New constraints on the tectonometamorphic evolution of the Fisher Terrane, central Prince Charles Mountains, eastern Antarctica

Abstract

The Fisher Terrane, located in the central region of the Prince Charles Mountains, east Antarctica, evolved during the Mesoproterozoic as a magmatic arc system. Metapelitic schists from the Fisher Massif were deposited after 1206 ± 49 Ma and yield metamorphic monazite ages of c. 538–505 Ma relating to the regionally recognized Neoproterozoic to early Cambrian Prydz Event. Calculated phase equilibria modelling constrains the metamorphic conditions during this event to 2.6–4.0 kbar and 553–576°C, relating to apparent thermal gradients of 144 °C/kbar to 212 °C/kbar. Conditions such as these are plausibly related to metamorphism taking place in an extensional back-arc setting which was then subsequently inverted and thickened via continental collision. A migmatitic felsic gneiss from Nilsson Rocks yields metamorphic monazite 206Pb/238U ages between 1115–1050 Ma with a weighted average age of 1085.0 ± 7.1 Ma. This metamorphic event may relate to a similarly aged episode of subduction related magmatism identified elsewhere in the central Prince Charles Mountains. This study therefore presents evidence of previously unidentified Prydz-aged high-thermal gradient metamorphism in the Fisher Terrane as well as identifying an older c. 1085 Ma metamorphic event not documented elsewhere in the Prince Charles Mountains.