Textural and mineralogical significance of the Granitic Xenoliths from the Central Volcanic Region, North Island, New Zealand

Abstract

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 consider...