to simulate the content of rock-forming minerals in these granites.The main rock-forming minerals and major elements of highly fractionated granites are the main criteria.This paper proposes the criteria for distinguishing highly evolved granites:(1)An value of plagioclase is less than 15,dark-colored femic mineral content is less than 8%;(2)CaO(wt%)<1%,FeOt+MgO+TiO2(wt%)<6%,SI(Solidation Index)<6,DI(Differ ential Index)>88.Large-scale tungsten and tin mineralization took place mostly in South China.This paper selects 13 typical tungsten-tin deposits associated with highly fractionated granite in this region,and these granites can be grouped into the following three groups: Sn-bearing granites,Sn-W-bearing granites,W-bearing granite.These ore-forming granites are characterized by enrichment in Rb,Th,U and Ta;depletion in Ba,Sr,Nb,Ti and Eu.In addition,we conducted a preliminary study on the difference between tungsten and tin mineralization,and proposed that the different degree of magma evolution is an important factor.The W-Sn-bearing granite has higher degree of magma evolution than Sn-bearing granite.This difference is also reflected in the trace element characteristics of three different types of granite.Compared with W-Sn-bearing granite,the Sn-bearing granite has a higher La/Yb.Compared with W-bearing granite,the Sn-bearing granite has higher Zr/Hf,Nb/Ta,and lower Y/Ho.Among the above three types of granites,the W-bearing granite has strongest tetrad effect.The difference in these trace elements can be used as a critdriom for distinguishing three types of granite.These differences can play an important role for prospecting tungsten-tin deposits.