NORMAL AND ABNORMAL GRAIN GROWTH IN CALCITE AND DOLOMITE MARBLES FROM THE DAMARA OROGEN, NAMIBIA

Coarsely crystalline metacarbonates, consisting of graphite-bearing sparry calcite marbles and sparry dolomite marbles are characterized by complex, normal and abnormal grain growth fabrics. The reduction of the interfacial free energy is the driving force behind the grain growth process, leading to an increase in grain size and a reduction of the grain boundary surface area. In the normal grain growth domain, the grain structure remains generally constant in regards to grain size distribution and grain shape. Some of the grains grew more rapidly at the expense of neighboring grains, generating an abnormal grain growth domain. This results in a bimodal grain size distribution which both rock types exhibit. The calcite matrix shows grain sizes of 0.5 mm to >1 cm, with sparry calcite rhombohedral blasts exhibiting sizes of >10 cm. The sparry dolomite porphyroblasts display sizes of 1 cm to >10 cm embedded in a relatively pure dolomite matrix (0.5-4 mm).

The coarsening in the calcite marble begins with an older equilibrium metamorphic grain fabric. Initial grain boundary mobility is indicated by the dismantling of triple-point grain boundaries. The original boundary configuration in the matrix is partially replaced with irregular and serrated grain boundaries. The presence of other phases (e.g. calc-silicates) hinders the coarsening. The final stage in this post-metamorphic process is the growth of the sparry calcite blasts, whose grain contacts with the matrix are irregular, suggesting that the dominant mechanism is grain boundary migration.

The host rock for the sparry dolomite marble is a fine-grained, strongly deformed mylonitic dolomite. Initial grain coarsening (normal grain growth with possible fluid enhancement) occurs in the mylonite with retention of its shape-preferred orientation. Matrix coarsening continues, with the sparry dolomite porphyroblasts forming by coalescence crystallization. Incorporation of matrix grains into the sparry dolomite is observable at the contact interface zone.