Petrogenesis of Eocene shoshonitic rocks from South-Central British Columbia: Melting of Proterozoic lithospheric mantle

Abstract

The Terrace Mountain complex is a part of a discontinuous north-west trending belt of Eocene volcanic centres, which stretches for about 1000 km in southern and central interior British Columbia and probably extends into the northwestern United States. The 55–45 Ma chain is related to east-vergent subduction of oceanic plates under the North American continent. The volcanic rocks of the belt are mainly of calc-alkaline intermediate to felsic composition, typical of distal continental arcs. However, close to the Canada-United States border, in the Terrace Mountain volcanic complex, there are alkaline (shoshonitic) mafic and intermediate rocks with an unusual isotopic composition compared to most other rocks of the belt. These rocks, which are hosted in an extensional basin/half-graben associated with regional strike-slip faulting in a continental arc setting, have highly negative ɛNd(t) values, Proterozoic neodymium depleted mantle model ages and elevated initial 87Sr/86Sr ratios. The two rock units, which are >700 m thick were emplaced within a short (<1–2 Ma) magmatic episode at ~52 Ma. The older volcanic pulse was composed of basaltic trachyandesites while the younger pulse included trachyandesites. The rocks are enriched in strongly incompatible trace elements and underwent fractional crystallization. Both suites are related to a parent magma, which was sourced during extension of the Cordilleran orogen by partial melting of heterogeneous subduction-modified sub-continental lithospheric mantle composed of amphibole-phlogopite-bearing peridotite. Nd isotopic characteristics require an old LREE-enriched slab in the mantle suggesting that the source was separated from convecting mantle since the Proterozoic.