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

摘要

Fluid-present partial melting has generally been regarded a poor candidate for effecting crustal differentiation. In this study we report on granulite-grade 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. Leucosomes occur in lens-shaped pods that are orientated at high angles to the regional stretching lineation that formed during orogen-parallel extrusion of the rocks during the main collisional event in the Damara Belt. Within these sites biotite underwent incongruent melting via the reaction Bt+Qtz+Pl+H2O=Melt+Grt+Crd. Cordierite nucleated on pre-existing crystals within the bounding gneiss; garnet nucleated within the fracture sites (leucosomes) and typically occurs as individual, large (50 to 120 mm) poikiloblastic crystals. Thermobarometry applied to the anatectic assemblage yields low-temperature, granulite-facies 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 dilational sites, suggests that anatexis occurred through water-present 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+Qtz+Pl+ H2O=Melt+Grt+Crd, between 700 and 750 °C, to produce melt, cordierite and garnet in the proportions 73:24:3. The garnet compositions produced in these low-temperature experiments match very well with the relatively high spessartine content (10%), low Mg# garnets formed with the leucosomes, confirming the water-saturated, low-temperature nature of anatexis in the study area. Mass balance calculations indicate that melting in typical dilational sites has produced 6 dm3 of melt This melt has been sourced from some 0.08 m~3 of rock, a volume that coincides well with the typical frequency of extensional fracture spacing. This was probably important in enabling efficient melt extraction which has caused a slight shift towards more refractory compositions in the granulite-facies metapelites over amphibolite-facies equivalents. However, the attendant expansion in chemical equilibration volume has resulted in a marked compositional homogenization of the high-grade gneisses over the rhythmically bedded amphibolite-facies metapelites.