Preliminary insights into lithological and chemical diversity in Mirdita Ophiolite peridotite massifs – Kukes and Puka case studies

The Mirdita Ophiolite in northern Albania forms a ~240 km long and ~40 km wide zone within Dinaric-Hellenic belt. It marks suture after Neo-Tethyan Ocean closure. The chemical diversity of volcanic crustal rocks led to its division into two zones: the eastern one is interpreted to have Supra-Subduction Zone (SSZ) origin, whereas the western zone exhibits Mid-Ocean Ridge (MOR) affinity. More than a dozen of ultramafic massifs occur along the entire length of the ophiolite.

In this study we focus on chemical diversity of peridotites from two adjacent massifs, Kukes and Puka, which have SSZ and MOR affinities, respectively. The Kukes Massif is composed of a sequence from harzburgites at its base to clinopyroxene-poor dunites at the top, followed by pyroxenitic and peridotitic cumulates at the mantle/crust transition zone. The Puka massif is a mantle dome, composed of harzburgites and plagioclase/amphibole lherzolites (locally mylonitzed) and it is interpreted as a former oceanic core complex (OCC; Nicolas et al. 2017). Both massifs are pervasively penetrated by pyroxenitic and gabbroic veins and are serpentinised to variable degree.

Chemical composition of minerals varies between samples and lithologies, as well as between massifs. Olivine from the Kukes harzburgites has higher Fo values and NiO contents than that from dunites (Fo_89.5-92 and NiO 0.31-0.52 wt.% vs. Fo_88.1-91.2 and NiO 0.15-0.30 wt.%, respectively). Clinopyroxene has Mg#_92.5-95.1 and Al=0.03-0.08 apfu in harzburgite, while interstitial dunite clinopyroxene has Mg#_94-98 and Al below 0.03 apfu. Harzburgite orthopyroxene has Mg#_90.1-91.8 and Al=0.03-0.08 apfu. Chromian-spinel has Cr#_0.55-0.72 and Mg#_0.46-0.56 in harzburgites and Cr#_0.63-0.86 and Mg#_0.25-0.48 in dunites, moreover in dunites it often exhibits chemical zonation with Cr# increasing to core. Chemical composition of minerals changes gradually in the scale of single outcrop, with Fe content increasing toward veins.

The Puka peridotites have more enriched composition. Olivine has Fo_87.8-90.8 and NiO=0.25-0.43 wt. %, clinopyroxene has Mg#_90.1-93.3 and Al=0.05-0.15 apfu, orthopyroxene has Mg#_88.5-91.0 and Al=0.03-0.1 apfu, while spinel has Cr#_0.38-0.55 and Mg#_0.42-0.57, with single sample of Cr#_0.60-0.75 and Mg#_0.33-0.52. Plagioclase is Ca-rich (77-95 An), amphibole – occurring in some lherzolites – has composition of pargasite-tremolite.

Differences in lithological and chemical composition are visible between peridotites from both massifs, which correspond with diversity of crustal rocks and suggest that also mantle sections of the ophiolite record different origin. Peridotites from Kukes are harzburgites and dunites pointing to their refractory nature. The depleted peridotites were further affected by intensive magmatic veining. Infiltration of the melt triggered gradual enrichment in Fe of the silicates and chemical zonation of spinel. This process is well visible in dunites, where changes of Fe contents can be followed on distances of few meters. As metasomatic modification has a limited range, most of chemical differences have to be related with different protolith, but further studies are required to reconstruct rocks evolution.

Protolith of Puka peridotites is more fertile compared with Kukes, but reaction between veins and host lherzolite was not observed, and mylonitization led to Al depletion in pyroxenes.

This study was financed from scientific funds for years 2018-2022 as a project within program “Diamond Grant” (DI024748).