| Southeast Inner Mongolia is located in the east part of the Central Asian Orogenic Belt?CAOB?,which is the key study region of late Paleozoic tectonic evolution of the CAOB.There are several ophiolite belts in this area,such as the the Hegenshan,the Solonker,Wenduermiao,Daqing pasture-Diyanmiao and the Xar Moron,These ophiolite belts likely represent the residual of the Palaeo-Asian Ocean.In this paper,we choose the Xingshuwa tectonic melange in the central of the Xar Moron ophiolite to Carry on the detailed field geological mapping and indoor analysis and test.Determine the rock composition,formation age and its tectonic nature of the tectonic melange,combined with the previous data,discussing the late Paleozoic tectonic evolution of the Southeast Inner Mongolia.Xingshuwa tectonic melange include a set of exotic block,such as limestone,serpentine,basalt,gabbro,silicalite and siliceous siltstone with the fine clastic sedimentary rock.It's not the normal sedimentary strata unit.The matrix is gray,greyish-black or greyish-green colored siliceous sandstone,argillaceous sandstone,tuffaceous siltstone and fine sandstone,suffered late tectonic deformation.The native medium and thin layer rhythmic layering can be seen in a hard siliceous siltstone,according to the characteristics of flysch,reflecting the characteristics of the deep-sea turbidite deposit.The serpentine?serpentinization peridotite?,basalt,gabbro andsilicalite make up the"steinman trinity",On behalf of dismembered ophiolite.Limestone rocks may represent the carbonate platforms scraped off in the process of the oceanic plate subduction,or the carbonate formation sedimentary in the stable continental margin.Zircon U–Pb dating of the gabbro in the north of the Banjiezigou serpentine mining pit reveals the Xingshuwa ophiolite formed at 273.9±1.3Ma,belonging to the late early Permian.This age is in keeping with the age of metamorphic basalt in the east of the Xiaoxingshuwa and that of the gabbro in the Jiujingzi,and it is also similar with the age of the radiolarian in the the siliceous rocks.It is suggested that ocean basin existed in the south-east Inner Mongolia the ocean basin at the time of late early Permian.The basalt in different locations of the melange shows different geochemical characteristics.The basalt in the Sorth of the Renjiayingzi has the characteristics of high MgO values?6.37-8.01%?,high Fe2O3T values?9.72-13.76%?and enriched in the light rare earth?LREE???La/Yb?N=2.36-3.61?,showing the enrichment of mid-ocean ridge basalt?E-MORB?characteristics.Banjiezigou basalt has the similar value of Mg?MgO=4.75-7.25%?and Fe?Fe2O3T=8.31-11.32%?with the Renjiayingzi basalt,it has a lower total alkali?N2O+K2O?content?2.84-4.42%?,belonging to the the alkaline basalt,depleted in the light rare earth?LREE?,showing the characteristics of normal mid-ocean ridge basalt?N-MORB?.The basalt in the north of the Beikundui is rich in large ion lithophile elements?LILE?,depleted in high field strength elements?HFSE?Nb,Ta,showing characteristics of island arc basalt.the white metamorphic basalt in the north of the Xiaoweitang serpentine mining pit has a relatively low value of?REE?10.72-29.90ppm?,and high value of Ni?144-842ppm?and Cr?260-2257ppm?,rich in LREE??La/Yb?N=1.07-3.72?.In the tectonic environment discrimination diagram,the white metamorphic basalt projections to the calc-alkaline volcanic rocks associated with subduction zone.The analysis shows that basalt rocks distribute in the tectonic melange with different chemical composition and genesis of the rock,which is further reflecting the"mixed structure"attribute.It is also indicated that a long-term development of the broad ocean basin existed until the late early Permian.Based on the previous data of the strata,paleontology and magmatic rocks,it is inferred that The Xar Moron ophiolite?including Xingshuwa,Jiujingzi and Kedanshan ophiolite?representative the final closed position of the Palaeo-Asian Ocean,closed at the time of late Permian.to early Triassic. |
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