Ibrahim Milushi

Structure and tectonics of lower crustal and upper mantle rocks in the Jurassic Mirdita ophiolites, Albania

The Jurassic Mirdita ophiolite crops out in the western branch of the Hellenide‐Dinaride 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.

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 ~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 ~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. LITHOSPHERE; v. 10; no. 1; p. 54–78 | Published online 14 July 2017 doi:10.1130/L606.1

Multi-stage Process of the Bulqiza Chromitites, Eastern Ophiolitic Belt, Albania

eastern ophiolitic belt of Albania, is the most important area for metallurgical chromitite ores. The massif consists of a thick (>4 km) rock sequence, with a generalized profile from the bottom to the top as follows. The tectonite sequence composed mainly of harzburgites with subordinate intercalations of dunites, where only scarce refractory chromitites occur within basal clinopyroxenebearing harzburgites, while abundant podiform chromitites of metallurgical type are hosted by middle-upper harzburgites. The transitional zone entirely composed of dunites with layered metallurgical chromitites; and, finally, the magmatic sequence composed of wehrlites, pyroxenites, troctolites and gabbros with scarce refractory chromitite occurrence. Several authors carried out studies of the petrological, mineralogical and structural features of the Bulqiza chromite deposits. The upper mantle and crustal units of the Bulqiza Ophiolite show major changes in thickness, rock types, and chemical compositions from west to east as a result of its complex evolution in a suprasubduction zone (SSZ) environment. These peridotites display numerous evidences of mantle metasomatism at various scales. Minerlogy of olivine, clinopyroxene, orthopyroxene and spinel show that they are formed as forearc peridotite. The composition of the melts gradually changed toward boninite due to melt– peridotite reaction. Both the massive and disseminated chromitites are high – Cr number (Cr=70-80), with diamond. Disseminated chromitites show systematic changes in olivine and magnesiochromite compositions from the dunite envelope to the massive ore, indicating melt-rock reaction without UHP minerals. Composition of orthopyroxene change between core and rim indicate melt interaction with cr and al element. Textural features such as the incongruent melting of orthopyroxenes and interstitial spinels, olivines and pyroxenes suggest that mantle metasomatism has affected the Bulqiza massif. These processes produced the chromite deposits and their dunitic envelopes in the upper levels of the mantle. These events suggest that the Bulqiza peridotites and chromitites were formed in a Multi-stage process.