Mineralogy and petrology of ultramafic section of Kukesi Massif, Mirdita Ophiolite (Albania) – preliminary results

Mirdita Ophiolite in northern Albania is a part of 30-40 km wide ophiolitic Pindos Zone in Dinaride-Hellenide part of the Alpine orogenic system (e.g. Dilek & Furnes 2009, Lithos). Mantle and crustal sections in the eastern part of this zone have Supra-Subduction Zone geochemical affinities. The goal of our study is to examine chemical diversity of rocks within Kukesi Massif and to decipher its evolution.

The Kukesi Massif is composed mostly of coarse- to medium-grained spinel harzburgites and dunite with chromite layers (e.g. Morishita et al. 2011, Lithos), locally  cross-cut by orthopyroxenite veins. Uppermost part of the sequence consist of cumulate pyroxenites and peridotites (composed of olivine, orthopyroxene, clinopyroxene and spinel). Most of the rocks are pervasively serpentinised, but degree of serpentinisation varies within the massive. Samples of peridotites and pyroxenites from over a dozen localities within the massif were collected.

Olivine occurring in the lower sections of the ophiolite has composition of Fo_89.5-91.2 (NiO 0.28-0.52 wt.%) in peridotites and Fo_90.6-92 (NiO 0.38-0.52 wt.%) in orthopyroxenite veins. Olivine forming cumulates has Fo_82.4-83.3 and NiO content=0.12-0.23 wt. %. Orthopyroxene (enstatite) in mantle peridotites is Al-poor (0.05-0.08 Al a.p.f.u.) and has Mg# 90.5-91.5. Orthopyroxene from peridotite cut by orthopyroxenite veins is even poorer in Al (0.03-0.04 a.pfu) and has lower Mg# 91.1-91.7 and is chemically indistinguishable from pyroxenitic orthopyroxene. Orthopyroxene forming cumulates has Mg#=82.3-84.0 and the highest Al content among all the lithologies (0.12-0.14 a.p.f.u.). Peridotitic clinopyroxene (diopside) has Al=0.02-0.08 a.p.f.u. which corresponds well to this in orthopyroxene, but Mg# is higher – 92.5-95.4. Clinopyroxene in cumulate rocks has Al content=0.13-0.16 a.p.f.u. and Mg#=87-88. Spinel in mantle peridotites has Cr#=0.47-0.80 and is negatively correlated with Mg# (0.38 to 0.56). The cumulative spinel has lower Cr# (0.18-0.27), but the  Mg# is similar to that forming peridotite (0.38-0.45). 

The orthopyroxene equilibration temperatures calculated with Witt-Eickschen & Seck (1991, CMP) algorithm, yield wide range of temperatures (800-950˚C in mantle peridotites and 950-1020˚C in cumulate peridotites suggesting its magmatic origin). Low Al content in orthopyroxene suggest that peridotites suffered from high degree of melt extraction.

Chemical composition of minerals forming rocks of Kukesi Massif is typical  for mantle sections of SSZ ophiolites (e.g. Troodos ophiolite, Batanova & Sobolev 2000, Geology). Our preliminary mineral chemical data for Kukesi ultramafics have a wider range than those previously obtained by Morishita et al. (2011, Lithos). The chemical composition of ultramafic rocks within this massif varies, which may result from variable geochemical history, but further studies are required to fully characterize the composition of Kukesi ultramafics and to reconstruct its geochemical and tectonic evolution.

This study was financed from scientific funds for years 2018-2022 as a scientific project within program “Diamond Grant” (DI 024748).