Mesoproterozoic deep K-magmatism recorded in a megacryst- and xenolith-bearing minette dyke, western Grenville Province

Abstract

The 1.07 Ga Rivard minette dyke transported thousands of exotic (xenoliths) and cogenetic (cognate nodules) clasts from deep lithospheric levels of the Grenville Province. Nodules related to the clinopyroxene- and biotite-phyric host consist of megacrystic clinopyroxene and K-feldspar and mica-rich pyroxenite. Clinopyroxene megacrysts record high-pressure and high-temperature crystallization, crystal recycling, or magma mixing, whereas Ba-rich K-feldspar megacryst possibly represent near-solidus phenocrysts crystallized from evolved K-rich magmas. Mica-pyroxenite xenoliths are interpreted as products of magma mixing or infiltration of K-rich melt in pyroxene cumulate. Partial replacement of pyroxenes by strained phlogopite attests to mica crystallization before or during plastic deformation and prior to xenolith incorporation in the minette. The minette is mafic, ultrapotassic, and enriched in large-ion lithophile elements and light rare-earth elements. It experienced limited fractionation and crustal contamination but has been exposed to magma mixing. High K, La, and Cr contents suggest partial melting of a K-metasomatized mantle source. The Rivard minette shares the age, mineralogy, and chemistry with the 1.09–1.07 Ga Kensington–Skootamatta potassic alkaline suite and forms part of a common K-rich magmatic event taking its source in an enriched mantle. Source heterogeneity, conditions of partial melting, crystal fractionation, magma mixing, and crustal contamination all contributed, to various extents, to the complex chemistry of the K-rich intrusions of the Kensington–Skootamatta suite. Collectively, this suite records extensive and diverse magmatic batches derived from partial melting of a mantle metasomatized during subduction events prior to emplacement.