Differentiation in Layered Diorite Intrusions, Ingonish, Nova Scotia

Abstract

Two differentiated intrusions (diorite to adamellite), located near Ingonish, Nova Scotia, are characterized by cryptic layering, weak igneous lamination, and scarce mineral layering. In their lower portions, layering is further marked by sharply bounded flows and pillows of hornblende gabbro. Variation in the layered dioritic rocks was controlled largely by gravitative settling and accumulation of plagioclase, hornblende, and magnetite. At higher levels in the intrusion sphene, biotite and quartz became cumulus phases. Plagioclase zoning and the relative abundances of interstitial minerals indicate that the diorites are orthocumulates. End-stage differentiates (quartz-rich tonalite, adamellite, and pegmatite) occur in a lens formed by filter-pressing intercumulus liquid from quartz diorite. Cryptic layering is defined by upward soda-enrichment in plagioclase. A few analyses of hornblende and biotite suggest the possibility of slight iron enrichment. Whole-rock chemical variation in the layered intrus...