Comparative geochemistry of some volcanic suites of the Solomon Islands and Bougainville : implications for metallogenesis

Abstract

The Solomon Islands arc a complex collage of crustal units or terranes (collectively called the Solomon Block) that have formed and accreted within an intra-oceanic environment during the Cretaceous and Tertiary Periods. The Solomon Block has recently being subdivided into five terranes based on: basement lithology and geochemistry, age, and development (or lack) of younger, arc-dominated, basaltic basement sequences. A combination of terrane assessment coupled with the ongoing magmatic and tectonic developments in the Solomon Islands has direct implications for understanding a number or metallogenctic processes. The study of glass inclusions (or formerly melt inclusions; MIs) in phenocrysts provide direct information about the chemical composition of the naturally-occurring. supra-subduction zone-derived melts within an overall transpressional environment. MIs have been trapped in various phenocrysts of clinopyroxene, plagioclase, quartz. amphibole, irontitanium oxides, and sulfides. This study was directed primarily at documenting and comparing bulk rock, MIs, and matrix glass compositions for calc-alkaline volcanoes of the islands of Bougainville, Fauro, Choiseul, and Guadalcanal. The MIs occur a isolated blebs and sometimes in groups with preferred alignments, and range in individual size from <1μm to 250 μm in length. Variable rates of crystal growth coupled with magmatic cooling are involved in the formation of such glass inclusions. Overall, the compositions of the MIs for each island range from basaltic (<53 wt% SiO) through intermediate Si01 (53-60 wt%) to highly silicic (>60 wt% SiO₂). For all MIs, the oxides of Al-Mg-Fe-Ti-Mn (and generally Ca) decrease while those of Na and K increase as the content of Si0₂ increases. The Gold Ridge rocks in particular have very high K concentrations. For most or the sample suites, the light rare earth elements (REE) arc enriched relative to the heavy REE, similar to many medium- to high-K volcanic arc suites world-wide. The fluid mobile elements (e.g., Sr, K , Rb. and Ba) are enriched relative to REE or similar melt-crystal incompatibilities, while the nominally fluid-immobile clement (e.g., Th, Ta. Nb, Zr, Hf, Ti, Y, and Yb) are similar in abundances to REE of equivalent incompatibility/compatibility. In the future, more refined targeting of potentially mineralised, volcanic rock-hosted ore occurrences will require knowledge or terrane (and hence basement geological) history, coupled with current tectonomagmatic settings.