Tectonic setting and paleoenvironment of terranes in the southern New England orogen, eastern Australia as constrained by radiolarian biostratigraphy

Abstract

Radiolarians are abundant in the Gamilaroi, Djungati and Anaiwan terranes of the New England orogen in eastern Australia. These microfossils present the first age constraints on the timing of development of hitherto undated lithologies within the various juxtaposed terranes of the orogen. They give a biostratigraphic framework for interpreting the history of marine sedimentation in these terranes and have significant implications for published tectonic models. Radiolarians also show that the stratigraphically lowermost rocks of the Gamilaroi terrane are most probably of Devonian age. Better stratigraphic resolution permits interpretation of these rocks as part of an intra-oceanic island arc succession which accreted to the eastern margin of Australia (Gondwana) around the end of the Devonian. This intra-oceanic arc is distinct from a younger, Carboniferous, superposed continental arc sequence which constitutes a successor basin that developed over the Gamilaroi terrane subsequent to its accretion to the margin of Australia. Radiolarians provide the basis for dating siliceous lithologies which dominate the Djungati terrane and permit a more detailed analysis of the history of this terrane. During the middle Silurian through Late Devonian the Djungati terrane was part of an oceanic basin which was isolated from any source of terrigenous sedimentation. The Djungati terrane was subsequently influenced by volcanic island arc activity and was tectonically disrupted during the latest Devonian to Early Carboniferous. Radiolarian age data show that age correlations inferred, on the basis of similarities in detrital sandstone petrography, between lithostratigraphic units in the Gamilaroi and Djungati terranes, are not always appropriate. Djungati terrane has been widely interpreted as a subduction complex related to the Gamilaroi terrane. Radiolarian data now elucidate much of the structural complexity of this terrane. Significant differences between the Djungati terrane and the style of well-documented subduction complexes include the close spacing of zones of radiolarian chert, the dominance of this lithology over others and the lateral extent of many of the chert horizons. The Anaiwan terrane has features which are characteristic of many well-documented subduction complexes. Radiolarians are abundant in this terrane and can be used to show that it developed along the eastern margin of Australia in response to Early Carboniferous subduction of Late Devonian to Early Carboniferous oceanic crust. Packages of chert accreted into the subduction complex are progressively younger towards the northeast indicating that subduction was directed to the southwest.