4. Petrography and fabrics of the Chinamora Batholith
4.1 Field observations
4.1.1 Gneissic granites
The gneissic granites in the marginal areas and the erosional windows of the Chinamora Batholith may be differentiated according to their amount of deformation into the southern, northern and western gneissic granites. They all show different intensities of their internal deformations and are hence described separately.
4.1.1.1 Southern gneissic granites
Field observations of the southern gneissic granites revealed numerous small scale shear zones (Plate 1D). The shear zones often cut xenoliths with offsets of a few centimeters (Plate 1A). Kinematic indicators in these small scale shear zones show sinistral as well as dextral movements and therefore probably originated from emplacement mechanisms rather than by movement along the Umwindsi Shear Zone (Jelsma pers. communication; Ramsay, 1989) that parallels the southern margin of the batholith. Strongly elongated xenoliths are abundant in the southern gneisses. Usually only 2D views of the xenoliths are available. Where a 3D view is possible they show strongly oblate forms and are flattened in the foliation plane (Plate 1B). Foliation is usually defined by small biotite flakes that form only weakly or unconnected layers and sometimes anastomose around
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feldspars (Plate 2 and 3). Feldspar megacrysts usually show a strong preferred alignment (Plate 3A), folded pegmatites are abundant and are frequently refolded (Plate1C).
Hand specimen all show a very high content of hornblendes and biotites giving the rocks a dark grey appearance. The well developed foliation plane (Plate 2) in most of the samples is defined by biotites and/or hornblendes. Feldspars are oriented in the foliation plane and sometimes appear to define augen gneisses, however, this texture is only weakly developed. Biotites of the southern gneisses are much smaller than biotites in the other suites, they hardly exceed ~0.5 cm in diameter.
Shear zones can be observed in some of the samples indicating a sinistral as well as dextral sense of shear. These shear zones sometimes crosscut large feldspars (Plate 3A) and hence must have been active after crystallization of the melt.
Lineations in the southern gneisses are defined by weakly aligned feldspars.
The southern gneissic contain quartz, plagioclase, K-feldspar, hornblendes and biotite as their main constituencies (see Table 4.1 for modal analyzes) and magnetite, titanite, zircon, apatite and garnet as accessories. The Streckeisen-diagram is shown in Fig. 4.1. Most of the samples plot in the granodiorite-field, still some of the samples plot as monzogranites or tonalites. Baldock (1991) found the same modal composition in his study.
Fig. 4.1: Streckeisen diagram of the samples in the southern gneissic granites.
Qtz
Plag K-fsp
90
60
20
Granite
Grano-diorite
Syenite Monzonite Monzo- Diorite diorite
Kf-Syenite
Ton alite Kf-granite
5
Syeno-granite Monzo-granite
Table 4.1: Modal composition calculated from geochemical analyses for the southern gneissic granites.
4.1.1.2 Western gneissic granites
In the western gneissic granites internal fabric mainly is defined by large biotite flakes (~2 cm) that define a well developed foliation (Plate 4). Feldspars are small and oriented in the foliation plane. In some samples hornblendes seem to define a weak SC-fabric indicating a dextral sense of shear. Generally, grain size is much coarser than in most of the southern gneissic granites. The massive occurrence of feldspars gives these rocks a light grey to white color, some ore minerals are embedded in the rocks that are easily recognized due to their small, brownish corona that probably originated during weathering (Plate 4A).
Fig. 4.2: Streckeisen diagram for the samples in the western gneissic granites.
CH 11 CH 18 CH 19 CH 21A CH 21B JB 210
quartz (%) 23 21 15 23 28 23
K-feldspar (%) 13 11 0 19 26 17
plagioclase (%) 45 48 40 43 32 45
hornblende (%) 9 11 28 6 0 3
biotite (%) 10 10 18 9 14 12
JB 211 JB 212 JB 213 JB 214A JB 214B JB 216
quartz (%) 24 33 15 33 29 28
K-feldspar (%) 20 13 22 5 18 14
plagioclase (%) 43 45 37 50 49 44
hornblende (%) 3 0 19 0 2 0
biotite (%) 10 12 7 12 2 14
Qtz
Plag K-fsp
90
60
20
Granite
Grano-diorite
Syenite Monzonite Monzo- Diorite diorite
Kf-Syenite
Ton alite Kf-granite
5
Syeno-granite Monzo-granite
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The western gneissic granites contain quartz, plagioclase, K-feldspar and biotite as the main constituencies. In most of the samples the hornblende content is fairly high and must be seen as a major phase as well. Accessories are magnetite, apatite and zircons. The modal content of the samples differs widely depending on the lithology (see Table 4.2 and Fig. 4.2), in the Streckeisen-diagram the samples plot in the granodiorite and tonalite field (see Fig. 4.2).
Table 4.2: Modal composition calculated from the geochemical measurements.
4.1.1.3 Northern gneissic granites
The northern gneissic granites can be divided into the central granodiorite (Musana Communal Land, see Fig. 4.3) and the marginal gneissic granodiorite and granite. The central granodiorite shows a weak foliation defined by small biotite flakes (Plate 5) while other samples in the same area are isotropic and do not show any macroscopically visible internal fabric. These samples are comparable to samples from the porphyritic granite but they lack the common occurrence of large feldspar phenocrysts. Ore minerals can frequently be recognized due to their brownish corona (Plate 5A + B). Grain size usually is small although some large quartz grains can be observed.
Outcrops in the marginal northern gneissic granites usually show small grains of feldspars and biotites defining a strong foliation. Folding of pegmatites is common but in contrast to the southern gneisses refolding of veins can only rarely be observed. These two subunits (sample JB200 and JB202 are from the Musana Communal Land) in the northern gneissic granites can not be distinguished according to their modal content (see Table 4.3, Fig. 4.4).
Table 4.3: Modal composition calculated from geochemical analyses.
CH 16A JB 217B JB 108 JB 217A
quartz (%) 37 18 29 21
K-feldspar (%) 8 0 14 5
plagioclase (%) 41 40 48 37
hornblende (%) 0 20 2 11
biotite (%) 14 22 8 25
CH 23 JB 77 JB 200 JB 202 JB 215
quartz (%) 36 37 34 33 31
K-feldspar (%) 0 3 18 18 15
plagioclase (%) 52 48 41 41 44
hornblende (%) 1 3 0 1 1
biotite (%) 11 10 7 8 9
Fig. 4.3: Location of the Musana Communal Land area
Fig. 4.4: Streckeisen diagram of the samples in the northern gneissic granites.
Their main constituencies are quartz, K-feldspar (although some samples show a very low K-feldspar content), plagioclase, biotite and minor hornblende, accessories include magnetite, haematite, zircon, apatite, titanite and sphene.