B and O isotopes as tracers of serpentinization along fossil oceanic detachments, Troodos ophiolite, Cyprus

Serpentine lubricated detachment faults strike parallel to two segments of a paleo spreading center that are separated by the Arakapas transform in the Troodos ophiolite, Cyprus. To the north of the transform, serpentinite faulted against gabbro shows bimodal spatial distribution and covariance of B-O isotope ratios. These data indicate overprinting of fault localized, 'high temperature' oceanic serpentinization (delta (super 18) O=4 to 6ppm; delta (super 11) B=-3 to 3ppm) by widespread late hydration at lower temperatures, forming abundant chrysotile veins (delta (super 18) O=10 to 12ppm; delta (super 11) B=7 to 13ppm). At the Limassol Forest complex, south of the transform, extensive talc-amphibole-chlorite metasomatic zones and rodingitized gabbro boudins occur within strongly foliated serpentinite shear-zones separating an ultramafic section from sheeted dykes. d (super 18) O values of serpentine from shear-zones in the Limassol Forest have a narrow range and are invariably lower than mantle values (1 to 5.7 ppm; n=26), consistent with serpentinization during seafloor spreading. delta (super 11) B (Srp) values are more scattered (5 to 27 ppm) and weakly correlate with boron contents (2 to 60 ppm), which might be accounted for by increase in pH of water as serpentinization progressed. Absence of the lower crustal section above the mantle and injection of gabbroic magma followed by localized serpentinization, metasomatism and deformation along this discontinuity are major characteristics of oceanic detachments. Isotope systematics strongly resemble those of serpentine recovered from modern oceanic core complexes such as the Atlantis Massif. The mantle sequence of the Limassol Forest is thus suggested to have been exhumed at the footwall of an oceanic core complex. This scenario sheds light on the location of the spreading axis south of the transform and explains the highly complicated structure of the fossil ridge-transform intersection of the Limassol Forest.