PLATE-MARGIN REACTIVATION OF ANCIENT CRATONIC SHIELDS

Basement-cored plate-margin orogens present two inter-related problems: recognizing early crust-forming events in polymetamorphic terrains, and determining conditions of reactivation in previously stabilized cratonic shields. As example, the Nimrod Group in Antarctica is a heterogeneous assemblage of high-grade para- and orthogneisses comprising a multi-stage cratonic terrain of the East Antarctic shield. Its initial Archean to Early Proterozoic history involved juvenile magmatic and metamorphic events at 3.0, 2.9 and 2.5 Ga, overprinted by 1.7 Ga Caledonian-type eclogite-facies metamorphism and magmatism. Continental rifting between 750-670 Ma, related to Rodinia breakup, was followed by thermal subsidence to 540-520 Ma, as recorded by rift-margin and platform deposits. The high-grade gneissic complex underwent major thermomechanical reactivation during the Ross Orogeny, which involved crustal thickening, deep-seated transpressional flow, and syn-kinematic magmatism. Igneous geochemical signatures suggest a continental-margin arc setting, but other evidence of convergence is absent. The terrain underwent complete structural and petrologic modification at this time, so that the pre-Ross history is recorded only by U-Pb ages from composite zircons. Equilibrium metamorphic assemblages and thermobarometry record high-P upper amphibolite to lower granulite facies metamorphism and high strain-rate ductile flow. These metamorphic rocks represent a ≥14 km-thick crustal assemblage of the middle-lower crust. Metamorphic assemblages and granitoid zircon geochronology suggest that dehydration melting was minor, possibly due to prior devolatilization, and that late-orogenic magmas originated by small fractions of crustal melting. However, the significant volumes of syn- to post-orogenic igneous rocks suggest an important contribution of advective heat sufficient to thermally homogenize mineral compositions, obliterate earlier parageneses, and facilitate penetrative flow at a scale of several km thickness. Post-peak cooling rates indicate late-orogenic denudation rates of ~1 mm/y, which preserved the Ross-age microfabrics and parageneses. Mineral chronometry and thermobarometry indicate a clockwise P-T path consistent with convergence and thickening.