Mineralogy of Crystallized Melt Inclusions from Gardiner and Kovdor Ultramafic Alkaline Complexes: Implications for Carbonatite Genesis

Abstract

Cumulus olivine, clinopyroxene, melilite and perovskite from silicate INTRODUCTION rocks and carbonatites of the Gardiner (East Greenland) and Kovdor Carbonatite-bearing ultramafic alkaline complexes gen(Kola Peninsula) ultramafic alkaline complexes contain primary erally exhibit chaotic field relations because of explosive melt inclusions crystallized into aggregates of daughter minerals. magmatism, metasomatic alteration and high activity of The petrography and homogenization temperatures of the inclusions volatiles. In these complexes the primary characteristics constrain the fractionation paths and the formation of carbonatites of the parental melts and the primary liquidus asin the complexes. Carbonated melanephelinite was parental to both semblages may be obscured by post-magmatic reactions, complexes and early cumulates (dunite, olivinite, peridotite and and volatile components are easily lost during crysmelteigite) are comparable. The common occurrence of phlogopite tallization together with components soluble in fluids. and amphibole in the inclusions and in the host rocks indicates Melt and fluid inclusions in liquidus minerals constitute that these were important liquidus phases. In both complexes the microscopic natural autoclaves and their study is espefractionation of phlogopite and amphibole drove the melts towards cially powerful in volatile-rich systems. The petrography Ca-rich (larnite-normative) compositions. At the ijolite stage the and composition of inclusions constrain petrogenetic evolutionary trends are believed to separate and the evolved larnitemodels for ultramafic alkaline complexes and carbonatite normative melt produced calcite-bearing ijolite in Kovdor and formation. Carbonatites and related silicate rocks have melilitolite in Gardiner. The two assemblages are related by debeen the subject of melt and fluid inclusion studies for carbonation reactions. A fractional crystallization origin is suggested several decades (e.g. Roedder, 1984, pp. 406–411), and for the Kovdor carbonatite, whereas the Gardiner carbonatite formed such studies have revealed traces of strongly alkaline by liquid immiscibility in the course of melilite fractionation. carbonatite melt associated with alkali-poor plutonic carNa–K–Ca and Na–Mg carbonates are common daughter phases bonatite (Aspden, 1981; Le Bas & Aspden, 1981; Kogarko and especially abundant in late-stage inclusions. Thus, all caret al., 1991). bonatite melts in the two complexes are alkaline. Calcite carbonatites Here we present results of an investigation of crysappear to be cumulates. tallized melt inclusions in rock-forming minerals from