| Beishan Terrane is located in the south of Central Asian Orogenic Belt(CAOB).It is the key section to understanding evolution of CAOB which went through a set of complicated arc-continental collage-collision and continental accretion accompanied several magmatisms,formed Cu,Ni and rare metal deposits.While,rare metal deposits have not been well studied.This paper presents petrology,zircon U-Pb isotopic dating and geochemistry data of Rb-bearing granite-pegmatite through field research and experiments,aimed to probe into the geochronology,magma source,petrogenesis,structural setting,and ore-forming mechanism,so as to provide the evidence of Beishan Terrane structure evolution in early Paleozoic,meanwhile seeking method of finding out new type of deposit.The following understandings and conclusions could be drawn:1.The U-Pb dating ages of most zircons from gneissic granite are 1853±16Ma and 152±2Ma,the former age is represented when the gneissic granite formed in Paleoproterozoic,while the latter one represented when a geothermal event took place in late Jurassic.The U-Pb dating ages of most zircons from amazonite granite are 427 ± 8Ma,466 ± 12Ma and 500± 9Ma.The first age of magma zircons is represented when the amazonite granite formed in,the latter two ages of captured zircons are represented when the tectonic events happened in middle Ordovician and middle-late Cambrian.The U-Pb dating ages of most zircons from pegmatite are 509±4Ma and 406±5 Ma.The former age is of captured zircons,the latter one of magma zircons is represented when the pagmatite formed.The evolution time from granite to pagmatite is lasting about 21Ma.2.The granitoid is belong to peraluminous high potassic calc-alkaline series which is evolved from biotite adamellite,amazonite granite to pagmatite.With evolution increasing,SiO2 content is decreased,Al2O3 and Na2O+ K2O content is increased,CaO content decreases first and then increases.Granitoid has a very low MgO,TiO2,Fe2O3 and P2O5 contents,rich sodium and poor potassium.Sr and Ba contents are low and Rb,Cs,Pb contents are high.Rb,Nb,Ta are increasing with evolution.Nb/U,Y/Nb,Rb/Sr ratio change within a wide range.On the primitive mantle standardized spider diagram of trace element,Ba,Sr,Nb,La,Ce,Eu are depleted,Y is slightly depleted,Cs,Rb and Pb are strongly enriched,Zr,Hf,U and Th are medium enriched,Ta is slightly enriched.?LREE is low while ?HREE is high.From the Chondrite standardized rare earth distribution pattern we can conclude that LREE evolve from loss to enrichment,but the HREE are opposite.(La/Yb)N is from 0.13 to 4.21.Eu has a strong negative anomaly.Comprehensive study of petrogeochemistry shows that the granitoid is A2 type of which the source rock is riched in muscovite.Fractional crystallization of allanite,monazite,plagioclase and potassium feldspar during magmatism happened after source rock melting and formed magma riched in F,Rb and fluid.Combined with the tectonic setting,Rb-bearing granite-pegmatite rock mass formed in extrusion-extension transformated environment after Nantianshan Basin collision.Another conclusion can be drawn that Nantianshan Basin closed before 427 ± 8Ma.3.Rb and mineral fluid mainly came from muscovite.Magma which was rich in Rb and fluid generated from muscovite rich source rock partial melting.F complexed Rb and migrated during magma evolution and then hydrolyzed at last phase.F-and Ca2+ formed fluorite,or enter into muscovite,Rb enriched in feldspar and became mineralized. |
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