Composition of Micas from Rocks and Melt Inclusions in Quartz of Li–F Granites of the Orlovka Massif in Eastern Transbaikalia as a Genetic Indicator

Based on the study of the chemical composition of rock-forming micas and micas from melt inclusions in quartz of a full range of differentiates of Li–F granites of the Orlovka massif in Eastern Transbaikalia, possible mechanisms of the massif formation are discussed. An early stage with a trend of the mica evolution (biotite–Li-containing aluminous annite–Li-rich phengite-muscovite) in rocks, which is manifested in the synchronous Li and F accumulation in the melt, mica from the rock, and melt inclusions (an ongonite trend of the melt evolution), has culminated in the formation of porphyroblast microcline-albite granites with Li-rich phengite-muscovite and snow-ball quartz. It was this porphyroblast granite melt that has undergone the subsequent evolution (crystallization fractioning, repeated manifestation of silicate-salt liquid immiscibility, and post-magmatic metasomatism), which determined the development of the ‘apogranite process’. Melt inclusions in quartz of porphyroblastic microcline-albite granites and later amazonite-bearing rocks contain exclusively lithium-free, high-alumina muscovite. The high Li and F contents in glasses of these melt inclusions obtained after the homogenization experiments indicate the muscovite crystallization in a heterogeneous system from a depleted aluminosilicate melt coexisting with an isolated Li–F-containing hydrosalt phase. The results obtained indicate the convergence of the Li–Fe mica formation mechanism, which makes probable their crystallization both from a fluid-saturated melt (two-mica granites, porphyroblastic microcline-albite granites, amazonite granites of the southwestern flank of the massif and their pegmatitoid bodies) and as a result of metasomatic reworking of a substance (amazonite granites of the main dome) at the late-postmagmatic stage of the Orlovka massif formation.