Multiple episodes of melting, depletion, and enrichment of the Tethyan mantle: Petrogenesis of the peridotites and chromitites in the Jurassic Skenderbeu massif, Mirdita ophiolite, Albania

The Jurassic Mirdita ophiolite in Albania displays a structural-geochemical transition from a mid-ocean ridge-type (MOR) oceanic lithosphere in the west to a suprasubduction zone (SSZ) type in the east across an similar to 30-km-wide fossil Tethyan oceanic domain. We investigated the upper mantle peridotites of the Skenderbeu massif, situated at this transition within the ophiolite, to document the geochemical fingerprint of the inferred tectonic switch. The peridotites comprise harzburgites and dunites with podiform chromitite deposits. We present new whole-rock major element, trace element, rare earth element (REE), and platinum group element chemistry to evaluate their mantle melt evolution and petrogenesis. Harzburgites have high average CaO, Al2O3, and REE contents, and contain Al-rich pyroxene and spinel with lower Cr contents. Dunites have low average CaO, Al2O3, and REE values, and contain Al-poor clinopyroxene and high-Cr spinel. Modeling of trace element compositions of the harzburgites suggests as much as similar to 10%-15% melting, whereas the trace element compositions of the dunites indicate similar to 20%-25% melting. The harzburgites and dunites and chromitites represent, respectively, the products of low-degree partial melting in a MOR setting, and the products of high-degree partial melting and refertilization in a forearc mantle. The harzburgites resulted from rock-melt interactions between ascending melts and residual peridotites beneath a MOR, whereas the dunites and the high-Cr chromitites formed as a result of interactions between boninitic melts and mantle peridotites in a mantle wedge. The Skenderbeu mantle units thus constitute a geochemical-petrological archive of a transition from MOR to SSZ melt evolution in space and time within the same ocean basin.