Subsolidus reaction textures in the anorthositic rocks of the southern part of the Kunene Intrusive Complex, NW Namibia
Drüppel, Kirsten; von Seckendorff, Volker; Okrusch, Martin
European Journal of Mineralogy Volume 13 Number 2 (2001), p. 289 - 309
published: Mar 27, 2001
This study concentrates on the cooling history of the anorthositic rock suite of the Proterozoic Kunene Intrusive Complex (KIC) in NW Namibia. All investigated samples display the typical magmatic mineral assemblage of plagioclase (An37-75) ± olivine (Fo54-65) ± orthopyroxene (En53-71) ± clinopyroxene (diopside with XMg: 0.66-0.76) + ilmenite + magnetite + apatite ± zircon. Ilmenite and pyroxene are surrounded by narrow reaction rims of biotite as well as pargasite. Thermobarometry studies on amphiboles yielded temperatures of 950-985°C and pressures of 7-9 kbar, respectively for the late-magmatic crystallisation of the anorthositic rocks. Isobaric cooling after emplacement in the lower crust has produced sporadic coronitic garnet (XMg: 0.31-0.42) + orthopyroxene (En62-71) + quartz assemblages around olivine, and garnet (XMg: 0.23-0.30) + biotite (XMg: 0.66-0.73) assemblages around ilmenite, both with magnetite exsolution, following the schematic reactions:Ca[Al2Si2O8] + (Fe, Mg)2[Si2O6] → Ca(Fe, Mg)2[Al2Si3O12] + SiO2 and, Ca[Al2Si2O8] + Ca2(Fe, Mg)3Al2[Al2Si6O22](OH)2 + Fe3O4 + SiO2 → 3 Ca(Fe, Mg)2[Al2Si3O12] + H2O + × O2 The chemical compositions of garnet and orthopyroxene and their textural relationships indicate re-equilibration of the leucotroctolites at conditions of 790 ± 60 °C and 8 ± 1 kbar, at the amphibolite to granulite facies transition. Subsequently, olivine was decomposed by various reactions, leading to conspicuous intergrowths of granular orthopyroxene with lamellar magnetite, displaying oriented ilmenite and gahnite exsolution. Ilmenitemagnetite geothermometry yields temperatures of 500-550 °C. Our results clearly indicate that the Kunene Intrusive Complex intruded into the lower crust and underwent nearly isobaric cooling.