Buchitic metagreywacke xenoliths from Mount Ngauruhoe,Taupo Volcanic Zone,New Zealand |
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| Institution: | 1. Volcanic Risk Solutions, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;2. School of Environment, The University of Auckland, New Zealand;1. IPATEC (Instituto Andino-Patagónico de Tecnologías Biológicas y Geoambientales), CONICET/UNCO, Av. de los Pioneros 2350, 8400 S. C. de Bariloche, Argentina;2. Universidad Nacional del Comahue, Centro Regional Universitario Bariloche, Quintral 1250, 8400 S.C. de Bariloche, Argentina;3. IADO (Instituto Argentino de Oceanografía), CONICET/UNS, La Carrindanga km 7, 8000 Bahía Blanca, Argentina;4. Universidad Tecnológica Nacional, F.R. Bahía Blanca, 11 de abril 46, 8000 Bahía Blanca, Argentina;5. CONICET, CONAE (Comisión Nacional de Actividades Espaciales), Av. Paseo Colón 751, 1063 CABA, Argentina;1. Dipartimento di Scienze, Università di Roma Tre, Largo San L. Murialdo 1, 00146 Roma, Italy;2. Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro, 5, 00185 Roma, Italy;3. Istituto Nazionale di Geofisica e Vulcanologia, sede di Roma, Via di Vigna Murata 605, 00143 Roma, Italy;4. Escuela Centroamericana de Geología & Red Sismológica Nacional, Universidad de Costa Rica, Ciudad Universitaria Rodrigo Facio, Costa Rica;1. Department of Earth Sciences, University of Geneva, Switzerland;2. School of Geography Earth and Environmental Sciences, University of Plymouth, UK;3. Montserrat Volcano Observatory, Montserrat |
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| Abstract: | Buchitic sedimentary xenoliths, a few centimetres to several decimetres diameter, occur in Recent andesite from Mount Ngauruhoe, Tongariro Volcanic Center, Taupo Volcanic Zone, New Zealand. Bulk chemistry and Sr isotope compositions of the xenoliths indicate that they are greywacke and argillite derived from Mesozoic Torlesse terrane basement that partly underlies the Taupo Volcanic Zone. The xenoliths contain up to 80% glass with quartz, apatite and zircon remaining as unmelted phases. Glasses within the xenoliths are peraluminous (A/CNK = 1.0 ? 1.4), have high normative corundum (2–7%), appreciable FeO (2–4 wt.%), MgO (0.2–1.5 wt.%), TiO2 (0.17–0.84 wt.%), relatively high normative An (1.0–5.3%), and do not represent S-type granitic melts. In the argillite the glass has higher amounts of AI2O3, FeO, MgO, CaO and K2O, and has less SiO2 and Na2O than glass in the greywacke. Silica-rich glass (up to 80 wt.% SiO2) surrounds partially melted quartz. Variable glass chemistry reflects the heterogeneous (layered) nature of the xenoliths. Cordierite (Mg/(Mg + Fe + Mn) = 0.78-0.58), orthopyroxene (En43–56), Mg-rich ilmenite, rutile, pleonaste, V-Cr-Ti spinel, and pyrrhotite occur in the glass of the xenoliths. The dominant cordierite, orthopyroxene, spinel assemblage can be accounted for by disequilibrium breakdown reactions under low oxidation conditions < QFM) involving phengite and chlorite which are abundant in Torlesse greywacke and argillite cropping out along the eastern side of the Taupo Volcanic Zone. Comparison with glass compositions and phase relations of disequilibrium melting experiments on Torlesse greywacke and argillite indicates a minimum temperature of 775°C and a maximum pressure of 1.5 kbar for fusion of the xenoliths that underwent a rapid rate of heating at a depth of less than 5 km and a cooling period constrained by the time of quenching when they were erupted. |
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