Geochemical Characteristics Of The Igneous Rocks From Zusileng-Hangwula Moutain And Huoresen In Alax Block And Their Tectonic Significance

The northern margin of Alax is an important part of the central Asian orogenic belt and an important area to study the tectonic evolution of the ancient Asian ocean,there late Paleozoic igneous rocks are important to limit the closure time of the ancient Asian ocean.This article through the research of the ZhuSileng-HangWula gabbro and moyite,and the Holsen gabbro by using chronology,rock geochemistry and isotope geochemistry study,combining previous research results,reveals the rock cause of the moyite and gabbro and its tectonic background,to provide a new basis for the tectonic evolution of the central Asia orogenic belts.This study mainly obtains the following understanding:(1)The age of holsen gabbro is 289±6.1ma,which belongs to the transition series of high K calc alkaline to k-basanite.The SiO₂ content is 45.73-48.31%,abundant with Na(Na₂O average3.79%),K₂O average 1.7%,medium Al₂O₃(15.50%),TiO₂(1.88%),high Mg^#(57.46-63.81),Eu anomaly is not obvious,high REE content(434.84×10^-6-534.61×10^-6),rich in Rb,Sr,Ba and other large ion lithophile elements,The loss in Zr,HF,Ti,Ta,Nb,in the Zr-Ti,Zr-Zr/Y and Ti/100-zr-3*y diagrams,are all projected in the intraplate tension environment,indicating that they formed in the intracontinental extension tectonic environment.(2)The LA-ICP-MS zircon U-Pb age of the moyite exposed in the eastern part of hangwula is 260.5±2.7 Ma,which is of Late Permian.The rocks are characterized by high SiO₂(74.78%-77.44%),high K₂O(K₂O/Na₂O>1.3),and low CaO(0.23%-0.70%),which belongs to peralkaline(A/CNK<1,A/NK<1).In the chondrite-normalized rare earth element diagram,light rare earth elements are enriched,and have strong negative Eu anomaly,?Eu is 0.16-0.17;In the the primitive-mantle normalized trace element spidergram,Rb,Th,K are relatively enriched,and Nb,Ta,Ti,P are depleted;at the same time,10000Ga/Al>4.2,(Na₂O+K₂O)/CaO>13, Zr+Nb+Ce+Y>374×10^-6,which is A-type granite.In the Y/Nb-Yb/Ta and Nb-Y-Ce map solutions,moyite fall into the A₂ type granite area,forming in the structure of post collisional extension Environmental.(3)The LA-ICP-MS zircon U-Pb age of the gabbro in the east of HangWula is 231.1±6.0Ma,which belongs to the Middle Triassic.The SiO₂ content of the gabbro is 45.19%-52.12%,rich in Na(Na₂O3.27%-3.88%),poor in K(K₂O 0.02%-0.98%,moderate Al₂O₃(15.27%),TiO₂(1.90%),and MgO content(6.42%),which are low potassium to medium potassium calcium alkali transition series,relatively enriched in light rare earth elements and large ion lithophile elements Rb,Sr,U,depleted heavy rare earth elements and Nb,Ta,and Zr/y ratio(5.22)>4,Hf/Th(3.67),Th/Nb(0.27),Nb/Zr(0.05),no Eu abnormality,positive?Nd(t)value(+4.6⁺7.5)and relatively high?Hf(t)value(+10.9⁺12.2).The results show that gabbro originated from the asthenosphere mantle,which is the product of partial melting of garnet lherzolite and has been contaminated by the crust,and formed in the tectonic environment of intracontinental extension.(4)Combined with the regional geological research,the Paleo Asian Ocean in Alax area should have been closed before the early Permian,and entered the post collisional extension stage in the early Permian.In the Middle Triassic,the extensional tectonic magmatic event occurred again in this area.