Quartz textures

In total five different samples have been measured, one for each of the different units within the Chinamora Batholith (see Fig. 4.10). From the northern gneissic granites a sample from outside the Musana Communal Land area has been chosen (sample JB71, see Fig. 4.10) since the samples from the Musana Communal Land are isotropic or nearly isotropic. Sample JB71 shows a very strong orientation pattern of the quartz c-axis with a subhorizontal N-S orientation which is parallel to the outer margin of the batholith and the inner boundary of the Musana Communal Land area. The quartz texture from the southern gneissic granites (sample JB305) shows a double maxima, a typical feature for quartz deformed in or nearby shear zones at higher temperatures (Riekels & Baker, 1977; Ramsay & Graham, 1970).

a-axis, n=66 max. dens. = 5.12 density lines at 1, 2, 3, 4, 5

b-axis,n=66 max. dens. = 8.58 density lines at 1, 2, 3, 4, 5, 6, 7, 8

c-axis, n=69 max. dens. = 5.94 density lines at 1, 2, 3, 4, 5 b-axis, n=57

max. dens. = 7.62 desnity lines at 1, 2, 3, 4, 5, 6 ,7 a-axis, n=57

max. dens. = 18.82 density lines at 1, 2, 3, 4, 5, 6, 7, 8, 9

c-axis, n=57 max. dens. = 12.03 density lines at 1, 2, 3, 4, 5, 6, 7, 8, 9 305

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N N N

N N N

Fig. 4.11: Pole figures of quartz textures obtained from U-stage measurements.

The measurements of the western gneissic granites (sample JB134) and the equigranular granites (sample JB146) only exhibit a random distribution of the crystallographic c-axes.

Measurements of the orientation patterns of quartz c-axes of the porphyritic granite (sample JB20) revealed a very weak, irregularly occupied crossed girdle.

This has been assigned by Tullis et al. (1973) to a preferred basal glide at higher temperatures. However, since the fabric is only very weakly developed its interpretation is ambiguous.

4.5 Conclusions

The microscopic analyzes showed that a division of the different units of the Chinamora Batholith according to their amount of deformation can be drawn. The southern gneissic granites show the highest deformational features in the solid state while the other gneissic granites only show a weak overprint of magmatic to submagmatic features in the solid state. The microstructures observed in the

JB134 n=235

max. dens.=3.24 (250/18) contours at 1,2,3

JB146 n=238

max. dens.= 3.63 (332/12) contours at 1,2,3

JB20 n=254

max. dens.=3.62 (000/66) contours at 1,2,3

JB305 n=219

max. dens. = 6.95 (300/42) contours at 1,2,3,4,5,6

JB71 n=225

max. dens. = 6.62 (185/12) contours at 1,2,3,4,5,6

N N N

N N

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equigranular granites are comparable to those of the western gneissic granites.

The northern gneissic granites shows two distinct units, the central Musana Communal Land area resembles isotropic or nearly isotropic rocks while the marginal areas of the northern gneissic granites shows a moderately to strongly developed fabric. This can be explained by two pulses of magma with the magma pulse now forming the Musana Communal Land being slightly younger than the marginal northern gneisses. The second magma pulse pushed the first pulse outwards producing the observed fabric. The central porphyritic granite shows no solid state overprint of the magmatic or submagmatic fabric. The macroscopic foliation in the granitoid rocks of the Chinamora Batholith is margin parallel in the gneissic granites and, except for the Musana Communal Land area, cross cuts internal lithological boundaries. This indicates an at least nearly coeval emplacement of the different lithologies in the different gneissic granites.

Macroscopic fabric in the porphyritic granite shows a stable trend that can not solely be related to the emplacement mechanism. The orientations of the whaleback domes in the porphyritic granite has been mapped using a SPOT (Système Pour l’Observation de la Terre) satellite image, they preferably are orientated in a WNW-ESE direction and probably eroded controlled by the orientation of the fracture network. This preferred orientation is subparallel to the macroscopic foliation. Some major fault zones can be recognized very well, they usually are pathways for rivers draining the area. Their strike is very consistent, either NNW-SSE or WNW-ESE. No other linear features are recognizable in the SPOT-image due to the extensive use of the area for farming. Only the northern gneissic granites and the porphyritic granite can be distinguished from the other units.

The biotite texture analyzes performed on samples from the southern gneisses showed that they all display a strong preferred alignment of biotite 001-poles. In the western and northern gneissic granites deformation was less distinct and hence the developed fabrics show a less distinct orientation pattern. Only some samples in the marginal areas of the northern gneissic granites exhibited a well-defined fabric which can be drawn back to the later intrusion of the granitoids in the Musana area. The orientation of biotite fabric in the analyzed samples does not reflect any common emplacement mechanism. Biotite foliation is not always parallel to the outer margin of the batholith or to internal lithological boundaries.

Given the observed structural patterns in the different gneissic granites it seems obvious that the different lithologies resemble different plutons that intruded during a distinct tectonic setting over a more or less longer period of time. During the time span of intrusions either the orientation of the regional stress-field has changed slightly or the different lithologies behaved different due to rheological differences and/or a changing amount of coupling between stress-field and intrusions (e.g.

intruding into the “shadow” of earlier intrusions) giving rise to the observed different fabrics. The southern gneisses have obviously suffered additional deformation as can be concluded from the more pronounced solid-state deformation, the subhorizontal orientations of biotites and the double maxima of the measured quartz texture. Orientations of the biotites in the porphyritic granite indicate a different textural genesis.

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