Garnet-forming Reactions in Mafic Granulites from Enderby Land, Antarctica--Implications for Geothermometry and Geobarometry

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Abstract
This study of mafic granulites from Enderby Land, Antarctica, demonstrates the control of rock compositions upon the reactions involved in the formation of garnet by cooling through part of the pyroxene granulite– eclogite transition. The rocks were originally a sequence of cumulates precipitated from an enclosing quartz tholeiite dyke which were then metamorphosed at granulite facies conditions. Subsequent mineral reactions and compositional zoning in minerals record an unusual period of cooling of this terrain from ˜900–600°C. No textural nor chemical evidence for further reaction between phases below this temperature has been detected. The formation of garnet in the metamorphosed quartz tholeiite dyke is consistent with previous descriptions of plagioclase feldspar reacting with opaque oxide or orthopyroxene to form garnet and clinopyroxene or quartz with decreasing temperature. The disappearance of plagioclase feldspar in the more mafic, quartz– free rocks involves a different series of reactions in which fassaitic clinopyroxene and garnet, with or without spinel or orthopyroxene, developed. A wide range of exsolution products is found within the different original aluminous clinopyroxenes. The wide variety of primary metamorphic mineral assemblages as well as reaction coronas and exsolutions provides an opportunity to test the internal consistency of several geothermometers and geobarometers. Mineral pairs of higher temperature ‘primary’ metamorphic character which persist as stable assemblages to lower temperatures retain a record of cooling through compositional zoning, and ‘closure temperatures’ for different mineral pairs can be inferred. In contrast, rocks which contain mineral assemblages such that cooling to even lower temperature produced newly formed secondary phases, record in these newly formed mineral pairs equilibration temperatures well below these closure temperatures for cation exchange between zoned, primary phases. Although individual thermometers indicate a wide range in temperatures for the different generations of assemblages (≈st 900– 600° C), there is excellent agreement in temperature estimates for each mineral assemblage using different geo–thermometers (Fe2³³ -Mg exchange in Gt-Cpx, Gt-Opx, Opx-Cpx; Ca-Mg exchange in Opx-Cpx). These features can be used to deduce a segment of the P-T-time cooling history of this granulite terrain, a distinctive feature of which is the considerable cooling interval near the base of the crust with only slight uplift following the peak metamorphism. The mineral assemblages at this one small outcrop are similar to the garnet pyroxenite and granulite suite of xenoliths found in some basaltic diatremes (e. g. Lovering & White, 1969) which is consistent with Lovering & White's (1969) and Irving's (1974) interpretation that such apparently varied suites of xenolith types could all have formed within a very spatially restricted area.

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