Petrochemistry and feldspar crystallisation in the silicic volcanic rocks, central North Island, New Zealand

Abstract

Chemical analyses of various andesitic, dacitic, and especially rhyolitic volcanic rocks are presented, including groundmass and phenocrystic feldspar components separated from a number of these rocks. The rhyolites are characterised by the early separation of phenocrystic plagioclase (An 25–46 ), a second potash-rich feldspar (Or 63–68 ) occurring only rarely. It is shown that in terms of the Ab−Or−An system, the rhyolitic total rock compositions all project into the plagioclase field, and that only a few of the groundmass compositions have reached the feldspar cotectic surface, thus explaining the low frequency of occurrence of sanidine. Analyses of 5 groundmass compositions which coexist with phenocrystic plagioclase, quartz, and sanidine provide information on the projection (within the ternary feldspar system) of the curve defining the intersection of the feldspar cotectic and quartz surfaces. These analyses plot close to the position of this curve as delineated by Carmichael (1963) . An apparent correlation is also noted between the compositions (in terms of the normative feldspar components) of coexisting plagioclasegroundmass pairs and the nature of their coexisting phenocrystic ferromagnesian assemblage i.e. hypersthene; hypersthene+hornblende; or biotite+hornblende±hypersthene.