Geochemical Characteristics Of Mafic Volcanics And Discussion Of Geotectonics Evolution From Carboniferous To Permian In Yin-E Basin

Yin-E basin, located in the Paleo-Asia Tectonic Domain,and in the intersection part of the Sibiria and Kasakstan plates, formed the mineral in Carboniferous-Permian. The tectonic evolution in this time has an important influence to the basin. This paper try to do a preliminary discussion of the tectonic evolution by mafic igneous rocks combining the petrology, mineralogy, geochemistry and isotope geology.Carboniferous mafic volcanic rocks in this study are pyroxene cumulate basalt, basalt and basalt-andesitic lavas.Pyroxene cumulate basalt has the feature of high TiO2(3.29% ~ 3.50%), high MgO(6.28%~6.56%) and low Al2O3(12.74%~13.22%) which belongs to alkaline series; in contrast, basalt and basalt-andesitic lavas have the feature of high Al2O3(15.40%~20.65%)and low TiO2(1.10%~1.78%)which belong to calc-alkaline series. The rock have low total rare earth content(∑REE=68.00×10-6~132.58×10-6) and the heavy rare earth is enrich than light rare earth(LREE/HREE=4.14~5.29), in which there are a degree of fractionation[(La/Sm)N=1.62~2.62,(Gd/Yb)N=1.49~2.66]. Geochemistry of Carboniferous volcanic rocks show that they formed in the transition mantle or enriched mantle then occurred fractional crystallization of olivine and assimilation and hybridization with crustal material during diagenesis. Pyroxene cumulate basalt was similar to OIB and basalt and basalt-andesitic lavas was formed in the plate tectonic setting.Permian volcanic rocks in this study are composed with olivine basalt、picritic basalt and latent diabase porphyrite, all of them have the feature of TiO2(1.91% ~ 2.48%), high Al2O3(13.65% ~ 15.67%), sodium-rich(Na2O / K2 O = 1.78 ~ 25.11). Olivine basalt and picritic basalt belong to alkaline series and latent diabase porphyrite belongs to calc-alkaline series. The rock have low total rare earth content(∑REE=68.00×10-6~132.58×10-6)and the heavy rare earth is enrich than light rare earth(LREE/HREE=2.51~7.32), in which there are a degree of fractionation[(La/Sm)N=1.09~2.66,(Gd/Yb)N=1.32~2.42]. Geochemistry show that primary magma of olivine basalt experienced weak fractional crystallization of olivine before consolidation diagenesis which was similar to OIB. While, picritic basalt and latent diabase porphyrite Experienced strong olivine fractional crystallization, weak clinopyroxene fractional crystallization and assimilation contamination before consolidation diagenesis which were formed in the plate tectonic setting.The mafic igneous rocks can be divided into HT basalt and LT basalt by the ratio of Ti/Y. HT basaltwas made directly from the original magma consolidation without crystallization differentiation which were similar to OIB. LT basalts were the basalts by continental crustal contamination which were formed in the plate tectonic setting. Comparing to Carboniferous-Permian volcanic rocks in the Tianshan and its adjacent areas we find that both of them have the same geochemistry, which shows that Yin-E basin have an affinity for the Central Asian Orogenic Belt.The study area entered the rift basin evolution stage from the late Devonian,the rift basin had been experiencing four stages in Carboniferous- Permian:① rift initial development stage in Early Carboniferous;② rift basins growth stage in Late Carboniferous- Early Permian;③ rift basin gradually contraction phase in Early- Middle Permian;④ rift basin shrinking-disappearing stage in late Permian. Maybe the driving force is the plume and Olivine basalt in Hal Hai group Late Permian suggested the end of the mantle plume activity, what we need further studies to prove.