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Browsing by Subject "oceanic crust"

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  • Maunu, Liisa (2023)
    Macquarie Island is a subaerial fraction of oceanic crust where lithology from mantle peridotites to crustal gabbro, dolerite, and extrusive rocks are present, thus providing a unique opportunity to study geochemistry and petrology of the oceanic crust. Macquarie Island represents a mid-ocean crust ophiolite in which the potential geochemical modification of continental crust and effects of subduction initiation are absent. A genetic link between plutonic and extrusive rocks and processes leading to formation of the ophiolite sequence were studied in this thesis. A set of samples representing different rock types of the oceanic crust were studied petrographically as well as for whole-rock major and trace element geochemistry. Selected samples were studied for chromian spinel, silicates, and apatite major and minor component geochemistry. Harzburgites of Macquarie Island are depleted in trace element composition and are not a straightforward residue for the source of the crustal section of the island. Melt infiltration of basaltic melt into potentially former lherzolitic mantle source has been dominating process leading to formation of Macquarie Island oceanic crust. As a consequence to melt infiltration plagioclase-bearing wehrlites recrystallized and these rocks probably acted as a more enriched source for crustal rocks.Major and trace element data show that fractional crystallization has not been significant process forming the island. Extrusive basalts are more primitive in MgO and SiO2 contents and more enriched in REE contents compared to gabbroic rocks of the island. This could be explained by porous fluids migrating through oceanic crust modifying both major and trace element compositions of the samples or extrusive and gabbroic rocks forming from different mantle sources. This study shows that formation of oceanic crust is much more complex than often assumed simple model of fractional crystallization from mantle melt leading to formation of the oceanic crust, or a hypothesized gabbro-dolerite-basalt plumbing system forming the genetically linked crustal rocks.