Spatial-Temporal Distribution And Sources Of Granitoids In The Middle Qinling Orogenic Belt,Central China:Implications For The Nature Of Deep Crustal Basement

The basement component of the Qinling Orogen is the kind of questions geologist focused on.Granitoid is an important part of the continental crust,it provides significant information on the formation of the continental crust,the mantle-crust interaction and evolution of the lithosphere.An increasing number of studies have used the isotopic compositions of granitoids to reveal the struture of continental lithosphere,distinguish the geochemical difference and correlation of the continent,and explore the evolution process of the deep crust.The middle Qinling Orogen(MQO),located in the conjunction area of eastern and western Qinling,is a key region for understanding the basement and evolution of the Qinling Orogen.The widely developed granitoids,from Proterozoic to Mesozoic,provide sufficient data to carry out the isotope mapping and deep crust research in the Qinling Orogen.On the bases of the published zircon U-Pb ages and Nd-Hf isotopic compositions of granitoids in the MQO,this paper carries out a systematic field investigation and laboratory analysis of the plutons that lack of research.These research data are used to constrain the spatial-temporal distribution and the nature of the crustal basement in the terranes where the granitoids are located.We compare the isotopic compositions of the Mesozoic granitoids in the MQO with those in eastern Qinling,in order to discuss basement differences between eastern and western Qinling.The major achievements and new recognizations in this study as follows.(1)Ninety-five published and nineteen newly acquired zircon ages indicate that granitoid magmatism in the MQO mainly occurred in five distinct periods:the Paleoproterozoic(1806-1741 Ma),Neoproterozoic(962?704 Ma),Early(470?424 Ma)and Late(417?365 Ma)Paleozoic,and Mesozoic(248?187 Ma).(2)The Paleoproterozoic granitoids are only sporadically present in North Qinling Belt(NQB),the main grantoid rock types are gneissic biotite-monzogranite,and are characterized by deformed S-type granites.The Neoproterozoic granitiods mainly occurred in the northern margin of the South China Block(N-SCB),are characterized by an evolution from S-type to I-type,and finally to I-A transitional type granitic magmatism.The Early Paleozoic granitoids are found in the southern margin of the North China Block(S-NCB)and NQB,and dominated by I-type granitoids,including biotite monzogranites and granodiorites.The Late Paleozoic granitoids are dominantly I type with poor developed in the S-NCB and NQB,they include monzogranites,granodiorites and syenogranite.The Mesozoic are found in all the tectonic of the MQO,these granitoids mainly formed from 222 to 206 Ma.The granitoids consist of various rock types,and most of them are classified as I-type characteristics,although a minor group has I-to A-type transitional features.(3)The Paleoproterozoic granitoids in the NQB have a negative sNd(t)and ?Hr(t)values,and their Nd and Hf model ages are ancient,it indicates that the Paleoproterozoic granitoids mainly derived from partial melting of Archean crust material,it also reveals the existence of Archean crustal material in the deep crustal composition of the NQB.(4)The Nd-Hf isotopic compositions of the Neoproterozoic granitoids indicate that the ?Nd(t)values of the granitoids increase and TDM decrease from the NQB to the N-SCB,indicating the sources and deep crustal compositions becoming younger in this direction.Besides,the ?Hf(t)values of Neoproterozoic granitoids in the N-SCB have large variability,indicating complex deep crustal composition.The dominant deep crustal material in the N-SCB is Mesoproterozoic in age,with some older(i.e.Archean and Paleoproterozoic)components.(5)The Paleozoic granitoids have negative ?Nd(t)and positive ?Hf(t)values,suggesting that they derived from the partial melting of the Neoproterozoic new crustal materials.The granitoids of the S-NCB have younger TDM and TDMc than that of the NQB,and show notable Nd-Hf isotope decoupling above the Terrestrial Nd-Hf Array,while the granitoids of the NQB show notable Nd-Hf isotope decoupling under the Terrestrial Nd-Hf Array.These characteristics above indicate that the source of the granitoids become older from the S-NCB to the NQB,and the deep crustal composition in the S-NCB is more juvenile than that in the NQB.(6)All the Mesozoic granitoids in these different terranes were mainly from old crust with some juvenile components,but their source components were relatively complex in the NQB and SQB.This suggests that the basement or deep crust components of the NQB and SQB are older and complex,whereas those of the S-NCB and N-SCB are slightly younger.(7)The continental growth in the MQO occurred mainly during the Mesoproterozoic.besides,the reworking of ancient crustal materials at the Archean and Paleoproterozoic are found in the NQB,SQB and N-SCB,it can be observed that the S-NCB may have a special evolution or tectonic attribute different from the NQB,SQB and N-SCB.(8)The basement components of the S-NCB are different from those in the MQO and eastern Qinling.The former is similar to that of the Qilian Orogen,whereas the latter is similar to that of the NCB.These results advance our understanding of the evolution of the Qinling Orogen.