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Geochemical evidence for the genesis and eruptive setting of lavas from Tethyan ophiolites

Pearce, Julian A. 1980. Geochemical evidence for the genesis and eruptive setting of lavas from Tethyan ophiolites. Proceedings of the International Ophiolite Symposium, Cyprus 1979 , pp. 261-272.

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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 'within-plate' setting. Ophiolites of the second category can be related to mantle which had first been depleted in incompatible elements, but had then been enriched by aqueous fluids and hydrophilic elements driven off subducted oceanic lithosphore in an arc-basin setting. The modelling also requires that ]eyes derived from the latter as compared with the former source were produced by a greater degree of partial melting followed by a greater proportion of olivine crystallization from the segregated magma. These conclusions are consistent with: 1)formation of the older ophiolites in small Red-Sea-type basins related to fragmentation of continental crust during initial opening of the Atlantic; and 2) formation of the younger ophiolites in marginal basins created at a time of major changes in the relative movement of the African and Eurasian plates.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Publisher: Ministry of Agriculture and Natural Resources, Cyprus
Last Modified: 04 Jun 2017 02:05

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