The Proterozoic P–T–t Evolution of the Kemp Land Coast, East Antarctica; Constraints from Si-saturated and Si-undersaturated Metapelites
1School of Geosciences, University of Sydney, Sydney, Nsw 2006, Australia
2School of Earth Sciences, University of Melbourne, Melbourne, Vic 3010, Australia
3Present Address: Institute of geoscience, University of Mainz, D-55099 Mainz, germany
4Continental Evolution Research Group, School of Earth & Environmental Sciences, University of Adelaide, Adelaide, Sa 5005, Australia
RECEIVED JUNE 29, 2006; ACCEPTED MARCH 29, 2007
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Abstract |
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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.
KEY WORDS: electron microprobe monazite dating; granulite facies; Rayner Complex; sapphirine; THERMOCALC
Minerals abbreviations: q, quartz; g, garnet; sill, sillimanite; ky, kyanite; opx, orthopyroxene; cd, cordierite; ksp, alkali feldspar; pl, plagioclase; bi, biotite; sp, spinel; ilm, ilmenite; mt, magnetite; ru, rutile; sa, sapphirine; cor, corundum; osm, osumilite; liq, silicate melt; mnz, monazite
*Corresponding author. Telephone: +61 2 93512919. Fax: +61 2 93510184. E-mail: jacqui_halpin{at}mac.com