Model for the genesis of Li-F granites in the Orlovka Massif, eastern Transbaikalia

The example of the Orlovka Massif in eastern Transbaikalia, a unique compli catedly differentiated intrusion of Li-Fe granites, is employed in developi ng 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 distribut ion of rare and trace elements in the most complete differentiation success ion of the "parental" Khangilai and the "daughter" Orlovka Massif. The reve aled 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, Nh , W, Sr, Zr, Ba, and Fl in the mineralizing medium during distinct stages i n the evolution of the Orlovka and Khangilai massifs. The effective hulk pa rtition coefficient of these elements are assayed, along with the partial K -feldspar-melt partition coefficients for K, Na, Rb, Ph, Sr, and Ba, mica-m elt coefficients for Li, Rb, Cs, Ta, Nh, F, Ti, Fe, Mn, and Mg, quartz-melt coefficients for Ta, Rb, and Fe, and zircon-melt coefficient for Zr. Our c alculation results indicate that a dramatic change in the crystallization c onditions occurred after the 65% crystallization of the original melt of th e Orlovka Massif, i.e., after the origin of the microcline-albite granites with pea-shaped quartz, a typical component of rare-metal granites, when th e H2O concentration of the melt attained 6 wt %. This event is marked by a contrasting change in the partition coefficients for all elements and, corr espondingly, a shift of the evolutionary trends of the compositional variat ions 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 postmagmati c metasomatism. This concept is confirmed by temperature estimates on the b asis of a variety of geothermometers, which point to a wide temperature int erval of 775-375 degreesC. Newly-obtained information on the geology, petro logy, and geochemistry of the rocks and minerals indicates that the Orlovka Massif of Li-F granites was produced by the differentiation of a single Kh angilai-Orlovka magmatic system.