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Chromian spinels from the Magura Unit (Western Carpathians, Eastern Slovakia) : their petrogenetic and palaeogeographic implications

Bónová K., Špisiak J., Bóna J., Kováčik M.

Geological Quarterly

2017

Vol. 61, No. 1

3--18

Detrital chromian spinels in sedimentary rocks provide much information concerning the tectonics of their parental ultrabasic rocks. Chromian spinels occurring in the Eocene to Oligocene depos its from the Magura Nappe were exam i ned to provide some constraints on the history of the Magura Basin. The Magura Nappe is a part of the Flysch Belt belonging to the External Western Carpathians. The Magura Nappe is separated by a narrow zone associated with the Pieniny Klippen Belt and is divided into three principal tectono-lithofacies units (from the S to N): the Krynica, Bystrica and Rača units. Cr-spinel is a common accessory mineral (2.3-5.9 vol% of heavy mineral spectra) in the siliciclastic rocks of the Rača and Krynica units. In terms of texture and chemical composition, two types of Cr-spinels were recognized: unaltered and altered. Unaltered spinels were found to contain silicate inclusions such as chromio-pargasite, enstatite, diopside, pargasite, plagioclase and olivine (forsterite). The chromian spinels show wide variations in compositional parameters such as Cr# (0.3-0.7), Mg# (0.3-0.7), TiO2 (<0.03-1.9 wt.%) and Fe2+/Fe3+ (2.5-13) whereas the differences between the Rača and Krynica units are in- significant. These parameters suggest a peridotitic and volcanic origin of the spinels, respectively. The ophiolite source consisting of harzburgitic mantle peridotites was developed mainly in a supra-subduction zone setting; volcanic spinels indicate an origin in mid-ocean ridge basalts, back-arc basin basalts and sporadically in ocean-island basalts. Concerning their geochem i cal features, we propose that during the Eocene to Early Oligocene, the ophiolitic detritus in the eastern part of the Magura Basin deposits may have been derived from a source area located in the Fore-Marmarosh Suture Zone (Eastern Carpathians) that is considered an equivalent of the Black Flysch and Ceahlau units. Some Cr-spinels found in the Eocene sedimentary successions may have resedimented from older Late Cretaceous-Paleocene formations of the Magura Unit, which are considered as reworked sedimentary material from the Pieniny Klippen Belt.

Heavy minerals from sedimentary rocks of the Malcov Formation and their palaeogeographic implications for evolution of the Magura Basin (Western Carpathians, Slovakia) during the Late Eocene–Late Oligocene

Bónová K., Bóna J., Kováčik M., Laurinc D.

Geological Quarterly

2016

Vol. 60, No. 3

675--694

Detrital heavy minerals separated from the Malcov Fm. deposits (Magura Nappe) indicate their source rocks and areas. The heavy mineral assemblages predominantly consist of garnet, zircon, tourmaline, rutile and Cr-spinel. EPMA analyses reveal a few groups of garnets: zoned and unzoned Grs almandines, Prp-Sps almandines, unzoned Prp almandines, almandines, Sps almandines and rare zoned spessartine grains (~85 mol% Sps). The garnet composition indicates that gneisses, mica schists, amphibolites and granites were their main source rocks, but low-grade metapelites with Mn mineralisation probably contributed as well. The detrital dravitic tourmalines were mostly derived from paragneisses and mica schists. Cr-spinel indicates a volcanic source. Based on heavy mineral assemblages, coupled with palaeoflow analysis, we conclude that the Marmarosh Massif and Fore-Marmarosh Suture are the most probable source areas. Aditionally, the Malcov sedimentary basin was supplied by material from the crystalline complexes of the Tisza Mega-Unit and Pieniny Klippen Belt (PKB). The bulk of the clastic deposits comprise classical turbidites. These lithofacies were deposited from either turbidity currents or from concentrated density flows. The palaeoflow record is varied and highlights the contribution of sedimentary material from several directions and/or diversion of gravity currents from the main flow direction (SE–NW). The marginal parts of the Malcov sub-basins were formed of deformed and uplifted older formations of surrounding units of the Magura Nappe and PKB (submerged ridges). Older (Late Cretaceous to Eocene) flysch sediments may have been redeposited from these ridges to neighbouring sub-basins in a transverse direction (NE–SW).