Chromite-hosted Silicate Melt Inclusions from Basalts in the Stravaj Complex, Southern Mirdita Ophiolite Belt (Albania)


Abstract: The Stravaj ophiolite compex, part of the western Mirdita ophiolite belt in Albania, is located east of the Shpati massif, and west of the Shebenik massif. The Stravaj ophiolite sequence itself consist of MOR-related and subduction-related volcanic rocks (Hoeck et al. 2007) formed by pillow lavas and various dykes. The deeper units are formed by gabbros and plagioclase-bearing peridotites. The pillow lavas are intersected by basaltic dykes with a rather primitive composition. The studied basaltic dyke contains former relics of olivine, fresh spinel and clinopyroxene phenocrysts in a glassy groundmass. The silicate phases are strongly altered. The spinels appear as fresh, opaque grains preserved in totally altered olivine phenocrysts. The spinels host negative crystal shaped, multiphase silicate melt inclusions. The inclusions consist commonly of clinopyroxene daughter minerals, glass and rare sulphide blebs. A series of heating experiments were conducted, using the furnace technique to homogenize the silicate melt, in order to obtain homogenized silicate melt inclusions for major and trace element composition analysis and to determine their homogenization temperatures. Therefore, samples were heated to and quenched from 1200±20°C to 1240°C. The melt inclusions homogenized between 1220-1240±20°C. The major element composition of the homogenized melt inclusions is 48.3-51.2 wt% SiO2, 5.4-6.7 wt% FeO, 9.9-12.6 wt% MgO, 14.5-17.3 wt% Al2O3, 1.9-2.4 wt% Na2O and 12.1-13.0 wt% CaO. This result is highly comparable with the host mafic rock composition. The trace element composition of the homogenized silicate melt shows characteristic LREE-depleted patterns (La: 0.24-0.35 ppm), while the MREE and HREE patterns are generally flat: average PM-normalized La/Lu is 0.094. The average contents of compatible trace elements such as Cr, Ni, V, Co are up to 621 ppm, 825 ppm, 235 ppm and 80 ppm, respectively. Based on the major composition, trace element characteristics and the calculated oxygen fugacity, the studied silicate melt inclusions show strong similarities to MOR-related volcanic rocks found commonly in the Stravaj Massif. These chromite-bearing basalt dykes define extreme primitive MORB related melts in the upper part of the pillow lava section.

Keywords: ophiolite, chromite, basalt, melt inclusions, Albania

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