Fluid-present melting of meta-igneous rocks and the generation of leucogranites - Constraints from garnet major- and trace element data, Lu-Hf whole rock-garnet ages and whole rock Nd-Sr-Hf-O isotope data

摘要

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 ~(207)Pb/~(235)U 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 Ma. P-T estimates obtained by conventional thermobarometry (c. 690-720 °C) and accessory mineral dissolution thermometry on the leucogranite (c. 730 °C) suggest that partial melting occurred through limited fluid present melting of biotite via the reaction: bt + kfs + plg + qtz + H2O<=>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 + plagiodase + K-feldspar+ quartz-bearing gneisses.