The impact of melt impregnation on the genesis of mantle peridotites from Puke Massif (Mirdita Ophiolite, Albania) revealed by geochemical data

The Mirdita Ophiolite (N Albania) consists of two meridional belts of different geochemical affinities: supra-subduction zone for the Eastern belt and mid-ocean ridge (MOR) for the western belt. Puke Massif described in this study is a mantle dome belonging to the MOR belt.

Structurally, the Puka Massif is interpreted as an Oceanic Core Complex formed of harzburgites cross-cut by dunitic channels grading to mylonitized plagioclase and amphibole bearing lherzolites with minor dunites and chromitites at the top of the section. The massif experienced an intense magmatic activity evidenced by gabbroic and pyroxenitic dykes. Field and petrographic evidences revealed that plagioclase, clinopyroxene and amphibole in lherzolitic mylonites crystallized from impregnating melts (Nicolas et al. 1999, 2017). Scientific question behind our study is whether this conclusion is confirmed by geochemical data.

Clinopyroxene from magmatic veins cross-cutting mylonites, has trace elements (TE) composition identical to that from the host peridotite. In general, 3 types of TE patterns can be identified in the veins and mylonites: 1. Strongly depleted (Yb=0.3-0.6x primitive mantle, PM, McDonough & Sun 1995); 2. Intermediate (Yb=1.1-4xPM); 3. Enriched (Yb=5-11xPM). The group 1 comprises only pyroxenites. Two relatively undeformed harzburgites occurring in the lowermost section of the mantle dome contain TE-poor clinopyroxene. One, which is amphibole-bearing, exhibits TE pattern resembling that in group 1, while the other one shows even more depleted signature, with Yb=0.8-1.3xPM and La <0.001xPM. Intrusive rocks from groups 2 and 3 are widespread in the whole massif while the occurrences of the depleted group are restricted to the lowermost sections. Rocks from different groups may occur within a single outcrop.

The TiO₂ content in clinopyroxene mimics the TE-based division of the rocks. Clinopyroxene in the group 1 and harzburgites has TiO₂<0.1 wt.%, whereas that from group 2 and 3 has 0.1<TiO₂<0.5 wt.% and TiO₂>0.5 wt.%, respectively. Similar relationships are observed in the composition of spinel, which has TiO₂<0.1 wt.% in group 1 rocks, 0.1 - 0.25 wt.% in group 2 and between 0.1 and 2.0 wt.% in the group 3 rocks.

As magmatic rocks and deformed peridotites share common clinopyroxene TE trends, as well as similar Ti variations in clinopyroxene and spinel, geochemical data support impregnating origin of mylonites. Impregnating melts, differing in enrichment level, were active within whole massif; only the most depleted seem to be restricted to some of its parts. Only internal or easternmost harzburgites could have escape magmatic impregnations; these samples are relatively undeformed and have depleted melting-like TE trends. These findings are in agreement with melt impregnation origin of mylonites. Presence of the depleted lithologies supports primarily harzburgitic origin of the massif, later followed by mylonitization of some of its part. 

This study was financed as a project within program “Diamond Grant” (DI 024748).