The origin of high-alumina basalts and their relationships to quartz tholeiites and alkali basalts

Abstract

Experimental crystallization of olivine tholeiite (20% olivine) at 9 kb shows that olivine and to a lesser extent, orthopyroxene are the early crystallizing phases, joined at lower temperatures by clinopyroxene and plagioclase. This contrasts with atmospheric crystallization of olivine first, joined at lower temperatures by plagioclase and clinopyroxene. In high-alumina olivine tholeiite (6% olivine) however, clinopyroxene is the liquidus phase at 9 kb, joined at lower temperatures by plagioclase, while from 0–6.8 kb plagioclase is the liquidus phase joined by olivine and clinopyroxene. Alkali basalt and olivine basalt have olivine and clinopyroxene as important near-liquidus phases at 9 kb, compared with olivine and plagioclase at atmospheric pressure. Quartz tholeiite contains olivine, together with plagioclase and clinopyroxene near the liquidus at atmospheric pressure, but at 4.5 kb and 6.8 kb plagioclase and clinopyroxene alone were evident. These results show that derivation of quartz-normative tholeiites from olivine-normative parent magmas is only possible at depths of less than 15 km, while at depths of 15–35 km high-alumina basalts of tholeiitic or alkalic affinities and only slightly enriched in silica are obtained from olivine tholeiite or olivine basalt parents. At depths of 35–60 km fractionation of olivine-rich magmas is largely governed by aluminous pyroxenes and the derivative liquids trend towards undersaturated alkali basalts. The inter-relationships of the three major magma types, quartz tholeiite, high-alumina basalt and alkali basalt, in such volcanic provinces as Japan are explained by magma segregation or fractional crystallization over specific depth ranges. Thus alkali basalts are derived at 35–60 km depth, high-alumina basalts at 15–35 km and quartz tholeiites at less than 15 km.