Reactivation of a rhyolitic magma body by new rhyolitic intrusion before the 15.8 ka Rotorua eruptive episode: implications for magma storage in the Okataina Volcanic Centre, New Zealand

Abstract

The 15.8 ka Rotorua rhyolite eruptive episode from the Okataina Volcanic Centre comprises a plinian pumice fall deposit followed by the extrusion of two rhyolitic lava domes or flows and their associated block-and-ash flows, with a total volume .1 km 3 (dense rock equivalent). Variations in mineralogy, wholerock, glass and mineral chemistry, and calculated magmatic properties suggest that two distinct rhyolitic magmas were sequentially tapped during the eruption. The first magma erupted (T1) is characterized by low SiO 2 (c. 76.5 wt% in glass), calcic feldspars (An 44 ), magnesian hornblendes (MgO c. 14.45 wt%), clinopyroxene, and high temperatures (c. 835 8C) and fO 2 (1.8-2.1˜FMQ (where FMQ is the fayalitemagnetite-quartz buffer)). The second magma (T2) was more evolved and is characterized by higher SiO 2 (c. 77.4 wt% in glass), Na-rich feldspars (An 24 ), less magnesian hornblendes (MgO c. 11.8 wt%), biotite, and low temperatures (c. 750 8C) and fO 2 (0.65-1.1˜FMQ). Both magmas are homogeneous, but evidence for some magma mingling indicates that they were in contact during eruption. However, there was only a minor degree of hybridization, perhaps reflecting the contrasting temperatures and viscosities of the two magmas. The crystal-rich, poorly vesicular nature of the T2 ejecta indicates that it originated from a cooling, high-level magma chamber that was reactivated by intrusion of hotter, volatile-rich T1 magma. The ponding of rhyolite magmas at shallow depth and their subsequent reactivation by later rhyolitic intrusion may be an important process in the compositional evolution and eruption dynamics of many Okataina Volcanic Centre rhyolite magma bodies.

Proximal stratigraphy and event sequence of the c. 5600 cal. yr BP Whakatane rhyolite eruption episode from Haroharo volcano, Okataina Volcanic Centre, New Zealand

“…[This process triggered the 15 700 cal. yr BP Rotorua eruptions (Smith et al 2004) from Okareka embay ment vents at the southwest end of the Haroharo linear vent zone (Fig. 1, 2). ]…”

Geochemistry and magmatic properties of eruption episodes from Haroharo linear vent zone, Okataina Volcanic Centre, New Zealand during the last 10 kyr

“…Position of Fig. 3a-c is shown 1992) and 15.8 ka Rotorua (Nairn 1980;Smith et al 2004) appear to be the earliest episodes. During these events much of the activity was from vents within the Okareka Basin at the southern end of the HLVZ (Figs.…”

“…The OVC rhyolitic episodes are extraordinary as most (seven of the nine) episodes in the last 26 ka, from the HLVZ and the Tarawera linear vent zone (situated 10 km south; Fig. 1), have tapped several distinct magmas along their length (Nairn 1992;Nairn et al 2004;Smith et al 2004Smith et al , 2005. The HLVZ eruption episodes are slightly different to those from Tarawera linear vent zone (e.g., Nairn et al 2004) as they erupt rhyolites with limited compositional diversity and display no evidence for mafic reactivation.…”

Bimodal pumice populations in the 13.5 Ma Harsány ignimbrite, Bükkalja Volcanic Field, Northern Hungary: Syn-eruptive mingling of distinct rhyolitic magma batches?

“…Nakamura 1995;Civetta et al 1997;Signorelli et al 1999;Smith et al 2004;Shane et al 2005Shane et al , 2007Shane et al , 2008Pabst et al 2007). Since the definition of the magma chamber is ambiguous in the literature (e.g.…”

“…The triggering mechanism of the violent volcanic eruption is often intrusion of basaltic magma into the more evolved felsic magma chamber (Sparks et al 1977;Pallister et al 1996;Murphy et al 2000;Nairn et al 2004), but in some cases the arrival of a new rhyolitic magma reactivates another rhyolite magma at depth Eichelberger and Izbekov 2000;Smith et al 2004;Shane et al 2008). In the Harsány eruption episode, no sign of basaltic intrusion can be observed.…”