PETROGRAPHY AND GEOCHEMISTRY OF LITHIC FRAGMENTS IN IGNIMBRITES FROM THE MANGAKINO VOLCANIC CENTER - IMPLICATIONS FOR THE COMPOSITION OF THE SUBVOLCANIC CRUST IN WESTERN TAUPO VOLCANIC ZONE, NEW-ZEALAND

The Mangakino Volcanic Centre is the westernmost and oldest rhyolitic caldera volcano in the Taupo Volcanic Zone, North Island, New Zealand. The largest eruptions from Mangakino occurred in two periods of calde ra-forming activity during 1.68-1.53 Ma (Period I), and 1.21-0.95 Ma ( Period IIA), producing several voluminous widespread welded and nonwel ded ignimbrites and minor fall deposits. Other activity from Mangakino generated fall deposits and rhyolitic lava domes. Lithic fragments ar e common in all Mangakino ignimbrites (1-10 modal %), and consist of d iverse lithologies including: rhyolite, dacite, andesite, and basaltic andesite lava, welded ignimbrite, tuff, volcanic breccia, biotite gra nite, granodiorite porphyry, siltstone, sandstone, greywacke, metagrey wacke, metaconglomerate, biotite and hornblende-biotite schist. Lithic populations in Period I ignimbrites are dominated by andesite lavas, suggesting that there was a pre-existing andesite volcano in the Manga kino area, geochemically distinct from Titiraupenga and Pureora, the n earest roughly contemporaneous andesitic volcanoes. Later ignimbrites that erupted during Period IIA, contain predominantly rhyolitic lava l ithics, implying that significant dome building activity occurred at M angakino, which represented greater volumes of rhyolitic lava than pre viously described from the area. Petrographic, geochemical, and geophy sical (density and magnetic susceptibility) data measured from the lit hic fragments are used to propose a model for the shallow crust below Mangakino Volcanic Centre. This model postulates eruptions through a b asement of Mesozoic biotite schists overlain by metagreywackes, a thin cover of Tertiary sandstones and siltstones, and an overlying volcani c succession of andesite, dacite and rhyolite lavas, welded ignimbrite s, and lacustrine sediments. Ignimbrite eruptions incorporated comagma tic biotite granite fragments from the crystallised margins of the sil icic magma chambers, and effectively sampled the subvolcanic crust and volcanic pile from the vent walls en route to the surface.