Gamilaroi Terrane: A Devonian rifted intra-oceanic island-arc assemblage, NSW, Australia.

The Gamilaroi terrane comprises part of the New England orogen of eastern Australia and contains Devonian strata preserving evidence of their development in a variety of arc-related environments. Radiolarian studies provide new and more precise constraints on stratigraphy and together with geochemistry allow development of a new model for evolution of the terrane. A diverse suite of volcaniclastic sedimentary rocks intercalated with regionally extensive extrusive and high level intrusive meta-felsic igneous rocks of low-K and calc-alkaline volcanic affinity is present. These rocks constitute the oldest in-situ volcanic rocks in the Gamilaroi terrane and are similar to those found in modern intra-oceanic volcanic island-arc settings. Basaltic intrusions and lavas occur at higher stratigraphic levels. Major and trace element compositions, together with relatively flat chondrite-normalized REE distributions, resemble those from rocks formed in modern intra-oceanic island-arc rift zones. The close spatial association of these rock types together with an absence of continent-derived detritus is consistent with interpretation of development of the Gamilaroi terrane in an intra-oceanic island arc which experienced local rifting. This interpretation is a departure from previous models for the New England orogen, which interpret the Gamilaroi terrane as a forearc basin that developed above a long-lived (Cambrian to Permian), west-dipping subduction zone fringing the continental margin of Gondwana. By analogy with modern arc-continent collision zones in which crust between colliding continents and island arcs is preferentially subducted beneath the approaching arc, we suggest that oceanic crust intervening between Gamilaroi terrane and Gondwana was subducted eastwards under the western margin of the Gamilaroi terrane arc. During a latest Devonian collision event the Gamilaroi terrane was obducted onto the Gondwana margin. This resulted in subduction flip and subsequent development of an east-facing continental margin arc system on top of the Gamilaroi terrane.