THE NATURE AND ORIGIN OF ECLOGITE BLOCKS IN SERPENTINITE FROM THE TAMWORTH BELT, NEW-ENGLAND FOLD BELT, EASTERN AUSTRALIA

Small eclogite blocks occur in a serpentinite mass in the eastern part of the Tamworth Belt of the New England Fold Belt. Mineral compositions are similar to those of group C eclogite from Oregon and California, suggesting metamorphic pressures of 1-1.4 GPa. Core compositions of garnet and clinopyroxene suggest temperatures of approximately 412-degrees-422-degrees-C whereas rims indicate temperatures of approximately 521-degrees-532-degrees-C. The blocks, which are of a composition similar to ocean floor basalt, are either accidental blocks incorporated into the ultramafic body during emplacement from a forearc substrate or were produced by high pressure metamorphism of a basaltic intrusion carried to depth in the ultramafic rocks of a downgoing slab. In both interpretations, rise of the ultramafic rocks probably occurred during crustal extension at the start of the Permian.