A MORB source for low-Ti magmatism in the Semail ophiolite Marguerite Godard ⁎ , Delphine Bosch, Florence Einaudi CNRS-UMR 5568 Laboratoire de Tectonophysique, ISTEEM, Université Montpellier 2, case courrier 49, Place E. Bataillon, 34095 Montpellier cedex 5, France Received 5 July 2005; received in revised form 10 March 2006; accepted 1 April 2006 Abstract We present the result of a trace element and Nd, Sr and Pb isotope study of a tholeiitic volcanic section in the Salahi massif, carried out in order to characterize the evolution from MOR-type (Geotimes) to the earliest stages of low-Ti arc-like (Lasail) magmatism in the Semail ophiolite. Because of the alteration of the extrusive section, as shown by the high 87 Sr/ 86 Sr ratios of the studied lavas, the two magmatic episodes have been characterized using trace elements considered to be immobile during alteration processes. Geotimes lavas have trace element compositions similar to N-MORB and Lasail lavas have lower trace element contents, light-rare-earth-element depleted patterns and low Zr/Hf ratios relative to Geotimes. Both magmatic units are characterized by a slight negative anomaly in Nb and Ta relative to Th and LREE on N-MORB normalized patterns. Similar to N-MORB, decompression melting of a convecting mantle can explain Geotimes chemical compositions with the exception of the slight Nb–Ta depletion. Further melting of the Geotimes mantle source and melt–rock interaction with shallow mantle, probably associated to intra-oceanic thrusting, accounts for the depleted trace element compositions of Lasail. Geotimes and Lasail highly incompatible element compositions overlap that of both MORB and back-arc lavas. Yet, Nd and Pb isotope data support a common mantle source for the two magmatic episodes and a Depleted MORB Mantle composition for their source reservoir. These compositions are consistent with the Semail ophiolite being formed either in a large back-arc basin, similar to present day North Fiji Basin, or at a mid-ocean ridge above an “abnormal mantle”, which has preserved older slab-derived signatures, the same as that sampled today by Indian Ocean MORB. © 2006 Elsevier B.V. All rights reserved. Keywords: Semail ophiolite; ICP-MS; Nd–Sr–Pb isotopes; Extrusive sequence 1. Introduction The Semail ophiolite samples a sliver of Tethyan oceanic lithosphere obducted onto the Arabian platform at the end of the Cretaceous. Structural and thermal evidence suggests that the obduction was initiated close to an active spreading centre (Coleman and Hopson, 1981; Nicolas et al., 1988; Ishikawa et al., 2002); the ophiolite was little deformed during and after emplace- ment and preserves the structure of the spreading centre from the mantle to the top of volcanic units (see Nicolas et al. (2000)). For this reason, it has been widely used as an analogue for understanding present day ridge processes and it is now one of the best-documented ophiolites, after the systematic work of several research groups (see, for example, Boudier and Juteau (2000) and Kelemen et al. (2004) for a compilation of the most Chemical Geology 234 (2006) 58 – 78 www.elsevier.com/locate/chemgeo ⁎ Corresponding author. Tel.: +33 4 67 14 39 37; fax: +33 4 67 14 36 03. E-mail address: margot@dstu.univ-montp2.fr (M. Godard). 0009-2541/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2006.04.005