The Baga-Gazriin Chuluu A-type granites of Central Mongolia compared with other igneous bodies nearby: a geochemical approach

Abstract

The subject of the study are the relationships between the Late Triassic A-type granites of the Baga-Gazriin Chuluu pluton in Central Mongolia and the magmatic bodies located either adjacent to or a short distance from this intrusion. We compare new geochemical data from the most representative samples of the Baga-Gazriin Chuluu pluton with published data from the Mesozoic granitoids of the Egiindavaa complex and Tsagduul massif in the Hotont area. The geochemical analyses of all these granitoids are also compared with the geochemical characteristics of trachyandesites of unknown age which neighbour the Baga-Gazriin pluton. The Baga-Gazriin granites, most of the Tsagduul granites and a few samples from Egiindavaa are A-granites with strong crustal affinity. By contrast, most of the Egiindavaa granitoids include considerable mantle-derived material and so seemingly formed in different conditions, with a higher water content in the magma. The trachyandesites adjacent to the Baga-Gazriin Chuluu pluton have been referred to a Permian volcano-sedimentary complex, making them distinctly older than the granitoids. All the A-type granites discussed were emplaced in extensional conditions, which developed after cessation of the compressional regime resulting from the closure of the Mongol-Okhotsk Ocean. The shallow-lying source for their generation was connected with slab break-off or the presence of a hot spot. The granitoids, in the late stage of crystallisation, were enriched in complex ions that were REE carriers, as shown by a distinct “tetrad effect”.