Geological Characteristics And Geochemistry Of Volcanic Rocks In Bilike，Inner Mongolia

There is a large expanse of roughly NW-SE distribution of the Cenozoic volcanicarea in the Xilin Gol League, Inner Mongolia. It is about800meters north-south and500meters east-west, and covers an area of about9300km2. This extensive lavaregion, linked with the Dariganga Plateau in SE Mongolia, straddling the borderbetween China and Mongolia, is the largest area of late Cenozoic intraplate lava fieldsin eastern Asia. The Abaga in the northwest and the Dalinuoer in the southeast hostnumerous (nearly300seats) volcanic cones and broad basalt platforms. In contrast theBeilike area, lying in the middle, is distinctive to them, for its characteristics of simplepetrology of tholeiite, no volcanic cone or vent surviving from aging, and mantlederived xenoliths free. Therefore, according to volcanic geology, the Cenozoic basaltsin the Xilin Gol League can be divided into three volcanic fields from northwest tosoutheast in proper sequence, namely, Abaga basalt, Beilike basalt and Dalinuoerbasalt．The Beilike basalt consists of three levels of lava platforms at differentelevations. The elevation increases from the first-level lava platform to third-levelplatform (the elevations are in turn:1034~1120m,1150~1200m, more than1280m),but their corresponding age gets old (the late Pliocene (2.31~2.41Ma), the earlyPleistocene (1.56~1.61Ma) and the middle Pleistocene (0.51~0.61Ma)). An abnormalcontact relationship exists between the elevation and the age with, can be related tononuniform episodic uplift of the Neotectonic movement. By the middle and lateperiod of the Middle Pleistocene Epoch（0.08-0.48Ma）, alkalic basaltic magma fromthe Dalinuoer volcanic field locally flew northwest and part covered the first-levellava platform and second-level lava platform.The Abaga and Dalinuoer volcanic rocks are composed dominantly of alkalibasalts with subordinate tholeiitic basalts. By contrast, volcanic rocks from theBeilike area are all tholeiitic basalt. In lithology, first-lever lava platform is olivinetholeiite, second-level lava platform and third-lever lava platform are quartz tholeiite. Phenocrysts of olive tholeiite is chrysolite (Fo=72-83), whose content is5%-8%, having out-of-shape, and development of corrosion-shaped harbor, andirregular cracks. Diameter of more than1000μm poly-phenocrysts are found locally.Interstitial structure can be seen in groundmass, which consists of fayalite and augiteand labradorite.Olivine tholeiite is distinctive to quartz tholeiite for olivine appears in bothphenocrysts and groundmass. On the contrary, Phenocrysts in quartz tholeiite is rare(＜3%), belonging to chrysolite-fayalite(Fo=55-81); matrix minerals with a widerange of changes: fayalite-hortonolite (Fo=35-67), augite, andesine-labradorite.In general, quartz tholeiite is different from olive tholeiite, for fewer phenocrystcontents and biggish mineral ingredients change.That more common olivine dissolutions are found in olive tholeiite implies thatstrong reaction between the early crystallization of olivine crystal and melt occured.Olivine does not appear in the reaction rim of orthopyroxene in tholeiite that is similarin most regions of eastern China. What more, lacking of hypersthene and pigeonite, sotholeiite has an excessive nature of the characteristics.Based on the main and trace element geochemistry, there does not exist theevolutionary relationship between the olive tholeiite and quartz tholeiite. Tholeiite isinterpreted to be resulted probably from the varying degrees of the garnet peridotitesource region and the depth of partial melting. From early stage of Quartz tholeiite tolate olive tholeiite, magma source gradually became deeper and the degree of partialmelting gradually reduced (14-16%,12-14%, respectively). When magma was risingto the surface, both olive tholeiite and quartz tholeiite were affected by crustalmaterial contamination. It is worthwhile to note that quartz tholeiite has obviouslybeen the impact of lower crust granulite contamination mainly reflected in the mildloss of Nb-Ta, and this phenomenon in the Cenozoic basalts was first discovered inNorth China.The history of lithospheric mantle beneath the south Xing’an-Mongolia OrogenBelt is complicated and it experienced multi-periods mantle metasomatism.Compared with the northern margin of North China Craton, they have a similar magma source and the lithospheric mantle thermal state, but different enrichmentlithospheric mantle types. In the Sr-Nd isotope chart, the mantle sources of the lateCenozoic basalts from Xing’an-Mongolia Orogen Belt display a DMM-EMⅡ arraydifferent from the northern margin of North China Craton, which showsDMM-EMⅠmixing trends. The interaction between the asthenosphere melts and theoverlying lithospheric mantle is a genetic model of tholeiite magma, and the differenttypes of interaction may be related to different ages and tectonic settings of thelithospheric mantle.Based on the asthenosphere-lithospheric mantle interaction model, lithosphericthinning phenomenon in North China is not confined to the craton, instead the southmargins of Xing’an-Mongolia Orogen Belt experienced lithospheric thinning too,which occurred alternatively. Overall, the lithospheric thinning of Jining was theearliest and strongest, while the Beilike region was the latest, and of the weakestdegree. It suggests that the degree of lithospheric thinning became weakened overtime.