Two belts of HTLP sub-regional metamorphism in the New England Orogen, eastern Australia: occurrence and characteristics exemplified by the Wongwibinda Metamorphic Complex

Abstract

Two north–south-trending belts of high-temperature–low-pressure (HTLP) sub-regional metamorphism have been identified in the New England Orogen of eastern Australia. Metamorphic complexes in the ∼1300 km long Early-Permian Inland belt have ages ca 300–290 Ma, and those of the ∼400 km long Mid-Permian Coastal belt ca 275–270 Ma. These periods correspond to the beginning and end of an extended (early–mid Permian) phase of subduction rollback and crustal thinning in eastern Australia. This paper describes and incorporates recent work on the Wongwibinda Metamorphic Complex in the southern New England Orogen as a basis for comparison with thirteen other HTLP sub-regional occurrences within the orogen. These are described in as much detail as is currently available. Some outcrops of HTLP rocks in difficult terrain have been subject to limited study and only conditional comparisons can be made. However, a significant number of characteristics shared between the complexes including: their location at the higher-temperature end of broad areas of very low-grade to greenschist facies metamorphic rocks, indicative of tilted crustal blocks; their association with major shear zones; the presence of migmatite at the high-temperature end of a steep metamorphic field gradient; the presence of two-mica granite formed by the melting of the local sedimentary pile; and temporal association with S-type granites; imply a common extension-related mechanism of formation for these HTLP belts. The connection with major faults and shear zones suggests the belts trace major crustal-scale extensional structures that migrated eastwards from ca 300 to 270 Ma.KEY POINTSTwo previously undocumented belts of HTLP subregional metamorphism are identified within the NEO.Available dating indicates that metamorphism occurred along the belts at the beginning and end of a major early–mid Permian extensional phase in eastern Gondwana/Australia.The characteristics of the HTLP complexes including their association with shear zones indicates they may delineate major loci of extension.