Pinwarian (1.50 Ga) volcanism and hydrothermal activity at the eastern margin of the Wakeham Group, Grenville Province, Quebec

Abstract

Volcanic belts developed along the southeastern continental margin of Laurentia between 1.70 and 1.30 Ga and subsequently metamorphosed at high grade are today largely concealed among gneiss complexes of the Grenville Province. At the eastern end of the Wakeham Group and in the La Romaine Supracrustal Belt to the east, four 1.50 Ga volcanic centres were found among gneissic synvolcanic intrusions typical of the 1.52–1.46 Ga Pinwarian continental magmatic arc. Upper amphibolite- to granulite-facies rhyolitic to dacitic lavas and coarse lapillistone overlie or are intimately associated with arenites typical of the Wakeham Group. Garnetite, ironstone, carbonate rock, calc-silicate rock, and sillimanite-bearing nodules, veins, and gneiss, locally preserving lapilli, are also present. The distribution, paragenesis, and modes of most of these latter units differ from those of normal metasediments but are diagnostic of metamorphosed exhalites and hydrothermal alteration zones. In the La Romaine Supracrustal Belt, they are associated with pyroclastic horizons and a mineralized composite amphibolite unit. Volcanic textures include flow banding, wispy lapilli moulding fragmented lapilli and rounded lapilli with quartz-feldspar mosaics (filled vesicles), and in situ shattering of lapilli. These textures and the presence of advanced argillic alteration point to vesicular volcanism and hydrothermal activity in a subaerial to shallow submarine environment. Rare mafic lapilli attest to coeval mafic magmatism. The pervasive calc-alkaline signature of the eruptive and intrusive felsic to mafic rocks and their distribution are compatible with the development of 1.50 Ga intra-arc volcano-sedimentary belts stemming from the Wakeham Group basin and extending eastward within the Pinwarian continental magmatic arc.