The volcanic and geochemical development of Sao 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 Sao 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 Sao 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 Sao 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 Sao Nicolau volcanism suggests that mantle plume material arrived under Sao Nicolau in batches separated by nonfertile material. (C) 2007 Elsevier B.V. All rights reserved.