Metamorphic evolution of the assemblage tremolite + talc + calcite + dolomite + quartz within a sample of siliceous dolomite from the southern Damara Orogen (Namibia)

Magnesian calcite temperatures of calcites formed during metamorphic reactions and fluorine distribution between OH-bearing minerals have been measured for the assemblage {ie180-1} within a sample of siliceous dolomite from the southern Damara Orogen. Two groups of temperatures were found: A group with higher temperatures in the range 583 to 545°C for reaction sites where tremolite was formed, and a second group with lower temperatures from 522 to 476°C for reaction sites where talc has grown. From special reaction textures it is evident that the reaction sites of the higher temperature group related with temolite is older, whereas talc has grown after the formation of tremolite with decreasing temperature. The transition from the tremolite stability field to the talc stability field occurred at a temperature between the two temperature groups. Each measured magnesian calcite temperature can be assigned to a microscopic fluid vein which connects single reaction sites either with tremolite or with talc. Single microveins developed at different times, because the prevailing temperature during the formation of individual fluid veins and the connected reaction sites are different. The mineral associations and the reaction textures of the sample were formed within a period of time necessary for cooling the rock at least from about 580 to 480°C. The textural relations after the successive reaction processes appear to be the result of a single univariant reaction. Distribution of fluorine between tremolite-phlogopite pairs and tremolite-talc pairs show that the fluorine content of the tremolite may be inherited in the latter case. A plausible P-T path is outlined explaining the evolution of assemblages in dolomites and metapelites of the same area which contain staurolite and kyanite. The first appearance of staurolite and kyanite in the field is situated on the lower grade side of the zone containing assemblage (I). The conversation of the present field occurrence of assemblage (I) is due to the fading of fluid infiltration into the dolomites.