| Central Asian Orogenic belts is a huge suture zone created by Paleo-Asian ocean subduction among Siberian Plate,Northern China Craton,Tarim Block,which is the most remarkably accret-ed orogenic belts during Phanerozoic and has undergone three geological stages-that is continental crust accretion,post-collisional stage,and intra-continent orogeny.All these geological events are accompanied by intense magmatism,such as the late Paleozoic granitoids in Alxa Block-which is located in the south of Paleo-Asian ocean and has recorded its subduction,collision to closure time,and geological implications.This article,based on former research in the region,has studied the petrogenesis of granitoids collected from western Alxa Block and also explored the tectonic evolution history of the northern margin of Alxa Block by field survey,petrology study,zircon U-Pb ages,major and trace elements analysis.The study results are listed blow:1.Four unites of granotoids LA-ICP-MS Zircon U-Pb ages data gained,which are respective-1y late Carboniferous(302.0Ma ± 9.2Ma)quartz diorite,late early Triassic(246.5:±4.6Ma)to-nalite,middle Triassic(235 ± 4.4Ma)monzonitic granite,middle Triassic(229.5 ± 5.6 Ma)bio-tite granite,and represent different time scales of granitoids and also has determined the chronolo-gyical structure of the magamatism in Alxa in late Paleozoic.2.The late Carboniferous(302.0Ma ± 9.2Ma)quartz diorite is high in Si(Si02 = 60.66%-61.37%)and Al(A1203 = 17.98-18.44%),rich in Ca(CaO = 6.08-6.59%)and Mg(MgO =2.13-2.51%,Mg#48-51),low in Ti(Ti02 = 0.58-0.63%),enriched in Na and depleted in K(Na2O = 4.20-4.46%,K20 = 0.67-0.92%,K20/Na2O = 0.16-0.22),high in Sr(582 X 10-6-620 × 10-6),lowin Y(11.4 × 10-6-13.7 X 10-6)and Yb(1.02 X 10-6-1.5×10-6),enriched in strong incompatible elements and depleted in Nb,Ta,Ti and HREE,lightly positive abnormal of Eu,and has high ratio of Sr/Y(35.15-53.25),Y/Yb(11.00-11.23)and La/Yb(7.28-15.10),low ratio of Rb/Sr(0.03-0.05),La/Ce(0.42-0.50)and Nb/U(4.70-8.93),all of the characteristics of which resemble Cenozoic Adakites in the circle of Pacific Ocean and indicate that it is form by partial melting of basaltic oceanic crust in the early stages of subduction towards the South.3.The late early Triassic(246.5 ± 4.6Ma)tonalite,which belongs to high-k calc-alkaline series,has characteristics of high Si(SiO2 = 62.14-71.18%)and Al(Al2O3 = 14.45-16.97%),rich(Na2O + K2O = 6.35-7.82%)and relatively high Mg(MgO = 0.85-2.3%.Mg#= 42-47),low Ti(TiO2 = 0.34-0.76),P(P205 = 0.13-0.26),enriched Rb,Th,K and LREE,depleted Nb,Ta,Ti,P,Sr,and lightly negative abnormal of Eu.Sample A YQ-3 is formed by young and hot oceanic crust,which is represented by Quagan Qulu ophiolitic belt,indicating that the subduction crust is close to the middle ocean ridge and back-arc basin is to be closed.Sample AYQ-1 is intensely assimilated by country rock(biotite schist)and is more enriched in Si and K while low in Mg.4.Middle Triassic(235 ± 4.4Ma)monzonitic granite has characteristics of high Si(SiO2?77.72-78.13%),low Al(Al2O3 = 11.56-14.07%),Ca(CaO = 0.93-2.32%),Mg(MgO ?0.2-0.56%,Mg#= 34-36)and Ti(TiO2 = 0.12-0.16%),the ratio of K20/Na2O = 0.97-1.28 and A/NKC = 0.94-1.01,comparatively depleted Ba,Sr,Nb,Ta,P,Ti,enriched Th,K and Pb,middle scaled enrichment of LREE,high ratio of Rb/Sr(0.81-1.40),evidently negative abnormal of Eu,which may be the product of the partial melting of continental crust caused by under-depleting and heating from the upwelling mantle basaltic magama under post-collisional background.5.Middle Triassic(229.5 ± 5.6 Ma)biotite granite has characteristics of high Si(SiO2=68.89-71.04%),A1(A1203 = 13.95-15.71%)and Ca(CaO = 1.75-2.54%),low Mg(MgO ?0.63-0.81%,Mg#= 34-36)and Ti(Ti02 = 0.35-0.44%),high K while low Na(Na20 = 3.63-4.81,K20 = 3.52-5.45,K20/Na2O = 0.73-1.5),high Sr and low Y(Sr = 309 × 10-6-364×10-6,Y = 8.38 × 10-6-11.9 × 10-6,Yb = 0.97 × 10-6-1.17 X 10-6),Sr/Y = 27-42,the ratio of Nb/U = 4-5,the ratio of Rb/Sr=(0.24-0.31),relatively enriched incompatible element such as Ba,Rb,Cs,Th,depleted Nb,Ta,Sr,P,Ti,which is the product of partial melting of intermediate rocks from middle and lower continental crust and resemble Northern China Adak-ites.6.Combined with geological setting studying,it is concluded that Quagan Qulu ophiolitic belt-represented back-arc basin subducted to Alxa Block from 302Ma all the way to 246Ma,during which Yabulai-Bayinnuoergong arc was formed.From 246Ma until 23 5Ma,the back-arc basin closed and started to undergo post-orogenic evolution,on the northern of which Paleo-Asian Ocean closed during the same period(before 250Ma). |
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