Early Palaeozoic cooling of the southern Prince Charles Mountains, East Antarctica: Synchronous cooling of three stratigraphic levels

Abstract

Dating of orogenic events on the East Antarctic Shield has generally involved mineral isotopoe systems with high closure temperatures (i.e. U-Pb). This has lead to the recognition of Archaean (c. 3.2-2.5 Ga), Meso-Neoproterozoic (c. 1.3-0.9 Ga) and early Palaeozoic (c. 550?450 Ma) orogenic events. Geochronological studies from the southern Prince Charles Mountains indicate both Neoproterozoic (Grenville) and early Palaeozoic (Pan-African) orogenesis has affected the Archaean to Neoproterozoic bedrock (Fig. 1a). Understanding the complex geological evolution of this region is crucial for constraining the geodynamics of Neoproterozoic East Antarctic/Indian continental reconstructions (Fig. 1b-c). 40Ar/39Ar data from the southern Prince Charles Mountains indicates that regional cooling occurred during early Palaeozoic orogenesis. Apparent ages obtained from this study indicate: (1) cooling of mica occurred between c. 500 and c. 480 Ma (Tc 350 ? 300°C), and (2) reset of hornblende occurred at c. 520 Ma (Tc 500°C). Samples targeted for argon thermochronology were from a temporally constrained vertical crustal section. Apparent age data from individual lithostratigraphic units indicate regional cooling occurred across all three stratigraphic levels in the early Palaeozoic. Affects of this event were: (1) total reset of biotite and mica from the Archaean- Mesoproterozoic strata, (2) partial reset of hornblende from the Archaean strata and (2) crystallisation of fine- grained sericite in the upper Neoproterozoic level. We attribute this synchronous cooling to reflect thermal resetting of the individual units that were already at similar vertical crustal levels. Coupled with structural and metamorphic observations we suggest the ?Pan-African? development of the region was related to large-scale reactivation and thin-skinned deformation. This attributes the gross crustal architecture of the southern Prince Charles Mountains to pre- `Pan-African' events (> c. 550 Ma). This period is coincident with the formation and break-up of Rodinia.