Metamorphic style of the Tabberabbera Zone, Lachlan Fold Belt

Abstract

IC and b spacing values of white micas in slates from the Tabberabbera Zone have been used to assess the P and T conditions of metamorphism. IC values range from 0.19 to 0.34, with upper anchizone conditions (T = 250?300°C) over much of the zone, and a belt of epizone rocks (T > 300°C) along the eastern edge of the zone. Average b spacing is 9.023?0.006 Å, with no discernible trends across the zone. This value is typical of medium-P metamorphic belts, with a geothermal gradient of about 30°C/km. For a temperature of 300°C, this equates to a pressure of ~0.3 GPa, and a depth of about 10 km, the same as migmatites of the Omeo Zone, adjacent to the east. The medium-P metamorphism is difficult to fit to an accretionary prism origin for the zone, unless an active spreading ridge was subducted under it.