Trace elements and Li isotope systematics in Zabargad peridotites: evidence of ancient subduction processes in the Red Sea mantle

Mantle peridotites exposed on Zabargad Island in the Red Sea (Egypt) record a range of features, most of which have been attributed to recent rifting events associated with the opening of this proto-oceanic basin. However, both the trace element data and Li isotope systematics presented here suggest an alternative hypothesis for some of these features, implicating them in the more ancient subduction zone processes that operated in the region during the Pan-African orogeny (700-600 Ma). High Li contents and other trace element characteristics of the exceptionally fresh spinel-lherzolites show they have been metasomatised by a hydrous fluid, but measured delta(7)Li values of around +4.9 are within the range reported for fresh MORB (+1.5 to +6.5) and OIB (+2.5 to +5.8). This suggests that these lherzolites and the metasomatic agent introducing the elevated Li contents are similar to typical upper mantle in terms of Li isotopes. In contrast, extensive modal metasomatism of these lherzolites to give as much as 20% of green pargasitic amphibole, is associated with the influx of melt or fluid with a relatively heavy delta(7)Li signature. As such heavy values are typical of the low-temperature weathering processes observed in altered oceanic crust (AOC), we propose that this metasomatic event involves the release of Li from a dehydrating, subducted slab, consistent with the tectonic setting during the Pan-African. Pyroxenite veins that cross-cut the peridotites and a lower crustal meta-basalt in tectonic contact not only display a similar, heavy AOC Li component, but also the trace element patterns characteristic of a slab release fluid, as observed in modem island arc basalts (IAB). In contrast, one of the IAB signature pyroxenites has a negative delta(7)Li value (-4.1%) that would require a different fluid source. We speculate that this could represent a later stage of fluid release, subsequent to the earlier dehydration processes that have preferentially removed Li-7 from the AOC. This study appears to indicate the presence of AOC signatures in a paleo-subduction zone mantle wedge, a feature that is predicted, but remains elusive in modem basalts collected from arc environments. (C) 2004 Elsevier B.V. All rights reserved.