Temporal-spatial Variations,sources And Their Tectonic Significances Of The Triassic Granitic Rocks In The Junction Part Of The East Kunlun And West Qinling,Central China

As major component of the Central China Orogenic Belt(CCOB),the tectonic connection relationship between the Kunlun and Qinling orogen have long-term been an interesting and questionable issue to Chinese geologists.The famous Gonghe basin,located in the junction region of the easternmost end of the East Kunlun orogen(EKO),as well as the westermost end of the West Qinling orogen(WQO),was considered as a crucial region to explore the tectonic relationship between the Kunlun and Qinling orogen.The Triassic granitic rocks,voluminously exposed in the eastern part of EKO(EEKO)and western part of WQO(WWQO),provide an important research role to explore the tectonic connection relationship between the two orogens.This paper reports zircon LA-ICP-MS U-Pb ages,bulk-rock major oxides,trace elements,and Nd-Hf isotopic compositions for the Triassic granitic rocks from the easternmost segment of the EKO and the westernmost segment of WQO.Besides,we have collected systematically the published data(zircon U-Pb ages and Nd-Hf isotopes)for the Triassic granitic rocks,and produced Nd-Hf isotopic mapping in order to compare the deep crustal structure as well as the basements of the EEKO and WWQO.Furthermore,we have established the chronology framework for the Triassic granitic rocks,and provided a detailed analysis on the isotopic variations and sources of the granitic magma,as well as provide a systematically contrast on the temporal variations of the Nd-Hf isotopic compositions.Based on the comparisons of the chronology,Nd-Hf isotopic composition and temporal variations of the magmatic sources for the Triassic granitic rocks in EEKO and WWQO,we provided a new evidence to recognize the magmatic evolution process,tectonic environment,and the isotopic composition of the basements on both sides of the Gonghe basin.The major achievements and new recognizations in this study as follws:(1)U-Pb analysis of 14 zircons from 12 granitic plutons in the easternmost segment of EKO and the westernmost segment of WQO yielded 252?224 Ma.Combined with the 138 zircon U-Pb ages collected from published literatures,we conlude that the Triassic magmatism in the conjunction part of the EEKO and WWQO are roughly similar,ranging from 252 to 203 Ma,and can be integrally divided into the early(252?238 Ma),middle-(238?226 Ma)and late-stage(226?203 Ma).The peak magmatism on both sides of the Gonghe basin is in the early-stage(252?238 Ma).After a careful analysis,it can be seen from the EEKO to WWQO and then to the East Qinling orogen in an E-W direction,the early-stage(252?238 Ma)magmatism is more intensive in the western part of the CCOB,and the late-stage magmatism is gradually escalated in the eastern part of CCOB.In addition,from north to south,the Triassic magmatism seems existing a minor trend to become younger,and it may reveal a northward subduction-accretion and collision process of the Palaeo-tethys in Mesozic.This establishment of the chronology and division of the magmatism not only provide a basis for us to analyze the tectonic-magmatic evolution of the Triassic magmatic rocks from the East Kunlun and West Qinlin orogen,but also reveal the Meso-Tethys' northward subduction-collison process during the Triassic period,as well as supply a chronology basis for further analyzing the Nd-Hf isotopes of the basement on both sides of the Gonghe Basin.(2)The mainl rock types of these granitic rocks are tonalite,granodiorite,monzogranite and syenogranite,and some diorite and rhyolite porphyry.Geochemically,these rocks are mainly high in SiO2,K2O and K2O+Na2O contents,low in P2O5 and TiO2 contents,depleted in Ba,Sr,P,Ti and enriched in U,Hf,Zr.Except the Late Triassic Rexue'ri monzogranites in the westernmost segment of the WQO display some features of S-type granites,the other granitic rocks are all I-type granites.From early(252?237 Ma)to late(234-224 Ma),the geochemical characteristics of these granitic rocks in the easternmost segment of EKO and the westernmost segment of WQO,show the change from a subduction-collision setting to a post-collision or within-plate setting.(3)Based on the combination of new measured data of 14 zircon Lu-Hf isotopic and 23 whole-rock Nd isotopes,as well as the collected data of 54 Lu-Hf isotopes(zircon and whole rock)and 179 whole-rock Nd isotopes,the statistical analysis of the Nd-Hf isotopes from Triassic granitic rocks on both sides of Gonghe basin shows:the ?Nd(t)are predominately negative and mainly ranged from-8.5 ro-2.5,Nd model ages(TDM)mainly concentrate between 1.8?1.0 Ga;?Hf(t)mainly ranged from-3.8 to+5.0,and crustal Hf model ages(TDMC)mainly concentrated between 1.7?0.9 Ga.As a whole,from the early-(250?237 Ma)to middle-(238?226 Ma)and then to late-stage(226?210 Ma),the Nd-Hf isotopes seems exhibit a certain evolutionary trend,especially the Nd-Hf isotopic compositons of Triassic granitic rocks from the EEKO.In the early-stage,the ?Nd(t)and ?Hf(t)values are high,and the component of old crustal material decreased;in the middle-stage,the high and low values of the ?Nd(t)and ?Hf(t)is coexisted together,and the old crustal material and younger mantle component also coexistedin this period;in the late-stage,the?Nd(t)and ?Hf(t)values is gradually minor increased,and the model age is also gradually became more younger.Through the Nd-Hf isotopic mapping,it reveales that the model age(TDM)on both sides of the Gonghe basin are similar and mainly concentrated between 1.80?1.20 Ga,which further indicates that the deep crustal material structure of EEKO and WWQO is substantially similar,and the basement of the EEKO and WWQO is similar or same.(4)The new recognization of the above research is that on both sides of the Gonghe basin,where is the conjunction region of the EEKO and WWQO in the middle part of CCOB,the Qinling and Kunlun orogen show a similar magmatic evolution trend in Triassic period and share a same or similar basement.Based on this recognization,we conclude that the Qinling and Kunlun orogen may be a same orogen in the Triassic period,and constituted a continuous orogenic belt system.The constraint of these three magmatic events in the EEKO and WWQO,is the results of the Meso-Tethys' northward subduction(252?238 Ma),syn-collision(238?226 Ma)and post-collison stage(226-203 Ma)in the Triassic period.