Contrib. Mineral. Petrol. 74, 35-43 (1980) Contributions to Mineralogy and Petrology 9 by Springer-Verlag. 1980 Origin of Arehean Migmatites From the Gwenoro Dam Area, Zimbabwe-Rhodesia Kent C. Condie and Philip Allen Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA Abstract. Archean migmatites in the vicinity of Gwen- oro Dam in Zimbabwe-Rhodesia are composed chief- ly of trondhjemite gneiss (TR), mafic tonalite (MT), amphibolite (AM), leuco-trondhjemite veins (LTR), and pegmatites. The gneiss is intruded in nearby areas with small tonalite plutons (TN). Geochemical model studies together with field relationships are consistent with the following model for migmatite production: AM is produced by partial melting of a partly deplet- ed ultramafic parent in which neither garnet nor am- phibole remain in the residue; TR and TN are pro- duced by partial melting of undepleted to variably depleted amphibolite in which garnet does not remain in the residue; MT is produced by mixing of plagio- clase-rich TR with AM ; and LTR represents the solid residue after fractional crystallization of TR. Introduction The origin of migmatites is a subject of considerable discussion and controversy. One or a combination of four processes are generally called upon to explain migmatite development: (1) magmatic injection at various stages of fractional crystallization ; (1) anatex- is of crustal rocks ; (3) metasomatism; and (4) subsoli- dus metamorphic differentiation. Injection has been suggested by Pitcher and Berger (1972) as the mecha- nism that produced migmatites found in parts of the Thorr and Fanad plutons of Donegal, Ireland. Winkler (1976) makes a distinction between "injec- tion migmatites" and" in situ migmatites" and favors an anatectic origin for the latter. Field, petrographic, and geochemical data are also interpreted to reflect a dominantly anatectic origin for migmatites by Meh- Reprint requests: K.C. Condie nert (1968), Tobschall (1971), and Dougan (1979). Brown (1967) concluded that metasomatism produced the Loch Coire migmatites in Scotland and a combi- nation of metasomatism and metamorphic differen- tiation was suggested by Misch (1968) for the origin of the Skagit gneiss in Washington. Metamorphic dif- ferentiation has also been suggested as a major pro- cess in migmatite formation based on geochemical and isotopic results (Hedge 1972; Olsen 1977; and Yardley 1978). It is clear from existing studies that more than one process is responsible for the formation of mig- matites. Furthermore, it is likely that more than one process operated in each of the terranes studied by the investigators cited in the previous paragraph. It is possible to rigorously test various models for the production of migmatites employing well-established geochemical modeling techniques (Hanson 1978; A1- legre and Minster 1978). In particular, it is feasible to evaluate possible parent-daughter-residue relation- ships between various migmatite components. This investigation presents results of such a study from an Archean migmatite complex exposed in the vicinity of Gwenoro Dam west of Selukwe in Zimbabwe-Rho- desia. This location is particularly well-suited for such a study in that field relationships are clearly exposed and the rocks are not altered. Field Relationships The Rhodesian Archean craton is composed of granite-greenstone terranes produced chiefly between 2.6 and 2.7 b.y. (Wilson et al. 1978). Older terranes (~3.5 b.y.) are recognized in the Selukwe- Shabani area. Stowe (1968) has subdivided the Archean rocks in the Selukwe area into five complexes including the Western Com- plex in which Gwenoro Dam is located. The Western Complex lies between the Selukwe and Ghoko schist belts (Fig. l) and is comprised chiefly of a tonalite-trondhjemite gneiss-migmatite ter- rane with minor remnants of greenstone belts. It is also locally 0010-7999/80/0074/0035/$01.80