Kónya Péter (szerk.): A Bakony-Balaton-felvidék vulkáni terület ásványai - TQS Monographs 1. (Miskolc - Budapest, 2015)
Kónya P.: A Bakony-Balaton felvidék vulkáni terület miocén-pleisztocén bazaltvulkazinumusa
Kónya P. (szerk.) (2015): A Bakony-Balaton-felvidék vulkáni terület ásványai. TQS Monographs 1. Miskolc-Budapest: Herman Ottó Múzeum és Magyar Földtani és Geofizikai Intézet, pp. 41-62. A Bakony-Balaton-felvidék vulkáni terület miocén-pleisztocén bazaltvulkanizmusa Miocene to Pleistocene basalt volcanism of the Bakony-Balaton Highland Volcanic Field, Hungary KÓNYA PÉTER Magyar Földtani és Geofizikai Intézet, 1143 Budapest, Stefánia út 14. e-mail: konya.peter@mfgi.hu Abstract The volcanic erosion remnants in the Balaton Highland and South Bakony belong to the Bakony - Balaton Highland Volcanic Field (BBHVF), which is located in the western Pannonian Basin, Hungary. The BBHVFhas approximately 50 monogenetic basaltic volcanoes in a relatively small (around 3500 km2) area. The volcanism was active from the Late Miocene (~8 My) until the Pleistocene (~2.3 My). The volcanic activity can be explained by the main plate tectonic events in the Carpathian-Pannonian Region. Early Miocene (~24 My) opposite-sense translations and rotations occurred between two small continental blocks. The Alcapa block, situated north of the Mid-Hungarian lineament, underwent counterclockwise rotation, whereas the Tisza-Dacia block began to rotate clockwise. Early to Middle Miocene (24-16.5 My) tectonics were characterised by NE to E-ward translations of the two blocks and collision of the Alcapa block with the European continent. This generated largescale contractions in the northern part of the Outer Carpathians and initiated back-arc extension in the Pannonian Basin. During Middle Miocene (16.5-11 My), the driving mechanism of extension was the subduction roll-back of the European lithosphere subducted beneath the Carpathian arc, to the north and east of the basin. The central Pannonian Basin was affected by pure E-W extension, resulting in the generation of deep basins and crustal thinning. In the Late Miocene to Pliocene (11-2.4 My) the syn- to post-rift phase accompanied by diapiric upwelling of asthenospheric mantle and active thinning of the mantle lithosphere. The basement of the volcanic filed consists of thick Palaeozoic to Mesozoic, and Neogene sequences. In the Neogene, just shortly before the volcanism started, a large lake occupied the Pannonian Basin, the Pannonian Lake. Then the area was most likely characterised by an alluvial plain. The volcanism of the BBHVF was intracontinental maar/diatreme volcanism. Based on thejuvenile glass content and the sedimentary structures the majority of the eruptive centres of the BBHVF have a phreatomagmatic history. The alkaline basaltic eruptive centres are eroded scoria cones, tuff rings, maar volcanic complexes, shield volcanoes and sill/dyke complex. Most of the explosions took place at depths of more than 100 metres. The phreatomagmatic explosions which created the maar craters took place at the lower levels of the volcanic subsurface, where the groundwater level was closer to the surface. The higher areas (northern part of the BBHVF) were typically characterised by Stromboli-type scoria cones and lava plateaus (Kab Hill, Agár-tető). The source of the water needed for the explosions was most probably the pore water of the Pannonian sediments, the karst water of the thick Permo-Triassic sequence, and overland flow of the South Bakony, respectively. The geographical distribution of vent types is in relationship with the general hydrogeological characteristics of the pre-volcanic rock units. In areas where thick Pannonian sandstone beds build up the underlying strata the so called “normal maar volcanic centres” have usually a thick late magmatic infill the maar basins (Tapolca Basin). In areas where relatively thin Pannonian sandstone beds cover the thick Mesozoic or Palaeozoic fracture controlled karst water bearing aquifer, large maar volcanic sequences are common, classified as Tihany-type maar volcanoes (Tihany Maar Volcanic Complex). According to the sum-alkalines-Si02 diagram, the alkaline basalts of BBHVF belong to the basanite-tephrite, trachybasalt and alkaline basalt ranges. The basalts are porphyritic and contain predominantly Mg-rich olivine and less clinopyroxene phenocrysts. The groundmass is composed of plagioclase, clinopyroxene, olivine, Fe-Ti oxides, apatite and occasionally glass, nepheline or leucite. Olivine and amphibole megacrysts and ultramafic, granulite and sedimentary xenoliths are common in the alkaline basalts. The rocks show bimodality of Si02. The boundary between the low-Si (e.g. Badacsony Hill, Gulács Hill, Hegyes-tű, Sarvaly Hill) and high-Si (e.g. Fekete Hill, Haláp Hill, Kovácsi Hill, Uzsa quarry) can be placed at about 48.3% of volatile-free Si02. The low-Si basalts have normative nepheline >4%, whereas the high-Si basalts are characterized by normative nepheline <6%, or normative hyperstene up to 9%. These basalts were generated by a mixing process of astenospheric and litospheric components. According to the recent 40Ar-39Ar ages the volcanism occurred in two broad periods: the first period is confined to
