Abstract
Abstract : This study is centred around the ca. 1,153 Ma Premier kimberlite pipe located on the central Kaapvaal Craton. The Premier kimberlite intruded into the rocks of the economically important Bushveld Igneous Complex. Previous studies have concluded that the Bushveld magmatic event has had a large thermochemical impact on the Kaapvaal cratonic mantle as sampled by the ca. 1,153 Ma Premier kimberlite. Through the petrological and geochemical investigation of a newly collected suite of peridotite xenoliths and clinopyroxene megacrysts from the Premier kimberlite, this study will seek to address the alleged impact of the Bushveld magmatic event on the Kaapvaal continental lithospheric mantle (CLM). Primary mineral major and minor chemical compositions were utilised in thermobarometery calculations in order to determine equilibration P-T for peridotite xenoliths. P-T estimations were used to construct a unique paleogeotherm for the Premier kimberlite. The newly defined Premier kimberlite paleogeotherm is determined to be ~42 mW/m2. The Premier kimberlite paleogeotherm (~42 mW/m2) is ‘warmer’ than geotherms from other Kaapvaal kimberlites and global kimberlites which have geotherms ranging from 38-35 mW/m2. Studies have attributed the warmer Premier geotherm to the thermal impact from the Bushveld magmatic event. However, this study has an alternative explanation for the ‘warmer’ Premier geotherm which is linked to higher mantle potential temperatures (Tp 1360, 1420 and 1500°C) at >1 Ga compared to present day mantle potential temperature (~1315°C). This study favours a warmer Tp ~1420°C which is intersected by the newly defined Premier paleogeotherm (~42 mW/m2) at depth of 205 km defining the petrological lithosphere-asthenosphere boundary (LAB) below the Kaapvaal Craton at 1.15 Ga. Results from this study have determined the width of the diamond window beneath the ca. 1,153 Ma Premier kimberlite at ~56 km. The geochemical data collected from peridotite xenoliths and megacrysts are applied into understanding if there has been any ii potential metasomatic impact by the Bushveld magmatic event on the Premier CLM. The Fe3+/ΣFe garnet compositions were used to calculate oxygen fugacity (fO2) of peridotite xenoliths to determine if there had been any oxidative metasomatism of the Kaapvaal CLM by a large magmatic event. The peridotite fO2 values range from -2.80 to -2.17 ΔlogfO2 units indicate samples originated from a reduced mantle, which is typical for Archean cratonic mantles. These reduced mantle signatures seen in peridotites from the Premier kimberlite indicates that the Kaapvaal CLM has not experienced a recent input from an oxidising event such as the Bushveld magmatic event. Clinopyroxenes in the peridotite xenoliths and megacrysts were analysed for 87Sr/86Sr and 143Nd/144Nd isotopic ratios in order to investigate the source of the Premier kimberlite. The initial 87Sr/86Sr and 143Nd/144Nd isotopic ratios (age corrected to 1,150 Ma) for the clinopyroxene in peridotites (0.703256-0.707224 and 0.511303- 0.511334 respectively) and clinopyroxene megacrysts (0.702235-0.706073 and 0.511164- 0.511345) have a genetic link to the host Premier kimberlite. The clinopyroxenes in peridotites, clinopyroxene megacrysts and the host kimberlite all originate from the same depleted mantle source possibly linked to influence by very low volume of asthenospheric melts near the lithosphere-asthenosphere boundary. When the 87Sr/86Sr and 143Nd/144Nd from the clinopyroxene in peridotites, clinopyroxene megacrysts and host Premier kimberlite are compared to the age-corrected (1,150 Ma) 87Sr/86Sr and 143Nd/144Nd whole rock results of the Rooiberg suite (Bushveld Complex) there is no overlap. These Sr-Nd values could indicate that the Bushveld magmatic event has had less of thermochemical impact on the Kaapvaal CLM than previously thought. The Sr-Nd from clinopyroxenes (peridotites and megacrysts) from the Premier kimberlite do show influence from very low volume of asthenospheric melt near the LAB. The clinopyroxene in peridotites and clinopyroxene megacrysts were analysed for δ44/40Ca and range from 0.78 to 1.0 ‰. The Ca isotopic ratios seen in clinopyroxenes from the Premier kimberlite are lighter than BSE (1.05 ‰). The lighter Ca isotopic signatures (<1.0 ‰)...
M.Sc. (Geology)