The petrology and geochemistry of the rossland volcanic rocks, southern British Columbia

Abstract

The Rossland Volcanic Group is a dominantly clastic succession that accumulated in a back-arc sedimentary basin in Lower Jurassic time. The lava-types preserved are predominantly augite- and plagioclase-phyric basalts and low-SiO(_2) andesites, but with some localised and conspicuous ankaramitic varieties. Amphibole is a less common but significant phenocryst in basaltic and andesitic compositions. Whole-rock and primary mineral (augite and amphibole) chemical data indicate that the Rossland volcanics are of island-arc affinity. However, rather than being a normal calc-alkaline association, they compare more favourably, in some respects, with the recent, ankaramitic associations of the New Hebrides and New Georgia island-arcs. The presence of primary amphibole in the basalts, and the rare occurrence of primary biotite in one low SiO(_2) andesite, also indicate a mildly alkaline, or high-K(_2)O affinity. Comparison with the sparse data on contemporaneous (Upper Triassic/Lower Jurassic) volcanics of the Canadian Cordillera, indicates that magmatism similar to that in the Rossland area was also operative along the Upper Triassic, Nicola-Takla axis. In the Lower Durassic, the bulk of the volcanics in north-central British Columbia were somewhat different, being more evolved in nature and representing the change of the continental margin to that of an Andean type. The chemistry of the Rossland representatives indicates an upper-mantle origin for the magmas, though only rarely were unmodified partial melts extruded. Rather, as would be expected from the strongly porphyritic nature of the volcanics, mostmagmas suffered a degree of polybaric olivine fractionation(during ascent from around 30Kb) and crustal level (2-7 kb)fractionation of olivine + Cr-spinel + clinopyroxene +amphibole + Ti-magnetite + plagioclase. Metamorphism of the volcanics occurred in Middle Jurassictime, during the early phases of the Columbian orogeny. Consideration of the secondary-mineral assemblages and chemistry, indicates that a progressive regional metamoprhism affected the volcanics, varying from the prehnite-pumpellyite facies, through the greenschist, to the epidote-amphibolite facies. This gradation is from south to north and probably culminates, north of the area, in the Valhalla gneiss dome of the Shuswap complex. Oxygen isotope ((^18)o/(^16)o) data further show that during metamorphism the volcanics exchanged oxygen considerably with a pervasive hydrothermal fluid, enriching them in (^18)0 (δ(^18)0=8-11 º/oo) relative to their original magmatic values (δ(^18)0=6º/00)), The origin of the hydrothermal fluid is enigmatic, but is most probably either extremely (^18)0-enriched meteoric water or is water released during prograde metamoprhism of deeper level sediments and which percolates upwards.