DBPapers
DOI: 10.5593/SGEM2014/B11/S1.015

GEOCHEMISTRY OF DETRITAL GARNETS FROM ALLUVIAL SEDIMENTS OF THE BISTRITA AURIE RIVER, ROMANIA

C. Ciortescu, G.O. Iancu, I. Sandu, V. Vasilache, G. Jigau
Wednesday 1 October 2014 by Libadmin2014

References: 14th International Multidisciplinary Scientific GeoConference SGEM 2014, www.sgem.org, SGEM2014 Conference Proceedings, ISBN 978-619-7105-07-0 / ISSN 1314-2704, June 19-25, 2014, Book 1, Vol. 1, 121-128 pp

ABSTRACT

The current study focused on the composition of detrital garnets from sediments of the Bistrița Aurie River, located in the Eastern Carpathians, Romania. A number of eight samples, collected from meanders and gravel bars, have been in situ separated with a gold pan. Garnets with a size between 0.5 and 1 mm were selected from the alluvial concentrates and mounted in epoxy resin, followed by polishing and carbon coating. So, 96 garnets and their inclusions have been used to determine the major element compositions with a SEM-EDX, in order to characterize the forming conditions and to track their sources. Additional investigation on the garnet grains were performed with a stereo microscope. Garnet crystals are the most abundant heavy minerals from the Bistrița Aurie River deposits. They often contain solid inclusions as: ilmenite, rutile, titanite, zircon, monazite, allanite, apatite, quartz and graphite. All the analyzed grains represent a mixture of four components: almandine, grossular, spessartine and pyrope. Mostly, according with the chemical composition, garnets from alluvial deposits can be divided in four groups with different sources. The first one is rich in almandine and (more than 75 %) has a low content of grossular or spessartine and pyrope. The second one has a high almandine component, moderate grossular (15-30 %), and low pyrope. These two groups have, as source, low and medium metamorphic rocks from the Bretila and Rebra Metamorphic Units belonging to The Crystalline-Mesozoic Zone. Not very frequent, the third group has a high spessartine (25-60 %), moderate almandine and very low pyrope. The source of this group is the low grade metamorphic rocks from Tulghes Metamorphic Unit. The most problematic were few garnet grains with high content in pyrope component (from 25 to 40%) and low grossular, sampled from three different areas of the Bistrița Aurie upper basin. This type of garnets, belonging to the 4th group, is believed to be derived from high-grade rocks with granulitic character. In the hydrographic basin of the Bistrița Aurie, metamorphic formations with such a high content of pyrope have not been described thus far. This could testify isolated hightemperature metamorphic events in the metamorphic history of the Crystalline- Mesozoic Zone, presumed also by other authors.

Keywords:garnet, alluvial deposit, source, pyrope, high-grade metamorphic rocks

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