The Line Rock Pegmatite Dike, Mont Saint-Hilaire, Quebec

Abstract The informally named Line Rock miarolitic pegmatite dike, briefly exposed in the Poudrette quarry at Mont Saint-Hilaire, Quebec, attracted our attention because of the unusual occurrence of aplitic “line rock”, not described from the quarry before. The upper zones of the thin (105 cm), sloping, tabular dike consist of elongate, outwardly flaring crystals of microcline perthite aligned toward the core and of sodalite (variety hackmanite), formed by replacement of nepheline. The primary feldspar, a sanidine solid-solution, formed at 800 °C or so, and eventually inverted successfully to orthoclase perthite, then to microcline perthite in this peralkaline environment. Below the core, the elongate crystals are oriented upward. In the core, microcline surrounded by schizolite is only partly ordered and contains relict orthoclase. Sodalite and nepheline are minor phases in the line rock interval above the base. There, microcline, albite, and aegirine define a subsolvus assemblage in a 15 cm stack of alternating light-colored feldspathic and dark aegirine-bearing aplitic layers. These formed by repeated buildup of rejected constituents at the rapidly advancing solid–melt interface. The peralkaline low-viscosity magma attained saturation in a saline aqueous fluid. Open space due to shrinkage of the zoned tabular dike was later filled by schizolite precipitated from a fluid phase, possibly a less strongly alkaline one.