Role of magma mixing in the petrogenesis of tephra

Authors: MarieChristine Gerbe JeanClaude Thouret

Publish Date: 2004/03/12

Volume: 66, Issue: 6, Pages: 541-561

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Abstract

The Nevado Sabancaya in southern Peru has exhibited a persistent eruptive activity over eight years following a violent eruption in May–June 1990 The explosive activity consisted of alternated vulcanian and phreatomagmatic events followed by declining phreatic activity since late 1997 The mean production rate of magma has remained low 106–107 m3 per yearThe 1990–1998 eruptive episode produced andesitic and dacitic magmas The juvenile tephra span a narrow range of compositions 60–64 wt SiO2 While SiO2 contents do vary slightly they do not show any systematic variation with time Phenocryst assemblages in the juvenile rocks consist of mainly plagioclase associated with highCa pyroxene hornblende biotite and irontitanium oxides Rare finegrained magmatic enclaves with angular to subrounded shapes are contained within some of the juvenile lava blocks which were expelled since 1992 They have a homogeneous andesitic composition 57 wt SiO2 and show randomly oriented interlocking columnar or acicular crystals plagioclase and amphibole with interstitial glass and a few voids which define a quenchtextured groundmassTextural mineralogical and chemical evidence suggests that the 1990–1998 eruptions have mainly erupted hybrid andesites except for the 1990 dacite The hybrid andesites contain a mixed population of plagioclase phenocrysts Carich clear plagioclase An40–60 Narich clear plagioclase An25–35 and inversely zoned “dustyrimmed” plagioclase with a sodic core An25–40 surrounded by a Carich mantle An45–65 Meltinclusions wavy dissolution surfaces and stepped zoning within the “dustyrimmed” plagioclases are compatible with resorption induced by magma recharge events Chemical and isotopic lines of evidence also show that andesites are hybrids resulting from magma mixing processes Repeated magma recharge incomplete homogenisation and different degrees of crustal assimilation may explain the extended range of isotopic signaturesOur study leads to propose an evolution model for the magmatic system at Nevado Sabancaya The main magma body consisted of dacitic magmas differentiating through extensive opensystem crystallization AFC Repeated recharge of more mafic magmas induced magma mixing leading to the formation of hybrid andesites A partially crystalline boundary layer formed at the interface between the andesites and the recharge magma The magmatic enclaves were produced by the disruption and dispersion of this andesitic layer as a result of new magma injection and/or sustained tectonic activityPeriodic magma recharge and interactions with groundwater are two processes that have enabled the explosive regime to remain persistent over an 8yearlong period What precise mechanism triggers the eruptive activity remains speculative but it may be related either to new magma injection or to the sustained tectonic activity that occurred at that time in the vicinity of the volcano or a combination of bothThe authors thank J Stix J Davidson and an anonymous reviewer for providing excellent and constructive comments We also thank G Wörner and B Barbarin for suggestions and comments on early drafts of the manuscript We are grateful to M Veschambre for assistance on the electron microprobe and C Perrache and C Bosq for invaluable help in REE and radiogenic isotopes analyses We also thank our peruvian colleagues O Macedo and M Uribe and PhD students A Finizola J Suni and P Navarro for their help in the field Fundings and logistical support for this project were provided from IRD Institut de Recherche pour le Développement from CRV Coordination de la Recherche Volcanologique and from IGP Instituto Geofisico del Peru

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