Granulite-facies mafic xenoliths in the Pliocene alkaline volcanics from western Sardinia (Italy): evidence for recent subduction-related lower crustal accretion in the Sardinia-Corsica microplate ?

MONTANINI, A., MELI, S., THONI, M. AND CASTORINA, F.

Granulite-facies xenoliths brought up very quickly to surface by recent eruptions represent an unique source of informations on the nature of the continental lower crust. In Western Sardinia an orogenic calc-alkaline magmatism related to formation of a NW-dipping subduction zone and counterclockwise rotation of the Sardo-Corsican microplate away from the European continental margin developed between 32 and 13 Ma. Subsequently, during Pliocene-Pleistocene (5.0-0.14 Ma) an intraplate (dominantly alkaline) volcanism took place in an extensional setting. Pliocene basanitic eruptions carried to the surface abundant xenoliths of ultramafic mantle rocks and rarer mafic xenoliths with variable degrees of recrystallization under granulite-facies conditions. According to their mode, these latter can be grouped into: 1) metagabbronorites; 2) spinel metagabbros. The metagabbronorites are characterized by orthopyroxene + clinopyroxene + plagioclase porphyroclasts (An_50-66) in a granoblastic matrix composed of pyroxenes + plagioclase (An_56-72) + Fe-Ti oxides ± alkalifeldspar ± biotite ± apatite; pyroxene porphyroclasts often bear mutual exsolution lamellae. Spinel-rich metagabbros are medium-grained rocks with xenoblastic textures composed of Ca-rich plagioclase (An_93-86) + Al-rich pyroxenes + Al-rich spinel or Ti-magnetite. Two-pyroxene geothermometry applied to both porphyroclastic and neoblastic opx-cpx pairs give similar results (~ 900°C); the spinel metagabbros equilibrated under similar T conditions. The metagabbronorites are quartz-normative rocks, with a narrow range of SiO2 content (48.5-53.2 wt.%); they mostly represent variably evolved mafic melts and their geochemical features (slight LREE enrichment, flat HREE patterns and HFSE depletion) are consistent with crystallization of their protoliths from magmas with orogenic affinity. Present-day Sr and Nd isotopic ratios of both types of xenoliths vary in a narrow range of values close to the Bulk Earth (⁸⁷Sr/⁸⁶Sr = 0.70578-0.70647, ¹⁴³Nd/¹⁴⁴Nd = 0.51240-0.51250). No Sm-Nd isochron nor meaningful correlation between ¹⁴³Nd/¹⁴⁴Nd and ¹⁴⁷Sm/¹⁴⁴Nd ratios have been found, in spite of the relatively wide range of ¹⁴⁷Sm/¹⁴⁴Nd (0.15-0.20). Sm-Nd data on mineral separates of metagabbronorites are close to the WR present-day ratios, in agreement both with a very young age of the protoliths and equilibration at the high (lower crustal) temperatures recorded by two-pyroxene thermometry. Calculated V_P on the basis of modal composition (7.1 km s^-1) are in good agreement with those reported for the lower crust of central-western Sardinia by seismic refraction surveys. On the basis of the reported data we therefore propose that the studied xenoliths represent evidence of underplating processes in the Sardinia-Corsica microplate related to the Oligo-Miocene subduction-related magmatic activity.