Andesites as Magmatic Liquids or Liquid-crystal Mixtures; Insights from Egmont and Ruapehu Volcanoes, New Zealand Research Article

Abstract

Late Quaternary andesitic magmas in New Zealand contain complexly zoned antecrysts and glomerocrysts that are not in equilibrium with either the host whole rock compositions or siliceous groundmass glass and glass inclusions. Glass inclusions represent partial melts of mafic to gabbroic cumulates in the lower crust that mix with restite crystals, or cumulates from earlier magma batches. Assimilation of partial melts of mid-crustal rocks, represented by glass in crustal xenoliths, contributes a crustal component to the andesites. Magmas at Egmont are stored at about the brittle/ductile transition at about 10 km depth and variability in the composition of erupted material is a function of the composition of the recharging magma, and which parts of the storage system are tapped during the eruption. At Taranaki recharge occurs on a c. 1400 year cycle while interactions within the storage give rise to shorter period events. A similar process on a less well constrained timescale operates at Ruapehu. Andesites are therefore complex mixtures of fractionated mantle basalts, siliceous partial melts of both the lower crust and underplated cumulates, restite and cumulate crystals. Further modification occurs by interaction with partial melts of lower to middle crustal basement as geotherms increase with time.

The evolution of hydrous magmas in the Tongariro Volcanic Centre: the 10 ka Pahoka-Mangamate eruptions

“…through one of the central vents of Ruapehu or Tongariro), no further 'crystal cargo' or stored magma batches will be encountered upon ascent. As a result, individual PM magmas are relatively homogeneous, discrete dacitic and andesitic melts, which is unusual for the TgVC (Price et al 2012;Stewart 2010) and perhaps for andesites in general (Reubi & Blundy 2009).…”

Petrological record from young Ruapehu eruptions in the 4.5 ka Kiwikiwi Formation, Whangaehu Gorge, New Zealand

“…These are generally at least an order of magnitude smaller than those reported for earlier eruptions at 10 ka (Moebis et al 2011;Pardo et al 2011). Kiwikiwi pyroclasts represent magma that developed in a crustal mush column below Ruapehu, as has also been inferred for other Ruapehu andesites (Nakagawa et al 2002;Stewart 2010;Price et al 2012). The column was frequently invaded by small magma batches, with eruptions inferred to be related to the arrival of these small batches being the norm for Ruapehu's activity since 10 ka, when the last Plinian eruptions took place.…”

“…A similar model has been proposed for Ruapehu's modern eruptions (Nakagawa et al 1999(Nakagawa et al , 2002Stewart 2010;Price et al 2012).…”

“…Thus no net fractionation occurred to separate the dacitic melt (Fig. 7) from a crystal residue (Price et al 2008(Price et al , 2012Stewart 2010). 5.…”

Eruptive history of the Late Quaternary Ciomadul (Csomád) volcano, East Carpathians, part I: timing of lava dome activity

“…Indeed, excess argon tends to be relatively uncommon in minerals from silicic volcanic rocks largely because argon is highly incompatible in all major igneous minerals (Kelley 2002). As already described for Ciomadul (Kiss et al 2014) and elsewhere (Singer et al 1998;Stewart 2010;Doherty et al 2012), the presence of glomerocrysts suggests a long-residence storage of silicic crystal mush in an upper crustal storage zone about 8-12 km below the surface. This may have been remobilized by any subsequent eruption of the dacitic magma (Kiss et al 2014).…”