STRATIGRAPHIC AND PETROLOGICAL VARIATION OF THE MOUNT SOMERS VOLCANICS GROUP, MID CANTERBURY, NEW-ZEALAND

Mt Somers, mid Canterbury, New Zealand, consists of seven temporally d istinct Upper Cretaceous volcanic formations which together form the M ount Somers Volcanics Group. These formations are the Surrey Hills Tuf f, Barrosa Andesite, Woolshed Creek Ignimbrite (new name), Somers Rhyo lite, Somers Ignimbrite (new name), volcanic conglomerates and tuffs, and Hinds River Dacite. Surrey Hills Tuff comprises both massive and b edded rhyolitic tuff and records the first explosive silicic volcanism at Mt Somers. Barrosa Andesite occurs as porphyritic pyroxene andesit e flows, breccias, and shallow intrusives in the north and northwest o f the area and is considered to represent an eroded stratocone. The ma in period of silicic volcanism began with the formation of Woolshed Cr eek Ignimbrite, a black, glassy, porphyritic vitrophyre containing up to 15% lithic fragments. Its massive, glassy nature is due to fusion b y the overlying Somers Rhyolite, which comprises nearly 1000 m thickne ss of coalesced rhyolite lava domes with minor lava flows and shallow intrusives. The rhyolites are porphyritic with phenocrysts of sanidine , quartz, plagioclase, and minor garnet and iron oxide in a devitrifie d groundmass. Somers Rhyolite is overlain by Somers Ignimbrite Formati on-high grade ignimbrites with an extant volume of >6.8 km(3) that wer e erupted from a source to the north of Mt Somers. The ignimbrites for m numerous sheets that represent major, temporally discrete deposits t hat ponded within a paleotopographic depression. They have the same mi neralogy as the rhyolite. The emplacement of successive sheets followe d rapidly with the overall eruption perhaps occurring over a time peri od of days to weeks. At the end of the ignimbrite eruptions, volcanic laharic conglomerates and tuffs were deposited, of which only small re mnants remain in a restricted area to the northwest side of Mt Somers. Finally, plagioclase-phyric and biotite-phyric andesite and dacite di kes, considered to be related to Hinds River Dacite, were emplaced thr ough earlier Mount Somers Volcanics Group units.