Subsolidus reaction textures in the anorthositic rocks of the southern part of the Kunene Intrusive Complex, NW Namibia

This study concentrates on the cooling history of the anorthositic rock sui te of the Proterozoic Kunene Intrusive Complex (KIC) in NW Namibia. All inv estigated samples display the typical magmatic mineral assemblage of plagio clase (An(37-75)) +/- olivine (Fo(54-65)) +/- orthopyroxene (En(53-71)) +/- clinopyroxene (diopside with X-Mg: 0.66-0.76) + ilmenite + magnetite + apa tite +/- zircon. Ilmenite and pyroxene are surrounded by narrow reaction ri ms of biotite as well as pargasite. Thermobarometry studies on amphiboles y ielded temperatures of 950-985 degreesC and pressures of 7-9 kbar, respecti vely for the late-magmatic crystallisation of the anorthositic rocks. Isoba ric cooling after emplacement in the lower crust has produced sporadic coro nitic garnet(X-Mg: 0.31-0.42) + orthopyroxene (En(62-71)) + quartz assembla ges around olivine, and garnet (X-Mg: 0.23-0.30) + biotite (X-Mg: 0.66-0.73 ) assemblages around ilmenite, both with magnetite exsolution, following th e schematic reactions: Ca[Al2Si2O8] + (Fe, Mg)(2)[Si2O6]-->Ca(Fe, Mg)(2)[Al2Si3O12] + SiO2 and, an in plag opx grt qz Ca[Al2Si2O8] + Ca-2(Fe,Mg)(3)Al-2[Al2Si6O22] (OH)(2) + Fe3O4 + SiO2--> an in plag ts in am mt qz 3 Ca(Fe,Mg)(2)[Al2Si3O12] + H2O + 1/2O(2) grt The chemical compositions of garnet and orthopyroxene and their textural re lationships indicate re-equilibration of the leucotroctolites at conditions of 790 +/- 60 degreesC and 8 +/- 1 kbar, at the amphibolite to granulite f acies transition. Subsequently, olivine was decomposed by various reactions , leading to conspicuous intergrowths of granular orthopyroxene with lamell ar magnetite, displaying oriented ilmenite and gahnite exsolution. Ilmenite -magnetite geothermometry yields temperatures of 500-550 degreesC. Our resu lts clearly indicate that the Kunene Intrusive Complex intruded into the lo wer crust and underwent nearly isobaric cooling.