School of Earth Sciences - Theses

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Subject: geochemistry × Subject: geochronology × Author: Chesler, Rachel × Type: PhD thesis ×

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    The geochemistry and geochronology of Tanzanian kimberlites
    Chesler, Rachel ( 2012)
    The Tanzanian Craton is host to numerous kimberlite intrusions, although few have been studied in any detail. Due to its proximity to the East African rift zone, this region also provides a unique opportunity to investigate the effect of rifting on kimberlite volcanism. To date, however, no geochemical studies have been conducted on Tanzanian kimberlites, largely due to their altered and weathered nature, and only four intrusions have published age constraints. This thesis seeks to redress these deficiencies by the application of modern high precision analytical techniques to the study of kimberlites from this previously neglected region. The study focused on 30 separate kimberlite intrusions located on the Tanzanian Craton. Due to their highly altered and weathered character, perovskite was used as a proxy to obtain geochemical and geochronological information on the host kimberlites. Perovskite is a common accessory mineral in kimberlites and contains substantial U, Sr and Nd and lesser amounts of Pb and Hf. Consequently, perovskite provides the means to determine both age constraints and initial isotopic signatures for the host kimberlites. Perovskite extracted from the Tanzanian kimberlites was utilised for U-Pb geochronology and analysed for trace element and Sr-Nd-Hf isotopic compositions. Similar to kimberlites worldwide, these samples show extreme enrichment in large ion lithophiles and LREE, whereas they are heavy REE depleted. They are characterised by moderately radiogenic 87Sr/86Sri compositions (0.70268 – 0.70476) and ɛSr values (-14.56 to 4.46), restricted 143Nd/144Ndi compositions (0.51138 to 0.51277) and ɛNd values (1.8 to 3.8), and very restricted 176Hf/177Hfi compositions (0.282058 to 0.282966) and ɛHf values (0.8 to 8.2). All samples plot within the range previously defined for Group I kimberlites. This study provides compelling evidence for the utility of perovskite analysis in highly altered kimberlitic samples that would otherwise be intractable to study. The ages obtained for these samples define two periods of kimberlite volcanism for the Tanzanian Craton. Early Mesoproterozoic kimberlites include the Itanana kimberlite (1083 Ma) and the previously dated Bubiki kimberlite (1097Ma). All other kimberlites for which ages were obtained are significantly younger, with ages ranging from 44-80 Ma. The latter group includes some of the youngest reliably dated kimberlites known; the Kikhomango (44 Ma) and X073 (47 Ma) intrusions. Ages from both the older and younger Tanzanian kimberlites coincide with periods of known kimberlite activity worldwide. Previous studies of the Tanzanian kimberlites have focused on the thermobarometry of garnet macrocrysts derived from disaggregated mantle xenoliths. Garnet compositions vary with proximity to the East African Rift, suggesting that the SCLM in this region was affected by rift related heating prior to kimberlite emplacement. If the source of these kimberlites was within the SCLM, systematic geographic variation in the isotopic and trace element compositions in relation to their proximity to the rift would be expected. This has not been observed in this study, with the entire region displaying no significant geographic variation in geochemical composition. Therefore the current results suggest a deeper source for the Tanzanian kimberlite magmas, below the influence of rifting in the region.