Petrology of metabasalts from the Chrystalls Beach accretionary mélange - implications for tectonic setting and terrane origin

Abstract

The Chrystalls Beach Complex is interpreted as an accretionary mélange within the Otago Schist on the South Island of New Zealand. Its stratigraphic position within the New Zealand Mesozoic accretionary prism is not well constrained, and the terrane affinity of the complex has remained enigmatic. Previous studies have focused on age relationships and sediment geochemistry. In this contribution we present geochemical data for metabasalts within the complex. Two distinctly different basalt compositions are found in the mélange. Metabasalts outcropping at the northern end of the complex have an inferred mid-ocean ridge affinity, while metabasalts in the central part of the assemblage are suggested to represent ocean island basalts. It is speculated that the northern mid-ocean ridge basalts represent blocks from the crust of a subducting slab, while the ocean island basalts are part of a subducting seamount. These tectonic affinities are in agreement with mélange formation during subduction of material below the Otago Schist protolith. In this setting, the mélange may contain material derived from both the Caples and Rakaia Terranes, or represent a different terrane fragment.