Geology and petrology of Raoul Volcano: Magma genesis and fractionation processes beneath the Tonga-Kermadec arc

Abstract

Intra-oceanic volcanic arcs are the simplest manifestation of subduction-related magmatism. Three key problems stand in sharp focus in this environment: crustal processing of primitive high-Mg magma to generate derivative low-Mg magmas, genesis of felsic magma, and mass transfer between the subducting slab and the mantle. New insights into these problems are obtained from a comprehensive petrological study of 311 samples from Raoul Volcano, Tonga-Kermadec arc. Raoul lavas are low-K series tholeiites with 48-70 wt.% Si02 and Mg#< 67. Few have 58-65 wt.% Si02. Glomeroporphyritic basalt-basaltic andesite predominates (5-48 % phenocrysts). Sparsely phyric basalt-basaltic andesite and dacite are less abundant. Common phenocryst phases are calcic plagioclase (-An84-90), augite, olivine, and orthopyroxene, with similar compositions in all lavas (and friable adcumulates). Phenocryst-magrna equilibrium is restricted to thin strongly zoned phenocryst rims. Geochemical differences between glomeroporphyritic and aphyric lavas reflect their phenocryst contents and proportions. Fractional crystallisation of primitive high-Mg magma must produce aphyric low-Mg basaltic-andesitic magmas by efficient crystal removal. These magmas accumulate unrelated crystals as they ascend through crystal mush. Eruptions of dacite commenced at 3.7 ka, and culminated with the 8-16 krn3 Fleetwood eruption (2.2 ka). Resurgent volcanism has occurred in Denham and Raoul Calderas. Each eruption vented a geochemically homogeneous and distinct magma batch. Fractional crystallisation cannot link these batches to each other or a common parent without major changes in the extract assemblage. They can be generated by dehydration melting (>20 %) of sub-arc crust at 0.6 GPa and >30 °C. Felsic magmatism may characterise arc adolescence, as primitive magma underplating sub­ arc crust converts it to dacitic melt and granulite within 1-2 m.y. Conservative elements in mafic lavas require 25 % fractional melting of mildly depleted (2 % melt extraction) homogeneous MORB-source mantle. The subduction-related component contributes -30 % Nd, 55 % La, 75 % Sr, 85 % Th, and >90 % K, Pb, Rb, Ba, and Cs. It has constant composition and magnitude, consistent with mass transfer from the slab to a sub-arc melting column by repeated episodes of arnphibole formation and decomposition. Remarkably similar subduction­related components in Raoul, South Sandwich, and Mariana lavas testify to efficient buffering of variable slab-derived fluxes by Dnuid-px and amphibole-saturation of mantle peridotite.