Geochronology and petrology of migmatites from the Proterozoic Damara Belt- importance of episodic fluid-present disequilibrium melting and consequences for granite petrology

The Oetmoed Granite-Migmatite Complex (OGMC), Central Damara Orogen, Namibi a, consists of grt- and crd-bearing S-type,granites and hbl- and titanite-b earing A-type granites that intruded into crd-sil-Kfs-bearing metasedimenta ry rocks, stromatic migmatites and nebulites, Stromatic migmatites formed b y limited in situ partial melting of metapelites under H2O-saturated condit ions at similar to 700 degrees C and 4-5 kbar. This partial melting event t ook place close to the peak of regional metamorphism at similar to 510 Ma a s revealed by Sm-Nd garnet-whole rock ages as well as U/Pb monazite ages. T he newly formed melt remained more or less at the site of origin. Melanosom es of the stromatic migmatites do not resemble true residues, instead they represent reaction zones between in situ melt and the metasedimentary host rock. Leucosomes from the stromatic migmatites are LREE- and HFSE-depleted which is typical for the low-melt fractions generally observed in migmatite terranes. Mass balance calculations suggest that these leucosomes may be d isequilibrium melts. Similar delta(18)O whole rock values between 13 parts per thousand and 14 parts per thousand observed in leucosomes and correspon ding melanosomes suggest homogenization of oxygen isotopes between the diff erent rock types via the melt and an internal fluid phase during melting. O ther leucosomes with a major element chemistry similar to the leucosomes fr om the stromatic migmatites but higher REE contents suggest significant inc orporation of LREE-enriched phases (monazite) from the country rocks. For t hese leucosomes Sm-Nd garnet whole rock ages are similar to 473 Ma indicati ng a second phase of melting. Nebulites mainly result from injection of gra nitic melts into the country rocks but their residual chemistry indicates t hat partial melting and limited melt removal must have occurred. Monazites from these nebulites record concordant U-Pb ages between similar to 540 Ma and similar to 470 Ma, indicating episodic migmatization during high-grade regional metamorphism. Intrusive peraluminous granites are likely generated by partial melting of pelitic sources in the lower crust, and different so urce rocks are probably not the controlling factor for the different chemis try of leucosomes and granites. Therefore, different melting conditions (fl uid-absent vs. fluid-present) and different modes of entrainment and solubi lities of accessory phases control the elemental budgets of the leucosomes. Large scale migmatite-granite complexes are a substantial part of some hig h-grade terrains but the distinctive geochemical composition of some leucos omes make these rocks unlikely precursors of large-scale granitic bodies. ( C) 2000 Elsevier Science B.V. All rights reserved.