A GEOCHEMICAL INVESTIGATION OF THE WOODLARK RIDGE-SOLOMON ISLANDS ARC TRIPLE JUNCTION

The triple junction formed by the subduction of the Woodlark spreading ridge beneath the Solomon Islands arc (New Britain and San Cristobal trenches) is an area of tectonic, volcanic, and geochemical complexity. Significant uplift of some New Georgia Group (NGG) islands and the presence of numerous arcuate faults near the triple junction suggest the area is under extreme tectonic stress, and the absence of a deep trench or strong Benioff zone beneath the NGG suggests that subduction is being stifled by the impingement of the high-standing ridge on the trench. The triple junction region is also notable for the presence of volcanism that is unusually close to the trench in what would normally be considered the forearc region, and the eruption of voluminous andesites and dacites from the Woodlark Ridge (Ghizo and Simbo Ridges) on the subducting plate. A previous episode (pre-Miocene) of south-directed Pacific Plate subduction, prior to the current north-directed subduction of the Australian Plate, may have geochemically enriched the regional mantle beneath the triple junction and may explain some of these observations. To identify the source(s) of enrichment for the lavas in the triple junction area, major and trace element concentrations and radiogenic isotopic ratios for rocks from the NGG and Woodlark Ridge have been measured. Woodlark Ridge lavas (MORB-like) become progressively more enriched in incompatible elements with proximity to the trench, culminating in enriched andesites and dacites on the Simbo and Ghizo Ridges. NGG lavas on the other side of the trench are generally more enriched than Woodlark lavas. NGG and Woodlark andesites and dacites may be enriched to some degree by partial melts from the subducting Australian Plate, as they exhibit high Sr/Y, depletions in HREE, and low ⁸⁷Sr/⁸⁶Sr.