U-, Pb-enrichment, Sr-depletion produced by water-rock interaction processes within the eclogitic oceanic crust of Ordovician age in NE Sardinia

Cruciani, G;Franceschelli, M;Puxeddu, M

2017-01-01

---

Abstract

A detailed analysis of geochemical data obtained from eclogite rocks outcropping within the Axial Zone, i.e. the most internal Variscan tectono-metamorphic zone in NE Sardinia, has revealed significant U, Pb and sometimes K and Rb enrichment and Sr depletion generated soon after emplacement or during descent at depth towards eclogite HT-HP conditions. These geochemical changes induced by water-rock interactions between hydrothermal fluids and oceanic crust caused depletion of Light Rare Earth Elements (LREE) with respect to Heavy Rare Earth Elements (HREE) and flat HREE patterns. Other concomitant cause of the altered geochemistry that affected the eclogite protolith could be the supply of continental sedimentary material to the uppermost mantle. Recently several authors have demonstrated that serpentinization of oceanic crust can increase the Pb contents in eclogites up to unexpectedly high values as observed in the Alpine External massifs in Western Alps. The serpentinization can accompany continuously the oceanic crust from depth at the eclogite facies P-T conditions up to surface.