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Abstract
The coarsely porphyritic biotite-granite of the Cape pluton, intrusive into the steeply dipping Malmesbury Series, extends the full length of the Peninsula. Structural features are feebly developed, primary flow structures being absent. The most prominent joint directions strike WNW.-ESE. parallel to the direction of stretching. They have been classified as longitudinal joints. Transverse NNW.-SSE. and NNE.-SSW. trending fractures are regarded as cross-joint directions. Secondary structures are undoubtedly present, but the primary age of these fracture systems is established by the fact that aplites follow them.
The Malmesbury Series in this area consists of psammitic strata with thinner pelitic intercalations. Both arenaceous and argillaceous strata have been converted into hornfels in the neighbourhood of the granite. Xenoliths of the Malmesbury sediments included in the granite show every gradation between normal hornfels and completely granitized fine-grained leucocratic granitoid rocks. Biotite-rich microgranitic xenoliths represent an intermediate stage.
In the initial stage of alteration the hornfels xenoliths were slightly enriched in alumina and cafemic constituents. This was followed by granitization effected chiefly by the introduction of potash-rich alkali material from the magma. Complete assimilation of downstoped sediments is believed to have modified the granite by causing an increase in the amount of biotite. The presence of cordierite in the granite is also ascribed to contamination.
The large microcline microperthite phenocrysts are considered to be the normal products of magmatic crystallization and not the result of late potash enrichment with the growth of replacement microcline.