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

Abstract

The Oetmoed Granite–Migmatite Complex (OGMC), Central Damara Orogen, Namibia, consists of grt- and crd-bearing S-type granites and hbl- and titanite-bearing A-type granites that intruded into crd-sil-Kfs-bearing metasedimentary rocks, stromatic migmatites and nebulites. Stromatic migmatites formed by limited in situ partial melting of metapelites under H 2O-saturated conditions at ∼700°C and 4–5 kbar. This partial melting event took place close to the peak of regional metamorphism at ∼510 Ma as revealed by Sm–Nd garnet-whole rock ages as well as U/Pb monazite ages. The newly formed melt remained more or less at the site of origin. Melanosomes 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 disequilibrium melts. Similar δ 18 O whole rock values between 13‰ and 14‰ observed in leucosomes and corresponding melanosomes suggest homogenization of oxygen isotopes between the different rock types via the melt and an internal fluid phase during melting. Other leucosomes with a major element chemistry similar to the leucosomes from the stromatic migmatites but higher REE contents suggest significant incorporation of LREE-enriched phases (monazite) from the country rocks. For these leucosomes Sm–Nd garnet whole rock ages are ∼473 Ma indicating a second phase of melting. Nebulites mainly result from injection of granitic melts into the country rocks but their residual chemistry indicates that partial melting and limited melt removal must have occurred. Monazites from these nebulites record concordant U–Pb ages between ∼540 Ma and ∼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 source rocks are probably not the controlling factor for the different chemistry of leucosomes and granites. Therefore, different melting conditions (fluid-absent vs. fluid-present) and different modes of entrainment and solubilities of accessory phases control the elemental budgets of the leucosomes. Large scale migmatite–granite complexes are a substantial part of some high-grade terrains but the distinctive geochemical composition of some leucosomes make these rocks unlikely precursors of large-scale granitic bodies.