Arc lavas on both sides of a trench: Slab window effects at the Solomon Islands triple junction, SW Pacific

Abstract

The Woodlark Spreading Center (WSC) is subducted at the San Cristobal trench, forming a triple junction at the New Georgia Group (NGG) arc in the Solomon Islands. WSC lavas are N-MORB at > 100 km from the trench, but with decreasing distance they have increasingly arc-like Sr–Nd–Pb isotopic ratios, enrichments in Rb > K > Pb > Sr, and depletions in HFSE and Y. Within 50 km of the trench on the Simbo and Ghizo Ridges, many recovered samples are island arc tholeiites to medium-K calc-alkaline andesites and dacites, and many have the same or similar major and trace element and isotopic characteristics as true arc lavas in the NGG on the other side of the trench. Previous investigations have concluded that these WSC lavas are the result of relic back arc mantle enrichments resulting from subduction of the Pacific plate prior to the late Miocene at the North Solomon trench, > 200 km to the north. However, the high-silica WSC lavas are more arc-like than those recovered from other distal back arcs, and are more voluminous, forming large submarine ridges and stratovolcanoes. We suggest that true arc mantle migrates across the plate boundary from the adjacent NGG arc through slab windows created by the subduction of the WSC. This leads to variable mixing between NGG arc and WSC N-MORB end-members, forming the transitional lavas recovered from the WSC. Lavas with similar arc-like characteristics have previously been recovered on the Chile Rise near where it is subducted at the Chile Trench, raising the possibility that such mantle transfer is a common phenomenon where active spreading centers are subducted. The presence of slab windows may also be responsible for the unusual forearc volcanism in the NGG, and melting of slab window margins may account for the presence of high-silica adakite-like lavas on the WSC.