Metamorphic evolution in external zones of the Dom Feliciano-Kaoko orogenic system. Examples from the Brusque Metamorphic Complex, Dom Feliciano Belt (Brazil) and the Central Kaoko Zone, Kaoko Belt (Namibia)

Abstract

The Brusque Metamorphic Complex in the Dom Feliciano Belt (Brazil) and the Central Kaoko Zone in the Kaoko Belt (Namibia) represent foreland fold- and thrust belts in external parts of the northern Dom Feliciano-Kaoko orogenic system. Petrographic and field observations of metapelitic samples from the Brusque Metamorphic Complex suggests that the western part of the complex can be subdivided into three different metamorphic zones: two zones corresponding to greenschist facies conditions, and a central zone corresponding to lower amphibolite facies conditions. Phase equilibrium modelling of one sample form the central zone yielded metamorphic conditions of ca. 560-570°C and pressures of 6-7 kbar. Assuming an average crustal density of 2.8 g/cm3, these conditions represent an apparent thermal gradient of ca. 22-26C/km at the time of peak metamorphism of the Brusque Metamorphic Complex. The Central Kaoko Zone consists of an inverted barrowian sequence of metapelitic rocks ranging from lower greenschist to higher amphibolite facies conditions, and has been subdivided from east to west into chlorite, biotite, garnet, staurolite, kyanite and kyanite-sillimanite-muscovite metamorphic zones. P-T modelling of three samples from to the Central Kaoko Zone (garnet, staurolite and kyanite zones) suggests apparent thermal gradient between 16 and 23°C/km. These results imply that despite a large difference in timing of metamorphism, the thermal conditions in the crust were similar during underthrusting of the western and eastern margin of the northern Dom Feliciano-Kaoko system.