Petrogenéza miocénno-pliocénnych granitoidov A-typu južného Slovenska: Petrogenesis of Miocene–Pliocene A-type granitoids of southern Slovakia

International audiencePetrogenetic association of post-orogenic A1-type granitoids genetically related to continental rifting and mantlederivedmafic magmas was recognized for the first time in the northern part of the Pannonian Basin. The variegated rocksuite comprises syenite, orthopyroxene-rich, charnockite-resembling granite (pincinite), and subalkalic granite stronglyenriched in Nb, Y and REE. Absence of rock-forming OH-bearing silicates in all rock types indicates water-deficient, hypersolvuscrystallization conditions and lacking overprint by deuteric hydrothermal alteration and/or subsequent metamorphic/metasomatic reactions. U-Pb-(Th) dating of monazite and zircon proved that all rocks crystallized within a narrow timeinterval between 5.2 and 5.8 Ma (Late Miocene – Early Pliocene boundary). The suite of A1-type granitoids is peraluminous,albeit individual rock types exhibit strongly fluctuating alkalis-to-calcium ratios: pincinite belongs to calcic rocks, granitesare calc-alkalic and syenites are alkalic. Except for magnesian pincinites, other rock types are ferroan. Syenites bear mostgeochemical features diagnostic of the A-type granites sensu stricto. They represent flotation cumulates from stronglydifferentiated alkalic basalt uncontaminated by the crustal material, as is indicated by the strontium and oxygen isotoperatios close to those in prevalent depleted mantle, strong enrichment in REE and Zr+Hf, moderate enrichment in Nb+Ta,and fluctuating distribution pattern of other elements. In contrast, a smoothed distribution pattern of trace elements inpincinites, enrichment in Ca, Ti, V, LREE, moderate depletion in Ba, Sr, absence of halogenides and siderophile elementsare interpreted as reflecting a high-degree dehydration melting of amphibole triggered by latent heat from alkali basaltunderplating the lower crust. The position within the field of OIB-like magmatic rocks rules out a substantial amount offelsic magmatic or sedimentary component in the zone of crustal anatexis. Sub-alkalic granites are provisionally interpretedas highly evolved residual melt originated by differentiation of alkalic basalt/trachybasalt modified by assimilation of aCa-rich contaminant, or as a low-degree partial melt from a mafic source similar to that in the case of pincinite. Position ofthe granites within the field for OIB-like magmatic rocks also excludes the substantial amount of felsic magmatic and/orsedimentary crustal component. In spite of the affinity to the A1-type subgroup, the sub-alkalic granites contain surprisinglyhigh concentrations of HFS elements, particularly Nb (up to 207 ppm). The coherent A1-subtype suite of early anorogenicgranitoids is different from the A2-subtype post-orogenic granitoids of the Western Carpathians formed dominantly bypartial melting of thin crust along continental margins during incipient Permian-Triassic rifting.Key words: A-type, granite, pincinite, syenite, xenolith, alkali basalt, Pannonian Basi

U-Pb geochronology of zircons from fossiliferous sediments of the Hajnáčka I maar(Slavakia) - type locality of the MN 16a biostratigraphic subzone

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U–Th–Pb geochronology of zircon and monazite from syenite and pincinite xenoliths in Pliocene alkali basalts of the intra-Carpathian back-arc basin

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Geochemistry, mineralogy, and zircon U–Pb–Hf isotopes in peraluminous A‑type granite xenoliths in Pliocene–Pleistocene basalts of northern Pannonian Basin (Slovakia)

International audienceAnorogenic granite xenoliths occur in alkali basalts coeval with the Pliocene–Pleistocene continental rifting of the Pannonian Basin. Observed granite varie - ties include peraluminous, calcic to peralkalic, magnesian to ferroan types. Quartz and feldspars are dominant rock-forming minerals, accompanied by minor early ilmenite and late magnetite–ulvöspinel. Zircon and Nb–U–REE minerals (oxycalciopyrochlore, fergusonite, columbite) are locally abundant accessory phases in calc-alkalic types. Absence of OH-bearing Fe, Mg-silicates and presence of single homogeneous feldspars (plagioclase in calcic types, anorthoclase in calc-alkalic types, ferrian Na-sanidine to anorthoclase in alkalic types) indicate water-deficient, hypersolvus crystallization conditions. Variable volumes of interstitial glass, absence of exsolutions, and lacking deu-teric hydrothermal alteration and/or metamorphic/meta -somatic overprint are diagnostic of rapid quenching from hypersolidus temperatures. U–Pb zircon ages determined in calcic and calc-alkalic granite xenoliths correspond to a time interval between 5.7 and 5.2 Ma. Positive εHf val -ues (14.2± 3.9) in zircons from a 5.2-Ma-old calc-alkalic granite xenolith indicate mantle-derived magmas largely unaffected by the assimilation of crustal material. This is in accordance with abundances of diagnostic trace elements (Rb, Y, Nb, Ta), indicating A1-type, OIB-like source mag-mas. Increased accumulations of Nb–U–REE minerals in these granites indicate higher degree of the magmatic dif-ferentiation reflected in Rb-enrichment, contrasting with Ba-enrichment in barren xenoliths. Incipient charnock-itization, i.e. orthopyroxene and ilmenite crystallization from interstitial silicate melt, was observed in many granite xenoliths. Thermodynamic modeling using pseudosections showed that the orthopyroxene growth may have been trig-gered by water exsolution from the melt during ascent of xenoliths in basaltic magma. Euhedral-to-skeletal orthopy-roxene growth probably reflects contrasting ascent rates of basaltic magma with xenoliths, intermitted by the stagna-tion in various crustal levels at a <3 kbar pressure. The Ter-tiary suite of intra-plate, mantle-derived A1-type granites and syenites is geochemically distinct from pre-Tertiary, post-orogenic A2-type granites of the Carpatho–Pannonian region, which exhibit geochemical features diagnostic of crustal melting along continental margins

Combined U/Pb and (U-Th)/He geochronometry of basalt maars in Western Carpathians: implications for age of intraplate volcanism and origin of zircon metasomatism

International audienceThe age of intraplate volcanism in northern Pannonian Basin of Carpathians is revisited using a combination of zircon U/Pb, zircon (U-Th)/He and apatite (U-Th)/He dating techniques, complemented by electron microprobe (EMP) characterisation of dated minerals. A total of six maar structures and diatremes in the South-Slovakian Volcanic Field (SSVF) were dated and the obtained new ages yielded the following key findings: Two isolated maars in SE part indirectly dated by geomorphologic constraints to Late Pleistocene are actually of Pliocene (2.8 ± 0.2 Ma) and Late Miocene (5.5 ± 0.6 Ma) ages. In contrast, two maars in NW part of the study area are of Late Pliocene age (4.1 ± 0.4 and 5.2-5.4 Ma), younger than the Late Miocene age (~6.5 Ma) inferred previously from K/Ar data on the proximal basaltic lava flows. These maars therefore belong to the second volcanic phase that was previously identified only in SE part of the SSVF. In the light of the new geochronologic data, it seems likely that the Pliocene phreatomagmatic eruptions may have occurred along extension-related, NW- and NE-trending orthogonal faults. EMP analyses and imaging revealed an extensive syn- and post-growth metasomatic replacement by dissolution-reprecipitation in the majority of zircons. Abundant silicate melt inclusions in porous metasomatised parts of the zircons are diagnostic of magmatic rather than hydrothermal metasomatism. Consistent ages of the metasomatised and non-metasomatised zones do not indicate disturbance of the U-Pb system during the metasomatism. Enrichment in U and Th loss in the metasomatised zircons are diagnostic of an increasing oxygen fugacity triggered by degassing of the volatile residual melt during the final stages of alkali basalt fractionation. Rare zircon-to-baddeleyite transformation was probably connected with lowered silica activity in carbonated basaltic magmas in south-eastern part of the study area