The Stratigraphical Subdivision And The Formation Of The Mesozoic Volcanic Sequence In The North Part Of Hulunbaier, Inner Mongolia

The volcanic rock series in the north of Hu Lunbaiershi, Inner Mongolia are partof the vast Mesozoic peri-Pacific mega-volcanic strip that spans along the east ofChina. They broadly lasted from late Jurassic into early Cretaceous in age. Limited bythe regional data, however, so far there are different opinions about the subdivisionand the correlation, especially the formation of the volcanic series.On the other hand, the analyses of the petro-chemistry, and the rare earth element(REE), as well as the trace elements show that the main of the Mesozoic volcanicrocks in this area are potassium-rich calc-alkali series; Most of them fall into theB-zone, i.e. the stretching environments, in the diagram of tectonic setting plot.Chondrite-normalized REE plots show that all the samples are relatively enriched inlight REE with strong fractionation of all elements; The intermediate and basic seriesare characterized without negative Eu anomaly and, however, the acidic ones withobvious negative Eu anomaly. Furthermore, the analyses of petro-chemistry also showthat the Tamulan’gou Fm. and the overlying three formations, i.e. Manketou’ebo Fm.,and Manitu Fm., as well as Baiyin’gaolao Fm., should belong to the sameevolutionary series of magmatic cycle and, however, the Meiletu Fm., the other cycle.Accordingly, it can be determined primarily that the volcanic series in the north of HuLunbaiershi come from the eruptions at the active margin to intra-continents setting,which mainly derived from the mantle and the mixture of crust and mantlecomponents.Based on the previous data, it can be determined further that the Type-I rhyoliteand the basalt in Tamulan’gou Fm., which consists of a continuous evolutionarygenetic series, should be derived mainly from the crystallization differentiation; TheType-II rhyolite and the basalt, not a genetic and co-magmatic series, should beresulted from the melting of the lower-crust of the lithosphere. Additionally, theappearance of the Late Yanshanian Type-A granite indicates that the regional tectonicsetting has been changed from the stretching stress patterns into the tensile or rift ones during the late Early Cretaceous.To sum up, the formation of the volcanic rocks series can be concluded asfollowed: At first, it should be derived mainly from the subduction process of thePacific plate toward the Eurasia continent. Later, it should be resulted partly from themantle plume uprising and the lower-crust delamination.