Geochemical evolution within a Devonian intra-oceanic island arc: The Gamilaroi terrane, southern New England orogen, Australia

Geochemical analyses of volcanic rocks in the Gamilaroi terrane reveal several phases of arc activity within an intra‐oceanic island‐arc terrane. Felsic volcanic rocks at the base of the section have rare earth element (REE) and trace element compositions which indicate that they were derived from an island‐arc source. Basalts immediately overlying the felsic volcanic rocks have a distinctive geochemical signature with low levels of Ti and Y and high levels of Ni, Cr and Mg. Low concentrations of REE and trace elements relative to mid‐ocean‐ridge basalts (MORB) indicate that they were also derived from an intra‐oceanic island‐arc source. Extensive basalts and basaltic andesites among the youngest rocks of the terrane have typically flat to enriched REE and trace element compositions, indicating a transitional arc‐back‐arc source. The change in basalt compositions indicates that rifting had occurred by this stage in the evolution of the arc. Confirmation of an intra‐oceanic setting for this terrane enables a more detailed comparison with similar intra‐oceanic rocks in the northern New England orogen. This study of the Gamilaroi terrane is an example of the potential use of geochemical data to identify other ancient intra‐oceanic island‐arc‐rift suites.