Fluid and deformation induced partial melting and melt escape in low-temperature granulite-facies metasediments, Damara Belt, Namibia.
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Fluid-present partial melting has generally been regarded a poor candidate for effecting crustal differentiation. In this study I report on anatectic metasediments from the Pan-African Damara Belt in Namibia that have undergone fluid-present biotite melting at a relatively low temperature, yet appear to have lost a significant volume of melt. In situ anatectic features have been identified on the basis of the existence of new generations of cordierite and/or garnet produced as the solid products of incongruent anatexis within or adjacent to leucosomes, that most commonly occur as lens shaped pods at a high angle to the lineation and formed during extension in a direction parallel to the long axis of the orogeny. Within these sites biotite underwent incongruent melting via the reaction Bt + Qtz + Pl + H2O = Melt + Grt + Crd. Cordierite nucleated on preexisting crystals within the bounding gneiss; garnet nucleated within the fracture sites (leucosomes) and typically occurs as individual, large (50 to 120 mm in diameter) poikiloblastic crystals. Thermobarometry applied to the anatectic assemblage yields low-temperature, granulitefacies peak conditions of 750 °C, 0.5 GPa. This temperature is approximately 100 °C lower than the accepted conditions for the onset of fluid-absent biotite melting. This, coupled to the focussing of anatexis on extensional fractures, suggests that anatexis occurred through waterpresent biotite incongruent melting. In order to better understand this process, both fluid-absent and water present partial melting experiments were conducted within the temperature interval 700 to 900 °C at 0.7 GPa. In the fluid-absent experiments, biotite incongruent melting started between 800 and 850 °C to produce melt coexisting with peritectic garnet and cordierite. In contrast, in water-saturated experiments, biotite melted via the reaction Bt + Pl + Q + H2O = Grt + Crd + Melt, between 700 and 750 °C, to produce melt, cordierite and garnet in the proportions 73:24:3.