Petrogenesis of basalts and gabbros from an ancient continent-ocean transition (External Liguride ophiolites, Northern Italy)

Remnants of a fossil continent–ocean transition similar to that of the modern non-volcanic continental margins are preserved in the Jurassic External Liguride units. They consist of fertile lherzolites of subcontinental origin, MOR-type basalts and rare gabbroic intrusives, together with continental crust bodies exhumed during the rifting phases preceding the oceanisation. The gabbroic rocks include troctolites, (olivine) gabbros, Fe–Ti oxide-bearing gabbros and diorites. Trace element and Nd isotope compositions indicate that these rocks were derived from N-MORB melts variably evolved through fractional crystallisation. In the gabbroic rocks, high temperature (ca. 900 °C) shearing along ductile shear zones is locally overprinted by amphibolite facies re-crystallisation (T ca. 650 °C), which was most likely assisted by seawater-derived fluids. Basalts crop out as lava flows and as dykes crosscutting mantle lherzolites and gabbroic rocks. They display nearly flat REE patterns and high Y/Nb values (5–14), similar to modern N-MORB. Basalts are also characterised by weak Zr enrichment relative to neighbouring REE (Zr/Zr*=1.1–1.7) and high (Sm/Yb)DM ratios (1.5-1.8). Their Nd isotope compositions are close to typical depleted mantle (initial εNd=+7.6 to+9.4). The geochemical features of parental melts of basaltic and gabbroic rocks may be attributed to melting of a MORB-type asthenospheric source. Trace element modelling shows that low-degree (≤6%) fractional melting of a depleted spinel peridotite cannot account for the elevated Sm/Yb ratios of basalts. Low degree melting of a mixed source of spinel peridotite with small amounts of garnet pyroxenite has been proposed to explain the trace element signature of basalts.