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.