Adaminaby Group west of Batemans Bay: Deformation and metamorphism of the Narooma accretionary complex, NSW

Abstract

This study provides new structural data that show that the Adaminaby Group is part of the Narooma accretionary complex and has been overprinted by HT/LP metamorphism associated with Middle Devonian Moruya Suite intrusions. The grade of metamorphism based on Kübler Indices is the same in the Wagonga and Adaminaby Groups at Batemans Bay inferring that these rocks were involved in the same accretionary event. White micas in slates of the Adaminaby Group record apparent K–Ar ages of 384.6 ± 7.9 Ma and 395.8 ± 8.1 Ma. These ages are believed to represent the age of Middle to Upper Devonian Buckenbowra Granodiorite. Kübler Index values indicate lower epizonal (greenschist facies) metamorphic conditions and are not influenced by heating in metamorphic aureoles of the plutons. All b cell lattice parameter values are characteristic of intermediate pressure facies conditions although they are lower in the metamorphic aureole of the Buckenbowra Granodiorite than in the country rock, defining two areas with dissimilar baric conditions. East of the Buckenbowra Granodiorite, b cell lattice parameter values outside the contact aureole (x = 9.033 Å; n = 8) indicate P = 4 kb, and assuming a temperature of 300°C, infer a depth of burial of approximately 15 km for these rocks with a geothermal gradient of 20°C/km. In the metamorphic aureole of the Buckenbowra Granodiorite, b cell lattice parameter values (x = 9.021 Å; n = 41) indicate P = 3.1 kb inferring exhumation of the Adaminaby Group rocks to a depth of approximately 11 km prior to intrusion. A geothermal gradient of 36°C/km operated in the aureole during intrusion. An extensional back-arc environment prevailed in the Adaminaby Group during the Middle to Upper Devonian.

KEY WORDS:

• subduction
• Kübler Index
• b cell parameter K-white micas
• MP/LT metamorphism
• Devonian
• LP/HT metamorphism
• Narooma accretionary complex
• Lachlan Orogen
• K–Ar

Acknowledgements

We thank Jenny Zobec for her assistance with XRD analyses, Richard Bale for providing preparation techniques, Jiang Zhiyu for making thin-sections, Yanyan Sun for making thin-sections and rock crushing, David Phelan for providing SEM analyses and assistance, and John Simpson for field assistance. Andrew Todd, CSIRO is thanked for technical assistance. We thank the reviewers, David Gray and Chris Fergusson, for their constructive comments that greatly improved the manuscript.