Characterisation of intra‐oceanic magmatic arc source terranes by provenance studies of derived sediments

Microprobe analysis of the detrital framework components of sedimentary rocks provides a powerful tool for determining both intra‐ and intersample variations in provenance. The interdependence of mineral composition with whole‐rock composition in igneous rocks means that this technique is particularly applicable to the study of magmatic arc derived volcaniclastic sediments, and can be used to establish the magma series of the source. The potential of this method is demonstrated with examples from the modern Tonga arc, where a direct link with the source terrane is still preserved, and the New England Fold Belt of eastern Australia, where the magmatic arc source terrane has now largely vanished. Petrography and phase chemistry of Miocene ‐ Recent volcanic detritus from the Tonga arc indicate derivation from a source of relatively uniform low‐K tholeiite composition. Plagioclase is the only feldspar phase and has compositions generally high in anorthite (An) and low in orthoclase (Or) components. Pyroxene grains and volcanic glass show Fe‐enrichment trends, and the pyroxenes have low TiO₂ contents. In the Paleozoic New England Fold Belt, low‐grade burial metamorphism has resulted in widespread alteration and replacement of primary detrital phases. However, relict clinopyroxene compositions, combined with petrographic data from the sandstones and whole‐rock geochemistry of igneous clasts in interstratified conglomerates, suggest derivation from a transitional low‐K to calc‐alkaline source during early phases of arc evolution. The incoming of detrital hornblende, biotite, and K‐feldspar during the latter stages of arc evolution indicate a change to a strongly calc‐alkaline or high‐K tholeiite source.