Journal of Petrology, Volume 40, Issue 6: June 1999.

Magmatism Associated with Orogenic Collapse of the Betic-Alboran Domain, SE Spain

S. P. TURNER¹, J. P. PLATT², R. M. M. GEORGE¹, S. P. KELLEY¹, D. G. PEARSON³ AND G. M. NOWELL³

¹DEPARTMENT OF EARTH SCIENCES, THE OPEN UNIVERSITY, WALTON HALL, MILTON KEYNES MK7 6AA, UK
²DEPARTMENT OF GEOLOGY, UNIVERSITY COLLEGE LONDON, GOWER STREET, LONDON WC1E 6BT, UK
³DEPARTMENT OF GEOLOGY, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DH1 3LE, UK

Extensional collapse of the Betic-Alboran Domain in the Miocene was accompanied by several discrete episodes of magmatism which provide insights into the relationships between the thermal and mechanical state of the lithosphere during late orogenesis. The tholeiitic Malaga dykes, emplaced at ~30-27 Ma, have flat rare earth element (REE) patterns and low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd 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 anomalies, elevated ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd, and represent increasing degrees of crustal contamination (20-50%) of rising asthenospheric magmas. The end of this phase of volcanism was marked by emplacement of crustally derived cordierite-garnet dacites suggesting very high Moho temperatures. Emplacement of alkali basalts at 10 Ma, which have smooth, convex-up, incompatible element patterns and low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd, is interpreted to reflect partial melting within the lower lithosphere. These lavas were succeeded by lamproites at 7 Ma, which have very high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd, negative Ta-Nb, Ti and Eu anomalies and extreme light REE enrichment. These formed by partial melting, in the presence of residual phlogopite, which buffered K and Nb concentrations, of shallow lithospheric mantle which had been enriched by 2-3% of a sediment component. The temporal sequence from the alkali basalts to the lamproites is interpreted to record the vertical progression of partial melting through the Iberian lithospheric mantle remaining at the margins of the Alboran Sea as convective removal propagated outwards in the orogen. The general cessation of magmatism at around 6 Ma presumably reflects thermal rethickening of the lithospheric mantle. Overall, the magmatic succession is more consistent with a geodynamic model involving convective removal of the majority of the lithospheric mantle than with models invoking wholesale delamination at the Moho, slab detachment or slab roll-back.

Keywords: Betic-Alboran Domain;magmatism;geochemistry; orogenic collapse; convective removal

Pages 1011-1036