TSK 11 Göttingen 2006 Gross Grain coarsening and hy- drothermal alteration in metacarbonates of the Damara Orogen, Namibia Poster Christian J. Gross1 Introduction and geological back- ground Grain coarsening is a process that oc- curs in a wide variety of rock types. The application of grain growth theory to natural geologic materials has its begin- nings in the theoretical foundations of the metallurgical and material sciences. Two types of grain growth can be statis- tically defined: 1) normal grain growth describing a uniform grain structure and 2) abnormal grain growth, where some grains grow more rapidly in size at the expense of matrix grains, thus creat- ing a bimodal grain size distribution. This study aims to understand the grain coarsening phenomenon in metacarbon- ate rocks, to determine the mechanisms involved and to elucidate the role of flu- ids (e.g. hydrothermal alteration). Metacarbonate units exhibiting coarsely crystalline fabrics, representing abnor- mal grain growth, and hydrothermal alteration have been investigated from two major geologic formations of the Damara Orogen (Namibia). The study sites are located in two distinct tectono- stratigraphic zones with different tec- tonic and metamorphic histories. The Central Zone (CZ) is characterized by crustal-scale dome structures, ex- tensive platform carbonates, numerous granitic intrusions, major shear zones and Karoo-age dolerite dike swarms. 1 GZG, Goldschmidstr. 3, 37077 Göttingen, Germany Figure 1: Large rhombohedral calcite blasts in a matrix of coarse-grained calcite with evenly distributed nests of graphite. Amphibolite to granulite facies meta- morphic conditions prevailed in the CZ. Shown in Figure 1 is a typical exam- ple of abnormal grain growth in a white graphite-bearing calcite marble of the Karibib Formation. Large rhombohe- dral calcite blasts are visible in a coarse- grained calcite matrix with graphite forming nests distributed throughout. Local lenses of grey calcite marble oc- cur showing similar grain growth char- acteristics, but without graphite nests. Peak metamorphic temperatures in the graphite-bearing calcite reached 760° C (Walter 2004). Late-stage cataclastic and mylonitic deformation accompanied by hydrothermal alteration overprints the coarse-grained calcite marbles. In the Southern Margin Zone (SMZ) where metadolomites of the Corona For- mation were investigated, greenschist facies conditions prevailed with dif- ferences in style of deformation and hydrothermal alteration. Carbon- ates are differentiated into siliceous dolomites, talc-bearing dolomites, a dark dolomite mylonite and a medium- grained dolomite marble with large 1 Gross TSK 11 Göttingen 2006 Figure 2: Metadolomite marble showing large dolomite porphyroblasts embedded in a matrix of medium-grained dolomite. dolomite porphyroblasts (Fig. 2). Field evidence for extensive hydrothermal al- teration is present as hydrothermal dolomite veins, hydrothermal quartz veins, extensive alteration in con- tact zones and bleaching of the dark dolomite mylonite. Fabrics and microstructures The graphite-bearing calcite marble of the CZ exhibits a wide variety of microstructures and fabrics. In the coarse-grained matrix domain, calcite shows irregular grain shapes character- ized by serrated, interpenetrating grain boundaries. Some microstructural fea- tures include leftover grains, twin-free bulges migrating into neighboring grains and examples of relic triple-point grain boundaries undergoing transformation. Twinning occurs in varying degrees, with twin-boundary migration also ob- servable. Contacts between matrix and blasts are curved and irregular. Thin flakes of graphite (3–4mm in size) oc- cur along grain contacts, in small clus- ters and occasionally in the large calcite blasts. Calc-silicate minerals occur in thin layers or clusters. When present in greater concentration, grain coarsening appears hindered. In optical cathodoluminescence mi- croscopy (CL), the coarse-grained cal- cite matrix and calcite blasts show ho- mogeneous red CL colors with very low to medium luminescence. Calcite show- ing very bright luminescence occurs as cross-cutting micro-veins. The etch ex- periments reveal a qualitative map of dislocation densities. In the calcite marble etching occurred primarily along grain boundaries and formed trains of etch pits aligned parallel to twin bound- aries. The dolomite marble from the SMZ ex- hibits a somewhat granoblastic fabric with a visible shape-preferred orienta- tion. The dolomite matrix grains range from flattened (a small percentage) to elongated in shape, where grain bound- aries can be straight or show fine irreg- ularities. Dolomite porphyroblasts ap- pear anhedral in thin section. Relatively large numbers of very fine-grained inclu- sions of albite clusters, talc, quartz and pyrite are found in the dolomite por- phyroblasts. The interface or contact zone between matrix and porphyblast is usually irregular and indentations of dolomite matrix grains into the porphy- roblast are locally observable as well as leftover grains. Bulk neutron texture analysis revealed a moderate to strong crystallographic preferred orientation in the dolomite matrix. The density of etch pits appears greater in the porphyroblast as compared to the matrix, with some etch grooves creating curved traces in the dolomite porphy- roblast. SEM also revels relatively open grain boundaries in the matrix dolomite after etching. Evidence for extensive grain-scale fluid- rock interaction is visible by CL in the dolomite marble. The CL color in un- 2 TSK 11 Göttingen 2006 Gross Figure 3: Classification of zonation types in the dolomite marble. A) CL image of the dolomite matrix depicting the two grain-boundary parallel Fe-rich zonations. B) Rhyth- mical growth zones indicating paleo-porosities. C) Local chaotic fabric of Fe-rich and Fe-poor dolomite. D) Fe-rich pathway (two generations) cut across the porphyroblast. Minor rhythmical zonations are also visible. Note: Pictures B-D represent back-scatter electron images. altered matrix grains as well as in the pophyroblast is yellow-orange, whereas the Fe-rich areas exhibit various shades of brown. Figure 3 shows a classi- fication of the types of zonation mi- crostructures present in the matrix and porphyroblast. In the dolomite matrix an interconnected network of Fe-/Mn- rich zones, creating two distinct genera- tions, occurs parallel the grain bound- aries (Fig. 3A). Complex rhythmical zonations with alternating bands and sharp boundaries occur in some parts of the matrix (Fig. 3B). Chaotic CL fabric patterns occur locally and destroy the matrix grains and the grain-boundary zonations (Fig. 3C). In the dolomite pophyroblast (Fig. 3D), the Fe-rich al- teration is represented by cross-cutting pathways also forming two generations. Furthermore, zoned albite clusters with dark cores and blue rims are always associated with the Fe-rich zonations. Those in the matrix are unzoned. Discussion and conclusion All the data indicates that the grain coarsening in the graphite-bearing cal- cite marbles is the result of regional metamorphism due to granitic intru- sions (e.g. static recrystallization, Evans et al. 2001). The dominant mechanism responsible for the grain growth in the calcite marble is grain boundary migra- tion, as is evident by the types of mi- crostructures and fabrics observable in thin section. Cathodoluminescence observations of the graphite-bearing calcite marble in- dicate that hydrothermal alteration was limited to a late-stage event. Micro- veins of brightly luminescent calcite (zoned and unzoned) are associated with the fluids accompanying cataclas- tic deformation and are not connected to the grain coarsening process. Fluid inclusions within the coarse calcite ma- trix and the calcite blasts have a compo- sition similar to that of seawater. Flu- ids released by the granitic intrusions must have played a role in the coarsen- ing process, since in theory the rate of 3 Gross TSK 11 Göttingen 2006 grain growth and grain boundary mo- bility would be much slower under dry conditions. The grain coarsening, porphyroblast growth and alteration in the dolomite marble is somewhat problematic. The dolomite porphyroblasts could be either younger or older than the coarsened ma- trix. The relic grain boundaries and possible leftover grains suggest the por- phyroblast is younger, whereas embay- ments of matrix grains into the por- phyroblast could suggest an older age. Furthermore, the grain coarsening and porphyroblast growth could have oc- curred earlier than the hydrothermal al- teration, since the Fe-rich zones cross- cut the porphyroblast. On the other hand, the CL zonations may represent evidence for fluid-enhanced grain coars- ening and thus the mechanism of growth is solution/precipitation. Fluid inclu- sion data from dolomite veins point to highly saline solutions present during hydrothermal alteration. The sequence of events for the hy- drothermal alteration represented by the Fe-/Mn-rich zonation network can be separated out into five stages of a continuous process. Stage I introduces two Fe-/Mn-rich fluid generations. In Stage II intergranular pore spaces are created which are then filled in by rhythmically zoned dolomite (Stage III). Formation of a second generation of pore spaces occurs in Stage IV caused by corrosion of the original rock fab- ric. This leads to the precipitation of dolomite (Stage V) represented by the local chaotic CL fabric. All these events represent episodic fluid influxes possibly caused by seismic events, since the car- bonate unit lies in close proximity to a thrust fault. References Evans B, Renner J & Hirth G (2001) A few remarks on the kinetics of static grain growth in rocks. Int J Earth Sci 90, 88–103 Walter J (2004) Fabric development, electri- cal conductivity and graphite formation in graphite-bearing calcite marbles from the Central Damara Belt, Namibia. PhD The- sis, Georg-August University of Göttingen, pp 289 4