Recent crustal accretion by underplating of gabbroic rocks in the perimediterranean domain: a case study from mafic xenoliths in Sardinia (Italy)

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

During Pliocene, basanitic eruptions in Western Sardinia brought to the surface xenoliths made of mafic granulites. According to their mode, they can be grouped into: 1) metagabbronorites; 2) spinel metagabbros.

The metagabbronorites are characterized by orthopyroxene, plagioclase and clinopyroxene porphyroclasts, surrounded by a granoblastic matrix composed of pyroxenes, plagioclase, alkalifeldspar, biotite, apatite and opaques; pyroxene porphyroclasts often bear exsolution lamellae. All pyroxenes have moderate Al₂O₃ contents. Plagioclase porphyroclast composition varies between An₅₀ and An₆₆, whereas neoblastic plagioclase is slightly Ca-richer (An56-72). Complex K-feldspar exsolutions occur in plagioclase with An<54.

Spinel-rich metagabbros are medium-grained, granoblastic rocks composed of Ca-rich plagioclase, Al-rich pyroxenes and Al-spinel or Ti-magnetite; the pristine igneous textures was not preserved.

In the gabbronorites, two-pyroxene geothermometry has been applied to neoblastic Opx-Cpx pairs: the equilibration temperatures span from 800 to 920° C. The spinel metagabbros equilibrated under similar T conditions.

The metagabbronorites are quartz-normative, with a narrow range of SiO2 content (48.5-53.2 wt.%). Whole-rock chemistry is sometimes affected by Opx/Pl cumulus; however, most xenoliths represent variably evolved mafic melts, and their geochemical features (iron enrichment, fractionated LREE and flat HREE patterns) are typical of evolved tholeiitic magmas with orogenic affinity. Crystallization probably took place at pressure > 5-6 kbar, as suggested by the occurrence of orthopyroxene + Ca-rich plagioclase as liquidus phases.

Sm-Nd data of mineral separates are close to the WR present-day ratios, suggesting a very young age of the protoliths (< 100 Ma), which in turn points to underplating processes related to the Oligo-Miocene calcalkaline cycle.