No abstract

During Cretaceous times, the northern Antarctic Peninsula was the site of an active ensialic magmatic arc. Volcanism was dominated by pyroclastic eruptions with rare lava flows. Marine conglomerates and sandstones formed a volcaniclastic apron along the eastern margin of the arc and represent the proximal deposits of an extensive back-arc basin. Volcanogenic material, redeposited by turbidity currents and other sediment gravity flows, forms an important part of the proximal basin fill. Air-fall tufts and eruption-induced sediment flows form a small but significant part of the succession and large exotic slideblocks of Jurassic sediment are a distinctive feature of the Lower Cretaceous strata on James Ross Island. Aeromagnetic data and regional geology indicate that the arc-back-arc basin boundarv was fault-controlled. Sedimentation within the basin was strongly influenced by both the steep, unstable nature of the faulted arc flanks and the coeval volcanism.

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.