Geochemical specifics of ongonites in the Ary-Bulak Massif, eastern Transbaikalia

A genetic model for magmatic rocks of the Ary-Bulak Massif is discussed based on a detailed geological map of the massif (prepared by the authors) and on original data of the authors on the petrography of the massif, its compositional zoning, trace-element geochemistry, physicochemical parameters of its crystallization, and melt inclusions in its minerals. The Ary-Bulak Massif was determined to be zonal, with the predominance (approximately 70% by area) of porphyritic topaz ongonites (central facies), which grade toward contacts into weakly porphyritic ongonites bearing topaz and, occasionally, fluorite (margin facies). Aphyric rocks with fluorite (inner-contact facies) occur as a stripe 50–80 m wide at the southwestern inner contact of the massif. Analysis of petrographic and geochemical data indicates that subvolcanic rocks of the Ary-Bulak Massif differ from typical elvanes (as they occur in the Cornwall province) but are similar to classic ongonites in the central and marginal facies of the massif. Rocks in the southwestern inner-contact zone are unusual high-F and high-Ca varieties, whose analogues have never been found in any rare-metal provinces with ongonites and which provide evidence of a complicated evolutionary history of the Ary-Bulak Massif. The geochemical evolution of this massif was determined to be characterized by the enrichment of the older inner-contact facies rocks in CaO, K₂O, F, and Rb, Cs, B, Ba, Sr, Sn, and Ta, whose concentrations decrease in the ongonites of the central facies. The central-facies ongonites thereby have much higher Na₂O and Li concentrations than those in the inner-contact facies rocks. It is demonstrated that the intense heating and melting of crustal material in this region at the Jurassic-Cretaceous boundary could have been induced by subalkaline basaltic magma. The chemical composition of the rocks, which is unusual for typical ongonites in, for example, high Ca and Sr concentrations, could be caused by the possible assimilation by the magma of limestones, which occur in the territory at a certain depth in the Ust’-Borzya Formation that hosts the Ary-Bulak Massif. The genesis of most rocks in the massif was controlled by the magmatic differentiation of crustal granitic magma, with the residual melts forming Li-F granites enriched in several trace elements (Li, Rb, Cs, B, Ba, Sr, etc.) and ongonites as their subvolcanic analogues.