Magmatic Evolution of the Salmagora Ultramafic-Alkaline-Carbonatite-Complex (UACC), Kola Alkaline Province, NW Russia

Abstract

The Salmagora complex, located in the southern part of the Kola Peninsula, NW Russia, shows a typical concentric zonation. This pluton consists of the sequence of rocks (dunite - wehrlite - ultramelilitolite - turjaite - pyroxenite - melteigite - ijolite - urtite - carbonatite) from the margin to the center of the complex. The Salmagora pluton comprises predominantly olivine-, clinopyroxene- and melilite-dominant ultramafic cumulates in the margin, clinopyroxene- and nepheline-dominant alkaline rocks in the center of the complex, calcite-dominant carbonatites intruded the complex at last. From the microscopic studies, dunite, wehrlite, ultramelilitolite, pyroxenite are consideres as cumulate facies, whereas the foidolites seem to have been formed from the residual magma of pyroxenite cumulates by crystal fractionation. Different from the simple chemical composition and mineral assemblage of the cumulate facies, pyroxenite-foidolite evolving series branched two types. They show different mineral assemblage, REE pattern, and chemical composition of minerals. One type is phlogopite, perovskite rich, the other is garnet, sphene rich in their minor mineral and whole-rock compositions with some HREE enrichment, which means they evolved from different sources. The spinel group minerals appear in all facies of the complex, and their chemical compositions varies from the spinel (sensu stricto) component to the pure magnetite component. The highest spinel component founded in the dunite cumulates evolves pure manetite component of the foidolite facies, which distinctly indicate inward crystallization sequence. The Al-Mg contents of spinel group minerals continuously decrease toward the later sequence, suggesting that all Salmagora rock units except the latter pyroxenite-foidolite series represent a continuum of a common magma. The latter pyroxenite-foidolite series seems to have been a different magma pulse which was derived from a lower-temperature regime of the source.