Study On The Pre-eruption State Of Cenozoic Volcanic Group Alkaline Basalt In Datong,Shanxi

The Datong volcanoes group is located in the Datong Basin,which is situated at the junction between North China Craton and Xing-Meng orogenic belt.There are ~30 volcanic edifice in the basin along faults.According to the studies of volcanic geology and geochronology,the Datong volcanic group has been divided into Southeast(Middle-Late Pleistocene)and Northwest(Late Pleistocene)groups.We conducted comprehensive studies of volcanic geology,geochronology,petrographic and geochemistry on the Datong volcanic rocks from NW group and those long the fault from SE group.All the volcanic rocks studied belong to alkaline basalt.NW volcanic rocks have phenocrysts of olivine,clinopyroxene and plagioclase as well as glomerocryst of olivine and clinopyroxene.This indicates that cumulus olivine + clinopyroxene had been disaggregated in the shallow chamber which had been triggered by magma replenishment and mixing before magma eruption.The magma recharging and mixing is also supported by the crystal size distribution(CSD)curves,and further supported by the variation of major element which are not consistent with simple and closed-system fractional crystallization model,but relevant to the amount of both phenocryst and glomerocryst content.The microprobe analysis show that the core composition of the phenocryst in NW samples have similar composition with the rim of those in SE samples.According to the geothermobarometer based on crystal-liquid equilibria,the range of the crystallization temperature of the southeast samples is 1060-1187°C,and the pressure is 6.2-8.4kbar,corresponding to depth of 21-27 km.The geothermobarometric results based on phenocryst rim-liquid equilibria in the NW samples show that temperature is ~1060°C,and pressure is 2.3-2.6kbar,corresponding to the depth of 8-9km,while those based on phenocryst core-liquid equilibria had yielded a temperature range of 1113-1158°C,and pressure of 6.6-8.3kbar,corresponding to the depth of 21-27 km.This clearly shows that there are at least two magma chambers beneath the northwest of Datong volcanoes group.Since samples from both groups display similar characteristics in term of trace element,we suggest that the volcanic rocks of both groups were derived from the same lower crustal magma chamber.Accordingly,during the middle-late Pleistocene,mantle-derived magma ascended into the above continental crust and formed crustal magma chambers.Erupted magma directly from the lower crustal chamber formed the SE volcanoes,and also resulted in magma stopping in the shallower crust thus forming upper crustal magma chamber.Magma replenishment and magma mixing in the upper crustal chamber,lead to the magma eruption forming volcanos in northwest during Late Pleistocene.