Zirconosilicate phase relations in the Strange Lake (Lac Brisson) pluton, Quebec-Labrador, Canada

Abstract Petrographic observations of subsolvus granites at Strange Lake indicate that the sodium zirconosilicate elpidite crystallized under magmatic conditions, but that the calcium zirconosilicates armstrongite and gittinsite are secondary. This interpretation is consistent with the extensive solid solution displayed by elpidite and the restricted compositions of armstrongite and gittinsite. Both calcium zirconosilicate minerals show textural evidence of having replaced elpidite, and in the case of gittinsite, with major volume loss. In the near-surface environment, gittinsite plus quartz fill the volume formerly occupied by elpidite. At greater depth, gittinsite and armstrongite partially replaced elpidite but are not accompanied by quartz, and abundant pore space is observed where gittinsite is the principal secondary phase. Below 70 m elpidite is generally unaltered. Replacement of elpidite by armstrongite is interpreted to have been a result of the cation-exchange reaction in which volume is nearly conserved, and replacement by gittinsite is thought to have resulted from the reaction which is accompanied by a 65% volume reduction. An alteration model is proposed in which external Ca-rich, quartz-undersaturated fluids dissolved elpidite and replaced it with gittinsite, where aH₄SiO⁰₄ was buffered mainly by the fluid (high water-rock ratio), and with armstrongite plus gittinsite, or armstrongite alone, where aH₄SiO⁰₄ was buffered to higher values by the rock (low water-rock ratio). The formation of gittinsite created extensive pore space that was subsequently filled, in the upper part of the pluton, when the fluid became saturated with quartz as its temperature decreased during the final stages of alteration.