High‐Ca boninites from the active Tonga Arc

Abstract

We report the first known occurrence of high‐Ca boninites within an active submarine island arc, at Volcano A within the Tonga Arc. Both the whole rock and a population of melt inclusions (in Fo86–92 olivines) from a dredged satellite cone have compositions classified as high‐Ca boninite. All samples from Volcano A, however, may be related to parental boninites, given the similarity in their rare earth element patterns and their coherency along a similar liquid line of descent. The primary high‐Ca boninite liquids were generated in the mantle wedge by high cumulative degrees of melting (>∼24%) at typical mantle wedge temperatures (<1300°C) driven by an influx of slab‐derived fluid (>4 wt % H2O in primary liquids). We propose a two‐stage model for generating primary boninite liquids at Volcano A: (1) melting of fertile peridotite within the Lau back‐arc basin, followed by (2) remelting of this residual peridotite with slab‐derived fluid beneath the Tonga Arc. The occurrence of high‐Ca boninites at Volcano A is related to the relative location and duration of back‐arc spreading. Here, the Eastern Lau Spreading Center has been processing mantle for ∼1 Ma, and corner flow circulation brings mantle from the back‐arc melting regime into the arc melting regime at a rate that is a significant fraction (>30%) of the convergence rate. On the basis of Si6.0 and Ti6.0 relationships, we argue that a significant portion of the central Tonga Arc near Volcano A, as well as several other arc volcanoes with active back‐arc basins, are also erupting basaltic andesites with boninite parentage.