Distribution of kimberlite and aillikite in the Diamond Province of southern West Greenland: A regional perspective based on groundmass mineral chemistry and bulk compositions

摘要

A regional study of dike and sill occurrences of kimberlitic rocks in terms of xenolith and megacryst assemblages from the diamond province in southern West Greenland demonstrates very systematic variation in the composition of melts across the margin of the Archaean core of the North Atlantic craton. Melt derived from the diamond window in the lithospheric mantle below the Archean core in Greenland is kimberlitic. Toward the Palaeoproterozoic Nagssugtoqidian orogen melts (also derived from the diamond window) gradually change to ultramafic lamprophyre compositions evolving toward an end-member akin to the aillikites of the penecontemporaneous Torngat region in Canada. The identification of the types of melts is based on the groundmass paragenesis including the presence or absence of clinopyroxene and chromite, and the compositions of groundmass ilmenite, spinel, and phlogopite. The systematic regional variation is also shown, although somewhat blurred, by the bulk rock major and trace element compositions. The change in melt composition is not a reflection of change in depth of initial melting, but relates to the composition of the deep SCLM. Kimberlite melts in the Archaean craton are suggested to be reaction products between a carbonate-rich astenospheric melt and an orthopyroxene-bearing and phlogopite-poor depleted SCLM, whereas the aillikite type melts across the craton margin are the products of reaction between a similar carbonate-rich asthenospheric melts and a subducted SCLM that had previously been subjected to metasomatism and veining.