New Types of Basic Rocks in the Korsun-Novomyrhorod Anorthosite-Rapakivi Granite Pluton as an Indicator of Its Petrogenesis

Abstract

Two species types of basic rocks contrasting in chemical and mineral composition were investigated within the Korsun-Novomyrhorod anorthosite-rapakivigranite pluton (KNP) of the Proterozoic age. In comparison to more typical basic rocks of KNP, the investigated rocks are presented by the extremely Fe-rich fayalite gabbroids and the most MgO enriched biotite gabbronorite. The first of them occupy a deeper level of the Horodishche massif in the central part of KNP. According to a high-Fe association of minerals and reduced or low An content in plagioclase they may represent the crystallization of a highly evolved basic melt (after anorthosite and gabbronorites separation), which were crystallized under abyssal conditions and low oxygen fugacity. Preliminary geochemical data indicate that trace elements concentration and negligible negative Eu-anomalies (0.72-0.95) are similar to most distributed basic rocks but unlike the last it is slightly differed by decreasing La/Yb and enriched in Sc (up to 118 ppm). Thus, we suppose those rocks might be crystallized as a result of mixing highly differentiated (iron and alkali enriched) melt with the early generation of anorthitic plagioclase, with subsequent dissolution of the last. Enrichment in iron of the mafic minerals and increasing of alkalinity of plagioclase in the basic rocks is consistent with the appearance of ferrodioritic melts as a product of prolonged crystalline differentiation of the initial melt. In contrast to fayalitic gabbroids, the pyroxene-biotite gabbronorites from the border zone in according to increased Mg# of the mafic minerals and rocks are obviously the least differentiated varieties of the anorthosite-gabbronorite series. The regularities in chemical composition in such type of rocks are consistent with the liquid line of dissent for basic rocks in KNP, which implies their crystallization at an earlier stage of magma ascending. By composition, such melt can be formed at an intermediate stage from slightly differentiated melt. This is indicated by enrichment in Sr (453-881 ppm) and Ba (910-930 ppm), Eu/Eu* (0.85-1.10), increased content MgO (up to 8 wt. %), Cr and V (59-193 and 169-350 ppm respectively). At the same time these rocks are enriched in Zr and Hf (378-478 and 10.3-12.02 ppm respectively), highly enriched in Rb (169-192 ppm), with moderate Nb and Ta content (14.6-18.1 and 0.91-2.84 ppm respectively) that point out to interaction and partial assimilation by crust material. Summarizing geological data of the deep drill-holes, it is possible to reveal a general direction of the mafic minerals evolution in the basic rocks and the evidences of cryptic layering. The last are quite clearly manifested both in the large gabbro-anorthosite massifs and individual intrusive bodies. We suppose that the evolution trend of mafic mineral composition are consistent with the tholeiitic trend differentiation of the primary melt with gradual increasing of iron content (under low oxygen fugacity) during differentiation and ascending in the upper crust. According to this interpretation of the inner structure of gabbro-anorthositic massifs, on the modern erosional level of KNP are often exposed the fragments of the upper (or lateral) layered series (Fe-enriched), less often the drill-holes reach up to the heads of the lower layered series.