MELTING OF MANTLE METASOMATIC PHASES DURING CRETACEOUS - CENOZOIC EXTENSION OF THE WEST ANTARCTIC RIFT SYSTEM: INSIGHTS FROM 1D GEODYNAMIC MODELING

The West Antarctic Rift System (WARS) underwent a late Cretaceous broad rift phase and a Cenozoic narrow rift phase that were separated by a period of relative tectonic quiescence. The majority of extension occurred during the Cretaceous phase, which is thought to be largely amagmatic. Cenozoic rifting focused on the margins of the WARS and was magmatic. The amount of extension (ca. 100%) makes it unlikely that melting of dry lherzolite mantle is responsible for most of the syn-extensional magmatism. Trace element compositions of Cenozoic syn-rift rocks suggest melting of a variety of fertile metasomatic mantle sources. Emplacement of these fertile sources has been attributed to metasomatism during the long period of subduction that preceded extension (550 - 100 Ma). We test this proposal by computing the pressure - temperature (P-T) paths in a 1-dimensional rift model. We use this to estimate the timing, amount, and extent of melting for dry lherzolite and various fertile phases (wet lherzolite, pyroxenite, and eclogite). Results show that fertile phases would melt at the onset of Cretaceous rifting if the mantle potential temperature was above 1200 °C. Temperatures above ~1530 °C would have led to 100% melting and thus depletion of mafic and ultramafic phases at the base of the lithosphere and at the top of the asthenosphere during the Cretaceous rifting episode. At temperatures between 1200 to 1530 °C, residual fertile phases are left in the lowermost lithosphere and uppermost asthenosphere after the Cretaceous phase of extension, and remain near their solidi during the latest Cretaceous through early Cenozoic period of tectonic quiescence. These upper mantle fertile components are thus capable of melting immediately after renewed extension in the late Cenozoic. The models predict that a normal to slightly warm mantle would have led to Cretaceous melting of metasomatic phases in the upper mantle, but unless the mantle was more than ~250°C above the global average, much of the metasomatic material would have survived to produce melt during late Cenozoic extension.