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Mineralogical and Geochemical Characteristics of the Middle Miocene Tuffs from Bukova Glava Locality, Krndija Mountain (NE Croatia)

Bukova Glava locality, in the vicinity of the town Našice, is a southern section of a large quarry owned by “Našicecement d.d.” company. The section consists of approximately 100 m thick deposits of middle Miocene age. The deposits are grouped into three main lithofacies: algal limestones, biocalcrudites and marls (PAVELIĆ et al., 2003) along with lithofacies of pyroclastites (KOVAČIĆ et al., 2015). Four layers of altered tuffs few centimetres in thickness were found on the Bukova Glava section of which three have been sampled: first, third and fourth in the sequence. Although all three samples are predominantly composed (~ 98%) of particles smaller than 63 μm, determination of volcanic origin was carried out by modal analysis of the 63 - 125 μm fraction. Mineral composition of light and heavy fractions was determined by optical microscopy and XRD analysis. Tuffs of the first and fourth layer in sequence are characterized by similar mineral association of light and heavy mineral fraction with different shares. Devitrified volcanic glass particles are dominant in light mineral fraction with less common quartz, mica, plagioclase, tridymite, cristobalite and sanidine, while heavy mineral fraction consists of garnet, zircon, apatite and phyllosilicate minerals like muscovite, biotite and chlorite. Tuffs of the third layer in sequence significantly differs from the other two analysed tuffs. About 90% of the light mineral fraction consists of gypsum with subordinate quartz, cristobalite, tridymite and mica, while the mineral phases that prevail in the heavy mineral fraction are opaque minerals and jarosite with weakly represented zircon, garnet, apatite, tourmaline, clinozoisite, muscovite, biotite, chlorite and barite. XRD analysis confirmed that the opaque minerals are mainly represented by ilmenite, assumed to have terrigenous origin. Also, presence of gypsum in the light mineral fraction may indicate the primary presence of sulfides whose oxidation could led to the formation of secondary sulfate minerals (ZIMBELMAN et al., 2005). Whole rock analysis of tuffs was obtained by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mass spectroscopy (ICP-MS) in Bureau Veritas Commodities Canada Ltd. The main characteristics of analysed tuffs are significant values of LOI (23.30 – 25.20 wt.%) and similar SiO2 (47.91 – 50.21 wt.%) content. Discrimination diagram based on the ratio of immobile elements Zr/Ti vs. Nb/Y (PEARCE, 1996) was used for general classification of tuffs due to highly altered layers of tuffs, so other discrimination diagrams based on mobile elements could possibly give incorrect data. According to the Zr/Ti vs. Nb/Y diagram, tuffs of the first and fourth layers correspond to the field of neutral series, i.e. tephriphonolite, and tuffs of the third layer plotted in the field of alkali basalts. In our opinion, presence of terrigenous ilmenite could reduce the value of Zr/Ti ratio, and thereby shift the result to the basic part of discrimination diagram. Discrimination diagram for granites based on immobile elements Y, Nb, Ta i Yb (PEARCE et al., 1984) was used to determine the geotectonic setting of tuffs formation. The diagram showed that the source for all three tuffs was the active continental margin. At that time, in middle Miocene, the closest active continental margin was in the middle part of Eastern Carpathians and presents the most likely source of volcanism (KONEČNÝ et al., 2002). This research confirmed volcanic origin and neutral geochemical character of altered tuffs. The potential volcanic source is the active continental margin located in the middle part of Eastern Carpathian. The radiometric method should be the focus of future research to determine the absolute age of tuffs.

Bukova Glava, Krndija Mt., NE Croatia, tuffs, middle Miocene, tephriphonolite

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