THE HYPABYSSAL 5034 KIMBERLITE OF THE GAHCHO KUE CLUSTER, SOUTHEASTERN SLAVE CRATON, NORTHWEST TERRITORIES, CANADA: A GRANITE-CONTAMINATED GROUP-I KIMBERLITE

Abstract

We present petrographic, mineralogical, geochemical and isotopic data on the 5034 kimberlite of the Cambrian Gahcho Kue kimberlite cluster (Slave craton, northern Canada). This Group-IA kimberlite is a multiphase intrusion composed of the diatreme-facies magmatic and hypabyssal kimberlite, which intruded in at least four magmatic phases. The dominant hypabyssal phase is composed of macrocrystal monticellite – phlogopite – serpentine kimberlite. Occasional samples with diopside ± melilite may reflect local heterogeneities in the bulk composition or separate intrusive phases. Aphanitic monticellite – serpentine kimberlite forms as early and contemporaneous small-volume magmas and is present in autoliths and in cross-cutting dykes. The subsequent explosive magmatic phase formed tuffisitic kimberlite breccia, which in turn is cut by macrocrystal phlogopite – serpentine kimberlite. The phlogopite in all recognized hypabyssal rock-types has distinct compositional trends, but within a given rock-type, it also varies significantly in response to local chemical heterogeneities and to the composition of minerals adjacent to it. The large variability in phlogopite compositions is due to very local diffusion-controlled crystallization, and not to changes in bulk composition of the magma. All varieties of the 5034 kimberlite show strong evidence of contamination by host granitic rocks. Pectolite, diopside, melilite and phlogopite have replaced and crystallized around partly assimilated granitic xenoliths from a hybrid melt enriched in Si and Na. A high activity of Si may be the major factor controlling the presence of melilite in kimberlites. The mineralogical character of the 5034 kimberlite, which is intermediate between Group-I and Group-II kimberlites, can be explained by its strong contamination by granites. The composition of the kimberlite in terms of Sr and Nd isotopes suggests an asthenospheric origin of the melt and allows for incorporation of no more than 8–12% granitic material into the parental magma.