Pétrologie du granite peralcalin du lac Brisson, Labrador central, Nouveau-Québec. II. Minéralogie et modalités de cristallisation

Abstract

Mineral zonation in the Québec–Labrador Brisson Lake peralkaline granite displays quartzose and feldspathic lithofacies arranged concentrically, the latter occupying the centre of the intrusion. The zonation is the result of successive magmatic pulses. In the feldspathic facies, agpaitic crystallization began under hypersolvus conditions around 720 °C with PF = 0.1 GPa. Subsolvus crystallization involving enrichment of the residual liquid in F continued to below 500 °C. The quartzose facies is more differentiated and its composition was controlled by feldspar fractionation. Early quartz crystallization is partly explained by the high content of F in the magma. The mafic mineral succession is, in both facies: Li- and Zn-rich arfvedsonite with an important ferrorichtérite component, which crystallized along with alkali feldspar under low [Formula: see text]; aenigmatite contemporary of amphibole or anterior, destabilized to form neptunite, astrophyllite, aegirine, or arfvedsonite; primary titaniferous aegyrine, contemporary with the amphibole and replaced by secondary aegyrine; neptunite and astrophyllite replacing aenigmatite. This succession is in accordance with the increase of Na and F in the fluid phase, and the increase of [Formula: see text] near the end of crystallization. Among the accessory minerals, euhedral zircon is indicative of the initial richness of the magma in Zr. Magmatic vlasovite, and elpidite formed from late fluid, are evidence that residual system entered the zirconium silicate stability field. Zircon with a fibrous, radiating texture, and gittinsite are indicative of the postmagmatic evolution of the pluton and the presence of a late stage residual fluid which was enriched in Ca and Sr.