Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy)

The pyroxenite-peridotite sequence from the External Ligurian ophiolites offers a snapshot on the evolution of recycled mafic crust within the mantle. We present new major and trace element compositions, and Nd-Hf isotopic compositions of garnet clinopyroxenites and garnet websterites from this mantle section. The garnet clinopyroxenites display clinopyroxene and bulk rock REE patterns with distinct positive Eu anomalies, which argue for the involvement of plagioclase-rich precursors in their origin. We propose that the garnet clinopyroxenites formed by crystallization of eclogite-derived melts that experienced negligible interaction with the host peridotites. The garnet websteritesare interpreted to have been produced by reactions between the eclogite-derived melts and peridotites, thereby giving rise to hybrid, second-stage pyroxenites with a crustal geochemical fingerprint. In our petrogenetic scenario, a rifting-related event at ~220 Ma caused melting of eclogites originated from a MORB-type gabbroic sequence. These mafic protholiths underwent a long-lived evolution of recycling into the mantle (1.5-1.0 Ga), with no evidence for substantial fractionation affecting the Lu/Hf and Sm/Nd isotopic systems. We show that heterogeneity of crustal protoliths, age of recycling and interaction with the host peridotites may lead to a significant compositional and isotopic diversity of crust-derived mantle pyroxenites.