Structure, Age, and Evolution of the Late Mesozoic East Mongolian Volcanic Belt

Abstract

Geochronological studies of volcanic rocks ascribed to the Late Mesozoic East Mongolian volcanic belt (EMVB) allow to distinguish several groups among them that differ in age and composition of igneous associations, as well as in the features of their distribution within the region. The earliest group (about 215 Ma) is represented by rocks of the bimodal association distributed in the western part of the EMVB. The next group (170–150 Ma) is predominantly formed by felsic lavas of the shoshonitic–latite association, which compose large volcanic fields in the eastern part of the EMVB. The rocks with an age of 140–105 Ma form the EMVB cover and are predominantly represented by trachyandesite basalts, whose fields are accompanied by small volcanoes of trachydacites and trachyrhyolites. Separate groups of extrusions of alkaline basaltoids were formed in the interval of 105–80 Ma. The rocks of different age associations are characterized by different geological position. The age and composition of the Late Triassic bimodal complex are compared with similar associations developed in central Mongolia and Western Transbaikalia framing the Khentey-Daur batholith. Their formation is associated with the evolution of the Early Mesozoic Khentey-Daur zonal magmatic area. Middle–Late Jurassic igneous complexes are distributed mainly in the eastern part of the EMVB. The fields of their outcrops fit into the distribution zone of the Late Jurassic volcanic fields of the Great Xing’an volcanic belt, with which they are close both in the formation time and composition of volcanic associations. Cretaceous magmatism determined the main history of the formation of the EMVB. Its development was accompanied by rifting of numerous depressions and grabens, as well as the regular evolution of its mantle sources, directed towards an increase in the proportion of the intraplate component. The dimensions, boundaries, and characteristics of the EMVB magmatism have been refined. The western boundary of the region is displaced to the east and is determined by a NW-trending end fault system that controls a chain of Early Cretaceous granite massifs, as well as rocks of the Late Cretaceous extrusive complex. The eastern boundary of the region is shifted to the west, where it coincides with the area of development of the rocks of the Early Cretaceous extrusive complex. Taking into account the data on the foreign position of the Late Triassic and Middle–Late Jurassic complexes in the structure of the EMVB, it has been established that the composition of the magmatic products of the belt is mainly determined by basic volcanic rocks. This is consistent with the idea that the EMVB is similar to other Late Mesozoic volcanic areas of the inland part of East Asia, in whose structure the proportion of felsic igneous rocks is subordinate.