3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Understanding magmatic timescales and magma dynamics in Proterozoic anorthosites: a geochronological and remote sensing investigation of the Kunene Complex (Angola)(2017) Brower, Alan MartinThe 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.Item Experimental constraints on crustal contamination in Proterozoic anorthosite petrogenesis(2017) Hill, Catherine MaryMassif-type anorthosites formed in the Proterozoic Eon are the most voluminous anorthosite occurrences on Earth, reaching tens of thousands of square kilometers in aerial extent. While they formed throughout the Proterozoic, most formed during a 700 Ma period between 1800 and 1100 Ma. The rocks are dominated by plagioclase (typically 70 – 95 volume %) of intermediate composition (An40-65). Olivine, orthopyroxene, clinopyroxene and Fe-Ti oxides make up the minor mafic proportion. While most researchers agree that the anorthosites formed from a high-alumina basaltic parental magma, there are disparate views on how that parental magma was generated. Whether the parental magma formed by partial melting of the lower crust, or by mantle melting, is a topic of much debate. The anorthosites commonly have crust-like isotopic signatures, but this could be produced by melting of the lower crust, or by crustal contamination of mantle-derived magmas. Many Proterozoic anorthosite complexes consist of both olivine-bearing and orthopyroxene-bearing anorthosites. This has been attributed to variable amounts of crustal contamination of mantle-derived magmas, based on evidence from isotopes and field relations. While geochemical and petrologic evidence for crustal contamination is plentiful, existing experimental work shows that a thermal divide exists for high-alumina basalts fractionating at lower crustal depths, casting doubts on whether fractionation of a mantle melt could produce anorthosite. Here I use high-pressure experiments to test whether the fractionation of high-alumina basalt can form anorthosites, and to what extent crustal contamination affects the fractionation sequence. The results are compared to new geochemical and petrologic data from the Kunene Anorthosite Complex (KAC), in Angola and Namibia. The KAC is one of the largest anorthosite complexes in the world, with an area of ~18 000 km2. The KAC (1438 – 1319 Ma) has an elongate shape and intruded into Palaeoproterozoic to Mesoproterozoic country rocks (~2200 to 1635 Ma) at the southern margin of the Congo craton. It is associated with a suite of granitoid rocks of variable composition, which are akin to the granitoids associated with nearly all Proterozoic anorthosites. The granitoids have been shown to be coeval with the anorthosites, but were from a chemically independent magma series. The most distinctive granitoids in the KAC are the Red Granites, which outcrop around the southern margins of the complex, and also cross-cut the complex in a NE-SW linear belt, dividing the complex roughly into northern and southern domains. The rocks of the KAC are highly variable in terms of mode, mineral chemistry, and texture, but there is a general trend of more olivine-bearing anorthosites north of the granite belt, and orthopyroxene-bearing anorthosites to the south. The olivine-bearing rocks (or leucotroctolites) typically contain plagioclase and cumulus and/or intercumulus olivine, with lesser interstitial orthopyroxene and/or clinopyroxene, Fe-Ti oxides, and biotite. The orthopyroxene-bearing anorthosites (or leuconorites) contain cumulus plagioclase ± cumulus orthopyroxene, and interstitial orthopyroxene, clinopyroxene, oxides and biotite. The leucotroctolites are characterized by more calcic plagioclase (An56-75), while the leuconorites contain more intermediate plagioclase (An48-56). The variability of the rocks across the complex suggests that the KAC consists of several coalesced plutons with different histories. The petrologic data and field observations in this study are consistent with the leuconorites of the complex being derived from a mantle-derived magma that experienced contamination by silica-rich rocks, crystallizing orthopyroxene rather than olivine, and less calcic plagioclase. The leucotroctolites experienced less or no contamination. To test whether the mineral dichotomy and the variations in plagioclase chemistry observed in Proterozoic anorthosites are due to variably contaminated mantle-derived magma, piston cylinder experiments were conducted on a synthetic high-alumina basalt (HAB) composition, as well as a mixture of this HAB with 30% of a Red Granite composition. Experiments were conducted at 10 kbar, to simulate the depth at which anorthosite differentiation most likely begins (based on Al-in-orthopyroxene geobarometry of highly aluminous orthopyroxene megacrysts that occur in many massifs). The uncontaminated experiments produced olivine as the first liquidus phase, followed by plagioclase (An65-68), and then by clinopyroxene, pigeonite and ilmenite at progressively lower temperatures. Residual liquids evolve towards more silica-rich compositions with decreasing temperature. The contamination experiments produced liquidus orthopyroxene, followed by plagioclase (An51-56), and then by pigeonite at lower temperatures. The experiments show that contamination of a primitive HAB magma by granitic material, most likely produced by partial melting of the lower crust during anorthosite formation, can shift the mineral assemblages of the crystallizing anorthosite from olivinebearing to orthopyroxene-bearing, and produce less calcic plagioclase than the uncontaminated HAB magma. This could explain the observation of olivine-bearing and orthopyroxene-bearing anorthosites in the KAC and many other Proterozoic anorthosites. Previous high-pressure experimental studies, using a slightly more evolved HAB composition, indicated the presence of a thermal divide, which causes liquids to evolve to more Si-poor compositions. The experimental results presented in this study however, do not show a thermal divide, indicating that small variations in experimental starting composition can cause large differences in the liquid line of descent. The results of this study indicate that partial melting of the mantle can produce anorthosite parental magmas, and that the range in mineral assemblages of the anorthosites can be accounted for by crustal contamination of a mantle-derived magma. Fractionation of the experimental starting compositions was also modeled using the MELTS algorithm. These calculations produce a close match to the experimental liquid trends. This allows for modeling of a variety of compositional and environmental variables. The MELTS modeling shows that as little as 10% contamination of HAB magma with a granitic composition may position the magma in the orthopyroxene stability field, forming orthopyroxene-bearing anorthosites. The modeling also shows that a variety of silica-rich contaminants, including granites, granodiorites and tonalities, produce similar results and liquid evolution trends, so a range of granitoid compositions may successfully produce the shift in mineral assemblages of the anorthosites. This suggests that crustal contamination of mantle-derived HAB could be a widespread process and the primary mechanism that produces the distinctive crust-like signatures in Proterozoic anorthosites. In summary, the mineralogical and chemical diversity observed in Proterozoic anorthosites can be produced by variable amounts of crustal contamination of mantle-derived, highalumina basaltic magma. The experimental results in this study combined with field observations, and geochemical and isotopic data, provide evidence for a model of massif-type anorthosite petrogenesis. Orthopyroxene-bearing rocks formed from an originally highalumina basaltic magma that experienced contamination by granitic partial melts of the lower crust, during ponding of the magma at the Moho. This process preconditioned the surrounding crust and possibly prevented further anatexis. Following emplacement of orthopyroxene-bearing anorthosites, subsequent magma pulses ponded at the Moho did not assimilate any/as much granitic material, as they were interacting with preconditioned crust, and formed olivine-bearing anorthosites. With better constraints on the parental magma composition, magma source, and crustal contamination processes, addressing aspects such as the tectonic setting and emplacement mechanisms of these massive intrusions should be prioritized. Understanding these enigmatic aspects of anorthosite petrogenesis is leading the anorthosite community towards answering the ultimate questions of why massif-type anorthosites are restricted to the Proterozoic.