Amphibole and biotite of melanocratic rocks from the Ural granitic massifs: composition, relationship, petrogenetic consequences

Abstract

Research subject. The article discusses the features of the relationship between biotite and amphibole on the example of magmatogene melanocratic rocks from a number of granitoid massifs of the Urals. These rocks form xenoliths and synplutonic intrusions of the calc-alkaline series of normal alkalinity: gornblenditam, gabbro, diorite, quartz diorite. They are composed of amphibole, acidic or middle plagioclase, in a subordinated quanitity they contain clinopyroxene, biotite, potassium feldspar, quartz.Materials and methods. The composition of the minerals of melanocratic rocks was determined on a JSM-6990LV electron microscope with an EDC-adapter of INCA Energy 450 X-Max 80 in the Geoanalytical Center of the IGG Ural Branch of the Russian Academy of Sciences.Results and conclusions. Wide variations in amphibole compositions and narrow biotite variations caused by exchange processes between the mineral and postmagmatic fluid are shown. The phenomenon of replacement of early magmatic amphibole with biotite is substantiated by the presence of a gap in the crystallization temperatures of minerals, indicating a lack of physicochemical equilibrium between them. Their structural relationships confirm the development of biotite as a result of the replacement of amphibole in accordance with the competent and incompetent phase boundaries. In the first case, the structural packets of biotite are embedded along the silicon-oxygen chains of amphibole, which is expressed in the parallelism of the (001) plane of biotite with the (100), (110) planes of amphibole. In the second case, the development of biotite occurs irregularly, inheriting the system of cracks in amphibole. The distribution of Mg/Fe between biotite and early magmatic amphibole was studied, showing that the magnesia value of biotite is higher than that of amphibole replaced by it. Inverse ratios of magnesia value occur between biotite and post-magmatic amphibole. The equality of the magnesia values of both minerals may reflect the conditions of subsolidus equilibration of the compositions. The problem of choosing amphibole compositions for calculating the PTparameters of the formation of massifs in the Earth crust is considered.