Nd And Hf Isotopic Characteristics Of Granitoids In The Altai-East Junggar-East Tianshan And Their Implications Of Deep Structure

The Central Asian Orogenic Belt is a typical accretionary orogenic belt,with significant crustal growth in Phanerozoic.However,the deep material composition structure of this belt is still unknown.Altai,East Junggar and East Tianshan are the typical areas of the CAOB.Their composition,especially the deep material composition,have been a major concern.Nd-Hf isotope mapping are effective tools to reveal the composition of deep crust.In this paper,we study the zircon U-Pb ages,rock geochemistries,all-rock Nd and zircon Hf isotopic compositions of granitoids from the Altai,East Junggar and northeastern Tianshan,and establish isotopic section of granitoids to explore the deep material composition,structure and the way of crustal growth.The following achievements and conclusions have been made.1.Zircon U-Pb(LA-ICP-MS)dating for four granitic samples from the southeastern Chinese Altai yield the ages of 382±4 Ma,381±4 Ma,385±5 Ma and 363±6 Ma.These Devonian granitic rocks belong to high-K calc-alkaline and metaluminous-weakly peraluminous series I-type granite.They have negative whole rock ?Nd(t)values of-2.42~-0.53 with Nd model ages(TDM)of 1.6~1.3 Ga and zircon ?Hf(t)values of-3.43~+12.89(mostly positive),with Hf model ages TDM-2 of 2.5~0.6 Ga,suggesting complex sources of mantle-derived substances and ancient crustal for these granitoids.2.The zircon U-Pb ages obtained from typical plutons in the northern and southern Junggar are 303 ± 3Ma,290 ± 3Ma,378 ± 3Ma and 301 ± 3Ma,revealing two(303 ~ 290 Ma and 378Ma)magmatic events.Devonian Wutubulak pluton belong to high-K calc-alkaline and weakly peraluminous series I-type granite,the granitoids from other three Late Paleozoic plutons belong to quasi-aluminum high-K calc-alkaline I-type or A-type granite.The granitoids from the northern Junggar have high positive zircon ?Hf(t)values of +11.7~+17.5,with Hf model ages TDM-2 of 2.5~0.6Ga and the southern Junggar of +10.2~+15.7 and 0.66~0.32 Ga,respectively.Both have similar Hf isotopic characteristics,which reflect the young material as the main component.3.LA-ICP-MS zircon U-Pb analysis of biotite monzongranite from Xiaopu and hornblende monzonitic granite from south Qincheng in the Harlic area yielded crystallizationages of 316±4 Ma and 320±3 Ma.Xiaopu granites belong to high-K calc-alkaline and weakly peraluminous series of I-type granite,but the granites from the south Qincheng pluton belong to high-K calc-alkaline and metaluminous A-type granite.Xiaopu granite have positive zircon?Hf(t)values of +8.0~+13.8 and south Qincheng granite have higher ?Hf(t)values of +10.8~+16.7,with corresponding two-stage Hf model ages of 0.82~0.45 Ga and 0.64~ 0.27 Ga,respectively.These isotopic composition indicates young crust as the main source for the granites generation.Combined with regional mafic rocks,which have similar isotopic characteristics,it indicates that these young substances may be related to the new mantle-derived mafic magma underplating,showing the post-collisional vertical crustal growth.4.The granitoids from Altai have ?Nd(t)values of-4.2 ~+8.1 with TDM(Nd)values of1.50~0.39 Ga,and they have ?Hf(t)values of-3.7~+12.4 with TDM2(Hf)values of 1.77~0.47 Ga.Comparatively,the granitoids from East Junggar have ?Nd(t)values of +2.4 ~+8.4with TDM(Nd)of 1.04~0.46 Ga,and these granites have ?Hf(t)values of +7.6~+14.7 with TDM2(Hf)of 0.81~0.38 Ga.There are significant differences in the Hf isotopic composition of granites between Altai and eastern Junggar.The value of ?Hf(t)from north to south is significantly increased from low negative ?Hf(t)to high positive ?Hf(t)value,Hf isotopic composition provides a deep evidence to divide tectonic unit.From the origin of the granite and the composition of the basements,the Erqis fault zone is the structural boundary for the Altai and Eastern Junggar.5.The Nd-Hf isotope mapping further confirmed that the different blocks of the Chinese Altai have different deep material composition.Unit 2 and 3 have similar ancient materials,which provide new evidence for the possible existence of the Precambrian base in the deep part of Altai.The eastern part of the Altai has older crustal material than the west.The eastern Junggar(Unit 6 and 7)has the characteristics of young material composition,which provides evidence for the basic properties of the young material,but the existence of the ancient material.East Tianshan(Unit 8)deep consists of young material.