Composition and origin of garnet from the Antarctic Peninsula Volcanic Group of Trinity Peninsula

Garnet is widely found as a minor constituent in rocks of the Antarctic Peninsula Volcanic Group (APVG) of Trinity Peninsula. It occurs as conspicuous megacrysts, or in xenoliths within volcanic rocks of andesitic-rhyolitic composition and as detrital grains in the associated terrestrial sediments. It is also found as an accessory mineral in many plutonic rocks from the E coast of the Antarctic Peninsula. Evidence is presented to show that the garnet can be divided petrographically and chemically into two main groups: Type A: almandine-rich primary igneous garnet, and Type B: less almandine- and more pyrope-rich garnet as xenocrysts or included in xenoliths within the volcanic rocks. Comparison with published experimental data on garnet occurrence in acidic igneous rocks suggests that high almandine-low spessartine garnet in volcanic rocks is a remnant phase of high pressure crystallization from magma at pressures of >7 kbar. Garnet with a higher pyrope content is regarded as xenocrystal in origin, having been derived from garnet-bearing country rocks at depth, either as accidental inclusions or through direct partial melting (restite) of the lower crust, and implies that a considerable thickness (>25 km) of crustal material was in existence before the generation of the Mesozoic magmatic arc. The origin of these calc-alkaline magmas may therefore be due, at least in part, to partial melting of pre-existing sialic crustal material.