Textural and Mineralogical Significance of the Granitic Xenoliths From the Central Volcanic Region, North Island, New Zealand

Abstract

Granitic boulders have been found at four localities in the central North Island: Mt Tarawera, Huka, Atiamuri, and Hinuera. They occur closely associated with the acid volcanic suite, in pumice breccias and associated water-laid deposits. Two distinct types of boulders are recognised, the true granodiorites of Tarawera and Huka, and those characteristically containing extensive graphic intergrowths, found at Atiamuri and Hinuera. Both types are characterised by a “high temperature” mineralogy, and are regarded as co-magmatic with the acid volcanic rocks of the region, and not part of the basement. The granodiorites are characterised by subhedral to euhedral zoned plagioclase, subhedral to euhedral embayed quartz, subhedral to interstitial sanidine, and the presence of biotite, hornblende, hypersthene, and magnetite. The granodiorites are considered to be representative of the magma from which they crystallised, but the second type show evidence of progressive potassium enrichment and are considered to be late-stage differentiates of their parent magma. Modal and chemical data are given. The granodiorite boulders are of special interest as they provide an insight into the end products of crystallisation of the Taupo acid volcanic suite of rocks. This is particularly important with regard to recent work on feldspar crystallisation, and much of the paper is devoted to this aspect. © 1967 Taylor & Francis Group, LLC. All rights reserved.