CHARACTERIZATION OF THE LOW-GRADE METAMORPHISM IN THE NAMBUCCA BLOCK (NSW, AUSTRALIA)

Investigation of the crystallinity of K-white micas in early Permian r ocks from the Nambucca Block, NSW, reveals a metamorphic pattern with the highest grade (epizonal) rocks in an east-west trending zone which changes transitionally to lower grade (anchizonal) rocks to the south and north. Chlorite crystallinity values show a similar pattern, but in some cases do not agree with the Kubler index values from the same location owing to peak broadening caused by the overlap of the (002) c hlorite-vermiculite peak and the (001) peak of chlorite. The usefulnes s of this parameter as a grade indicator is thus limited, unless decon volution programs are applied. b(0) cell parameters of K-white micas i n slates indicate intermediate P facies series metamorphism (b(0)=9.02 3; sd=0.020), which accords with an interpretation based on mineral as semblages in metabasites. The much wider range in b(0) values noted in these rocks, compared to those formed under similar metamorphic condi tions in other terrains, is attributed to a wide range in bulk chemica l composition. Temperatures of formation based on Al-V contents of chl orites and chlorites in mixed-layer clays, range from 216 degrees-231 degrees C in the anchizonal rocks to 244-349 degrees C in epizonal roc ks.