Stratigraphic and petrological variation of the Mount Somers Volcanics Group, mid Canterbury, New Zealand

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Abstract
Mt Somers, mid Canterbury, New Zealand, consists of seven temporally distinct Upper Cretaceous volcanic formations which together form the Mount Somers Volcanics Group. These formations are the Surrey Hills Tuff, Barrosa Andesite, Woolshed Creek Ignimbrite (new name), Somers Rhyolite, Somers Ignimbrite (new name), volcanic conglomerates and tuffs, and Hinds River Dacite. Surrey Hills Tuff comprises both massive and bedded rhyolitic tuff and records the first explosive silicic volcanism at Mt Somers. Barrosa Andesite occurs as porphyritic pyroxene andesite flows, breccias, and shallow intrusives in the north and northwest of the area and is considered to represent an eroded stratocone. The main period of silicic volcanism began with the formation of Woolshed Creek Ignimbrite, a black, glassy, porphyritic vitrophyre containing up to 15% lithic fragments. Its massive, glassy nature is due to fusion by the overlying Somers Rhyolite, which comprises nearly 1000 m thickness 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 devitrified groundmass. Somers Rhyolite is overlain by Somers Ignimbrite Formation—high grade ignimbrites with an extant volume of >6.8 km3 that were erupted from a source to the north of Mt Somers. The ignimbrites form numerous sheets that represent major, temporally discrete deposits that ponded within a paleotopographic depression. They have the same mineralogy as the rhyolite. The emplacement of successive sheets followed rapidly with the overall eruption perhaps occurring over a time period of days to weeks. At the end of the ignimbrite eruptions, volcanic laharic conglomerates and tuffs were deposited, of which only small remnants remain in a restricted area to the northwest side of Mt Somers. Finally, plagioclase‐phyric and biotite‐phyric andesite and dacite dikes, considered to be related to Hinds River Dacite, were emplaced through earlier Mount Somers Volcanics Group units. The Somers ignimbrites and rhyolite are geochemically related and are both derived from a deep crustal source by partial melting of quartzofeldspathic Torlesse Supergroup sediments. Geochemical zonation in ignimbrites reflects extraction from a compositionally zoned magma chamber comprising more differentiated magma overlying less differentiated magma. Andesite dikes that postdate Somers Ignimbrite Formation are geochemically related to Hinds River Dacite.