Grain size control of mineralogy and geochemistry in modern river sediment, New Guinea collision, Papua New Guinea

Abstract

The New Guinea collision includes diverse sediment source areas associated with arc-continent collision to the west of the Solomon Sea triple junction, and with subduction of oceanic crust to the southeast and northeast of the triple junction. Twenty-seven sediment suites, each comprising up to seven gravel, sand and mud samples from the same locality, were analysed to determine mineralogical and geochemical variation with changing grain size. This paper deals primarily with the mineralogical variation. Compositional variation from coarse to fine grained fractions is generally >50% of each variable's abundance, with many variables showing greater than 100% variation. In contrast, variation due to analytical error is generally less than 10%, between-year variation generally ranges from −15% to +25% and downstream variation is generally less than 50%. Petrographic results indicate that sediment composition is fundamentally dependent on grain size. The principal control on sediment composition is mechanical weathering of rock fragments and compositional sorting of the constituent mineral phases. While the Gazzi-Dickinson method for point counting sediment reduces modal variability and, in most cases, provides robust provenance discrimination, it is unsuitable for discerning faint source rock signatures often revealed by grain size-dependent compositional variation. Only the upper plate magmatic arc provenance signature is evident from recalculated modal and geochemical data, and that signature does not reflect the modern upper plate magmatic arc. Detailed petrographic analysis of grain types, as opposed to recalculated modal or geochemical data, is required to elucidate both upper and lower plate provenances, and discriminate between ancient and contemporary arc provenances.