Petrology of adamant pluton, British Columbia

Abstract

Adamant pluton is an elongate, east-trending mass surrounded by amphibolite facies metamorphic rocks of the Proterozoic Horse thief Creek Group. The pluton is a composite body consisting of a hypersthene-augite monzonite core enclosed by hornblende quartz monzonite. The core grades into the peripheral rock through a transition zone in which the monzonite core is progressively recrystallized to hornblende quartz monzonite. Chemical analyses show that this transition is isochemical. Internal structure is characterized by foliation and lineation patterns consistent with two fold systems found in the northern Selkirk Mountains, and a prominent marginal foliation locally associated with zones of migmatite. Plagioclase foliation and poorly developed lineation in the monzonite core are obliterated and locally truncated by a younger hornblende and biotite foliation in the recrystallized periphery of the body. Analyses of augite and hypersthene show a constant Fe:Mg ratio for this mineral pair. Coexisting hornblende and biotite have variable F e:Mg ratios which are correlated with changes in oxidation state due to a thermal gradient from the core to margin of the pluton. High ?.HzO in rocks at the margin caused a decrease of the hornblende: biotite ratio. The internal structure, petrology, and mineralogy of the pluton indicate a metamorphic origin for the peripheral rock, and a magmatic one for the older monzonite core. It is concluded that the pluton was originally emplaced as a single igneous intrusion of pyroxene monzonite. Subsequently, this body underwent metamorphism and reintrusion during the late Mesozoic, the most recent episode of deformation and metamorphism in the northern Selkirk Mountains.