Acid Miocene volcanism in the Eastern Slovakia, variable sources and magma forming processes: constraints from petrology and geochemistry

Young Middle Miocene (Badenian to Sarmatian) highly silicic volcanism is evolved in the region of the East Slovakia Depression (ESD). Four areal regions were sampled: 1) extrusions outcropping on the north of the ESD (Merník and Beňatina), 2) few bodies on the northern part of the ESD around Lesné, 3) small bodies on the south in the region of Zemplin and 4) bodies near southern continuation of the Slanské vrchy mts (Byšta). Variability in mineralogy reflects magma composition and stages of magma evolution. Highly siliceous rocks (70.7-77.3 wt. % SiO₂) occupy rhyolite field in TAS diagram. Rhyolite mineralogy is in part regionally dependent. The most striking feature is presence of almandine garnet (Alm₇₆Pyr_9.6Gross_6.9Spess_5.2) in extrusions at Merník, Beňatina and Lesné. Almandine phenocrysts with fairly homogeneous composition across the grain and with inclusions of zircon and apatite are presumed to be of magmatic origin. Presence of highly to moderately corroded garnet is suggestive of its instability at low pressure during ascent. High pressure garnet fractionation is recorded by intensive whole rock HREE depletion and steep REE profiles. Suppressed plagioclase fractionation indicated by absence of Eu negative anomaly and low Rb/Sr = 0.66 and corroded quartz phenocrysts is consistent with quick magma evacuation without essential low pressure fractionation/assimilation in the upper crustal magma chamber(s). Rhyolites from Byšta can be recognized by presence of orthopyroxene. Plagioclase, biotite, K-feldspar and ±quartz are surrounded by fully crystallized matrix (feldspars and quartz) with only minor glass abundance. Complex plagioclase zonality (hybrid cores resulted from mixing of two contrasting magmas, events of drastic corrosion, development of more basic in composition thick rims with numerous melt pockets entrapped), presence of large relic clinopyroxene (presumed to be a remnant after fractionation from more basic magma), disintegrated intergrowths of biotite-plagioclase-ilmenite cumulates indicate evolution of magma likely via AFC in shallow magma chamber which developed in the upper crust. Rhyolites from Zemplin area are compositionally heterogeneous, but neither contains garnet or orthopyroxene. Almandine garnet is common in banded metapelitic rocks trapped as xenolits. Inclusions of quartz, ilmenite, Mg-chlorite, graphite, biotite and typically spessartine enriched margin of host garnet are evidence of its metamorphic origin. Most of extrusions or dykes are autometamorphosed. Matrix is replaced by K-feldspar, quartz and clay minerals; Fe released from mafic biotite fills cavities or armor phenocrysts. Rhyolite bodies have variable mineral proportions, crystallinity of the matrix and phenocrysts that is unique for each body. Peraluminous rhyolites from Eastern Slovakia are characterized by low content of Nb (10-19 ppm), Rb (120-159 ppm) and Y (5-37 ppm). Position on tectonic discrimination diagram corresponds to felsic magmatites evolved on the volcanic arc. Negative Nb, P, Ti anomalies and Pb peak on multi-element diagram indicate evolution with contribution of continental crust. Almost identical shape as for rhyolites from Central Chilean Andes suggests their origin in subduction regime with volcanic arc developed on thin continental crust. Rhyolites were analysed for Nd and Sr isotopic ratios (¹⁴³Nd/¹⁴⁴Nd 0.51223-0.512484 and ⁸⁷Sr/⁸⁶Sr 0.708163-0.715491). Variations in isotopic ratios are compatible with crustal component involved in petrogenesis of rhyolites. Three domains can be identified: 1) more ¹⁴³Nd/¹⁴⁴Nd and less radiogenic ⁸⁷Sr/⁸⁶Sr rhyolites from Merník (δNd -3.0, δSr 52), 2) lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr ratios from Beňatina and Byšta (δNd -7,5, δSr 92,9), 3) ¹⁴³Nd/¹⁴⁴Nd as with 2-nd domain but more radiogenic ⁸⁷Sr/⁸⁶Sr from Zemplin area (δNd -7,5, δSr 156). Position on the δNd vs δSr diagram scatters along trajectory from MORB to upper crust. Merník with the least crustal influence overlaps with fields of A-type and I-type granites; Beňatina and Byšta cluster in the intersection of I-type and S-type granites and Zemplin area is in the field of S-type granites. Dependence of variations in Nd-Sr isotopic ratios on areal distribution is attributed to different source composition and/or type and intensity of interaction with country rock during formation of partial melts.
   Acknowledgment: This work was supported by the project 15 06 supported by Ministry of Environment of Slovak Republic and by the Slovak Research and Development Agency under the contract No. APVV-0549-07.