Petrological Features of Korsun'-Novomyrhorod Anorthosite-Rapakivi Granite Pluton

Abstract

The Korsun’-Novomyrhorod pluton is the second after the Korosten one in terms of the scale of Proterozoic (1757-1748 Ma) anorthosite-rapakivi-granite magmatism in the Ukrainian Shield. According to geochronological data, pluton was formed as a result of multiple ascending and crystallization of basic to acidic melts. Differentiation of initial melts because to be responsible for gabbro-anorthosite and monzonites series crystallization. Whereas rapakivi granites, which are predominate in the modern erosion level, were formed from felsic magma not directly related with differentiation of basic melt. In view of the current level of mineralogical research, it is difficult to use modern geobarometry methods to reliably estimate the depth of rocks crystallization. At the same time, a number of factors (absence of volcanic and dike analogues of basic rocks, insignificant distribution of pegmatite bodies, predominance of high-Fe mafic minerals, absence of primary magnetite, etc.) indicate deeper conditions for rocks disclosed by modern erosional cut in comparition to similar Korosten pluton. Therefore, the liquid line of dissent, petrological and mineralogical features of the rocks can be explained by the reducing (low fO2) or abyssal conditions of their formation. It is possible that the deeper conditions of crystallization of parental melt are due to more distinctly developed syenitic trend of evolution with the appearance of high-Fe syenites during final stages. Preliminary data indicate on possibility of vertical layering of gabbro-anorthosite massifs, which manifested by increasing proportion of high-Fe basic rocks with depth. Available isotope-geochemical studies do not provide unambiguous data on regarding reservoirs of primary melts implaying both mantle and mixed mantle-crustal their origin. The evolution of the petrochemical features of basic rocks, in our opinion, is in better agreement with their formation as result of differentiation of the primary high-alumina tholeiitic melt, significantly contaminated by lower crustal material. This determined the subalkaline nature of basic rocks and a significant predominance of norites, in comparition to more typical gabbros, and monzonites. In contrast to the previously proposed hypotheses of the formation of intermediate rocks because of partial melting of felsic rocks by basic intrusions, or mingling of basic and acidic melts, some of petrochemical features and geological position can be satisfactorily explained by their crystallization from the residual melt.