Formation of rhyolite at the Okataina Volcanic Complex, New Zealand: New insights from analysis of quartz clusters in plutonic lithics

Abstract

Granitoid lithic clasts from the 0.7 ka Kaharoa eruption at the Tarawera volcano (Okataina Volcanic Complex, Taupo Volcanic Zone, New Zealand) provide insight into the processes of rhyolite formation. The plutonic lithic clasts of the Kaharoa eruption consist of (1) quartz phenocrysts, which are often grouped into clusters of two to eight quartz grains, (2) plagioclase phenocrysts (mostly ~An 40 with up to An 60 cores), and (3) interstitial alkali feldspar. Quartz orientations obtained through electron backscatter diffraction (EBSD) methods show that 78% of the 82 analyzed clusters have at least one pair of quartz grains with the dominant dipyramidal faces matched. Variations in cathodoluminescence (CL) zoning patterns of the quartz suggest that quartz clusters came together after initial crystal growth and that many quartz crystals were subject to one or more resorption events. The process of quartz crystals with different magmatic histories coming together into common relative orientations to form clusters is indicative of oriented quartz synneusis and suggests a history of crystal accumulation. The quartz clusters are interpreted to have formed as part of a crystal cumulate mush within a shallow magma chamber where quartz crystals rotated into contact along their dominant dipyramidal faces during hindered settling and/or compaction. The preservation of oriented quartz clusters from the Kaharoa plutonic lithics thus provides evidence for synchronous, shallow pluton formation from a cumulate mush during active silicic volcanism. This result is consistent with models whereby meltrich, high-silica rhyolite formation occurs via interstitial melt extraction from a low-silica rhyolite mush in the shallow crust.

Variation in rhyolitic magma composition linked with fractionation from a common source: insights from the Rotoiti eruption, Taupo Volcanic Zone, New Zealand

“…andesite) composition data, but as outlined in Wilson and Rowland (2016), andesitic eruptions are mostly confined to the southern TVZ. Rotoiti volcanic glass data measured here lie between the TVZ basalt field and the rhyolite using Rb and Ba (Figure 7), which may represent Rotoiti pre-eruption rejuvenation from the Rb, Sr, U, and Ba poor crustal source, that is, 'crystal mush' (Deering et al 2008;Graeter et al 2015).…”

“…A comparable model of magmatic pulsing over a period of <3 Myr has been proposed by Gagnevin et al (2011) in regard to the Tuscan Magmatic Province (Italy). In the case of the active (~2 Ma to the present) TVZ, it is possible magmatic pulsing functioned to trigger the 'crystal mush' (Deering et al 2008) or 'granodiorite mush' (Graeter et al 2015) zone residing in the upper crust. This study proposes that periodic upwelling of mantle material, on the scale of thousands of years as a result of incremental subduction, would melt the upper crust and create an intermediate (Rb-, Sr-, U-, and Ba-poor) 'crystal mush' zone where AFC processes may initiate (Figure 8).…”

“…Furthermore, immediately prior to eruption, these ponded highly fractionated melts were triggered to erupt via thermal rejuvenation. However, not only is the TVZ associated with subduction of the Australian and Pacific plates, it is broadly located in an intra-continental rift zone (Cole et al 2014;Graeter et al 2015), a tectonic setting that places doubt on a model of prolonged ponding and fractionation. Once again, geochemical data presented here cannot definitively rule out and/or support either model, but taking into account the intra-continental rift zone, geochemical data that support fractionation and pre-eruption rejuvenation in an interconnected chamber network, and the consistent pressure estimates from the nearby Oruanui eruption, this author is inclined to support a model for a rapid fractionation/extraction processes described in Allan et al (2013).…”

“…The transition from mafic to felsic melts associated with large caldera-forming complexes such as the Taupo Volcanic Zone (TVZ), New Zealand, how these magma chamber systems interact, what triggers their eruption, and how large volumes of differentiated rhyolitic compositions are generated in the upper crust, remain a highly debated topic (Johnson et al 2013;Bégué et al 2014;Cole et al 2014;Graeter et al 2015;Wilson and Rowland 2016). In order to explain the origin of voluminous rhyolite melts, traditional magma chamber models whereby batches of parental mafic magmas rise and then undergo in situ fractionation in single chemically zoned magma chambers (e.g.…”

“…A number of more recent studies (Bachmann and Bergantz 2008;Deering et al 2008Deering et al , 2010Cole et al 2010Cole et al , 2014Smith et al 2010;Klemetti et al 2011;Shane and Smith 2013;Bégué et al 2014) suggest a process of lower crustal mafic magma injection where crustal assimilation and fractionation drives the melt compositions towards intermediate silica levels. These magmas are periodically tapped to pond in plagioclase-rich (±pyroxene and hornblende) 'crystal mush' (Deering et al 2008) or 'granodiorite mush' (Graeter et al 2015) zones higher in the crust. Interstitial melts here evolve towards dacite and rhyolite and these felsic melts are periodically extracted either to directly erupt or to pond temporally in shallow chambers.…”