Offler, Robin, and Huang, Hui-Qing (2018) Middle-Late Devonian nascent back arc formation, southern New England Orogen, NSW, Australia. Gondwana Research, 63. pp. 250-267.

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Abstract

Dolerite dyke swarms are common in Devonian sequences of the Tamworth Belt, southern New England Orogen but have not been studied in detail. We present new geochemical and LA-ICP-MS U-Pb zircon data obtained from dolerites in the Glenrock Station area, NSW. Our study shows that most dolerites have tholeiitic to transitional basaltic compositions, a few boninitic features. Most have chondrite normalised patterns that exhibit slight or no LREE enrichment and PM normalised patterns showing varying degrees of Nb depletion and Th enrichment. Th/Yb and Nb/Yb ratios are variable and indicate moderate to no subduction zone enrichment suggesting the magmas have been emplaced in a back arc basin (BAB) setting. N-MORB type magmas are rare leading to the conclusion that a nascent back arc had developed. This contrasts with dolerites of similar age in the Nundle area that commonly show N-MORB like characteristics suggesting that greater extension had occurred resulting in a well developed back arc basin. U-Pb zircon dating of dolerites in the study area reveals ages varying from 383 to 386 Ma, confirming ages determined from radiolarian studies of associated sediments.

Trace element geochemistry suggests the majority of the magmas have been derived from depleted lithospheric and mixed lithospheric-asthenospheric sources. Extraction of the melts responsible for the dolerites appears to have taken place at moderate depths from a primitive mantle-eclogite source. The Middle Devonian rifting event recognised in the Glenrock Station area appears to have been widespread affecting both continental and oceanic regions adjacent to and within the Australian continent. It is attributed to subduction rollback that has been impeded locally possibly as a result of subduction of an oceanic plateau and ridge. Extension was then able to take place in both oceanic and continental regions. (C) 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

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