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) – the world’s most productive region where rhyolite volcanism is currently active – give 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 with anorthitic cores and albitic rims, and (3) interstitial potassium 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 matched dipyramidal faces that are in either parallel or Esterel twin orientation. Variations in cathodoluminescence (CL) zoning patterns of the quartz suggest that quartz clusters in the plutonic lithics formed after initial crystal growth and that many quartz crystals were subject to one or more resorption events. Thus, individual quartz crystals with different magmatic histories came together into preferred orientations to form clusters; this process 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 dipyramidal faces during hindered settling and/or compaction. Therefore, the preservation of oriented quartz clusters from the Kaharoa plutonic lithics provides direct evidence for synchronous, shallow pluton formation from a cumulate mush during active volcanism. This result is consistent with a model whereby melt-rich, high-silica rhyolite formation occurs via interstitial melt extraction from a low-silica rhyolite mush in the shallow crust.