The volcanic and geochemical development of São Nicolau, Cape Verde Islands: Constraints from field and 40Ar/ 39Ar evidence

We present 34 new age results from 40Ar/ 39Ar incremental heating analyses of groundmass separates from volcanic rocks from São Nicolau, Cape Verde. Combining the age results with field observations, we show that the volcanic activity that formed the island occurred in four separate stages: 1: >...

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Bibliographic Details
Published in:	Journal of volcanology and geothermal research Vol. 162; no. 1; pp. 1 - 19
Main Authors:	Duprat, Helene Inga, Friis, Jacob, Holm, Paul Martin, Grandvuinet, Tanja, Sørensen, Rikke Virring
Format:	Journal Article
Language:	English
Published:	Lausanne Elsevier B.V 01-04-2007 Amsterdam Elsevier New York, NY
Subjects:	40Ar/ 39Ar dating Cape Verde Islands Crystalline rocks Earth sciences Earth, ocean, space Exact sciences and technology geochemistry Geochronology Igneous and metamorphic rocks petrology, volcanic processes, magmas Isotope geochemistry. Geochronology primitive melts volcanic rocks Cape Verde Islands geochemistry Cape Verde Islands volcanic rocks 40Ar/ 39Ar dating primitive melts plumes phonolites alkali olivine basalt alkali basalts olivine Atlantic Ocean Islands basanite magma chambers volcanism hyaloclastites lava cyclic processes nepheline basalt basalts mantle origin silicates dating electron probe data nesosilicates 40Ar/39Ar dating igneous rocks magmas chemical analysis pyroclastics volcanic breccia Ar-40/Ar-39 nephelinite
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Summary:	We present 34 new age results from 40Ar/ 39Ar incremental heating analyses of groundmass separates from volcanic rocks from São Nicolau, Cape Verde. Combining the age results with field observations, we show that the volcanic activity that formed the island occurred in four separate stages: 1: > 6.2–5.7 Ma, 2: 4.7–2.6 Ma, 3: 1.7–1 Ma and 4: < 100 ka. The first stage is dominated by marine hyaloclastite breccias, whereupon several generations of dominantly subaerial lavas form the younger stages. In stage 2, far the greatest volume of lavas was extruded. The major and trace element geochemistry of 207 volcanic rocks representing all volcanic activity stages is presented. The rocks are alkaline and mostly primitive (MgO > 8 wt.%) basic to ultrabasic ranging from nephelinites through basanites to picrobasalts. Evolved rocks range to phonolites. During all four volcanic stages predominantly high MgO rocks were erupted with 8–19 wt.% MgO. Olivine microprobe data from eight highly porphyritic of these rocks had Fo 86–88 demonstrating that even crystal-enriched rocks represent primitive melts, and, in general, it can be concluded that mantle melts reached the surface after only little modification during much of the development of São Nicolau. Only during stage 2 magma chambers were established and highly developed magmas erupted. At this time the major part of the island was formed. The new results constrain the evolution of São Nicolau to have taken place during a later time interval than previously assumed. With more ages of only around 50 ka the youngest volcanic rocks found on the island indicate that stage 4 may still be active. The cyclicity of São Nicolau volcanism suggests that mantle plume material arrived under São Nicolau in batches separated by nonfertile material.
ISSN:	0377-0273 1872-6097
DOI:	10.1016/j.jvolgeores.2007.01.001