PRIMITIVE MAGMAS IN ARC-TYPE VOLCANIC ASSOCIATIONS - EXAMPLES FROM THE SOUTHWEST PACIFIC

In widely accepted models for the origin of volcanic arcs at convergen t plate margins, magmas originate in subduction-modified peridotitic m antle. Primary are magmas are predictably unique in different arcs bec ause of the variety of potential components and processes involved in their generation, but they should share the common characteristics of high concentrations of Mg, Ni and Cr, reflecting their mantle origin. An examination of suites of samples taken from three southwest Pacific volcanic arcs (the Kermadec, New Zealand, and Papuan arcs) shows cont rasting geochemical patterns that correlate with different tectonic se ttings. Magmas with primitive chemical characteristics are comparative ly rare, and appear to occur where an extensional tectonic setting has allowed paths of relatively rapid ascent. In typical are settings, ma gma ponds at one or more positions above its source and is modified by a combination of fractionation, eruption, assimilation and recharge p rocesses, so that the most primitive magma erupted is a chemically evo lved high-Al basalt. The factors that dictate the geochemical nature o f magmas in volcanic arcs are tectonic setting together with the envir onment (depth) at which primary mantle-derived magmas are modified.