Mineral chemistry and crystallization sequences in kimberlite and lamproite dikes from the Sisimiut area, central West Greenland

Abstract

Precambrian and Eocambrian ultramafic and ultrapotassic dike rocks from the Sisimiut (Holsteinsborg) area of West Greenland can be grouped into three main types each showing characteristic mineral parageneses. Micaceous kimberlites contain macrocrysts and phenocrysts of olivine and phlogopite set in a groundmass of olivine, phlogopite, diopside, perovskite, and spinels. Leucite lamproites contain olivine, leucite (pseudomorphed), and phlogopite macrocrysts and phenocrysts in a matrix of phlogopite, leucite, diopside, and rutile. Amphibole lamproites contain phenocrysts of diopside in a fine-grained matrix of phlogopite, diopside, sanidine, richterite, ilmenite, rutile, and quartz. Olivine in the leucite lamproites have compositions Fo 93 to Fo 89, while olivine in the kimberlites have compositions Fo 90 to Fo 80. Diopside in the kimberlites are calcic and contain relatively high Al when compared to the diopside in the lamproites. Phlogopite in the kimberlites contain relatively high Al and low ti and are deficient or low in calculated Fe 3+. Phlogopite in the lamproites contain relatively high Ti and significant amounts of Fe 3+. Ilmenite xenocrysts from the kimberlites are magnesian with relatively high hematite contents, whereas the groundmass ilmenites from the amphibole lamproites are manganoan ilmenites with low hematite and magnesium. The spinels range compositionally from titaniferous magnesiochromite to titanomagnetite. Amphiboles are mainly potassium-titanian richterites. Leucite is mostly pseudomorphed to K-feldspar. Other phases present in variable amounts in the groundmass of the dikes are carbonate minerals, chlorite, serpentine, apatite, amphiboles (actinolite, arfvedsonite, and a riebeckite-like phase), priderite, sulphides, and zircon. Mg-ilmenites and pyrope garnets are considered xenocrystic while most olivine, phlogopite, and leucite macrocrysts are believed to be cognate. The primary kimberlite and lamproite magmas originated in the mantle and were emplaced at a low level in the upper crust where they consolidated at contrasting levels of T, ϱ H 2O , ϱ CO 2 , ƒ O 2 , a Al 2O 3 , and a SiO 2 . The kimberlites crystallized olivine, phlogopite, spinels, and perovskite as early phases. The two lamproite types are related to different levels of volatile pressure. A low ϱ H 2O type crystallized leucite, phlogopite, and olivine at a pressure below 0.5 kbar and liquidus temperatures ranging from 1260° to 900°C. A relatively high ϱ H 2O type crystallized diopside, phlogopite, richterite, rutile, and ilmenite at a pressure above 1 kbar and liquidus temperatures most likely from 1150° to 950°C. The crystallization of perovskite and Al-rich phlogopite points to a relatively low a SiO 2 in the kimberlite magmas. Higher a SiO 2 appears to have prevailed in the leucite lamproites, constrained by the leucite-orthoclase and perovskite-rutile silica buffers. The sanidine-bearing amphibole lamproites record a relatively high a SiO 2 with zircon and quartz crystallization in the groundmass. The oxygen fugacities in the kimberlites are suggested, from the appearance of olivine and magnetite in the groundmass, to have been controlled by a FMQ oxygen buffer. The compositions of the richterites, ilmenites, and rutiles in the lamproites point to a relatively low Fe 3+ and ƒ O 2 in the melts.