Ar-39-Ar-40 ages and geochemistry of the basaltic shield stage of Tenerife, Canary Islands, Spain

We report the first Ar-39-Ar-40 ages from the three early basic shield-like massifs of Tenerife, Canary islands, and couple these with detailed major and trace element chemistry to constrain the nature and timing of the mantl e melting processes. The massifs have chemically different sources, and ind ependent evolutionary histories. The Teno and Rogue del Conde massifs appea r chemically to represent the products of single mantle melting cycles, wit h progressive decrease in mean melt fraction and increase in mean melting d epth in younger rocks. The Teno massif(NW) was erupted in a short time peri od around 6.0-6.4 Ma, while at least the lower half of the Rogue del Conde massif (SW) is older than 11 Ma. In contrast, the Anaga massif (NE) is poly genetic, with Ar-39-Ar-40 ages ranging from 8.0-4.2 Ma. and no simple strat igraphic chemical progression. These ages run counter to published suggesti ons of progressive younging of Canary shield stages to the southwest. Basic rocks in all three massifs are the result of much deeper melting and small er melt fractions than equivalent units in Gran Canaria, bur nevertheless t he melting column must have extended significantly into the spinel facies, requiring substantial disruption of the local lithosphere. The age and melt ing relationships broadly support the mantle blob model for Canary magmatis m proposed by Hoernle and Schmincke (Hoernle, K., Schminke, H.-U., 1993. Th e role of partial melting in the 15-Ma geochemical evolution of Gran Canari a: a blob model for the Canary hotspot. J. Petrol. 34, 599-626). In all thr ee massifs, extensive fractional crystallisation has taken place at crustal levels so that mean MgO contents are only some 6-7%. The fractionation seq uence is olivine-clinopyroxene-magnetite in basaltic compositions, with the involvement of plagioclase. amphibole and apatite only to generate the inf requent more evolved hawaiites to benmoreites. Despite the abundance of bas anitic magmas in the Tenerife older massifs, these follow a differentiation trend towards weakly undersaturated benmoreite rather than to phonolite. T his probably reflects early crystallisation of magnetite, perhaps resulting from somewhat high oxygen fugacity. The chemical evidence for replenished magma chambers in Tenerife described by Neumann et al. (Neumann, E.R., Wulf f-Oedersen, E,, Simonsen, S.L., Pearson, N.J., Marti, J,, Mitjavila, J., 19 99. Evidence for fractional crystallisation of periodically refilled magma chambers in Tenerife, Canary Islands. J. Petrol. 40, 1089-1123) is a conseq uence of treating as a single cogenetic suite the products of several magma tic systems that differ in parental melt fraction. (C) 2000 Elsevier Scienc e B.V. All rights reserved.