TY - JOUR

T1 - The volcanic and geochemical development of São Nicolau, Cape Verde Islands

T2 - constraints from field and 40 Ar/39 Ar evidence

AU - Duprat, Helene Inga

AU - Holm, Paul Martin

AU - Sherson, Jacob Friis

AU - Grandvuinet, Tanja

AU - Virring Sørensen, Rikke

PY - 2007

Y1 - 2007

N2 - We present 34 new age results from 40 Ar/39 Ar 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 fonn 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 demonstrating that even crystal-enriched rocks represent primitive melts, and, in general, it can be concluded that mantle melts reached the surface afler 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 cyclieity of São Nico volcanism suggests that mantle plume material arrived under São Nicolau in batches separated by nonfertile material.

AB - We present 34 new age results from 40 Ar/39 Ar 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 fonn 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 demonstrating that even crystal-enriched rocks represent primitive melts, and, in general, it can be concluded that mantle melts reached the surface afler 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 cyclieity of São Nico volcanism suggests that mantle plume material arrived under São Nicolau in batches separated by nonfertile material.

KW - Faculty of Science

KW - Cap Verde øerne

KW - geokemi

KW - Cape Verde Islands

KW - volcanic rocks

KW - 40 Ar/39 Ar dating

KW - geochemistry

KW - primitive melts

M3 - Journal article

VL - 162

SP - 1

EP - 19

JO - Journal of Volcanology and Geothermal Research

JF - Journal of Volcanology and Geothermal Research

SN - 0377-0273

ER -