ZIRCONOSILICATE PHASE-RELATIONS IN THE STRANGE LAKE (LAC-BRISSON) PLUTON, QUEBEC-LABRADOR, CANADA

Petrographic observations of subsolvus granites at Strange Lake indica te that the sodium zirconosilicate elpidite crystallized under magmati c conditions, but that the calcium zirconosilicates armstrongite and g ittinsite are secondary. This interpretation is consistent with the ex tensive solid solution displayed by elpidite and the restricted compos itions of armstrongite and gittinsite. Both calcium zirconosilicate mi nerals show textural evidence of having replaced elpidite, and in the case of gittinsite, with major volume loss. In the near-surface enviro nment, gittinsite plus quartz fill the volume formerly occupied by elp idite. At greater depth, gittinsite and armstrongite partially replace d elpidite but are not accompanied by quartz, and abundant pore space is observed where gittinsite is the principal secondary phase. Below 7 0 m elpidite is generally unaltered. Replacement of elpidite by armstr ongite is interpreted to have been a result of the cation-exchange rea ction [GRAPHICS] in which volume is nearly conserved, and replacement by gittinsite is thought to have resulted from the reaction [GRAPHICS] which is accompanied by a 65% volume reduction. An alteration model i s proposed in which external Ca-rich, quartz-undersaturated fluids dis solved elpidite and replaced it with gittinsite, where alpha(H4SiO40) was buffered mainly by the fluid (high water-rock ratio), and with arm strongite plus gittinsite, or armstrongite alone, where alpha(H4SiO40) was buffered to higher values by the rock (low water-rock ratio). The formation of gittinsite created extensive pore space that was subsequ ently filled, in the upper part of the pluton, when the fluid became s aturated with quartz as its temperature decreased during the final sta ges of alteration.