The proterozoic P-T-t evolution of the Kemp Land coast, east Antarctica; Constraints from Si-saturated and Si-undersaturated metapelites

Copyright © The Author 2007. Published by Oxford University Press. All rights reserved.Integrated metamorphic and geochronological data place new constraints on the metamorphic evolution of a Neoproterozoic orogen in east Antarctica. Granulite-facies rocks from a 150 km stretch of the Kemp Land coast reflect peak conditions involving T 870–990°C at P 7·4–10 kbar, with pressure increasing westward towards an Archaean craton. Electron microprobe-derived (Th + U)–Pb monazite ages from metapelitic assemblages indicate that the major mineral textures in these rocks developed during the c. 940 Ma Rayner Orogeny. Complex compositional zoning in monazite suggests high-T recrystallization over c. 25 Myr. Diversity in metapelitic reaction textures reflects silica and ferromagnesian content: Si-saturated Fe-rich metapelites contain garnet that is partially pseudomorphed by biotite and sillimanite, whereas Si-saturated Mg-rich metapelites and Si-undersaturated metapelitic pods have reaction microstructures involving cordierite enclosing orthopyroxene, garnet and/or sapphirine, cordierite + sapphirine symplectites around sillimanite and coarse-grained orthopyroxene + corundum separated by sapphirine coronae. Interpretations based on P–T pseudosections provide integrated bulk-rock constraints and indicate a clockwise P–T–t path characterized by a post-peak P–T trajectory with dP/dT 15–20 bar/ °C. This moderately sloped decompressive-cooling P–T path is in contrast to near-isothermal decompression P–T paths commonly cited for this region of the Rayner Complex, with implications for the post-collisional tectonic response of the mid- to lower crust within this orogenic belt.J. A. Halpin, R. W. White, G. L. Clarke and D. E. Kelse