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Petrogenesis of basalts and gabbros from an ancient continent–ocean transition (External Liguride ophiolites, Northern Italy) |
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| Authors: | Alessandra Montanini Riccardo Tribuzio Luigi Vernia |
| Institution: | aDipartimento di Scienze della Terra, Università di Parma, Parma, I-43100, Italy bDipartimento di Scienze della Terra, Università di Pavia, via Ferrata 1, I-27100 Pavia, Italy cCentro di Studio per la Cristallochimica e per la Cristallografia, CNR, via Ferrata 1, I-27100 Pavia, Italy |
| Abstract: | 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 oceanization. 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 ( 900 °C) shearing along ductile shear zones is locally overprinted by amphibolite-facies recrystallization (T 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  Nd = +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. |
| Keywords: | Ocean–continent transition Ophiolites Northern Apennine MORB Mantle sources |
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