ACCRETION AND POST-ACCRETION METAMORPHISM IN SUBDUCTION COMPLEX TERRANES OF THE NEW-ENGLAND FOLD BELT, EASTERN AUSTRALIA

Regional metamorphic rocks that form Late Palaeozoic subduction comple xes in central Queensland, Australia, are products of two metamorphic episodes. Synaccretion metamorphism (Ml) gave rise to prehnite-pumpell yite and greenschist facies rocks, whereas a subsequent episode (M2) a t about 250 Ma formed upper greenschist to upper amphibolite facies ro cks of both intermediate- and low-pressure type, probably in a compres sive arc or back-arc setting. A similar pattern can be recognized for 1000 km along the New England Fold Belt, although at several localitie s, where higher grade rocks are exposed, metamorphism was essentially continuous over the MI-M2 interval, with a rapid rise in geothermal gr adient at the end of accretion. Where out-stepping of tectonic element s has occurred at long-lived convergent margins elsewhere, similar ove rprinting of high- by lower-pressure facies series is anticipated, com plicating the tectonic interpretation of metamorphism. The discrete ch aracter of metamorphic events may be blurred where conditions giving r ise to a major episode of accretion and out-stepping are followed by t he subduction of a major heat source.