Igneous Petrology of Cape Granites

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Geological Setting

The Cape Granite Suite formed about 560-520 Ma during the Saldanian orogeny which occurred between 780 to 510 Ma (Harris & Vogelli, 2010; Villaros, Stevens & Buick, 2009). This orogeny formed as a result of the convergence of the Rio de la Plata and Kalahari cratons during the formation of Gondwanaland (Harris & Vogelli, 2010). The granitoids of the CGS were intruded into the three terranes of the metasedimentary and metavolcanic Malmesbury Group which are divided by distinct north-west trending shear zones (Scheepers, 2000). These terranes include the southwestern Tygerberg terrane, Swartland terrane at the centre and Boland terrane to the north east (Fig. 1) (Scheepers, 1995).

The Saldania mobile belt comprises of structural features that are characterised by the occurrence of ductile and dextral strike-slip shearing that preceded the orogenic event which occurred approximately 600 Ma (Scheepers, 2000). This led to the first phase of magmatism about 560-540 Ma (Scheepers & Nortje, 2000). This phase involved the syn- to post tectonic S- type granites intruding the Tygerberg terrane (Scheepers & Nortje, 2000). The second phase involved the intrusion of the I-type granites. Like the S-type granites, the strike-slip movement affected the intrusion of the I- type granitoids which occurred along the Colenso fault in the Swartland and Boland terranes about 540- 520 Ma (Scheepers, 1995; Scheepers & Nortje, 2000). The intrusion of the post-orogenic to anorogenic A- type granites occurred at approximately 520- 500 Ma, representing the third phase of magmatism (Scheepers & Nortje, 2000). They are intrusive in the Tygerberg and Swartland terranes (Scheepers & Nortje, 2000). The final phase of magmatism is often negle...

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...ircon ages of the Cape Granite Suite: implications for the magmatic evolution of the Saldania Belt.", South African Journal of Geology, vol. 105, no. 3, pp. 241-256.

Scheepers, R. & Nortjé, A. 2000, "Rhyolitic ignimbrites of the Cape Granite suite, southwestern Cape province, South Africa", Journal of African Earth Sciences, vol. 31, no. 3, pp. 647-656.

Scheepers, R. & Ozendaal, A. 1995, "Phosphorus as a typological and mineralization potential indicator: the Cape Granite Suite of the Saldania belt as a case study", Journal of African Earth Sciences, vol. 21, no. 1, pp. 127-140.

Villaros, A., Stevens, G., Moyen, J. & Buick, I.S. 2009, "The trace element compositions of S-type granites: evidence for disequilibrium melting and accessory phase entrainment in the source", Contributions to Mineralogy and Petrology, vol. 158, no. 4, pp. 543-561.