Trace element ratios as indicators of source mixing and magma differentiation of alkali granitoids and basites of the Haldzan-Buregtey massif and the Haldzan-Buregtey rare-metal deposit, western Mongolia

Abstract

The Haldzan-Buregtey group of alkali granitoid massifs with an age of 391–395 Ma is located among the Early Caledonides of the Ozernaya zone of western Mongolia and consists of seven intrusive phases, including two rare-metal phases with Zr, Mn, Y, and REE mineralization. In order to identify the magma sources of the massifs, the abundances and canonical ratios of incompatible trace elements in the rocks of various intrusive phases are analyzed and compared with those in the volcanic rocks of Pantelleria island. The latter rocks were taken as the reference association of rocks linked through crystallization differentiation. The rocks of the Haldzan-Buregtey Complex were formed by mixing an OIB source (with participation of MORB) and host ophiolites, while alkali granitoids of phase 2 originated via mixting these sources with the host non-alkaline granitoids. Practically all rocks have mixed sources, with all transitional varieties from OIB, MORB to ophiolites. OIB was the main source for the rocks, while the host ophiolites could serve as sources for anatectic magmas or contaminants of the magmas of other considered rocks. The rare-metal granitoids were produced from the same sources as the barren magmatic rocks of the Haldzan-Buregtey Complex. The rocks of the Haldzan-Buregtey Complex show a bimodal distribution, with the practically complete absence of intermediate varieties between basite dikes and syenite-granite rocks. This seems to be related to the formation of the least differentiated sialic rocks (nordmarkites, pantellerites, some alkali granites) by anatexis of their own parental basite rocks (dolerites and basites), their cumulates, or ophiolites. Most of the phase-2 alkali granites likely resulted from the differentiation of the phase-1 nordmarkites coupled with assimilation of the host ophiolites. Ekerites are geochemically similar to the nordmarkites and can be interpreted as their residual in situ melts or their anatectic melts.