The Geochronology Petrography And Geochemical Characteristics Of Mesozoic Granitoids From Shanghang Area In SW Fujian And Their Implications

The studying area is located in Shang Hang County of Fujian Province ,which is famous for the copper-gold polymetallic deposit of the Zijinshan,and several places of copper-gold orefields has been discovered which are connected with the Mesozoic magatism within this area.There are many intrusive rocks in the studying area,mang of which belong to acid or intermediate-acid granites of Mesozoic,the most important intrusive plutons include Zijingshan pluton,Caixi pluton,Sifang pluton and Luoboling pluton.The Zijinshan pluton is a intrusion and only occurs in the south-west of the studying area,and be intruded by the Caixi pluton and the Sifang Pluton,but the Luoboling pluton only are found in the middle part of the Sifang pluton.Though the predecessors had already done thorough research in Petrology and Mineral Deposits Geology,there still short of systematic research in Geochronology and Geochemistry about Mesozoic magmatic rocks.In this paper,the studies of geochronology,petrology,mineralogical characteristics,element and isotopic geochemical characteristics have been conducted which stepped on the former studies, to discuss the source characteristics and their tectonic setting then reach the following conclusions:1.There are four representative plutons in Zijinshan area include:Zijinshan pluton(168±4Ma),Caixi pluton(150±3Ma),Sifang pluton(108Ma) and Luoboling pluton(105Ma).The Zijinshan pluton and Caixi Pluton blongs to Middle and Late Jurassic,respectively in Early Mesozoic;the Sifang pluton and Luoboling pluton blongs to Early-Cretaceous of Late Mesozoic.The SHRIMP U-Pb date has shown that there are three groups of age in the Zijinshan granite,the first group is about 1000Ma,the second group about 168±4Ma(n=13,MSWD=0.97)which is interpreted as the crystallization time of the pluton,the third group ages is 119±15Ma which reflected the timing of the late ore-forming process.2.Electron probe microanalysis prove that the muscovite of Zijinshan granite rich in Si,Al,K,Fe,which reflects the strong-peraluminous characteristics of the magma.The biotite of Caixi monzonitic granite is characterized by rich in Al, Mg,the values of Mg/Fe+Mg vary from 0.44-0.50,Averaging in 0.46,and belong to Mg-Fe-biotite.The biotite of Sifang granodiorites and Luoboling granodiorites are characterized by rich in Al,Mg,the Al and Mg;the values of Al and Mg in the biotite of Luoboling granodiorites are more than Sifang realtively.The values of Mg/Fe+Mg vary from 0.57-0.61,0.56-0.74,averaging in 0.59 and 0.64,respectively, and belong to Mg-biotite.3.The Zijinshan granite are rich in Al and K,and relatively poor in Fe,Mg,the values of A/CNK vary from 1.19-1.24,averaging in 1.22,strong peraluminous,with standard mineral coruudum molecules universally.The Zijinshan pluton is rich in HREE and displays negative Eu anomalies moderately,poor in Nb,Ta,Ti,P,Ba.Due to the contents of Rb and Sr approach to that of the S-type granite in south-China,the Zijinshan pluton is considered as a typical S-type granite rock.The Caixi monzonitic granites are rich in Si and alkali,poor in Fe,Mg,Ti,P,the values of A/CNK vary from 0.96-1.14,averaging in 1.02,weakly peraluminous.Rich in LREE and displays negative Eu anomalies low-Moderate.Have the geochemical behavior of elements which between Zijinshan and Sifang pluton,be considered transitional granite from S-type to I-type granite. And compared with Caixi monzonitic granites,Sifang granodiorites have the lower contents of Si and alkaline,and the higher contents of Fe,Mg,Ti and P,the values of A/CNK vary from 0.95-1.01,averaging in 0.97,metaluminous.Rich in LREE and display weakly negative Eu anomalies,relatively rich in Sr and be considered typical I-type granite rock. And compared with Sifang granodiorites,Luoboling granodiorites have the lower contents of Si and alkali,the lower contents Al ,K,and the higher contents of Fe,Mg,the values of A/CNK vary from 0.93-1.07,averaging in 0.97,metauminous.Relatively richer in Sr,Ba,Ti,P,considered as typical I-type granite rock;REE and displays negative Eu anomalies weakly,relatively rich in Sr which is considered typical I-type granite rock,there are more mantle components contaminatedin the formation process.4.The(87Sr/86Sr)i rations in Zijinshan granite vary from 0.708192 to 0.710912,theεNd(t) values ranging from -9.6 to -10.16,and the two-stage Nd Model ages T2DM range from 1.61 to 1.72Ga,which above match the characteristics of S-type granite.The(87Sr/86Sr)i rations in Caixi monzonitic granite vary from 0.710462 to 0.712265,averaging in 0.711114,theεNd(t) values ranging from -8.52 to -9.81,averging in -8.99,and the two-stage Nd Model ages T2DM range from 1.51 to 1.61Ga,averaging in 1.54 Ga,both of which proved are formed by partial melting of the crust-derived rocks correlative to Proterozoic crust,there are some materials contaminated by mantle to some extent in the late stage of formation process of later.The(87Sr/86Sr)i rations in Sifang granodiorites vary from 0.707056 to 0.708005,averaging in 0.707433,theεNd(t) values ranging from -5.07 to -4.74,averging in -4.88,and the two-stage Nd Model ages T2DM range from 1.20 to 1.22Ga,averaging in 1.22 Ga,Th(e87Sr/86Sr)i rations in Luoboling granodiorites vary from 0.706729 to 0.706416,average value is 0.706573,theεN(dt) values ranging from -5.13 to -4.15,averge value is -4.64,and the two-stage Nd Model ages T2DM range from 1.15 to 1.23,average value is 1.19 Ga,both of which proved are formed by partial melting of the meta-basic rocks or greywacke correlative to Proterozoic crust in the basaltic magma underplating,there are more materials mixed from mantle in the formation process.Consequently, as time goes,crust-mantle evolution is increasing,more and more mantle components contributed to the formation of granodiorite magma.5.The Sifang granodiorites,Luoboling granodiorites and dacite volcanic rocks which have strong close relationship with Zijinshan copper-gold polymetallic ore deposit are the host rocks of metallogeny.The isochron age of dacite volcanic rocks is 125±9.8Ma,the K-Ar age about sericite and alunite of altered-rock are 94Ma and 82Ma,respectively.In our research,the third group SHRIMP U-Pb age is considered the record of large-scale Cu-Au mineralization.Maybe this stage of hydrothermal process was the commence of the large-scale,long-time,multistage mineralization,and made the foundation of the ultra-large scale copper-gold deposit in the studying area.