Origin of the Intra-Oceanic Silverwood Block (New England Orogen, Australia): Evidence From Radiolarian Biostratigraphy and Detrital Zircon Petrochronology

Cambrian to Triassic subduction processes in eastern Australia produced a complex and highly contorted assembly of supra-subduction units. However, regardless of the long history of oceanic subduction, relatively little evidence exists on oceanic terranes whose accretion onto the continental margin may have accompanied subduction processes. We present new radiolarian and petrochronological data from Devonian and Carboniferous rocks exposed in the middle of a tight orocline. Based on radiolarian biostratigraphy and U-Pb dating of detrital zircon grains, we show that volcaniclastic rocks from the Silverwood Group and Alice Creek beds were deposited during the Late Devonian, which is later than previously thought. Trace-element compositions of the Devonian zircons are characteristic of crystallization of magmas in an oceanic environment, thus supporting previous suggestions that the Silverwood Group was derived from an intra-oceanic arc system. Carboniferous zircons from a nearby forearc basin unit (Mount Barney beds) exhibit a continental affinity, indicating that accretion of the Silverwood Block likely occurred before the establishment of this continental arc. A serpentinite belt that occurs adjacent to the Silverwood Block might represent a remnant ophiolitic suture of the now-consumed oceanic domain, which once separated the Silverwood Block from the eastern Gondwanan margin. Based on the assumption that such an intervening ocean existed, we discuss alternative scenarios for the origin and accretion of the Silverwood Block. The most likely scenario involves intra-oceanic magmatism in a marginal oceanic basin whose development was driven by trench retreat, and a subsequent accretion in response to trench advance and/or slab flattening.