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1 Geological Quarterly

1 2012

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

Machowiak K., Stawikowski W.

Geological Quarterly

2012

Vol. 56, No 3

457--474

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 T sagduul 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 thus 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”.