Petrography and trace-element geochemistry of metabasalts on Diego Ramirez Islands, southern Chile
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Date
1988-03
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The Ohio State University
Abstract
The purpose of this paper is to determine the original igneous setting of metabasalts on the Diego Ramirez Islands, which are a group of islands located about 100 km southwest of Cape Horn, in the southernmost part of Chile. These islands are located where the western edge of South America's continental shelf intersects the Shackleton Fracture Zone. The islands, which are thought to represent material accreted in an ancient subduction zone along the continental margin, are complicated because of metamorphic and tectonic effects. The islands consist of two main rock units - metabasalt and tectonic melange. The presence of pillow structure and high vesicularity in the metabasalts indicates these rocks were formed in a shallow-marine environment. The petrography, based on the presence of titanaugite and absence of orthopyroxene, indicates the original igneous material was alkali-olivine basalt. This composition provides evidence for a seamount environment. Metamorphic assemblages include crossite, pumpellyite, chlorite, white mica, epidote, albite, and prehnite, but lawsonite is absent. The assemblages indicate high pressure/low temperature metamorphism that is transitional between blueschist, greenschist, and prehnite-pumpellyite facies. Metamorphic conditions were approximately 5.5 kbars and 270°C. The geochemical analysis of the elements Rb, Sr, Zr, Y, Cr, Ni, and Ti, using x-y graphs to test mobility and discrimination diagrams to test environmental setting, indicates that these rocks were originally erupted in a seamount setting. Rb and Sr were mobilized during low-grade metamorphism.