Magmatic and volcanological evolution of the Desertas rift zone (Madeira Archipelago, NE Atlantic)

Abstract

Madeira and the Desertas Islands show two well-developed volcanic rift zones which intersect near the eastern tip of Madeira (São Lourenço peninsula). To examine a possible genetic link, volcanic structures, rock ages and rock compositions were investigated at the Desertas and São Lourenço. Barometric data derived from fluid inclusions and clinopyroxene-melt compositions indicate fractionation at multiple levels within the mantle beneath the rift zones, followed by temporary magma stagnation within the crust. Depths of crustal stagnation beneath São Lourenço and the Desertas differ significantly, and there is no evidence for a shallow magma reservoir feeding both rifts. This suggests that Madeira and the Desertas have separated magma supply systems. 40Ar/39Ar age determinations revealed that Desertas volcanism lasted from >5.1 to 1.9 Ma overlapping in age with the shield phase of Madeira. Eruptive quiescence between 4 and 2.5 Ma at São Lourenço coincides with the main period of volcanism on the Desertas. Major and trace elements and Sr-Nd-Pb isotope compositions of volcanics from São Lourenço and the Desertas overlap with those from Madeira, indicating a common magma source. São Lourenço samples include the most depleted Sr-Nd-Pb isotope ratios, whereas Desertas rocks of similar age show the most enriched ratios, not being compatible with mixing of Desertas and Madeira melts or with lateral magma transport from Madeira to the Desertas. Madeira and the Desertas rather represent two separate volcanoes that root in discrete sectors of the Madeira source. The Desertas volcanism was caused by flexural stresses within the lithosphere due to the loads of Madeira and Porto Santo, and an extensive stress field in-between the overlapping Madeira and Desertas edifices provoked the formation of the Desertas rift. The shift of volcanic activity between Madeira and the Desertas possibly reflects an irregularly shaped plume causing lateral variations in melt production.