Kariba porphyritic biotite granitoid gneiss
Submitted by Andrew du Toit on Tue, 24/11/2015 - 10:34 | Last updated: Tue, 24/11/2015 - 10:34
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Roadside on the way into Kariba from Makuti
The Kariba porphyritic biotite granitoid gneiss is coarse-grained, with a weak gneissic fabric, and contains large K-feldspar phenocrysts, and numerous decimetric slab-like mafic xenoliths. It contains small garnets rimmed and replaced by biotite. The zircons dated have oscillatory zoning in the centre (of igneous origin), but the margins have been affected by metasomatic alteration zones, and in some cases have discrete overgrowth rims. The zircons have yielded ages of 1962.9 ± 8.5 Ma and 2.1-2.17 Ga. The age of intrusion is regarded as 1.963 Ga, while the older zircons reflect inheritance from older crust (Master et al., 2015).
The Kariba porphyritic biotite gneiss has been included within the Kariba Paragneiss (Broderick, 1976), together with some porphyroblastic biotite gneisses containing euhedral microcline and microperthite porphyroblasts, which are associated with small plugs of adamellite and granodiorite. The new SIMS U-Pb zircon age is slightly younger than the very imprecise recalculated WR Rb-Sr date of 2050 ± 32 Ma obtained by Loney (1969) for a metagranite intrusive into Kariba Parageisses. The new age of 1962.9 ± 8.5 Ma is indistinguishable, within error from the older of two ages of 1962.7 ± 0.4 Ma and 1920.2 ± 0.4 Ma, obtained on porphyritic garnetiferous granodiorite gneisses by Dirks et al. (1999).
Stop 7 on the attached filed guide