Varicolored Marble in the Central Metasedimentary Belt, Grenville Supergroup. I. Circumstantial Evidence of Melting

Abstract

Abstract Large expanses of granulite-facies white marble occur in the Central Metasedimentary Belt of the Grenville orogen in western Quebec and in contiguous areas of eastern Ontario and New York. Locally, the white rock is transformed into “varicolored” marble, ranging from pink, orange, yellow, green, gray to blue marble, in some cases closely juxtaposed. We have dissected specimens from three target areas in western Quebec. A powerful analytical workflow including high-resolution ZEISS Atlas 5 large-area imaging was employed in combination with detailed energy-dispersive spectroscopy analyses to create digital map-like online datasets of entire samples to display the textures and characteristics in their entirety. We describe in detail the texture and mineralogy of marble specimens collected from exposures created during the construction of Autoroute 5 near Wakefield and of Autoroute 50 near Grenville-sur-la-Rouge and two specimens collected near Bryson, Quebec. At the inferred pressure and temperature and in the presence of H2O, the white marble seems to have melted. A carbonate melt, a powerful flux, can dissolve silicate wallrocks along a mutual contact. In this way, a marble-derived melt has assimilated wallrocks ranging from peridotite to granite. This melting event may have occurred more than once in the 350-million-year lifespan of the Grenville event; the thermal event associated with the Rigolet stage, during gravitational collapse of the orogen approximately one billion years ago, was the most intense. The marble samples contain no vestiges of older deformed assemblages. Whereas the varicolored marble undoubtedly is metasedimentary, a coeval influx of mantle-derived carbonatitic melt and related fluids from a subjacent complex may have caused localized metasomatism of the marble prior to and after melting.