Submarine alkalic through tholeiitic shield-stage development of Kïlauea volcano, Hawai’i

Abstract

The submarine Hilina region exposes a succession of magma compositions spanning the juvenile "Lō‘ihi" through tholeiitic shield stages of Kïlauea volcano. Early products, preserved as glass grains and clasts in volcaniclastic rocks of the 3000 m deep Hilina bench, include nephelinite, basanite, phonotephrite, hawaiite, alkali basalt, transitional basalt, and rare alkali-poor Mauna Loa-like tholeiite. Transitional basalt pillow lavas overlie the volcaniclastic section and record an early phase of subsequent subalkaline magmatism. Rare degassed tholeiitic pillow lava and talus above the volcaniclastic section are products of subaerial shield volcanism. Major and trace element variations of clasts and pillow lavas point to a factor of 2 - 2.5 increase in degree of melting from juvenile alkalic to modern tholeiitic Kïlauea. Progressive changes in element ratios that distinguish Hawaiian shield volcanoes, without commensurate changes in elements fractionated by partial melting, also signal increased contributions from Mauna Loa-type source regions as Kïlauea matured from its juvenile alkalic to its tholeiitic shield stage. Ancestral Kïlauea basanites and nephelinites were not primitive magmas but might have evolved from plume-derived alkali picritic parents by lithospheric- level crystallization differentiation, or solidification and remelting, involving pyroxene and garnet, similar to the subcrustal differentiation origin of hawaiites [Frey et al, 1990]. Low magmatic productivity early in Kïlauea’s history sustained a poorly integrated trans-lithospheric conduit system in which magmas stalled and differentiated, producing evolved hawaiites, nephelinites, and basanites. This contrasts with shield-stage Kïlauea where high magmatic productivity flushes the conduit system and delivers primitive magmas to shallow levels.