Nomenclature, mineralogy, and geochemistry of a plagioclase kersantite lamprophyre suite, South Eastern Canadian Cordillera; British Columbia.

Abstract

The newly named Minerva kersantite lamprophyre dikes of the Monashee complex represent the final igneous event in the area. The dikes were emplaced in a north-trending subvertical orientation, where they bisect the acute angle of two sets of regional extensional fractures (D5). This magmatic episode is coincident with Eocene extension (~50.0 Ma), based on the flat 40Ar/39Ar step-heating plateau of contact metamorphic muscovite on the margin of a Minerva dike. Some of the dikes can be delineated with airborne magnetic data and can be correlated with major north-trending extensional faults. These weakly altered mafic dikes have a panidiomorphic texture with phenocrysts (0.5 - 2.0 mm) of phlogopite, augite, amphibole (pseudomorphed by clays), labradorite and both primary and secondary carbonates set in a similar tine-grained groundmass. This is generally consistent with their classification as plagioclase-bearing kersantite lamprophyre. They are weakly silica-undersaturated, ultrapotassic (2.8 wt. % K2O: 7.7 wt.% MgO), with very high large-ion-lithophile element contents(-~300 times primitive mantle) and low highfield- strength element contents similar to E-MORB, as well as a prominent negative Nb anomaly: these geochemical attributes are consistent with the Three Valley Suite being lamprophyres of calc-alkaline affinity that are related to subduction zone magmatism. These dikes are geochemically similar to mafic rocks of the Roman igneous province. On the basis of this relation, it is inferred that subduction of the Juan de Fuca Plate influenced subcrustal mantle wedge metasomatism in the region. Later Eocene extension resulted in decompression melting of this metasomatised mantle, and a short episode of ‘melt ponding’ below the continental lithosphere. Continuing extension enabled the emplacement of this suite of kersantite dikes.