A volcanological and geochemical investigation of Boa Vista,Cape Verde Islands; 40Ar/39Ar geochronology and field constraints |
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| Authors: | Charlotte T Dyhr Paul M Holm |
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| Institution: | 1. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. Center for Global Tectonics, China University of Geosciences, Wuhan 430074 China;3. Institute of Geological Survey, China University of Geosciences, Wuhan 430074, China;4. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;1. Departamento de Geodinámica y Paleontología, Facultad de Ciencias Experimentales, Universidad de Huelva, Campus de El Carmen, Avda. 3 de Marzo, s/n, 21071 Huelva, Spain;2. Facultad de Ciencias Marinas, Universidad Autónoma de Baja California, Ensenada, BC 22800, Mexico;3. Department of Geosciences, Williams College, Williamstown, MA 01267, USA;4. Institut für Geophysik, Westphälishe-Wilhelms Universität, Corrensstraße 24, 48149 Münster, Germany;5. Faculdade de Ciências da Universidade de Lisboa, Departamento de Geologia e Centro de Geologia, Campo Grande, 1749-016 Lisboa, Portugal;1. Marine and Coastal Survey Division, Geological Survey of India, Cochin 682037, India;2. Center for Tectonics, Resources and Exploration, Department of Earth Sciences, University of Adelaide, Australia;3. School of Earth Science and Resources, China University of Geosciences Beijing, 100083, China;4. Geochemistry Division, CSIR-National Geophysical Research Institute, Hyderabad, 500007, India |
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| Abstract: | Boa Vista, the easternmost island in the Cape Verde archipelago, consists of volcanic products, minor intrusions and a thin partial sedimentary cover. The first 15 age results from 40Ar/39Ar incremental heating analysis of groundmass separates from volcanic and plutonic rocks from Boa Vista are presented. The combination of age results and field observations demonstrates that the volcanic activity that formed the island occurred in three main stages: (1) > 16 Ma, (2) 15–12.5 Ma and (3) 9.5–4.5 Ma. The first stage, restricted to the north eastern part of the island, is dominated by ankaramitic lavas. The second stage, consisting of evolved lavas of phonolitic–trachytic compositions and nepheline syenites, makes up large central parts of the island. The large volume of evolved rocks and the extended eruption period of several Ma make stage 2 in Boa Vista unique to Cape Verde. Mainly basanites and nephelinites were erupted during the third stage, initially dominated by eruption of subaerial mafic lavas around 9 Ma. Pillow lavas are erupted around 7 Ma whereupon dominantly subaerial mafic lavas were erupted. Stage 3 saw volcanism in many centres distributed mainly along the present coastline and with activity partly overlapping in time. The volcanic evolution of Boa Vista constrains the initiation of volcanic activity in the Cape Verde archipelago to the eastern islands. Major and trace element geochemistry of 160 volcanic and plutonic rocks representing the entire exposed chronological sequence on Boa Vista is presented, revealing an extremely well developed Daly Gap. Only a little was modified from the mafic magmas that rose in small batches from the mantle. Compositional variation distinguishes each volcanic complex and was to a large extent present in the mantle melts. The highly evolved stage 2 phonolites and trachytes are related through the fractional crystallization of three compositionally distinct magmas. Two of these may have been derived by crystal fractionation of primitive Boa Vista melts, whereas the third was not. |
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