THE ARCHITECTURE, ERUPTIVE HISTORY, AND EVOLUTION OF THE MOUNT GRACE CARBONATITE (SOUTHEASTERN CANADIAN CORDILLERA)

The pyroclastic Mount Grace carbonatite (Late Devonian) is a thin, laterally discontinuous marble layer in the Monashee Mountains northwest of Revelstoke, BC, Canada, on the NW flank of the Frenchman Cap dome and the NE margin of the Shuswap Metamorphic Core Complex. The Mount Grace carbonatite occurs near the top of a transgressive marine sequence deposited on a low-relief basement complex. A phreatomagmatic eruption occurred when carbonatitic magma interacted with groundwater within a permeable and friable metasedimentary marble unit, creating the Mount Grace carbonatite. The initial phase of the eruption involved a pulse of dilute pyroclastic base surges. The main vent-clearing phase of the eruption produced concentrated pyroclastic density currents, followed by a waning in concentration and energy represented by another period of dilute base surges, ultimately accompanied by fallout. The initial surge deposits are dominated by thin (1–50 cm), finely laminated, moderately sorted carbonatitic ash tuffs that commonly occur at the stratigraphically lowest and highest positions. The main density current deposits are dominated by thick (10 cm – 1.2 m) massive to poorly bedded, very poorly sorted tuff breccias with carbonatitic-matrices and fenite lithic clasts up to 1.5 m in diameter. Final stage fallout deposits are dominated by 5–80 cm thick juvenile, massive, well-sorted carbonatitic ash tuffs. Accretionary lapilli and soft sediment deformation were absent or rare from all outcrops, suggesting that most of the external water was converted to steam either at the site of magma-water interaction or shortly after. The presence of the metasedimentary marble unit below, interbedded with, and above the Mount Grace carbonatite indicates that the influence of external water at the vent did not fluctuate significantly through the eruption. Most of the lithic clasts in the tuff breccia originate from a fenite unit ~300 m below the current position of the Mount Grace carbonatite, indicating that the crater excavated to approximately these upper crustal depths.