Fluid and deformation induced partial melting and melt escape in low-temperature granulite-facies metasediments, Damara Belt, Namibia.

[摘要] Fluid-present partial melting has generally been regarded a poor candidate for effecting crustaldifferentiation. In this study I report on anatectic metasediments from the Pan-African DamaraBelt 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 beenidentified on the basis of the existence of new generations of cordierite and/or garnet produced asthe solid products of incongruent anatexis within or adjacent to leucosomes, that most commonlyoccur as lens shaped pods at a high angle to the lineation and formed during extension in adirection parallel to the long axis of the orogeny. Within these sites biotite underwent incongruentmelting via the reaction Bt + Qtz + Pl + H2O = Melt + Grt + Crd. Cordierite nucleated on preexistingcrystals within the bounding gneiss; garnet nucleated within the fracture sites(leucosomes) and typically occurs as individual, large (50 to 120 mm in diameter) poikiloblasticcrystals. Thermobarometry applied to the anatectic assemblage yields low-temperature, granulitefaciespeak conditions of 750 °C, 0.5 GPa. This temperature is approximately 100 °C lower thanthe accepted conditions for the onset of fluid-absent biotite melting. This, coupled to thefocussing of anatexis on extensional fractures, suggests that anatexis occurred through waterpresentbiotite incongruent melting. In order to better understand this process, both fluid-absentand water present partial melting experiments were conducted within the temperature interval 700to 900 °C at 0.7 GPa. In the fluid-absent experiments, biotite incongruent melting started between800 and 850 °C to produce melt coexisting with peritectic garnet and cordierite. In contrast, inwater-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.

[发布机构] Stellenbosch University