Kariba sillimanite quartzite Kariba Heights
Submitted by Andrew du Toit on Tue, 24/11/2015 - 10:38 | Last updated: Tue, 24/11/2015 - 10:38
Grid Ref (WGS84 Lat/Long in decimal degrees)
Road down from Kariba Heights
The Kariba sillimanite quartzite Kariba Heights, Sample ZMB13/11 is an aluminous quartzite consisting mainly of quartz and sillimanite. The zircons have yielded concordant ages of 2.018 Ga, 2.172, 2.220 and 2.70 Ga (Master et al., 2015). The maximum age of the quartzite is 2.018 Ga, the age of the youngest concordant detrital zircon, while the other zircons reflect a provenance from older crust dated at 2.17, 2.22, and 2.70 Ga. Three youngest zircons have (discordant) ages between 1.955 and 1.963 Ga, reflecting possible resetting during intrusion of the 1.96 Ga granitoids.
The Kariba sillimanite quartzite is part of the Kariba Paragneiss succession which is a northern continuation of the Chipisa Paragneisses The Chipisa Paragneisses were dated by Loney (1969), and yielded a recalculated Rb-Sr WR age of 2443 ± 90 Ma. This imprecise result was interpreted by Master (1991, 1996) as indicating a Neoarchaean to Palaeoproterozoic age for the metamorphism that affected these rocks. The Kariba Paragneisses consist of foliated biotite paragneisses with calc-silicate bands and thin leucogneisses. The Kariba sillimanite quartzites occur interbedded within the Kariba Paragneisses, and are regarded as an arenaceous facies of the paragneiss. Loney (1969) obtained a (recalc. WR) Rb-Sr age of 2368 ± 92 Ma for the Kariba Paragneisses. Master (1991, 1996) considered this imprecise age as a minimum, metamorphic age, with the age of sedimentation of the protoliths being unknown, but most probably Neoarchaean or earliest Palaeo-proterozoic.
The new precise U-Pb zircon ages from the Kariba sillimanite quartzite indicate that the protoliths had a maximum sedimentary age of 2.018 Ga, and that they were metamorphosed at c. 1.96 Ga, during intrusion of the Kariba porphyritic granitoids. This means that the Kariba Paragneiss protolith was a sedimentary succession that was synchronous with the intrusion of the Andean arcs on the western edge of the Magondi Belt and Zimbabwe Craton.
Stop 8 on the attached guide