MODEL FOR THE GENESIS OF LI-F GRANITES IN THE ORLOVKA MASSIF, EASTERN TRANSBAIKALIA

Abstract

The example of the Orlovka Massif in eastern Transbaikalia, a unique complicatedly differentiated intrusion of Li-Fe granites, is employed in developing a model for the genesis of mineralized Li-F granites. The model involves magmatic and metasomatic stages and is based on the study of the distribution of rare and trace elements in the most complete differentiation succession of the "parental" Khangilai and the "daughter" Orlovka Massif. The revealed regularities are correlated with the results of numerical simulation. Mass-balance calculations with the use of the actual compositions, volumes, and spatial distribution of all rock types were conducted with the aim to evaluate the concentrations of major and trace elements (Li, Rb, Cs, Ta, Nb, W, Sr, Zr, Ba, and F) in the mineralizing medium during distinct stages in the evolution of the Orlovka and Khangilai massifs. The effective bulk partition coefficient of these elements are assayed, along with the partial K-feld-spar-melt partition coefficients for K, Na, Rb, Pb, Sr, and Ba, mica-melt coefficients for Li, Rb, Cs, Ta, Nb, F, Ti, Fe, Mn, and Mg, quartz-melt coefficients for Ta, Rb, and Fe, and zircon-melt coefficient for Zr. Our calculation results indicate that a dramatic change in the crystallization conditions occurred after the 65% crystallization of the original melt of the Orlovka Massif, i.e., after the origin of the microcline-albite granites with pea-shaped quartz, a typical component of rare-metal granites, when the H2O concentration of the melt attained 6 wt %. This event is marked by a contrasting change in the partition coefficients for all elements and, correspondingly, a shift of the evolutionary trends of the compositional variations in the rocks and minerals. The transformation in the conditions and/or mechanisms of crystallization might have been related to the appearance of a new phase (fluidized melt) in the system and/or the onset of postmagmatic metasomatism. This concept is confirmed by temperature estimates on the basis of a variety of geothermometers, which point to a wide temperature interval of 775-375°C. Newly-obtained information on the geology, petrology, and geochemistry of the rocks and minerals indicates that the Orlovka Massif of Li-F granites was produced by the differentiation of a single Khangilai-Orlovka magmatic system.