Accretion and post‐accretion metamorphism in subduction complex terranes of the New England Fold Belt, eastern Australia

Abstract

Abstract Regional metamorphic rocks that form Late Palaeozoic subduction complexes in central Queensland, Australia, are products of two metamorphic episodes. Synaccretion metamorphism (M1) gave rise to prehnite‐pumpellyite and greenschist facies rocks, whereas a subsequent episode (M2) at about 250 Ma formed upper greenschist to upper amphibolite facies rocks of both intermediate‐ and low‐pressure type, probably in a compressive arc or back‐arc setting. A similar pattern can be recognized for 1000 km along the New England Fold Belt, although at several localities, where higher grade rocks are exposed, metamorphism was essentially continuous over the M1‐M2 interval, with a rapid rise in geothermal gradient at the end of accretion. Where out‐stepping of tectonic elements has occurred at long‐lived convergent margins elsewhere, similar overprinting of high‐ by lower‐pressure facies series is anticipated, complicating the tectonic interpretation of metamorphism. The discrete character of metamorphic events may be blurred where conditions giving rise to a major episode of accretion and out‐stepping are followed by the subduction of a major heat source.