School of Earth Sciences - Theses
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ItemThe petrogenesis of Brazilian kimberlites and kamafugites intruded along the 125° lineament: improved geochemical and geochronological constraints on magmatism in Rondonia and the Alto Paranaiba Igneous Province( 2014)Despite a number of studies, the origin of ultrapotassic magmatism in Brazil remains contentious. In particular, problems relate to the timing of Cretaceous kimberlite and kamafugite magmatism within the Alto Paranaiba Igneous Province (APIP), the origin and relationship between the Cretaceous kimberlites and kamafugites, and the geochemical nature of the Permo-Triassic kimberlite magmatism. Many of these uncertainties arise as a consequence of either the implementation of now superseded analytical techniques or the use of highly altered samples. This study seeks to address these issues by performing state-of-the-art geochemical and geochronological analyses on a number of fresh kimberlite and kamafugite drill core samples situated along the 125° lineament. The first part of the thesis provides a comprehensive geochronological study of Brazilian kimberlites and kamafugites. This is built around a combination of the most recent U/Pb perovskite, Rb-Sr phlogopite, and Ar-Ar phlogopite dating techniques that together provide 33 age estimates, representing 22 separate intrusions, of which 11 are kimberlites and 11 kamafugites. The results are in broad agreement with previously published data, identifying a Permo-Triassic age for the Amazonian kimberlites, and kimberlites and kamafugites of Cretaceous age in the APIP. The new data from this study also support an age decrease from NW to SE within the APIP. However, data from this study suggest that this is not a continuous younging, instead indicating that two distinct magmatic episodes may exist. The data also reveal the presence of an “older” mid Cretaceous cluster (~88 Ma) encompassing Goias and the NW APIP (Southern Goiás and NW Minas Gerais) and a slightly younger Cretaceous cluster (~80 Ma) in the Central/Southern APIP. The importance of these new findings is discussed with reference to previously proposed petrogenetic models and casts doubt on the applicability of the plume-related hypothesis for ultrapossic magmatism in the region. In the second part of the thesis, an in-depth geochemical study of 15 kimberlites and 16 kamafugites from the APIP (27 samples) and Rondonia/Mato Grosso (4 samples) is presented. Bulk rock major and trace element data, together with Nd and Hf isotope determinations are combined with perovskite trace element, Sr and Nd isotope analyses in order to provide a comprehensive geochemical dataset. The elemental and isotopic similarity of the APIP kimberlite to South African transitional kimberlites, the APIP kamafugites to Ugandan and Chinese kamafugites and, for the first time, the Amazonian kimberlites to a mixture of South African Group I and transitional kimberlites is established. The overlapping range in Sr isotopic signatures identified between the APIP kimberlite and kamafugite magmas during previous bulk rock studies is confirmed by in situ perovskite analysis, with APIP kimberlites ranging between 70496 and 0.70596 and APIP kamafugites ranging between 0.70499 and 0.70574. Modelling suggests that this range can not result from contamination and must represent a heterogeneous source component, common in the formation of both rock types. It is argued that the combination of geochemical results is best explained by derivation of both rock types from the heterogeneous subcontinental lithospheric mantle. Kimberlites are believed to have been sourced from carbonated garnet lherzolite whereas the kamafugites were generated from phlogopite and clinopyroxene rich vein assemblages, with small volume MARID type metasomes indicated by the kamafugite Hf signatures. Finally, the geochronological and geochemical information is combined into a petrogenetic model: it is argued that the Cretaceous magmatism is likely not the result of plume heating but is instead a consequence of adiabatic decompression melting of the heterogeneous SCLM caused by extension. In parallel with the study of Brazilian ultrapotassic magmatism the thesis also investigates technical aspects of the U-Pb perovskite dating technique. Using a subset of Indian and Brazilian kimberlite samples, in situ perovskite dating was performed and combined with isotope dilution analysis of the co-magmatic, low uranium phase, titanomagnetite, in an attempt to improve precision of the Tera-Wasserburg regression. The inclusion of the titanomagnetite phase improved precision in all cases and enabled ages to be determined for the Indian samples without recourse to an assumed Stacey-Kramers common Pb value. The investigation also highlights some of the shortcomings associated with using an assumed Stacey-Kramers Pb value for young unradiogenic perovskite samples, whilst also highlighting the circularity of the commonly employed 207Pb correction method.