Author: Šime Bilić, Ph.D.
In the Banovina area, peridotites and pyroxenites are rocks formed as parts of the former earth's upper mantle, representing material that comes from deep parts of the earth (up to 100 km), which cannot be reached by drilling. Farther more, the appearance of such rocks is evidence of the existence of the former oceanic realm, so their exploration is extremely important for all those involved in petrology, mapping, regional and structural geology. With the help of modern, but also classical analytical and petrological research methods, very interesting information related to the genesis of these rocks can be obtained, such as: how old are these rocks; what temperatures and pressures prevailed during their formation; from what depths do they come; how these rocks came to the surface, etc. Based on the results of detailed field research, petrographic and geochemical analysis of the studied rocks, it was found that in the Banovina area there are structural and chemical characteristics two different types of peridotites that also geographically belong to different localities and can be classified into two belt, northern and southern belt. Within the northern belt (N-belt), serpentinite breccias and serpentinized spinel lherzolites predominate, which are recognizable in the field by the structure characteristic of ophiolite mélange. The geochemical characteristics of N-belt peridotites indicate its origin from the suboceanic mantle that underwent melting processes in the mid-ocean ridge area. The southern belt (S-belt) contains spinel lherzolites, dunites, and pyroxenites that vary within spatially very confined spaces. The geochemical characteristics of S-belt peridotites indicate subcontinental origin and were most likely formed in the initial rift phase during which they ascended to the upper crust as parts of the continental mantle and at the same time underwent a relatively low degree of melting. Pyroxenites located exclusively within the S-belt show completely different petrographic and geochemical characteristics. They were most likely formed separately, by crystallization from a melt of unknown origin that was circulating through the mantle. Petrological characteristics of peridotite and pyroxenite from Banovina show a very good correlation with similar rocks found in Bosnia and Herzegovina, so they can be declared as a part of the Central Dinaric Ophiolite Belt (CDOB; Lugović et al, 1991), and a new division of peridotite from our study can therefore be translated to CDOB. Petrological research in the Banovina has shown that the Peridotites and Pyroxenites from the Banovina record three different phases of ocean evolution: a) the early stage of the initial rift and ocean opening (S-belt peridotites); b) later phase of already developed ocean (N-belt peridotites) and c) phase of ocean closure which is evident from the geologic structures and metamorphic rocks in contact with peridotites.
Figure 1 Outcrops of mantle rocks, Zrinska gora.
Figure 2 Talc minerals (vivid colors) fill cracks in gray serpentinite (seen through a petrographic microscope).
Reference:
Lugović, B., Altherr R., Raczek I., Hofmann A. W., Majer V. (1991): Geochemistry of peridotites and mafic igneous rocks from the Central Dinaric Ophiolite Belt, Yugoslavia, Contributions to Mineralogy and Petrology, 106(2), 201–216
Šime Bilić, dr. sc. is a postdoctoral fellow at the Department of Mineralogy, Petrology and Mineral Resources at the Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb. He successfully defended his doctoral dissertation on 23 July 2021 at 12:00 entitled Petrogenesis of Peridotite and Pyroxenite in the Banovina area, Croatia.
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