Petrography of ultrabasic and basic rocks from the Ozren ophiolite complex in Bosnia and Herzegovina

The Ozren ophiolite complex (OOC) is the second largest ophiolite complex in Bosnia and Herzegovina, and it is a part of the huge Dinaride ophiolite belt [1 and reference therein]. This contribution comprises mineralogical-petrographical descriptions of representative rocks of the OOC which were determined from polished sections by polarized light microscopy and introductory EPMA. The investigated harzburgites are composed of Ol (55%), Opx porphyroclasts with Cpx exsolution lamellae (35%), Cpx with Opx exsolution lamellae (5%) the latter following Ol-Opx boundaries or ingrowing Opx and Ol matrix. Spinel occurs in the form of anhedral grains. Lherzolites contain Ol (55%), Opx (25%) and Cpx (15%). Anhedral Opx porphyroclasts have Cpx exsolution lamellae. Similarly, porphyroclastic Cpx contains Opx exsolution lamellae. Late magmatic Cpx and Opx aggregates are ingrowing the Ol matrix and these are also surrounding deformed Opx porphyroclasts. Spinel is immersed in the Ol matrix. Dunites are rare. A remnant of the gabbroic layer is inferred only from a borehole core. This gabbro has ophitic texture and contains primary magmatic porphyric Pl, Cpx and green Amp1. Pyroxene and Amp1 are partially replaced by Amp2 and Chl. Plagioclase is weakly altered. Moreover, we found cross-cutting dykes of gabbros (micro-gabbros, called dolerites, to gabbro-pegmatites, and dunite-associated troctolite) in peridotite. These dykes have randomly oriented minerals, only locally showing mylonitization signatures. Most dykes have a discordant relationship to the peridotite structures. Dolerites and basalts also occur in the form of relatively thicker lens-like bodies in serpentinites. A basaltic dyke cross-cuts layered gabbro (in borehole). Most dolerites are composed of Cpx, Amp and Pl but we also found an exceptional Ol-dolerite dyke cross-cutting peridotite. It has well preserved magmatic ophitic texture composed of Ol, Opx, Cpx, Amp, Pl, Ilm and Ap. Pyroxenes and amphiboles are weakly chloritized and Ol is serpentinized. Dolerites and basalts have an ophitic texture, defined by fine-grained prismatic Pl, Px and Amp. Secondary Amp2 and Chl follow the grain boundaries of magmatic minerals. Ophiolitic breccias cover some peridotite parts. These breccias contain 1cm – 10m fragments of all lithological sequences of the OOC including reddish radiolarite sediments. Gabbros from ophiolitic breccia have coarse grained Pl and Px. Exsolution lamellae in Px and kink-banding are characteristic features from subsolidus magmatic conditions. A rare plagiogranite intrusion in peridotite is composed of Qz, Pl and needle-like Amp aggregates after Bt. Such an association of ultrabasic and basic rocks may indicate percolating gabbroic magmas through the peridotites. Amphibolites were found only at one locality so far and these are composed of oriented Amp and Pl aggregates in the metamorphic texture, most likely indicating metamorphic sole of an ophiolitic thrust sheet. These preliminary results shed light on the lithology and petrography of the OOC and have arisen problems for further research.

References:

[1] Babajić E. (2019) Krivaja-Konjuh ophiolite complex – petrology, geochemistry and geotectonics of mafic sequences. (Monograph), MIT-ALEX, Tuzla (Bosnia and Herzegovina)

Acknowledgement: APVV Agency Project No. APVV-19-0065 (M.P.) is acknowledged.