Uranium as an indicator of the origin of the Tethyan ophiolites

The distribution of uranium in minerals and rocks of the ophiolitic sequences from the Northern Apennines (Italy) and the Hellenides (Greece) of the Tethyan mountain belt has been investigated by fission-track mapping. Whole-rock U abundances, ranging from 11 to 74 ppb in ultramafic rocks, from 13 to 54 ppb in gabbroic rocks, from 73 to 281 ppb in basalts and from 279 to 900 ppb in spilites, partially reflect the post-crystallization history of the rocks. Among the fresh relics of primary rock-forming minerals, clinopyroxene has the highest U content. The estimated original U concentration of ultramafic rocks is about 1 – 10 ppb. These rocks do not appear to represent the primary undepleted mantle material. U distribution in the analyzed suites of ophiolites is consistent with a model for alpine-type ophiolites, where harzburgites and lherzolites of the basal ultramafic zone represent the residuum after extraction of oceanic tholeiitic magma. The segregated basaltic liquid either extruded rapidly as basalts or underwent magmatic differentiation in a quiet environment producing mineral accumulates as troctolites and gabbros.