Refertilization of subcontinental mantle recorded by the lherzolite-websteritehorneblendite association from St. Lucia (Corsica)

The St. Lucia nappe from Alpine Corsica consists of late-Variscan lower crust exhumed along the European rifted margin of the Ligurian Tethys and Mesozoic sediments (Beltrando et al., 2013). The basement includes a High Grade Mafic Complex and a Granitoid Complex of Early Permian age. The base of the Mafic Complex is associated with mantle slices consisting of mylonitic spinel-bearing lherzolites and mm- to cm-thick websterite and hornblendite layers concordant with the foliation of the host rocks. The mylonite microstructure in the peridotites is characterized by aligned porphyroclasts of pyroxene (± spinel) in a fine-grained polyphase matrix composed of olivine + pyroxenes + spinel. Large exsolved orthopyroxene (Opx) porphyroclasts mantled by neoblastic Cpx + Opx + spinel testify a lowstrain spinel tectonite stage predating the mylonite deformation. Major element compositions of olivine, Cpx and spinel porphyroclasts of the lherzolites indicate a fertile geochemical signature. The websterites are Opx-poor rocks containing Al-Ti-rich Cpx porphyroclasts, Cr-poor spinel, disseminated kaersutite and accessory Fe-Ni sulphides. Spinel-facies mylonite recrystallization in both lherzolites and websterites occurred at ~850-900°C. The hornblendites are composed of K2O-rich kaersutite + Ti-rich phlogopite + ilmenite. Cpx porphyroclasts from the lherzolites have heterogeneous trace element compositions pointing to four different geochemical signatures. Type 1 Cpx is strongly LREE-depleted (CeN/SmN down to 0.05) with nearly flat MREE-HREE at 8-10 times chondrite. Type 2 Cpx is moderately LREEdepleted (CeN/SmN = 0.31-0.49), whereas Type 3 has nearly flat LREE. Type 4 Cpxs are weakly LREE depleted, peak at MREE and have variable HREE (YbN = 7-12). The websterite Cpxs display convex-upward REE patterns and higher concentrations of incompatible trace element. The kaersutite from the hornblendites are enriched in Rb, Ba, U, Nb, Ta, LREE and depleted in HREE with respect to the websterite ones. The peridotite protholiths are consistent with residual rocks after low degrees of near-fractional melting of spinel facies DM. Type 4 Cpx from the lherzolites likely reflect equilibrium with percolating MORB-type melts. Putative liquids in equilibrium with Cpx and kaersutite from the websterites and the hornblendites have transitional to alkaline affinity. Our study has evidenced that the St. Lucia mantle slices underwent injections of melts with MORB to alkaline affinity forming different kinds of magmatic layers, which were associated with metasomatic haloes in the ambient peridotite. Beltrando M., Zibra I., Montanini A. & Tribuzio R. 2013. Crustal thinning and exhumation along a fossil magma-poor distal margin preserved in Corsica: a hot rift to drift transition, Lithos, 168-169, 99-112.