PETROLOGY OF AN INTRUSION-RELATED HIGH-GRADE MIGMATITE - IMPLICATIONSFOR PARTIAL MELTING OF METASEDIMENTARY ROCKS AND LEUCOSOME-FORMING PROCESSES

Intrusion-related migmatites comprise a substantial part of the high-g rade part of the southern Damara orogen, Namibia which is dominated by Al-rich metasedimentary rocks and various granites, Migmatites consis t of melanosomes with biotite + sillimanite + garnet + cordierite + he rcynite and leucosomes are garnet-and cordierite-bearing. Metamorphic grade throughout the area is in the upper amphibolite to lower granuli te facies (5-6 kbar at 730-750 degrees C). Field evidence, petrographi c observations, chemical data and mass balance calculations suggest th at intrusion of granitic magmas and concomitant partial melting of met asedimentary units were the main processes for the generation of the m igmatites. The intruding melts were significantly modified by magma mi xing with in situ partial melts, accumulation of mainly feldspar and c ontamination with garnet from the wall rocks. However, it is suggested that these melts originally represented disequilibrium melts from a m etasedimentary protolith. The occurrence of LILE-, HFSE- and LREE-enri ched and -depleted residues within the leucosomes implies that both qu artzo-feldspathic and pelitic rocks were subjected to partial melting. Isotope ratios of the leucosomes are rather constant (Nd-143/Nd-144 ( 500 Ma): 0.511718-0.511754, epsilon Nd (500 Ma): -3.54 to -5.11) and S r (Sr-87/Sr-86 (500 Ma): 0.714119-0.714686), the metasedimentary units have rather constant Nd isotope ratios (Nd-143/Nd-144 (500 Ma): 0.511 622-0.511789, epsilon Nd (500 Ma). -3.70 to -6.93) but variable Sr iso tope ratios Sr (Sr-87/Sr-86 (500 Ma): 0.713527-0.722268). The most res titic melanosome MEL 4 has a Sr isotopic composition of Sr-87/Sr-86 (5 00 Ma): 0.729380. Oxygen isotopes do not mirror the proposed contamina tion process, due to the equally high delta(18)O contents of metasedim ents and crustal melts. However, the most LILE-depleted residue MEL 4 shows the lowest delta(18)O value (<10 parts per thousand). Mass balan ce calculations suggest high degrees of partial melting (30-40%). It i s concluded that partial melting was promoted by heat transfer and rel ease of a fluid phase from the intruding granites. High degrees of par tial melting can be reached as long as the available H2O, derived from the crystallization of the intruding granites, is efficiently recycle d within the rock volume. Due to the limited amounts of in situ meltin g, it seems likely that such regional migmatite terranes are not the s ources for large intrusive granite bodies. The high geothermal gradien t inferred from the metamorphic conditions was probably caused by exhu mation of deep crustal rocks and contemporaneous intrusion of huge mas ses of granitoid magmas. The Davetsaub area represents an example of m igmatites formed at moderate pressures and high temperatures, and illu strates some of the reactions that may modify leucosome compositions. The area provides constraints on melting processes operating in high-g rade metasedimentary rocks.