Reverse age relations of talc and tremolite deduced from reaction textures in metamorphosed siliceous dolomites of the southern Damara Orogen (Namibia)

Abstract

The regional metamorphosed siliceous dolomites of the southern Damara Orogen (Namibia) show a distinct talc-bearing zone on the lower metamorphic grade side, and a distinct tremolite-bearing zone on the higher metamorphic grade side, separated by the occurrence of the fivemineral assemblage: (I) tremolite + talc + calcite + dolomite + quartz.

The direction of increasing metamorphic grade is also established by a common sequence of mineral parageneses within the metapelites accompanying the marbles. The succession of talc to tremolite in siliceous dolomites with increasing metamorphic gradient is quite common, and is interpreted by analogous age relations assuming progressive metamorphism: initial formation of talc, followed by tremolite. By examining the reaction textures within and just below the five mineral paragenesis in the metamorphic siliceous dolomites of the Damara Orogen, it can however be shown that tremolite was initially formed, followed by talc. This means that talc is younger than tremolite. Tremolite may be formed at higher pressure on the P-T-path, followed by talc during the erosion-induced downward grade.

Some typical textures of the critical zone illustrating the age relationship between talc and tremolite will be described here and shown by photomicrographs.