Abstract
Furnas is one of three active central volcanoes on Sao Miguel Island, Azores, and is considered to be one of the most hazardous in the archipelago. In this study, the pre-eruptive magma plumbing system of the 10 young (<5 ka), intra-caldera, sub-Plinian, trachytic eruptions of the Upper Furnas Group (UFG) is investigated via whole rock major and trace element geochemistry, mineral chemistry, thermobarometry, and petrogenetic modelling. The main aim of this work is to elucidate the petrogenesis of the Furnas trachytes, constrain the P–T–fO2 conditions under which they evolve, and investigate the temporal evolution of the magma plumbing system. Results indicate that the trachytes are derived predominantly from extended fractional crystallisation of alkali basalt parental magmas, at depths between ~3 and 4 km. This is considered to take place in a density-stratified reservoir, with alkali basalt magmas at the base and hydrous trachytes forming an upper cap or cupola. The presence of this reservoir at shallow crustal depths beneath the caldera likely inhibits the ascent and subsequent eruption of mafic magmas, generating a compositional Daly Gap. Rare syenitic ejecta represent in situ crystallisation of trachytic magmas in the thermal boundary zone at the top of the reservoir. Trachytic enclaves within these syenites, in addition to banded pumices and ubiquitous clinopyroxene antecrysts in the UFG pumice falls, provide evidence for mingling/mixing processes within the magmatic system. Despite relatively uniform major element compositions, systematic trace element variations within individual eruptions highlight the importance of fractional crystallisation during late-stage evolution of the trachytes. This is facilitated by the accumulation of water and the development of mild peralkalinity, which contribute to low pre-eruptive melt viscosities and efficient crystal settling. Compositional zoning patterns between individual eruptions cannot be accounted for by periodic tapping of a single magma batch undergoing fractional crystallisation. Instead, up to four individual cycles are recognised, in which a zoned cap of eruptible trachytic magma, formed at the top of the reservoir, was erupted in one or more eruptions and was re-established via intermittent replenishment and subsequent fractional crystallisation.
Highlights
Whilst peralkaline volcanic rocks are volumetrically less significant than their subalkaline counterparts, the petrogenetic processes that lead to their formation may be more complex (e.g. Mahood 1984; Mungall and Martin 1995; Scaillet and Macdonald 2001; White et al 2009; Markl et al 2010)
Recent studies of peralkaline silicic magmatic systems have led to the general consensus that mafic magmas either act as a parental magma, linked to the peralkaline silicic magmas primarily by fractional crystallisation (e.g. Barberi et al 1975; Self and Gunn 1976; Civetta et al 1998; Peccerillo et al 2007; Macdonald 2012), or alternatively act as a heat source generating crustal-derived partial melts which may differentiate via fractional crystallisation (e.g. Bohrson and Reid 1997; Trua et al 1999; Avanzinelli et al 2004)
Furnas volcano is an example of an oceanic island volcanic centre that exhibits many of the petrogenetic problems associated with peralkaline magmatic systems, including a volumetric abundance of felsic products with a peralkaline affinity, an apparent Daly Gap, and a spatial concentration of felsic products within the volcano’s caldera complex, with mafic products on the flanks (Booth et al 1978; Moore 1991)
Summary
Whilst peralkaline volcanic rocks are volumetrically less significant than their subalkaline counterparts, the petrogenetic processes that lead to their formation may be more complex (e.g. Mahood 1984; Mungall and Martin 1995; Scaillet and Macdonald 2001; White et al 2009; Markl et al 2010). Recent studies of peralkaline silicic magmatic systems have led to the general consensus that mafic magmas either act as a parental magma, linked to the peralkaline silicic magmas primarily by fractional crystallisation Bohrson and Reid 1997; Trua et al 1999; Avanzinelli et al 2004) Both of these models can be affected by varying degrees of crustal assimilation and magma mixing (Macdonald et al 2015). Furnas volcano is an example of an oceanic island volcanic centre that exhibits many of the petrogenetic problems associated with peralkaline magmatic systems, including a volumetric abundance of felsic products with a peralkaline affinity, an apparent Daly Gap, and a spatial concentration of felsic products within the volcano’s caldera complex, with mafic products on the flanks (Booth et al 1978; Moore 1991). The evolution of peralkaline trachyte has not been quantitatively demonstrated at Furnas, and constraints upon the plumbing system are currently limited to geophysical studies (e.g. Machado 1972; Camacho et al 1997; Montesinos et al 1999), which broadly point towards a magma reservoir at shallow depths in the crust (~5 km)
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