Recognition of Pan-African-aged metamorphism in the Fisher Terrane, central Prince Charles Mountains, East Antarctica

The Precambrian tectonic architecture of the East Antarctic Shield is subject to considerable uncertainty because of limited outcrop. The Fisher Terrane, located in the central region of the Prince Charles Mountains, evolved during the Mesoproterozoic as a volcanic arc system and provides key data about the evolution of this region. Our study provides evidence that the Fisher Terrane formed in proximity to a Proterozoic plate boundary and was subsequently metamorphosed during the late Neoproterozoic to early Cambrian during the final stages of Gondwana amalgamation. U-Pb detrital zircon geochronology reveals that metasedimentary rocks from within a metasedimentary-metavolcanic package cropping out at Fisher Massif were deposited after c. 1300 Ma, and contain detritus derived from the Rayner-Eastern Ghats Terrane. This suggests that the Fisher Terrane was not an isolated oceanic arc but rather formed on the same tectonic plate as the Rayner Complex. Metapelitic schists from within the same metasedimentary package yield metamorphic U-Pb monazite ages of c. 512-509 Ma, corresponding to a regionally recognized Pan-African-aged event. This event has not been previously identified in the Fisher Terrane, and demonstrates that Pan-African-aged metamorphism affected all parts of the Prince Charles Mountains. Calculated phase equilibria modelling constrains the metamorphic conditions during this event to 2.5-4.0 kbar and 550-615 degrees C, corresponding to apparent thermal gradients of 146-220 degrees C kbar(-1). Such conditions plausibly relate to metamorphism taking place in an extensional setting.