Anomalous Cr-Rich Zones In Sector-Zoned Clinopyroxene Macrocrysts In Gabbro, Mont Royal, Montreal, Quebec, Canada

Abstract

We document unusual zones of Cr enrichment (up to 0.9 wt.% Cr 2 O 3 ) in sector-zoned clinopyroxene crystals from gabbro sampled near the outer contact of the Mont Royal intrusive complex, of Early Cretaceous age, located in the city of Montreal. Mont Royal is one of the Monteregian Hills, but by no means the best-known mineralogically. We describe the compositional variation in macrocrysts of Ti-rich clinopyroxene by means of element-distribution maps and a detailed 0.37 mm traverse across the contact between a relatively homogeneous core and a mantle showing sector and oscillatory zoning. A Cr- and Mg-enriched zone appears in that interval and is also recorded in the coeval ferri-kaersutite. Both elements, being highly compatible, are not expected to build up in a boundary layer. After a period of slow growth in a hotter part of the reservoir, the macrocrysts seem to have migrated near the outer contact, where more rapid loss of heat and degassing contributed to the development of sector and oscillatory zoning. Such zoning was accompanied by an overall rise in f (O 2 ). The anomalous Cr-Mg-enriched zones seem more likely attributable to local cooling and oxidation of the magma than to an episode of replenishment in the magma reservoir.