The southern part of the Burin Peninsula of southeast Newfoundland is marked by extensive Carboniferous granite magmatism, intruding Late Precambrian to Middle Cambrian volcanic and sedimentary rocks. The St. Lawrence pluton, representative of this Carboniferous magmatism, ranges from equigranular to porphyritic in texture, displays much evidence of gas brecciation, and contains no pegmatites or schlieren, all features of high-level intrusion. The bulk chemistry corresponds to cotectic compositions in the synthetic Q–Ab–Or–H2O system for 500 bar graphic, likewise suggesting shallow levels of intrusion and crystallization. The granite is alkaline to peralkaline and of commenditic affinity, with agpaitic indices around 1 and modal aegirine and riebeckite. Its chemical affinity with less silicic alkaline granites of the same age in southeast Newfoundland suggests an origin by differentiation of such granites. A high initial Sr87/Sr86 ratio of 0.722 suggests substantial interaction with or derivation from continental crust. The St. Lawrence fluorite deposits occur as veins confined to and derived from the granite during the final stages of cooling.

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