Trace element geochemistry of the rhyolitic volcanic rocks, Central North Island, New Zealand. Phenocryst data

Abstract

Data are presented for K, Ba, Sr, Rb, Li, Ga, Mg, Mn, and Fe for twelve rhyolitic plagioclases (An₂₈-An₄₆), one dacitic (An₅₃), and three andesitic plagioclases (An₆₈-An₈₁). Additional data are presented for Ga, Gr, V, Ni, Co, Sc, Y, La, Sr, and Ba for two augites, nine hypersthenes, and five hornblendes separated from the same rocks. Distribution factors have been calculated, using these data, and previously published results for coexisting groundmass compositions (=“liquids”).

The plagioclases show a positive correlation between, and a progressive increase in K and Ba (range 0.09–0.58% and 61–610 p.p.m. respectively) with increasing Ab-content. Sr (range 465–880 p.p.m.) shows a well defined maximum between An₄₀-An₅₅. The plagioclases have extremely high K/Rb ratios (mostly > 1,000).

This volcanic series is characterised by relatively Mg-rich pyroxenes and hornblendes. The augites contain higher Sc, Cr, Y, Sr, and Y relative to their coexisting hypersthenes, while the hornblendes exhibit higher Sc, V, Ba, Sr, Y, and La relative to coexisting hypersthenes. Very marked differences in concentrations of these elements exist between the rhyolitic and andesitic ferromagnesian phenocrysts. There is also evidence of a systematic distribution of Sc, V, Cr, Y, Co, and Ni between coexisting hypersthenes and hornblendes, and between these minerals and their coexisting whole rock and groundmass compositions.

The data are discussed from a petrological viewpoint, as they are interpreted to indicate that the phenocrysts crystallised in the magmas in which they are found, and are not xenocrystic. No evidence of hybridisation or contamination, subsequent to the onset of crystallisation, is found.