Browsing by Author "Fest, Helena Michaela"

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    An investigation of magma mingling and potential magma mixing during the intrusion of mafic melts into metasediments undergoing partial melting in the Dalradian series of eastern Scotland
    (Stellenbosch : Stellenbosch University, 2022-04) Fest, Helena Michaela; Stevens, Gary; Moyen, Jean-Francois; Gerdes, Axel; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: In order to understand the evolution and differentiation of the Earth, the petrogenesis of chemically evolved intermediate to felsic rocks which are characteristic of Earth’s continental crust, is of fundamental importance. The generation of granitic melts by melting of source rock, and the segregation and upwards migration of these melts are main mechanisms driving the differentiation of the crust. The frequent exposure of granitoids in close relation with mafic rocks has led many studies to discuss the involvement of mantle derived mafic melts in the petrogenesis of granitic material. Processes of mingling and mixing of magmas from different sources are popular hypothesised mechanisms to explain chemical and isotopic variabilities of co-magmatic granitoids, often inferred to occur in mid- to lower crustal zones where mantle derived mafic magmas interact with crustal derived felsic melts. However, geological exposures of partial melting zones and granitic melt production in corelation with mafic intrusions with indicative textures for mingling and mixing processes are scarce and poorly constrained. A coastal section in north-east Scotland is interpreted to expose a crustal sequence of in situ anatexis, interspersed with numerous mingled intrusions varying from mafic to felsic in composition. The magmatic rocks along the coastal section of Inzie Head show a variety of field and textural evidence indicative of magma interaction processes, ranging from interconnected vein networks, pillow-like structures, enclaves and schlieren, to gradual changes in colour and grain sizes along contact domains. In order to investigate the potential of processes such as magma mingling and mixing in the petrogenesis of granitoids, this study presents a combination of different geo- analytical techniques: field and petrographic observations of the different rock types, whole rock and mineral chemistries, combined in situ U-Pb and Lu-Hf isotopic analyses of zircon, as well as in-situ apatite and whole rock analyses of Sm- Nd and Rb-Sr isotopic signatures. Magma mixing models were formulated to investigate if additions of chosen end- members can account for the chemical and isotopic compositions observed in the hypothetical mixed rocks. Geochronological constrains suggest co-magmatic behaviour over a period of 30 myr coinciding with crustal anatexis of the metasedimentary sequence and the in situ derivation of leucosomes within the crustal domain. The intrusion of multiple mafic magma batches is suggested to have promoted crustal anatexis and maintained a partially molten state over a prolonged period in the crustal domain, promoting processes of magma interaction. The zircon Hf and apatite Sr and Nd isotopic compositions of the different rocks are characterized by a high degree of variability within the individual samples. The mixing models can account for the variable chemical and isotopic compositions of the different rocks. The study concluded that the first addition of dioritic magmas resulted in the elevation of the overall heat budget in the crustal sequence, whereby magma interaction was limited to diffusional processes along sharp contact domains. Succeeding intrusions of dioritic material into the crustal zone resulted in gradual contact domains and larger scale mixing processes shaping the exposed igneous rocks. It was furthermore concluded that mixing and hybridisation was more efficient where granitic material invaded the mafic intrusions, forming hybrid vein networks.