PETROLOGICAL AND GEOCHEMICAL EVOLUTION OF THE MOHO IN A JURASSIC SUPRASUBDUCTION ZONE OCEANIC LITHOSPHERE: EASTERN MIRDITA OPHIOLITE, ALBANIA

We examined the peridotite massifs, mafic-ultramafic cumulate sections, and late melanocratic intrusions in the Eastern Mirdita Ophiolite (EMO, Albania), which includes ~700 meters of volcanic rocks in the upper extrusive sequence showing island arc affinities. Clinopyroxene porphyroclast-bearing harzburgites (Cpx-harzburgite hereafter) occur structurally in the lower parts of the peridotite massifs, whereas harzburgitic and dunitic peridotites with chromitite bands and layers are more abundant in the structurally upper levels. Orthopyroxenite occurs as dikes and/or vein networks in all structural levels within the peridotites, although more abundantly in the uppermost sections. Orthopyroxenite commonly crosscuts the foliation planes in the peridotites and lithological boundaries between the chromitite-bearing dunites and harzburgites, suggesting its late stage formation. Major and trace element compositions of minerals in the Cpx-harzburgites indicate that these peridotites were formed as residue after partial melting. Harzburgites have more depleted signatures in major element compositions in comparison to the Cpx-harzburgites. Light rare earth element (LREE)-enrichment in clinopyroxene coupled with silicate mineral inclusions in spinels indicate that their host harzburgites were produced as a result of enhanced partial melting of depleted peridotites that was facilitated by infiltration of LREE-enriched, slab-derived fluid flux. Dunites and chromitites were produced by melt-mantle interactions, whereas orthopyroxenites by the reaction of pre-existing olivine with hydrous, orthopyroxene-saturated melts that were produced by assimilation of dissolved pyroxene during formation of dunites. The ultramafic-mafic cumulate sequence just above the peridotite massifs mainly consists of dunite, wehrlite and gabbro-norite, indicating early crystallization of olivine and clinopyroxene before plagioclase. This crystallization sequence can be explained by magma crystallization at crustal pressures under hydrous conditions (i.e. island-arc setting). Melanocratic, massive intrusive rocks occur in gabbroic rocks and are characterized by large, poikilitic orthopyroxene crystals containing numerous, rounded olivine grains. Chemical compositions of spinels in the melanocratic intrusive rocks are relatively high in the Cr#(=Cr/Cr+Al) atomic ratio) (Cr#＞0.53) and low in TiO₂ contents, and are similar to those in arc-related rocks. The crystallization sequence combined with high Cr# spinel suggests that the melanocratic intrusive rocks were formed from melts which were produced with high degrees of partial melting of peridotites. We infer that orthopyroxenites and late melanocratic intrusions are genetically associated with the late stage boninitic dikes and lavas in the uppermost crustal section of the EMO. The lower crustal and upper mantle sections of the EMO appear to have formed first in a seafloor spreading setting and then to have undergone metasomatism and repeated melting due to the infiltration of slab-derived fluids in an island arc setting.