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MINERALOGICAL AND PETROLOGICAL CHARACTERISTICS OF MIDDLE TRIASSIC VOLCANIC AND PYROCLASTIC ROCKS FROM THE KUNA GORA MTN. (NW CROATIA)

Along the northern slopes of Kuna Gora Mtn., situated in the junction area of the three major geodynamic units – southern Alps, the Dinarides, and Tisza, the outcrops of volcanic and volcanoclastic rocks associated with middle Triassic carbonates are reported. This area belongs to the south-western part of the Zagorje-Mid-Transdanubian Shear Zone (PAMIĆ & TOMLJENOVIĆ, 1998). The surface manifestations of volcanic and associated pyroclastic rocks are relatively modest totalling about 0.3 km2. Preliminary results on petrography, mineral chemistry and bulk rock chemical composition of these rocks are provided in this contribution. Furthermore, K-Ar age on the K-feldspar mineral separate from basalts is also determined. Their K-Ar age is 241.1±5.2 Ma, which in the geological time scale of OGG et al. (2016) corresponds to late Anisian-early Ladinian. Analysed volcanites occur in the form of massive lavas and are interstratified with minor layers of vitro-crystalloclastic tuffs. They are of porphyritic texture and homogenous structure and are dominantly consisted of homogenous to slightly sericitized K-feldspar (An0.0Ab1.1-3.7Or96.3-98.9), while albitized plagioclase (An0, 21-0, 67) and clinopyroxene (augite ; Wo43.5-44.9En43.5-43.9Fs7.3-11.2.) are rarely encountered. Minor phases are chlorite, Ti-bearing magnetite (TiO2 = 15.58 wt.% ; FeO = 74.08 wt.%), Fe-bearing Ti-oxide (rutile? ; TiO2 = 98.12 wt.%), ilmenite, hematite and illite. In pyroclastic rocks major crystaloclast is K-feldspar (sanidine ; An0.0Ab1.0-3.2Or96.5-98.7). Volcanic rock microcrystalline matrix is holocrystalline or, alternatively, can be made of devitrified volcanic glass. The latter is the dominant component of the matrix of pyroclastic rocks. Millimetre-scale vesicules are often filled by flaky chlorite. Chemical composition of volcanic and pyroclastic rocks is characterized by high amounts of K (K2O = 8.36-9.58 wt.%), low Na (Na2O = 0.13-0.81 wt.%), and Ti (TiO2 = 0.91-0.96 wt.%), while silica varies between 48.40 and 50.29 wt.%. High K content is explained by elevated modal abundances of K-feldspar (~ 55-60%). The loss of ignition value is between 5.2 and 6.9 wt.%, which points to the medium level of alteration. In the classification diagram Nb/Y vs. Zr/TiO2 x 0.0001 (WINCHESTER & FLOYD, 1977) analysed rocks are projected in the field of sub-alkaline basalts. High K content coupled with high values of the ratio Th/Yb (1.6-2.0) indicate their high-K calc-alkaline affinity. The lavas show an evolved geochemical character, and are moderately fractioned in terms of Mg# and Cr content (61-64 and 27-41 ppm, respectively). All lavas and tuffs show moderate enrichment of LREE over HREE [(La/Lu)cn = 2.64-4.42] at ~ 38-50 times chondrite relative concentrations. Negative Eu anomaly (Eu/Eu* = 0.81–0.87) indicates early feldspar accumulation or fractionation at low pressure. All rocks show the pronounced negative anomalies of Nb-Ta and Ti relative to La [(Nb/La)n = 0.32-0.52 ; (Ti)n = 0.71-0.76], which is typical for subduction zone magmas. High values of Th/La (0.29-0.32) and Th/Ta (15.1-16.0) ratios may indicate a continental crust contamination (e.g., TAYLOR & MCLENNAN, 1985), but low positive values of initial εNd(241Ma) (2.08) and low values of 147Sm/144Nd ratio (0.137967) point to the influence of the subducted juvenile material (subducted slab with little pelagic sediment ; SWINDEN et al., 1990). In addition to the enrichment from the fluids derived from the subducted slab it appears that the contamination of magma chamber was facilitated by sediment flux from the ancient, Palaeotethyan, subduction slab, as suggested by the linear trend of Th/Nb values (0.98-1.14) along with a constant Ba/Th ratio (21-27 ; e.g., LEAT et al., 2000). The origin of the volcanic and pyroclastic rocks of Kuna Gora Mtn. is therefore complex resulting from the above-mentioned processes and events. In the Hf/3-Th-Nb/16 diagram (WOOD, 1980) the lavas and tuffs are plotted within the orogeny field, unique for calc-alkaline arc-related volcanic rocks, which also suggests a subduction-related nascent environment. Highly-K calc-alkaline character of these rocks and their characterization based on Th/Yb vs. Ta/Yb (GORTON & SCHANDL, 2000) and La/Yb vs. Sc/Ni (BAILEY, 1981) ratios clearly support the origin Kuna Gora Mtn. volcanic and pyroclastic rocks in an ensialic mature volcanic arc setting developed in an active continental margin environment analogue to the Andean-type volcanism. This is in favour of geodynamic evolution that hypothesizes the existence of an active, ensialic and mature volcanic arc developed along the southern active continental margins of Laurussia during late Anisian-early Ladinian northward subduction of Paleotethyan lithosphere. Therefore, such arc-related explosive volcanism from Kuna Gora Mtn. is furthermore featured by inherited geochemical signatures of the Paleotethyan subducted plate.

mineralogy, petrology, Middle Triassic, volcanic rocks, Kuna Gora Mtn., Croatia

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