Stable isotopic composition of atypical ophiolitic rocks from east Liguria, Italy

Abstract

In east Liguria, Italy, pelagic sediments and discontinuous pillow lava formations (< 300 m thick) commonly rest with depositional contact upon serpentinite, gabbro or ophiolitic breccias that lack a sheeted dyke complex. Stable isotopic data on these unusual ophiolites indicate that basalts and, unexpectedly, gabbros are significantly enriched in 18O due to moderate- to low-temperature alteration, that veins cutting the basalts formed at ∼ 150–200°C (quartz-epidote) to ∼ 100–150°C (calcite), that serpentinization took place at ∼ 100–200°C, and that the ophicalcitic upper surfaces of the serpentinites formed in contact with normal seawater at ∼ 80–135°C. During uplift of deep-seated gabbro to a near-seafloor position, mafic phases were progressively transformed to lower-temperature phases such as tremolite then chlorite, whereas plagioclase underwent low- T retrograde isotopic exchange, probably with 18O-enriched fluids. Following the formation of a discontinuous volcanic carapace to these lithologies, moderate- to low-temperature hydrothermal activity produced vein systems and general 18O enrichment in the volcanics. If the 18O enrichment of the east Ligurian ophiolite is typical of other stratigraphically similar ophiolites in the western Tethys, then waters in this ocean basin may not have been buffered to a δ 18O-value of ∼0%.