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Felsic xenoliths assimilation in the Renard 65 kimberlite pipe, Québec, Canada

University of British Columbia

This study constrains the nature of the kimberlite-xenolith reactions, melt temperatures in the crust, and the origin of the fluids in the Renard 65 kimberlite, a Kimberley-type pyroclastic kimberlite (KPK) by focusing on the reaction textures and mineralogy assimilated felsic xenoliths. The kimberlite pipe is emplaced into granitoid and gneiss of the Superior Craton and is infilled by pyroclastic and hypabyssal kimberlite. The zonal mineralogical and textural changes that resulted from reactions with the host kimberlite were characterized by petrography, bulk composition, conserved element ratio analysis, and thermodynamic Perple_X modelling. To better replicate the observed mineral assemblages, the Perple_X database was extended to include new thermodynamic data on pectolite, calculated and measured using density functional theory methods. Theoretical models accurately reproduced the zoning at the kimberlite-xenoliths contacts and correctly predicted the distinct mineralogy in reacted granitoid and gneiss. The assimilation of xenoliths is a process that starts at high-temperatures (1200–600 ºC) with the formation of clinopyroxene and wollastonite, continues at 400–200 ºC with the further growth of clinopyroxene and the formation of garnet and phlogopite, and finishes at temperatures < 300 ºC when pectolite and prehnite join in. The bulk of the new mineral growth occurs sub-solidus, at temperatures below 600 ºC. Crustal xenoliths experienced a strong outflow of Si, a strong inflow of Ca, and a moderate inflow of Mg. The host kimberlite experiences an ingress of Si and an outflow of Ca, resulting in the formation of the distinct “contaminated kimberlite halo” surrounding the xenolith. The major elemental flows at the xenolith-kimberlite contacts, confirmed by the conserved element ratio analysis, and the sub-solidus temperatures of the reactions support the metasomatic origin of the mineralogy. The analogy of xenolith-kimberlite contacts with skarns is highlighted by the first recognition in kimberlites of such traditional skarn minerals as wollastonite, andradite and K-feldspar. The low-temperature mineralogy of the fluid-limited thermodynamic model, where H₂O and CO₂ are equal to their contents in the kimberlite, reproduces the observed mineralogy better than the fluid-saturated model where an H₂O-CO₂ fluid (9:1 composition ratio) is in excess. This implies a deuteric origin of the fluids in KPK pipes rather than post-emplacement hydrothermal meteoric waters controlling the kimberlite mineralogy and texture.

Text; Dataset

2020-12-09

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/76712

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/