Exploration Of Quaternary Basaltic Magma Evolution Process From The Wulanhada Area,North China

The volcanic activity records the whole process of magma origin,evolution and eruption,and is an important object in the study of the deep process of the magma system.In the magma storage and transportation system,there are partial melting,separation and crystallization,magma mixing,crustal mixing,etc.Magma mixing exists in most magma evolution processes,and the glomerocrysts,CSD curves in the petrography,the geochemical characteristics of the whole rock can indicate whether the volcanic rock formation process has magma mixing.Wulanhada located in the central part of Inner Mongolia Autonomous Region,is the only Quaternary volcanic group that has erupted in the Holocene.Under the dual influence of the NE trending basement fault and the Wulanhada-Gaowusu NE Great Fault,the volcanic group consists of more than 30 volcanoes that have erupted.In the past few decades,domestic and foreign scholars have focused on the eruption era,volcanic characteristics and tectonic evolution.This paper studies the neglected pre-eruption magma evolution process and the pre-epilation state.The chronological study shows that the Quaternary volcanic group can be divided into two types: Late Pleistocene and Holocene.The two types of volcanic edifices are well preserved,mainly composed of volcanic slag cones and lava flows.Rock is mainly tephrite.The rock is taupe-black,with stomatal structure and almond structure,porphyritic structure.Phenocryst have plagioclase,pyroxene and olivine.The average content of stomata in the late Pleistocene is 28%.The average content of the stomata in the Holocene is 14%.The plagioclase phenocryst is mainly labradorite,the pyroxene is sahlite and augite;the matrix is intergranular-intersertal texture,with a small plagioclase,alkaline feldspar,monoclinic pyroxene,olivine,magnetite,and volcanic glass.The CSD curves and the geochemical characteristics of the whole rock indicate that the volcanic rock formation process has magma mixing.The trace elements indicate that the Holocene deep magma has different evolution processes,which are derived from different magma chambers.The Nanliandanlu volcano is obviously different from the other Holocene volcanoes.The Late Pleistocene volcano originates from the same magma and has a similar evolution process.The characteristics of rare earth elements indicate that the volcanic rocks of the Holocene and Late Pleistocene originated from similar source regions,and the degree of Eu anomaly indicates that the magma evolution process of the two-stage volcanic rocks is similar during the formation process.The CSD characteristics show that the plagioclase in the Holocene volcanic rocks has a two-stage crystallization process,one of which exists in the Nanliandanlu volcano;the calculation by the clinopyroxene-melt thermobarometer indicates that the Holocene volcanic magma chamber depth is 14-24 km,there are two shallow magma that is 9-12 km and 4km respectively;the depth of the deep magma chamber in the late Pleistocene is 23-30 km,and the position of the upper magma chamber is about 15-20km;plagioclase-melt thermobarometer results show that the Holocene volcano and the Late Pleistocene volcano have a common deep magma chamber,which is located at a depth of 32-35 km.There are two stages of plagioclase crystallization in Nanliandanlu volcano.The shallow depth is 25-30 km,corresponding to the characteristics of the CSD curves.