| Summary: | Most Tethyan ophiolites in the Mediterranean region fall into two categories: 1) fragmentary sequences of late Tri-assic to early Cretaceous age; and 2) fully-developed complexes. probably of late Cretaceous age. Lavas of the first category (from Calabria, Northern and Southern Apennines, Eastern Alps and Greece) are characterized by 'immobile’ trace element ratios, such as Zr/Y, Nb/Zr and Ce/Yb, which vary from slightly lower to significantly greater then chondritic values. These ratios and the absolute abundances of the component incompatible elements are similar to or greater then those of 'typical MORB' although abundances of the compatible element, Cr, are identical. Lavas with these characteristics are usually associated with 'anomalous' areas of oceanic crust, such as the Gulf of Aden near Afar or the North Atlantic near Iceland. ‘Non-ophiolite' lavas of similar age have the composition of within plate alkali basalts. By contrast, lavas of the second category (Cyprus (Troodos Massif), Baer Bassit, Oman (Semail Nappe)) exhibit Zr/Y ratios which are significantly lower thanchondritic and similar to or lower than 'typical MORB'. Compared with 'typical MORB', such lavas contain similar or lower contents of incompatible elements, are strongly depleted in Or and contain much lower Ce/Sr ratios (even when alteration has been taken into account). Lavas with these characteristics have occasionally been recovered from back-arc spreading centres but virtually never from major oceans. 'Non-ophiolite' lavas of this age have the composition of island arc tholeiites. The results of petrogenetic modelling of ophialite lava compositions suggest that, prior to partial melting beneath Tethyan spreading centres, the mantle source regions for the two lava types had undergone different histories. Ophiolites of the first category can be related to mantle which had first been depleted in incompatible elernents relative to 'bulk earth', and had then been enriched in the same elements by migrating fluids or interstitial melts in a ...
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