Microstructural evidence of time-relationships between metamorphism and deformation in the metasedimentary sequence of the Northern Hill end trough, New South Wales, Australia

Microstructural evidence of time-relationships between regional metamorphism and deformation has been obtained in the higher-grade, biotite-bearing parts of the metasedimentary sequence of the northern Hill End Tough, New South Wales. The rocks have only one prominent foliation (slaty cleavage) and one mineral-streaking lineation, which are of consistent orientation through the area. Therefore, they should be suitable for obtaining clear evidence of time-relationships. Microstructural criteria in many of the rocks are ambiguous, but relatively clear evidence is shown by porphyroblastic biotite phyllites (locally containing garnet), in which some of the porphyroblasts show microboudinage, new biotite occurring with quartz in the interboudin areas. These rocks indicate that biotite was stable during the later stages of deformation. The study shows that careful microstructural studies can be diagnostic of time-relationships in low-grade metamorphic areas, where critical rocks are available.