Granitic Magmatism And Crustal Evolution Of Baoji-Taibai Area,Qinling Orogen

The Qinling Orogenic Belt is located in middle China mainland,and experienced a multi-stage complex tectonic evilution,including Neoproterozoic plate rifting and convergence,Early Mesozoic continent-continent collision,and Late Mesozoic intraplate evolution.Each stage accompanied by the corresponding granitic magmatism.The petrogenesis and source rocks of granitoids can constrain the evolution of oregen.In this thesis,three plutons in Baoji-Taibai area were selected to be studied.On the basis of systemic petrological,geochemical,zircon U-Pb chronological study,we establish the magma regularities in Baoji-Taibai area and discuss the source nature as well as the geodynamic significance.Combined with data from literature,this thesis obtained zircon U-Pb ages of three plutons by LA-ICP-MS.The Lianghekou pluton formed in 940-930 Ma and underwent strongly deformation.The Baoji pluton invaded during three episodes over 20 million years:granodiorites formed at～217 Ma,monzogranites and monzodiorites formed at～212 Ma and K-feldspar granites at～196 Ma.The Taibai late Mesozoic granites formed at～120 Ma.The Lianghekou pluton mainly composes of augen granite,gneissic granite and monzogranite.Augen granite contains aluminum minerals of garnet and muscovite.Most granites show the characteristics of A/CNK>1.1,high silica and aluminum content as well as high-K calc-alkaline series.Chondrite-normalized REE patterns show a rightward inclination with medium negative Eu anomalies.The trace element geochemistry is characterized by enrichment in LILEs(such as Rb,Ba,K),depletion in HFSEs(such as Nb,Ta,Ti)and obviously negative anomalies of Ba,P,Sr.Mineralogical and geological characteristics indicate that the gneissic granites and augen granites of Lianghekou pluton are typical S-type granites.Lianghekou granites show large range of initial B7Sr/86Sr(t)ratios of 0.701067-0.739451,lower εNd(t)values varied from-5.7 to-3.3 with two stage Nd modal ages of TDM2=1.9-2.1 Ga and highly radiogenic Pb composition,indicating a crustal magmatic origin and the source rocks may be amphibolite and gneiss of Qinling Group.Combined with the regional geological background,we conclude that granites of Lianghekou pluton originate from melting of ancient middle-lower crust under tectonic transition stage of late continental crust collision during Neoproterozoic and this is the response to the assemblage of Rodinia supercontinent.The Late Triassic Baoji pluton consists of granodiorites,monzogranites,quartz monzodiorites,and K-feldspar granites.The granitoids have high-K calc-alkaline,metaluminous to weakly peraluminous composition(s).Silica contents increase and Mg,Fe,Ti,Ca,and P contents decrease from quartz monzodiorites through granodiorites to monzogranites and K-feldspar granites.Combining our findings with evidence from whole-rock Sr-Nd and zircon Hf isotopes,we suggest that the granodiorites formed by magma-mixing processes involving metasomatized lithospheric mantle and Mesoproterozoic crustal material.The monzogranites record the contribution of an asthenospheric mantle source.The quartz monzodiorites were derived from a metasomatized lithospheric mantle source.Partial melting of crustal lithologies generated the K-feldspar granites.We suggeste that the Baoji pluton formed in a late-collision setting.The late mesozoic Taibai granites have high silic and alcali contents and are defined into high potasium calc-alcali and weakly peraluminous series.The granites are enriched in LILEs and depleted in HFSEs.They have relatively depleted Sr-Nd compositions,suggesting a crustal source with contribtion of mantle material.The Taibai granites have similar age and geochemical composition to the second stage granitoids in southern margin of North China block,indicating they both formed in an extension setting.The granitoids occurred in North Qinling have different isotopic compositions,indicating that they have different sources.The basement of North Qinling and mantle beneath it are inhomogeneous,and the granitoids are product of different crustal reworking or crust-mantle reaction.