A MORB source for low-Ti magmatism in the Semail ophiolite

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 Sr-87/Sr-86 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. (c) 2006 Elsevier B.V. All rights reserved.