Understanding Magmatic Timescales and Magma Dynamics in Proterozoic Anorthosites: a Geochronological and Remote Sensing Investigation of the Kunene Complex (Angola)

• Main Author: Brower, Alan Martin
• Format: Others
• Language: en
• Published: 2018
• Online Access: http://hdl.handle.net/10539/23653

A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, in fulfillment of the requirements for the degree of Master of Science, 2017 === The Kunene Anorthosite Complex (KAC), located in southwest Angola, is one of the largest Proterozoic anorthosite intrusions on Earth, with an areal extent of at least 18 000 km2. The KAC is composed of anorthosite, leucotroctolite, leuconorite, leucogabbro and granitoids. Many aspects of Proterozoic anorthosite petrogenesis are still unknown with debates including the parental magma source, temporal restriction of anorthosite, and anorthositic magma emplacement mechanisms. Very little research has been conducted on the Angolan portion of the KAC, and published maps lack detail and are often inconsistent. Previous studies considered the Kunene Complex to be have formed as a layered intrusion and, more recently, as a series of coalesced plutons. As one of the largest and least studied Proterozoic anorthosites in the world, the KAC provides a unique opportunity to test recent ideas surrounding Proterozoic anorthosite petrogenesis related to the KAC. Its linear geometry, make-up of multiple plutons and large age range also create a great desire for study. This study allows for the development of new models for the emplacement dynamics, timescales, and tectonic setting of the KAC. This study makes use of the interpretation of remote sensing datasets (Landsat 8 and SRTM 3 – Shuttle Radar Topography Mission) as well as U-Pb TIMS (Thermal Ionisation Mass Spectrometer) geochronology to analyse the composition, structure and age variations of the KAC. In order to extract maximum compositional data from this magmatic complex, various image processing techniques have been performed, including false colour composites, a minimum noise fraction, a principle component analysis, and band ratioing for the Landsat 8 data. To best identify structural data, hill-shading and an automatic lineament extraction was used for the SRTM and Principle Component 1 images. The results of the Landsat Image processing enable identification of different spectral signals and allow for the differentiation of the KAC from country rocks, in addition to separating the anorthositic rocks of the KAC itself. From the SRTM imaging, lineament data were extracted and various structural features identified throughout the KAC. In combination with ground truthing, these lineaments are classified into either magmatic foliations, subsolidus planar structures or fault structures. Using these techniques, this study reiterates the batholitic appearance of the KAC and identifies two main magmatic entities of distinct crystallization age, composition and Landsat spectral response, making up the KAC. In combination with ground truthing, a new interpretative lithological map for the KAC and adjacent country rocks has been produced. Understanding the relative timing of the anorthosite emplacement is crucial for understanding how these enigmatic magmas form and how they rise through the crust. The ages and relative emplacement sequence of the individual batches forming the KAC are unknown. New high precision U-Pb ID-TIMS ages on zircon and baddeleyite for many of the newly defined spectral domains across the anorthositic complex are presented. These new geochronological results reveal subtle variations in crystallisation age within the KAC on the order of 10 Ma. There is no gradual age progression between potential distinct magmatic batches but distinct groupings of ages. Mean age clusters of 1379.8 ± 2.0 Ma (n=5) occurring to the north of the NE – SW striking Red Granite intrusions, whereas in the south there is an older age grouping of 1390.4 ± 2.3 (n=3). Two additional ages of 1400.5 ± 1.3 Ma and 1438.4 ± 1.1 Ma have been obtained in the centre and southeast of the complex, respectively. These results indicate that the Kunene anorthosites were emplaced over 60 Ma. The 40 Ma difference between emplacement of the first anorthositic body and the remaining anorthositic emplacement suggests two possibilities for the long-lived magmatic system: 1) Magma differentiation occurred slowly over an extended period of time with anorthositic mushes reaching their final emplacement depth at a faster rate. 2) Differentiation occurred at a faster rate but the mushes ascended slowly to their final emplacement depths. A link has been found between spectral domain composition and age. In general, leuconoritic domains are older than the leucotroctolitic domains. This may imply that the first pulses of magma received a greater degree of crustal contamination, forcing the initial broadly basaltic magma to produce orthopyroxene as the main mafic phase. The later pulses received less contamination as they ascended through the already partially melted and refractory crust, producing olivine as the mafic phase and deforming the older domains. This study reiterates the multiphase petrogenesis of Proterozoic anorthosites and sheds light on the assembly of crystal-rich magmas as they ascend through the crust. Utilizing the remote sensing data and the geochronological results, a new model for the petrogenesis of the KAC been developed and it is suggested that the most valid setting for the KAC is a continental arc. === XL2018