Time constraints on deformation and metamorphism from EPMA dating of monazite in the Proterozoic Robertson River Metamorphics, NE Australia

EPMA dating of monazite confirms the relative timing of a succession of Foliation Intersection Axis trends in porphyroblasts (FIAs) and two periods of progressive metamorphism intervened by retrogression in the Robertson River Metamorphics (NE Australia). A complete pressure-temperature-deformation-time (P-T-D-t) path has been deciphered involving an early clockwise P-T loop followed by high temperature, low-pressure metamorphism.Successive generations of monazite were identified using microstructure and FIA trends and then dated. The succession of four FIAs trending ENE–WSW (FIA1), E–W (FIA2), N–S (FIA3), NE–SW (FIA4) plus four matrix foliations, S1–S4 were accompanied by three periods of garnet and two periods of staurolite growth. Monazite grew in response to most of these events.Isopleth thermobarometery, using P-T pseudosections in the MnNCKFMASH system for garnet porphyroblasts selected based on FIA trends in their core, reveal that pressure increased progressively from 3–4 kb at 530–550 °C to 6–7 kb at 600–620 °C during medium pressure-temperature metamorphism. This was accompanied by changing bulk shortening directions from NNW-SSE to N–S to E–W (perpendicular to FIAs 1, 2 and 3). The development of FIA1 and FIA2 occurred between 1655 ± 2.2 Ma and 1625 ± 15 Ma. FIA3 developed over 30–50 Ma periods from 1625 ± 15 Ma to 1586 ± 6 Ma. Retrogressive metamorphism, decompression and exhumation occurred over a ∼45 Ma period. Further metamorphism accompanied regional granite intrusion and generated lower pressure–higher temperature metamorphism during the development of FIA4 (NW-SE bulk shortening) at 1542 ± 8 Ma. This period possibly extended for another 30–50 Ma based on the monazite ages obtained from the youngest foliations in the matrix. Correlation of these data with other NE Australian Proterozoic Inliers suggests that they were all once part of a single orogen that developed from ca. 1650 to 1500 Ma.