| The southern Great Xing'an Range,in which the widely distributed Early Cretaceous granites are of closely genetic relationship with the tin-polymetallic mineralization,is an important tin-polymetallic orefield in Northern China.However,disagreements still exist concerning the petrogenesis and tectonic setting of the Mesozoic granites in the southern Great Xing'an Range and their relations with mineralization.In this paper,with Beidashan granites in the southern Great Xing'an Range as the research object,we analyzed the genetic mechanism,tectonic setting and mineralization of the Early Cretaceous granites in this area from the perspectives of petrology,mineralogy,geochemistry,isotope chronology and other relevant researchs.The major obtained achievements in this thesis are following:1.The emplacement sequence of the Beidashan granitoids was as follows:Xiaobeidashan(includes porphyritic quartz monzobiorite and dioritic enclaves),Kulongshan(porphyritic quartz-syenite)and Mopanshan(includes porphyritic biotite-bearing syenogranite,intermediate-fine grain biotite-bearing syenogranite,fluorite-bearing alkalic feldspar granite).LA-ICP-MS zircon U-Pb dating results show that the porphyritic quartz monzobiorite,dioritic enclaves,porphyritic quartz-syenite,intermediate-fine grain biotite-bearing syenogranite in the Beidashan granitoids were formed at 142.6±1.8Ma(2?,n=16),143.6±2.7Ma(2?,n=28),141.4±2.3Ma(2?,n=16)and 138.5±2.1Ma(2?,n=15),respectively.2.Electron microprobe analysis results show that plagioclase includes oligoclase,andesine and labradorite in the dioritic enclaves,and plagioclase are albite in the porphyritic quartz-syenite and syenogranite.Moreover,potassium feldspar is orthoclase,and biotite includes lepidomelane,siderophyllite and ferrimuscovite in the syenogranite.3.The Beidashan granitoids are rich in alkali,REE and LILE(e.g.,Rb,Th and K),but poor in HFSE(e.g.,Nb,Ta,Ti and Zr),Sr and P,and have higher ratios of104*Ga to Al,with strongly negative Eu anomalies.The features indicate that the Beidashan granitoids have a pronounced A-type affinity.4.The results of the zircon Hf isotope analysis in the Beidashan granitoids show that the dioritic enclaves have positive?Hf(t)ratio range from+3.45 to+10.57,with a peak value of+8.7,the two-stage model ages(518?974Ma);the porphyritic quartz monzobiorite also have positive ?Hf(t)ratio range from+4.29 to+8.92,with a peak value of+6.7,the two-stage model ages(629?916Ma);the porphyritic quartz-syenite have positive?Hf(t)ratio(+4.62?+11.05),the two-stage model ages range from 488Ma to 900Ma,with an average age of 714Ma;the intermediate-fine grain biotite-bearing syenogranite have positive?Hf(t)ratio range(+5.70?+10.24)as well,the two-stage model ages range from 541Ma to 825Ma,with an average age of 652Ma.This indicates that sources of Beidashan granitoids may be derived from a mixture of underplated juvenile lower crust and Paleozoic subduction-accretionary assemblages.Furthermore,the?Hf(t)values(+10.24?+11.07)of the Beidashan granitoids are higher than those of the underplated juvenile lower crust.This implies that sources of these granitoids may be more juvenile than underplated materials.5.These dioritic enclaves have magmatic structure,quench boundary,reverse veins and unbalanced textures,such as acicular apatite,quartz ocellar texture,plagioclase oscillatory zoning in the Xiaobeidashan complex.Moreover,unified formation age and varied zircon Hf isotope of the dioritic enclaves and host rocks.These features indicate that the magma mixing between mafic magma and felsic magma is the primary mechanism responsible for the Xiaobeidashan granitoids of crust-mantle mixed source.6.High iron indexes and low oxygen fugacities,rich in alkali and volatile,and higher tin values in the intermediate-fine grain biotite-bearing syenogranite of the Mopanshan are favorable for tin mineralization.Therefore,the Mopanshan area has great potential for finding tin polymetallic deposits.7.With comprehensive consideration of the regional geologic features,the Beidashan granitoids may be originated from Early Cretaceous in lithosphere extension setting,which may reduced by the subduction and rollback of the paleo-Pacific plate and subsequent upwelling of the asthenosphere. |
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