Trace element abundances in garnet from a polyphase migmatite were measured by secondary ion mass spectrometry (SIMS) in order to identify some of the effective variables on the trace element distribution between garnet and melanosome or leucosome. In general, garnet is zoned with respect to REE, in which garnet cores are enriched by a factor of 2-3 relative to the rims. For an inclusion-rich garnet from the melanosome, equilibrium distribution following a simple Rayleigh fractionation is responsible for the decreasing concentrations in REE from core to rim. Inclusion-poor garnet from the same melanosome located in the vicinity of the leucosomes shows distinct enrichment and depletion patterns for REE from core to rim. These features suggest disequilibrium between garnet and the host rock which, in this case, could have been an in-situ derived melt. This would probably indicate a period of open-system behaviour at a time when the garnet, originally nucleated in the metamorphic environment reacted with the melt. In addition, non-gradual variation in trace element abundances between core and rim may suggest variable garnet growth rates. Inclusion-free garnet from the leucosome, interpreted to have crystallised in the presence of a melt, has a small core with high REE abundances and a broad rim with lower REE abundances. Here, crystal-liquid diffusion-controlled partitioning is a likely process to explain the trace element variation.

Pan-African high-grade metamorphism in the Proterozoic Damara orogen (Namibia) led to formation of garnet-bearing leucosomes in potassic meta-igneous gneisses producing a meta-igneous migmatite. In addition, the migmatite (gneiss (mesosome) plus leucosome) was intruded by small-scale leucogranitic melts with a high amount of accumulated biotite and garnet. U-Pb zircon ages obtained on the mesosome and the leucosome indicate late Proterozoic (ca. 850 Ma) concordia upper intercept ages which are interpreted as minimum ages of the precursor rock of the migmatite. U-Pb monazite ages obtained on the leucogranite give a concordant age of 512 +/- 1 Ma and two reversely discordant ages with Pb-207/U-235 ages of 544 +/- 1 and 534 +/- 1 Ma, indicating the growth of monazite before or close to the age of high-grade metamorphism in the Damara orogen. High precision Lu-Hf garnet-whole rock dating gave ages of 492.6 +/- 1.7 Ma for the mesosome, 497.6 +/- 1.7 Ma for the leucosome and 494.0 +/- 1.7 Ma for the garnet- and biotite-bearing leucogranite indicating that the growth of garnet postdates the growth of monazite during high-grade metamorphism. In addition, it is suggested that melting and intrusion was coeval and occurred probably shortly after the main peak of metamorphism which occurred at c. 512 +/- 1 Ma. P-T estimates obtained by conventional thermobarometry (c. 690-720 degrees C) and accessory mineral dissolution thermometry on the leucogranite (c. 730 degrees C) suggest that partial melting occurred through limited fluid present melting of biotite via the reaction: bt + kfs + pig + qtz + H2O double left right arrow grt + melt. Outcrop evidence (diffuse relationship between the gneiss domain and the leucosomes, similar size of the leucosomes, homogeneous distribution of leucosomes on the sample scale) suggests that minor melt segregation had occurred. Whole rock Sr, Nd, Hf and O isotope data of the mesosome indicate that it belongs to basement rocks from this area. Geochemical and isotope data obtained on the leucosomes argue for derivation by in-situ melting of the mesosome. Both, leucosome and leucogranite originated from the same source rock but the leucogranite represents an accumulated melt that was able to segregate and to intrude the gneiss domain. The similar isotope features of the mesosome, leucosome and leucogranite indicate a direct relationship for the gneiss and the melts. Chemical and mineral data favour a derivation of both types of melt through fluid-present melting of isotopically and chemically comparable biotite + plagioclase + K-feldspar + quartz-bearing gneisses. (C) 2008 Elsevier B.V. All rights reserved.

Major element, trace element and Nd–Sr–Pb–O isotope data for a suite of Neo-Proterozic, pre-orogenic, rift-related syenites from the Northern Damara orogen (Namibia) constrain their sources and petrogenesis. New U–Pb ages obtained on euhdreal titanite of inferred magmatic origin constrain the age of intrusion of the Lofdal and Oas syenites to ca. 750 Ma compatible with previous high-precision zircon analyses from the Oas complex. Major rock types from Lofdal and Oas are mildly sodic nepheline-normative and quartz-normative syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Primitive samples from Lofdal and Oas show depletion of Rb, K and Th relative to Ba and Nb together with variable negative anomalies of P and Ti on a primitive mantle-normalized diagram. Evolved samples from Oas develop significant negative Ba, Sr, P and Ti anomalies and positive U and Th anomalies mainly as a function of crystal fractionation processes. The lack of a pronounced negative Nb anomaly in samples from Lofdal suggests that involvement of a crustal component is negligible. For the nepheline-normative samples from Lofdal, the unradiogenic Sr and radiogenic Nd isotope composition and low δ18O values suggest derivation of these samples from a moderately depleted lithospheric upper mantle with crustal-like U/Pb ratios (87Sr/86Sr: 0.7031–0.7035, ε Nd: ca. +1, δ18O: 7‰, 206Pb/204Pb: ca.18.00, 207Pb/204Pb: 15.58–15.60). Primitive samples of the Oas quartz-normative syenites have identical isotope characteristics (87Sr/86Sr: 0.7034, ε Nd: ca. +1, δ18O: 6.5‰, 206Pb/204Pb: ca.18.00, 207Pb/204Pb: 15.59) whereas more differentiated samples have higher 87Sr/86Sr ratios (0.709–0.714), slightly higher δ18O values (7.0–7.1‰), less radiogenic ε Nd values (−1.1 to −1.4) and more radiogenic 206Pb/204Pb ratios up to 18.27. These features together with model calculations using Sr–Nd–Pb isotopes suggest modification of a primary syenite magma by combined AFC processes involving ancient continental crust. In this case, high Nb abundances of the parental syenite liquid prevent the development of significant negative Nb anomalies that may be expected due to interaction with continental crust.

Geochemical and Nd-Sr-O whole rock isotope data and Pb isotope data from leached feldspars for a suite of synorogenic (ca. 540 Ma) syenites and granites from the northern part of the Proterozoic Damara orogen ( Namibia) constrain their sources and petrogenesis. Major rock types are mildly potassic nepheline syenites and quartz syenites that were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Even the most primitive samples show pronounced depletion in Nb, Ti, Sr, and P on a primitive mantle - normalized diagram, indicating the involvement of a recycled crustal component in the source. Extrapolation of the Sr-Nd-Pb-O isotope composition of the syenites back to a hypothetical parental melt with 8 wt% - 10 wt% MgO suggests derivation from a moderately enriched lithospheric upper mantle (Sr-87/Sr-86 = 0.7055, epsilon(Nd) = -4, delta(18)O = 5 parts per thousand, Pb-206/Pb-204 = 17.60, Pb-207/Pb-204 = 15.58, Pb-208/Pb-204 = 37.50). More evolved quartz syenites show virtually unchanged Sr-87/Sr-86, Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 and Pb-208/Pb-204 ratios and epsilon(Nd) values. The most distinctive feature of the quartz syenites is their higher delta(18)O values up to 10.5 parts per thousand. These features seem to indicate that, unlike most other Damaran igneous rocks, the syenites described here were entirely formed within the lithospheric mantle and were not modified by assimilation processes of basement and/or Damaran metasedimentary rocks. In order to account for the substantial variation in O isotopes, the most likely source for these rocks is a heterogeneous association of mantle rocks and subducted upper crustal material, most likely altered basaltic rocks from subducted ocean floor. Additionally, low temperature alteration may have affected the quartz syenites. In contrast, although coeval granites with A-type affinities are interpreted as fractionation products of the syenites, their distinctly more negative epsilon(Nd epsilon) values ranging from - 9 to - 10 indicate that deep crustal assimilation processes of local Proterozoic basement rocks play an important role in the evolution of these granites.

The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous homblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U-Pb zircon age of 540 +/- 3 Ma.Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (epsilon(Nd) ((540) (Ma)) = -6.3 to - 19.8; delta(18)O = 9.0-11.6parts per thousand) but rather low and uniform initial Sr isotope compositions (Sr-87/Sr-86(initial) = 0.70794-0.70982). Two leucogranites and one aplite have higher initial Sr-87/Sr-86 ratios (0.70828-0.71559), but similar initial epsilon(Nd) (-11-9 to -15.8) and oxygen isotope values (10.5-12.9parts per thousand). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K2O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (< 0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material. (C) 2003 Elsevier Science B.V. All rights reserved.

The Oetmoed Granite–Migmatite Complex (OGMC), central Damara orogen, Namibia, consists of restite-rich, grt- and crd-bearing S-type granites and grt–crd–sil–Kfs-bearing metasediments, stromatic migmatites and nebulites. Both types of migmatites formed by limited in situ partial melting of metapelites under H2O-saturated conditions at ∼700°C and 5 kbar. Melanosomes of the stromatic migmatites do not resemble true residues, instead they more probably represent reaction zones between in situ melt and the metasedimentary host rock. Leucosomes of the stromatic migmatites have LREE- and HFSE-depleted disequilibrium compositions, typical of low-melt fractions generally observed in migmatite terranes. Similar δ18O values in the melanosomes and leucosomes suggest that partial melting occurred under fluid-present conditions. Nebulites are more residual than melanosomes and metasediments, indicating that separation of melt and residue must have occurred. Cordierite- and grt-bearing xenoliths in the granites do not represent residue from the site of origin of the intrusive granites; their depleted chemical composition is best explained by extensive degrees of partial melting of incorporated country rocks. Chemical variations among the grt- and crd-bearing granites are explained by fractional crystallization processes and xenolith entrainment. Major and trace element data and high δ18O values suggest that the grt- and crd-bearing granites were derived from H2O-undersaturated melting of metapelitic rocks.

Intrusion‐related migmatites comprise a substantial part of the high‐grade part of the southern Damara orogen, Namibia which is dominated by Al‐rich metasedimentary rocks and various granites. Migmatites consist of melanosomes with biotite+sillimanite+garnet+cordierite+hercynite and leucosomes are garnet‐ and cordierite‐bearing. Metamorphic grade throughout the area is in the upper amphibolite to lower granulite facies (5–6 kbar at 730–750 °C). Field evidence, petrographic observations, chemical data and mass balance calculations suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites. The intruding melts were significantly modified by magma mixing with in situ partial melts, accumulation of mainly feldspar and contamination with garnet from the wall rocks. However, it is suggested that these melts originally represented disequilibrium melts from a metasedimentary protolith. The occurrence of LILE‐, HFSE‐ and LREE‐enriched and ‐depleted residues within the leucosomes implies that both quartzo‐feldspathic and pelitic rocks were subjected to partial melting. Isotope ratios of the leucosomes are rather constant (143Nd/144Nd (500 Ma): 0.511718–0.511754, ε Nd (500 Ma): −3.54 to −5.11) and Sr (87Sr/86Sr (500 Ma): 0.714119–0.714686), the metasedimentary units have rather constant Nd isotope ratios (143Nd/144Nd (500 Ma): 0.511622–0.511789, ε Nd (500 Ma): −3.70 to −6.93) but variable Sr isotope ratios Sr (87Sr/86Sr (500 Ma): 0.713527–0.722268). The most restitic melanosome MEL 4 has a Sr isotopic composition of 87Sr/86Sr (500 Ma): 0.729380. Oxygen isotopes do not mirror the proposed contamination process, due to the equally high δ18O contents of metasediments and crustal melts. However, the most LILE‐depleted residue MEL 4 shows the lowest δ18O value (<10). Mass balance calculations suggest high degrees of partial melting (20–40%). It is concluded that partial melting was promoted by heat transfer and release of a fluid phase from the intruding granites. High degrees of partial melting can be reached as long as the available H2O, derived from the crystallization of the intruding granites, is efficiently recycled within the rock volume. Due to the limited amounts of in situ melting, it seems likely that such regional migmatite terranes are not the sources for large intrusive granite bodies. The high geothermal gradient inferred from the metamorphic conditions was probably caused by exhumation of deep crustal rocks and contemporaneous intrusion of huge masses of granitoid magmas. The Davetsaub area represents an example of migmatites formed at moderate pressures and high temperatures, and illustrates some of the reactions that may modify leucosome compositions. The area provides constraints on melting processes operating in high‐grade metasedimentary rocks.