Petrography of the rhyolite lavas of Tarawera Volcanic Complex

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Abstract
The rhyolite domes and associated lava flows of the Tarawera Volcanic Complex, North Island, New Zealand, can be divided into four rock types: hypersthene-hornblende rhyolite; hornblende-biotite-rhyolite; hypersthene rhyolite, and biotite rhyolite. Two types of biotite are found within the lavas; “normal” biotite with a γ refractive index of 1.65–1.67 and “ferrian” biotite with a γ refractive index of 1.72–1.74. Four types of hornblende occur: a pale one (similar to cummingtonite), and green, brown, and basaltic ones. The concentration of K2O in the lava was probably the controlling factor which determined whether biotite or hornblende (or both) crystallised. Plagioclase occurs in all lavas, and quartz in most. Where they both occur initial crystallisation appears almost synchronous and subsequent resorption was probably due to variations in water pressure within the magma reservoir.