Magmatism associated with orogenic collapse of the Betic-Alboran Domain, SE Spain

Extensional collapse of the Betic-Alboran Domain in the Miocene was accompa nied by several discrete episodes of magmatism which provide insights into the relationships between the thermal and mechanical state of the lithosphe re during late orogenesis. The tholeiitic Malaga dykes, emplaced at similar to 30-27 Ma, have flat rare earth element (REE) patterns and low Sr-87/Sr- 86 and high Nd-143/Nd-144 indicative of shallow decompression melting (5-15 %) within the asthenosphere following removal of lithospheric mantle. Calc- alkaline magmas, erupted between 15 and 6 Ma, have negative Ta-Nb and Ti an omalies, elevated Sr-87/Sr-86 and low Nd-143/Nd-144, and represent increasi ng degrees of crustal contamination (20-50%) of rising asthenospheric magma s. The end of this phase of volcanism was marked by emplacement of crustall y derived cordierite-garnet dacites suggesting very high Moho temperatures. Emplacement of alkali basalts at 10 Ma, which have smooth, convex-up, inco mpatible element patterns and low Sr-87/Sr-86 and high Nd-143/Nd-144, is in terpreted to reflect partial melting within the lower lithosphere. These la vas were succeeded by lamproites at 7 Ma, which have very high Sr-87/Sr-86 and low Nd-143/Nd-144, negative Ta-Nb, Ti and Eu anomalies and extreme ligh t REE enrichment. These formed by partial melting, in the presence of resid ual phlogopite, which buffered K and Nb concentrations, of shallow lithosph eric mantle which had been enriched by 2-3% of a sediment component. The te mporal sequence from the alkali basalts to the lamproites is interpreted to record the vertical progression of partial melting through the Iberian lit hospheric mantle remaining at the margins of the Alboran Sea as convective removal propagated outwards in the orogen. The general cessation of magmati sm at around 6 Ma presumably reflects thermal rethickening of the lithosphe ric mantle. Overall, the magmatic succession is more consistent with a geod ynamic model involving convective removal of the majority of the lithospher ic mantle than with models invoking wholesale delamination at the Moho, sla b detachment or slab roll-back.