Contrib Mineral Petrol (1984) 88:269-275 Contributions to Mineralogy and Petrology 9 Springer-Verlag 1984 Dehydration melting and the granulite transition in metapelites from southern Namaqualand, S. Africa D.J. Waters 1 * and C.J. Whales 2 Dept. of Earth Sciences, Downing Street, Cambridge, U.K. 2 Dept. of Geology, University of Cape Town, Rondebosch, South Africa Abstract. In a prograde amphibolite-granulite transition zone in the Namaqualand Metamorphic Complex, metape- lites show an interbanding of the amphibolite facies associa- tion biotite + sillimanite+ quartz with the granulite facies association garnet + cordierite + K-feldspar. Relict graded bedding shows that compositional banding is of sedimenta- ry origin. The garnet-cordierite-K-feldspar gneisses contain quartzofeldspathic segregations surrounding garnets, and have more Fe-rich bulk compositions than the biotite-silli- manite schists. The contrasting asemblages could have formed at the same pressure and temperature provided that a(H20 ) was systematically lower in the garnet-cordierite-K-feldspar layers. The a(H20 ) reduction resulted from the production of silicate melt by a vapour-absent continuous Fe- Mg re- action such as biotite + sillimanite + quartz = garnet + K-feldspar + liquid which affects Fe-rich compositions before vapour-absent melting occurs in more Mg-rich rocks. The segregations represent the solid and liquid products of the reaction. Such processes imply local control of a(H20), and indi- cate that this granulite transition did not result from a re- gional influx of metasomatising fluids. Introduction There currently exists a variety of hypotheses to explain the development of anhydrous, granulite facies mineral as- semblages. Observed transitions between upper amphibolite facies and granulite facies rocks have been interpreted as reflecting a lowering of water activity a (H20), with or with- out a concomitant increase in temperature (Phillips 1980; Glassley and Sorensen 1980). From fluid inclusion studies it has been suggested that the granulite transition approxi- mately corresponds to a change from H20-dominated to CO2-dominated trapped fluids (Touret 1971). Two recently proposed models account both for a low- ering of a(H20) and for the presence of CO2-rich fluid inclusions, but differ in their implications for fluid behav- iour during high grade metamorphism. Theoretical treat- * Present address. Dept. of Geology, University of Cape Town, Rondebosch, 7700 South Africa Offprint requests to. D.J. Waters ments of melting equilibria under water-undersaturated conditions (Thompson 1982; Powell 1983) sugggest that formation of melt would progressively decrease a(H20), and could allow a(H20 ) to be locally controlled by the extent of the melting process. The differential solubility of H20 and COg in a silicate liquid could progressively enrich a coexisting fluid phase in CO2. A second model proposes that low a(H20 ) is externally imposed on rock systems undergoing granulite facies metamorphism. Newton et al. (1980) envisaged a metasomatic flux of CO2-rich fluid from a deep source, which lowered a(HzO ) by dilution. The difference in fluid behaviour between these two fun- damentally different models should be detectable by petro- logical study of granulite facies transitions (Powell 1983). Patchy charnockitisation of amphibolite facies gneisses in southern India (Janardhan et al. 1982; Friend 1983) was ascribed to the introduction of CO2-rich fluids along anas- tomosing channelways, evidence which apparently supports the second, open system model. In contrast, this study of dehydration and melting processes in metapelitic rocks from southern Namaqualand reveals features consistent with local control of a(H20 ). Regional setting The Proterozoic Namaqualand Metamorphic Complex in western Namaqualand consists of a sequence of para- gneisses occurring in roughly east-west trending belts, set among voluminous granitic gneisses (Tankard et al. 1982). A central granulite facies zone is bordered to the north and to the south by areas of upper amphibolite facies as- semblages. The southern amphibolite facies zone has recently been delineated by field studies in the Bitterfontein area (Fig. 1 ; Moore, in prep., Waters et al. 1983). Mafic rocks in the south consist of granoblastic green hornblende-plagioclase and clinopyroxene-hornblende-plagioclase amphibolites. Further north, orthopyroxene appears, frequently forming oval poikiloblasts at the expense of hornblende, and from Bitterfontein northwards the commonest assemblage is or- thopyroxene-clinopyroxene-brown hornblende-plagioclase (Joubert 1971). The principal transformation in metapelitic rocks is from biotite-sillimanite schist into garnet-cordierite gneiss. In the southernmost part of the Complex pelitic schists contain the assemblage Qtz + Kfs + P1 + Bt + Sil ! Grt