The Jurassic Mirdita ophiolite crops out in the western branch of the HellenideDinaride ophiolite belt in the Balkan Peninsula; it represents a remnant of the Tethyan oceanic lithosphere developed between the Apulian and Pelagonian microcontinents. Emplaced during the Cretaceous on the western margin of Pelagonia, the Mirdita ophiolite was involved in southwestward thrusting of the Hellenide-Dinaride tectonic units during the Eocene alpine tectonics. It was little affected, however, by these alpine events as marked by nearly horizontal Cretaceous limestone deposits overlying the ophiolite. A continuous section of Middle Jurassic oceanic crust, including thin gabbros, a N-S-trending sheeted dike complex, and extrusive rocks, is exposed in the central part of the ophiolite and thickens eastward within a regional, N-S-trending synform. Peridotite massifs exposed along the western and eastern edges of this synform show major structural and petrological differences. The eastern ultramafic domain has a harzburgitic mantle exhibiting a high-temperature asthenospheric foliation dipping steeply to moderately to the west. Major chromite deposits are restricted to these eastern ultramafic massifs. The western ultramafic domain has a zoned mantle, with asthenospheric harzburgite exposed at its western margin, progressively replaced eastward by plagioclase-peridotites that were highly strained at low temperatures, and that are bounded by amphibole-peridotites along with crustal rocks. The occurrence of plagioclase is ascribed to melt impregnation processes that occurred immediately before or during intense lithospheric deformation. The gabbros in the eastern massifs are thicker (1-2 km and layered, whereas they are highly thin and discontinuous in the western massifs. Mantle peridotites of the western massifs locally are in direct contact with the overlying diabase and volcanic rocks along ductile shear zones. Development of epidote-amphibolite facies metamorphism in upper-crustal rocks was produced by hydrothermal alteration in the oceanic realm, and was intense and widespread in the western domain. The Mirdita ophiolite likely originated in a short-lived, narrow ocean basin, which was closed during the Middle Jurassic by eastward overthrusting of the western domain, and westward subduction of the eastern domain. This constrictional phase was followed by dextral oblique convergence between the Pelagonian and Apulian microcontinents. This model implies a parautochtonous origin of the Mirdita ophiolite between these two microcontinents.

Various combinations of diamond, moissanite, zircon, corundum, rutile and titanitehave been recovered from the Bulqiza chromitites. More than 10 grains of diamond have been recovered, most of which are pale yellow to reddish–orange to colorless. The grains are all 100–300 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm−1 and 1333 cm−1, mostly at 1331.51 cm−1 or 1326.96 cm−1. This investigation extends the occurrence of diamond and moissanite to the Bulqiza chromitites in the Eastern Mirdita Ophiolite. Integration of the mineralogical, petrological and geochemical data of the Bulqiza chromitites suggests their multi–stage formation. Magnesiochromite grains and perhaps small bodies of chromitite formed at various depths in the upper mantle, and encapsulated the ultra–high pressure, highly reduced and crustal minerals. Some oceanic crustal slabs containing the magnesiochromite and their inclusion were later trapped in suprasubduction zones, where they were modified by tholeiitic and boninitic arc magmas, thus changing the magnesiochromite compositions and depositing chromitite ores in melt channels.

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 ~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, Al 2 O 3 , and REE contents, and contain Al-rich pyroxene and spinel with lower Cr contents. Dunites have low average CaO, Al 2 O 3 , and REE values, and contain Al-poor clinopyroxene and high-Cr spinel. Modeling of trace element compositions of the harzburgites suggests as much as ~10%-15% melting, whereas the trace element compositions of the dunites indicate ~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.

The Bulqiza ultramafic massif, which is part of the eastern Mirdita ophiolite of northern Albania, is world renowned for its high‐Cr chromitite deposits. High‐Cr chromitites hosted in the mantle section are the crystallized products of boninitic melts in a supra‐subduction zone (SSZ). However, economically important high‐Al chromitites are also present in massive dunite of the mantle‐crust transition zone (MTZ). Chromian‐spinel in the high‐Al chromitites and dunites of the MTZ have much lower Cr# values (100Cr/(Cr+Al)) (47.7–55.1 and 46.5–51.7, respectively) than those in the high‐Cr chromitites (78.2–80.4), harzburgites (72.6–77.9) and mantle dunites (79.4–84.3). The chemical differences in these two types of chromitites are reflected in the behaviors of their platinum‐group elements (PGE). The high‐Cr chromitites are rich in IPGE relative to PPGE with 0.10–0.45 PPGE/IPGE ratios, whereas the high‐Al chromitites have relatively higher PPGE/IPGE ratios between 1.20 and 7.80. The calculated melts in equilibrium with the high‐Cr chromitites are boninitic‐like, and those associated with the high‐Al chromitites are MORB‐like but with hydrous, oxidized and TiO2‐poor features. We propose that the coexistence of both types of chromitites in the Bulqiza ultramafic massif may indicates a change in magma composition from MORB‐like to boninitic‐like in a proto‐forearc setting during subduction initiation.

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