Title

A widespread compositionally bimodal tephra sourced from Volcán Melimoyu (44°S, Northern Patagonian Andes): Insights into magmatic reservoir processes and opportunities for regional correlation

RIS ID

130895

Publication Details

Geoffroy, C. A., Alloway, B. V., Amigo, A., Parada, M. A., Gutierrez, F., Castruccio, A., Pearce, N. J. G., Morgado, E. & Moreno, P. I. (2018). A widespread compositionally bimodal tephra sourced from Volcán Melimoyu (44°S, Northern Patagonian Andes): Insights into magmatic reservoir processes and opportunities for regional correlation. Quaternary Science Reviews: the international multidisciplinary research and review journal, 200 141-159.

Abstract

We describe the stratigraphy, age, constituent geochemistry and phenocryst thermobarometry of a closely spaced Holocene tephra couplet from Volcán Melimoyu (VMm), located in the Northern Patagonian Andes. The lower tephra unit (La Junta Tephra, Mm-1) is distinctly banded comprising a dominant lower layer of rhyodacitic (∼70% SiO2) pumiceous ash and lapilli (Mm-1p) that abruptly transitions to a subordinate upper scoriaceous layer (Mm-1s) of basaltic andesite composition (∼53% SiO2). This bimodality within Mm-1 contrasts significantly with the closely overlying Santa Ana Tephra (Mm-2) that has a homogeneous trachyte-dacite (∼63% SiO2) composition and is intermediate between the two magmatic end members of Mm-1. We propose a genetic affiliation between Mm-1 and Mm-2, and that the latter event likely represents a hybridised-remnant of those discrete magmas involved in the earlier Mm-1 eruption. To test this hypothesis we applied whole rock elemental mixing and fractional crystallisation model to reproduce the composition and crystallinity of Mm-2. Results indicate that Mm-2 can be reproduced by mixing ∼70% Mm-1p with ∼30% Mm-1s, with subsequent ∼13% fractional crystallisation of plagioclase, and minor amphibole, orthopyroxene, magnetite and biotite. Equilibrium P-T conditions calculated from Mm-1p phenocrysts point towards magma residency at moderately shallow depths (200-290 MPa, ∼7-10-km depth, 850-1000 °C), whereas Mm-1s phenocrysts indicate higher overall P-T conditions (240-480 MPa, ∼8.5-17-km depth, 1080-1150 °C). P-T conditions determined for Mm-2 (∼290 MPa, ∼10-km depth, 930-1000 °C) are similar to those of Mm-1p. There is no physical and/or geochemical evidence of mafic magma involvement in the Mm-2 eruption. Similar compositionally bimodal tephra are known from other Northern Patagonian Andean centres (i.e. Playas Blanca-Negra Tephra, Antillanca; Lepue Tephra, Michimahuida; Ho and H3 eruptions of Hudson) suggests that the intrusion of mafic magma into more silicic magma bodies is a common occurrence throughout this Andean sector. These widely dispersed, compositionally bimodal tephra not only provide key insights into pre-eruptive magmatic conditions and triggering processes, but can also be readily identified geochemically, and thereby be more fully utilised within future hazard- and paleoenvironmental-related studies.

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Link to publisher version (DOI)

http://dx.doi.org/10.1016/j.quascirev.2018.09.034