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Depleted arc volcanism in the Alboran Sea and shoshonitic volcanism in Morocco: geochemical and isotopic constraints on Neogene tectonic processes |
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| Authors: | RCO Gill A Aparicio M El Azzouzi J Hernandez MF Thirlwall J Bourgois GF Marriner |
| Institution: | aDepartment of Geology, Royal Holloway, University of London, Egham TW20 0EX, UK bDepartamento de Volcanologia, Museo Nacional de Ciencias Naturales CSIC, José Gutierrez Abascal 2, 28006 Madrid, Spain cFaculté des Sciences, Université Mohammed-V, av. Ibn Batouta, BP1014 Rabat, Morocco dInstitut de Minéralogie et Géochimie, Université de Lausanne, CH-1015 Lausanne, Switzerland eLaboratoire Géodynamique, Tectonique et l'Environnement, Université P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France |
| Abstract: | Samples of volcanic rocks from Alborán Island, the Alboran Sea floor and from the Gourougou volcanic centre in northern Morocco have been analyzed for major and trace elements and Sr–Nd isotopes to test current theories on the tectonic geodynamic evolution of the Alboran Sea. The Alborán Island samples are low-K tholeiitic basaltic andesites whose depleted contents of HFS elements ( 0.5×N-MORB), especially Nb (0.2×N-MORB), show marked geochemical parallels with volcanics from immature intra-oceanic arcs and back-arc basins. Several of the submarine samples have similar compositions, one showing low-Ca boninite affinity. 143Nd/144Nd ratios fall in the same range as many island-arc and back-arc basin samples, whereas 87Sr/86Sr ratios (on leached samples) are somewhat more radiogenic. Our data point to active subduction taking place beneath the Alboran region in Miocene times, and imply the presence of an associated back-arc spreading centre. Our sea floor suite includes a few more evolved dacite and rhyolite samples with (87Sr/86Sr)0 up to 0.717 that probably represent varying degrees of crustal melting. The shoshonite and high-K basaltic andesite lavas from Gourougou have comparable normalized incompatible-element enrichment diagrams and Ce/Y ratios to shoshonitic volcanics from oceanic island arcs, though they have less pronounced Nb deficits. They are much less LIL- and LREE-enriched than continental arc analogues and post-collisional shoshonites from Tibet. The magmas probably originated by melting in subcontinental lithospheric mantle that had experienced negligible subduction input. Sr–Nd isotope compositions point to significant crustal contamination which appears to account for the small Nb anomalies.The unmistakable supra-subduction zone (SSZ) signature shown by our Alboran basalts and basaltic andesite samples refutes geodynamic models that attribute all Neogene volcanism in the Alboran domain to decompression melting of upwelling asthenosphere arising from convective thinning of over-thickened lithosphere. Our data support recent models in which subsidence is caused by westward rollback of an eastward-dipping subduction zone beneath the westernmost Mediterranean. Moreover, severance of the lithosphere at the edges of the rolling-back slab provides opportunities for locally melting lithospheric mantle, providing a possible explanation for the shoshonitic volcanism seen in northern Morocco and more sporadically in SE Spain. |
| Keywords: | Andesite Shoshonite Alboran Sea Rif Subduction Post-collisional |
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