Thesis (PhD (Earth Sciences))--University of Stellenbosch, 2009. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1226 |
| Date | 03 1900 |
| Creators | Ward, Robert Alexander |
| Contributors | Stevens, Gary, Kisters, Alex, University of Stellenbosch. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
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