Geological Features Of The Ultrabasic Rocks In Dashizhai Area,Inner Mongolia And Its Tectonic Implications

The study area is located in Xing’anmeng, Inner Mongolia,between the NorthChina Craton and the Siberian Craton, and belongs to the Palaeo-Asian Oceanstructural domain. It has been discussed a long time for the age of Paleo-AsianOcean’s closing and the location of suture zone between North China craton andSiberia craton, two scenarios have been proposed for the location: some scholarsthought Hegenshan suture is the two cratons collided location, while the otherscholars thought Suolun mountain-Xilamulun river is the location of the suture. Inrecent years, more and more scholars began to agree Hegenshan suture is the locationof the two cratons collided. However, I t’s eastern part is not well exposed becausecovered by the Cenozoic strata or the rock bodies intrusion. When did the Hegenshansuture formed is also controversial, some scholars proposed it’s in the period fromDevonian to Carboniferous,Others thought it’s in late Permian.In order to solve theabove problems related,This article has done some research about Dashizhaiultrabasic rocks in Inner Mongolia.In this paper, on the basis of field geological mapping, combined with theanalysis of the samples in the laboratory, we explored the field geological,petrological, petro-geochemical and isotopic chronology characteristics of theDashizhai ultrabasic rocks, and made the following main achievements:1,Through the geological mapping, we have found the adjacent rocks of theseultrabasic rocks which emplaced structurally are sandy slate, siltstone and siliceousmudstone, which belongs to Wujiatun formation of Permian age.2,These ultrabasic rocks are composed of serpentinized pyroxene peridotite,dunite and peridotite with m/f about9which belong to magnesia ultramafic rocks,they have high content of MgO and Mg#and all of them locate in the transition zonebetween ultramafic cumulate and metaperidotite in the ACM and AFM figures,suggestting these rocks are remnants of ancient oceanic plate.3,The REE distribution patterns show slightly right-dipping, the total amout ofREE are relatively low and the LREE are slightly enriched, suggestting these rocks’ differentiation are not strong, δEu are between0.88and2.93, indicating fractionalcrystallization of the partial melts of the early mantle. Trace element diagrams showthese rocks are enriched in Cr、Ni、Ti and depleted in Nb、Rb, indicating they camefrom depleted mantle.4,Through the isotopic dating of Re-Os, we achieved their ages between475and313Ma which was the formation age of the ocean basin.5,By the comparition of the petrological, geochemical and isotopic chronologycharacteristics of these ultrabasic rocks with the Hegenshan’ultrabasic rocks in thewest,and they have the very high similarity, we believe that this area is probably theeastward extension of the Hegenshan suture. So this paper provide important newevidence for the problem of the eastern of the Hegenshan suture and clarify the timeof the ultrabasic rocks formation.