Phase relations of talc and tremolite in metamorphic calcite-dolomite sediments in the southern portion of the Damara Belt (South West Africa) |
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| Authors: | D Puhan E Hoffer |
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| Institution: | 1. Mineralogisch-Petrologisches Institut der Universit?t G?ttingen, Deutschland
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| Abstract: | The occurrence of talc and tremolite in a temperature gradient was investigated in siliceous calcite-dolomite sediments exposed along a strip in the southeastern part of the Damara Orogen. Five bivariant reactions may lead to the formation of talc and tremolite: - 3 dolomite+4 quartz+1 H2O ? 1 talc+3 calcite+3 CO2
- 5 talc+6 calcite+4 quartz ? 1 tremolite+6 CO2+2 H2O
- 2 talc+3 calcite ? 1 tremolite+1 dolomite+1 CO2+1 H2O
- 5 dolomite+8 quartz+1 H2O ? 1 tremolite+3 calcite+7 CO2
- 2 dolomite+1 talc+4 quartz ? 1 tremolite+4 CO2.
The common paragenesis of four mineral assemblages tc+cc+dol+qtz1 and tre+tc+ cc+qtz with increasing temperature over an extended area show that the reactions must have taken place along the equilibrium curve or when fluid pressure is not constant along the equilibrium plane of reactions (1) or (2). The described occurrence of the five mineral assemblage tre+tc+cc+dol+qtz can be stable only on the isobaric intersection point, or when P f is variable on the univariant intersection curve of the equilibrium planes of all five reactions. The genetic relations of the described parageneses are illustrated with the help of a phase diagram. Minimum P-T conditions which prevailed during metamorphism in this part of the Damara Orogen have been estimated to be about 590° C and 5 kb. |
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