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

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 87Sr/ 86Sr 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.