Origin of high-Si dacite from rhyolitic melt: evidence from melt inclusions in mingled lavas of the 1.6 Ga Gawler Range Volcanics, South Australia

Abstract

¶Part of the Mesoproterozoic (1.6 Ga) Gawler Range Volcanics in South Australia is composed of mingled feldspar- quartz- phyric dacite, rhyodacite and rhyolite lavas. Field relationships suggest that dacite erupted first, locally grading into rhyodacite, followed by mingled dacite and rhyolite or rhyodacite and rhyolite, and finally in some areas rhyolite, and imply that the three lithofacies co-existed in a compositionally stratified magma chamber. Data on the bulk rock, groundmass and melt inclusion compositions suggest that post-eruption alteration has had very little effect on the original rock compositions. Melt inclusions in quartz from rhyolite and rhyodacite-dacite, respectively, belong to two compositional populations. Inclusions in the rhyolitic quartz have less evolved compositions with lower SiO2 (72–76.4 wt %) and higher Al2O3 (13.2–15.6 wt%) and Na2O (2.5–4.2 wt%) abundances. In contrast, melt inclusions in quartz from the rhyodacite-dacite are more “evolved” (i.e., 75.5–78.3 wt% SiO2, 11.2–12.7 wt% Al2O3 and 1.7–2.2 wt% Na2O). The two melt populations define a single compositional trend towards groundmass compositions, which are essentially similar in all three lithofaci es (77.8–80.5 wt% SiO2, 9.9–11.1 wt% Al2O3 and 2.2–2.4 wt% Na2O). This trend is consistent with the derivation of the groundmass melt from a single precursor melt of rhyolitic composition by means of crystallisation of dominant plagioclase, K-feldspar and minor quartz. Plagioclase-enriched dacite-rhyodacite magma comprises a mixture of the residual melt and plagioclase phenocryst s that accumulated in the upper part of the magma chamber and erupted first. Similar residual melt containing quartz and K-feldspar phenocrysts was present deeper in the magma chamber and erupted later to form quartz-, K-feldspar-phyric rhyolite.