Oxygen and hydrogen isotope geochemistry of S- and I-type granitoids: the Cape Granite suite, South Africa

The Late Precambrian Cape Granite suite of the southwestern Cape, South Africa, can be subdivided into six major batholiths (Saldanha-Vredenburg, Darling, Malmesbury, Paarl-Wellington-, Stellenbosch-Kuils River, and Peninsula). Individual plutons have been classified as A-, I- and S-type granites on the basis of petrography and chemical composition. Quartz and feldspar from the Cape granites have δ18O values which range from 8.6 to 13.4‰ and 5.0 to 13.6‰, respectively. The δ18O value of quartz is a very effective discriminator between S- and I-type granites, and provides the best means to estimate the δ18O value of the original magma. The S-type granites crystallized from magmas having δ18O values between 9.5 and 11.4‰ (mean = 10.6‰). The A- and I-type granites crystallized from magmas having δ18O values between 6.6 and 9.9‰ (mean = 7.8‰). There is almost no overlap of δ18O values between the S- and the I-type granites with the boundary at 9.5‰. Of the 31 quartz-feldspar pairs analysed, the vast majority have values of Δqtz-fsp between 0 and 4‰. However, only eight samples have values of Δqtz-fsp between 0.7 and 1.55‰, which would be consistent with equilibrium under closed-system conditions; the remaining samples were affected by minor fluid-rock interaction. Biotite δD values range from −113 to −47‰ and show no correlation with estimated magma δ18O value, biotite-water content or whole-rock loss on ignition. The variation in biotite δD is probably caused by a combination of varying degree of magma degassing and minor low-temperature alteration (< 300°C). No stable isotope evidence was found to suggest that extensive interaction of the granites with hydrothermal fluids occurred and this is consistent with the lack of large-scale base-metal mineralization. The S-type granites were intruded first and are found to the west of a NW-SE fault line dividing basement terranes in the area. The I-type granites were intruded later to the east of this line. No systematic regional difference in the δ18O value of the Malmesbury Group basement in the area was found, whose average δ18O value is 12.7‰. The S-type granites probably represent partial melts of the Malmesbury Group possible mixed with minor amounts igneous source material and are not variously contaminated I-type magmas. The I-type granites are partial melts of igneous basement not exposed in the area.