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The Renard 4 kimberlite : implications for ascent of kimberlites in the shallow crust Gofton, Emma Lindsay

Abstract

Kimberlite rock types that compose the Renard 4 kimberlite in northern Quebec, Canada, record the transition between the root and diatreme zones of a Class 1 kimberlite pipe. Analysis of lithologic textures and the componentry (clasts, macrocrysts and groundmass) of these rock types provides information pertinent to describing the evolution of the kimberlite pipe as well as to understanding the dynamics of its emplacement. This study defines six rock types present in the Renard 4 kimberlite. Five of these are kimberlitic. The rock types include two volcaniclastic facies, coherent kimberlite, transitional kimberlite and country rock breccia. The two volcaniclastic facies are: 1) a tufiisitic kimberlite breccia facies which is further subdivided into four phases and 2) an 'accretionary' magmaclastic facies. Coherent kimberlite is divided into two subfacies: 1) massive coherent macrocrystic kimberlite and 2) macrocrystic kimberlite breccia. The coherent breccia is a contact zone of the massive coherent kimberlite with the rock it intrudes and comprises non-kimberlitic clasts in a matrix of coherent kimberlite. A multi-stage emplacement process is invoked to account for the presence and the geometries of the Renard 4 rock units. Prior to the kimberlite eruption, coherent kimberlite dykes were emplaced. The eruption and filling of the diatreme were succeeded by emplacement of late-stage coherent kimberlite dykes and the emplacement of a late-stage volcaniclastic facies. Textures in the volcaniclastic facies as well as the juxtaposition of volcaniclastic, transitional and coherent rock types indicates that the present day surface of the Renard 4 kimberlite represents a level between the lower diatreme and the root zone of a Class 1 kimberlite pipe. It is proposed that concentration of country rock clasts occurred in areas of highest ascent velocity in the volcanic conduit. The presence of each of the Renard 4 rock types and their spatial distribution has important implications for the emplacement model of this kimberlite and provide important insight for general models pertaining to the formation of the root/diatreme transitional zone in other kimberlite pipes. The accretionary magmaclastic textures seen in some volcaniclastic rock types in Renard 4 indicates that fragmentation of the kimberlite magma occurs as deep as 2 kilometres below the earth surface and that during emplacement these magmas are inflated to many times the volume they currently occupy. It is furthermore suggested that the kimberlite magma is inflated, and the fragmental textures are formed in systems which may never have breached the surface.

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