Petrology of Granitoids of the Mayorka Massif (Gorny Altay): Contribution to the Problem of High-Silica Magma Formation

Abstract

Abstract ––The paper presents data from a comprehensive study of granitoids identified in the Mayorka intrusion that is located in the western part of the Altai Mountains. It is shown that the massif is composed of rocks of four intrusive phases, the age of these rocks ranges from 391 to 372 Ma, and the intrusion of the main volume of granitoids dates back to a relatively short interval of 386–384 Ma. The massif contains rocks of two geochemical types. The first type is differentiated calc-alkaline granite-leucogranites with near-clark contents of high-field-strength elements and rare earth elements: ɛNd(T) = + 4.3…+ 4.5 and σ18O V-SMOW = +10.7…+11.2 ‰. The second is alkaline and moderately alkaline A-type alyaskites, strongly enriched in high-field-strength elements and rare earth elements, having ɛNd(T) + 5.3 and σ18O V-SMOW = +11.6 ‰. Granitoids of the first group are of crustal source, while the rocks of the second group contain a significant portion of mantle material. The near-simultaneous introduction of these melts to the level of formation of the intrusion causes their interaction and the formation of hybrid magmas. Low crystallization temperatures of granitoids (<700 °C) and the presence of syngenetic melt and fluid inclusions in most rock varieties indicate a high fluid saturation of the melts. The abundance of leucogranites, whose geochemical characteristics cannot be explained from the standpoint of shallow differentiation of primary magmas, indicates the leading role of fluid-magmatic interaction processes in the formation of high-silica magmas.