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

Abstract

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 δ 18O values which range from 8.6 to 13.4‰ and 5.0 to 13.6‰, respectively. The δ 18O value of quartz is a very effective discriminator between S- and I-type granites, and provides the best means to estimate the δ 18O value of the original magma. The S-type granites crystallized from magmas having δ 18O values between 9.5 and 11.4‰ (mean = 10.6‰). The A- and I-type granites crystallized from magmas having δ 18O values between 6.6 and 9.9‰ (mean = 7.8‰). There is almost no overlap of δ 18O 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 Δ qtz-fsp between 0 and 4‰. However, only eight samples have values of Δ qtz-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 δ 18O 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 δ 18O value of the Malmesbury Group basement in the area was found, whose average δ 18O 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.