High-grade metamorphism in Turkana, Kenya: implications for the evolution of the East African Orogen
Date
2023
Authors
McIntyre, A.
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Abstract
The Lokhone Block metasedimentary belt in Turkana, NE Kenya, lies within an understudied region of the Mozambique Belt on the margin of the Congo-Tanzania craton. Phase equilibria modelling indicates that this high-grade belt comprising intercalated pelitic metasediments and calc-silicate rocks underwent peak granulite-facies metamorphism at pressures of 8.4– 10.6 kbar and temperatures of 750–830°C. Peak metamorphic conditions have been directly dated by Lu – Hf isotopic analysis of garnet (625 ± 7 Ma and 621 ± 5 Ma), and by U – Pb isotopic analysis of metamorphic titanite (604 ± 8 Ma and 622 ± 16 Ma) and metamorphic zircon rims (612 ± 14 Ma). This is coeval with high-grade metamorphism recorded throughout the Mozambique Belt associated with the formation of the East African Orogen during the Neoproterozoic amalgamation of Gondwana. P-T conditions indicate a moderate geothermal gradient of 78 – 98°C/kbar (17-30°C/km), consistent with metamorphism during crustal thickening. U – Pb apatite geochronology yields ages between ca. 540 and ca. 560 Ma, interpreted as post-peak cooling after metamorphism. These data suggest slow cooling of 2.5 - 9.5°C/Myr, consistent with slow exhumation over several tens of million years. U – Pb dating of detrital zircons suggest that at the time of formation, the Lokhone region was not connected to any pre-Neoproterozoic terrane. These findings highlight the complexity of the processes shaping the East African Orogen and provide important constraints with which to improve plate tectonic reconstructions for the Neoproterozoic Earth.
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School of Physical Sciences
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Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, YEAR
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