Uncommon Ca- and F-rich rocks in the ongonite massif Ary-Bulak (Eastern Transbaikalia, Russia) as a result of crystallization of fluoride-calcium and alumosilicate immiscible melts

摘要

The zonal massif Ary-Bulak forms a domal stock, sized 0.7x1.2 km, cross-cuting sedimentary-volcanogenic rocks of the Devonian age. The rocks of the massif were formerly described as ongonites, the subvolcanic analogs of rare-metal lithium-fluorine granites. The analyses made for 31 samples of the Ary-Bulak rocks revealed prevalence of K over Na, what is typical of elvans. The center of the massif is composed of porphyric ongonites holding phenocrysts of quartz, sanidine, albite, sometimes topaz and mica. The matrix of rocks consists of quartz grains (20-30 μm) and tabular albite (30-80 μm) rimmed with feldspar (5-20 μm). The micro-porous spots of rocks and quartz microlites contain the aggregates and separate inclusions of spicular topaz respectively. In the endocontact zone porphyric rocks are replaced by glassy aphyric rocks lying 50-200 m from the contact with hosting schists. The porphyric ongonites incorporate varieties with anomalously high CaO (3.3-21.8 wt%) and F (2.7-16 wt%). The rocks of this type occur sometimes in the central part of the massif, but they are mostly abundant in the marginal zone. High CaO (7.8-18 wt%) and F (7.1-15.5 wt%) are characteristic of all investigated samples of aphyric rocks (table). The porphyric and aphyric Ca-rich rocks display direct correlation between Ca and F.