Huraiová M., Hurai V. & Paquette J.-L., 2015: Petrogenéza miocénno-pliocénnych granitoidov A-typu južného Slovenska. Acta Geologica Slovaca, 7, 1, 37–50.

Petrogenéza miocénno-pliocénnych granitoidov A-typu južného Slovenska

Petrogenesis of Miocene–Pliocene A-type granitoids of southern Slovakia

Monika Huraiová¹, Vratislav Hurai² & Jean-Louis Paquette^3,4,5

¹Katedra mineralógie a petrológie, Prírodovedecká fakulta, Univerzita Komenského v Bratislave, Mlynská dolina, Ilkovičova 6, 842 15 Bratislava, huraiova@fns.uniba.sk
²Geologický ústav, Slovenská akadénia vied, Dúbravská cesta 9, 845 36 Bratislava
³Laboratoire Magmas et Volcans, Clermont Université, Université Blaise Pascal, BP 10448, 63000 Clermont-Ferrand, France
⁴CNRS, UMR6524, LMV, 63038 Clermont-Ferrand, France
⁵IRD, R 163, LMV, Clermont-Ferrand, France

Abstract

Petrogenetic association of post-orogenic A1-type granitoids genetically related to continental rifting and mantle-derived mafic magmas was recognized for the first time in the northern part of the Pannonian Basin. The variegated rock suite comprises syenite, orthopyroxene-rich, charnockite-resembling granite (pincinite), and subalkalic granite strongly enriched in Nb, Y and REE. Absence of rock-forming OH-bearing silicates in all rock types indicates water-deficient, hypersolvus crystallization 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 time interval 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, granites are calc-alkalic and syenites are alkalic. Except for magnesian pincinites, other rock types are ferroan. Syenites bear most geochemical features diagnostic of the A-type granites sensu stricto. They represent flotation cumulates from strongly differentiated alkalic basalt uncontaminated by the crustal material, as is indicated by the strontium and oxygen isotope ratios 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 in pincinites, enrichment in Ca, Ti, V, LREE, moderate depletion in Ba, Sr, absence of halogenides and siderophile elements are interpreted as reflecting a high-degree dehydration melting of amphibole triggered by latent heat from alkali basalt underplating the lower crust. The position within the field of OIB-like magmatic rocks rules out a substantial amount of felsic magmatic or sedimentary component in the zone of crustal anatexis. Sub-alkalic granites are provisionally interpreted as highly evolved residual melt originated by differentiation of alkalic basalt/trachybasalt modified by assimilation of a Ca-rich contaminant, or as a low-degree partial melt from a mafic source similar to that in the case of pincinite. Position of the granites within the field for OIB-like magmatic rocks also excludes the substantial amount of felsic magmatic and/or sedimentary crustal component. In spite of the affinity to the A1-type subgroup, the sub-alkalic granites contain surprisingly high concentrations of HFS elements, particularly Nb (up to 207 ppm). The coherent A1-subtype suite of early anorogenic granitoids is different from the A2-subtype post-orogenic granitoids of the Western Carpathians formed dominantly by partial melting of thin crust along continental margins during incipient Permian-Triassic rifting.

Key words: A-type, granite, pincinite, syenite, xenolith, alkali basalt, Pannonian Basin

Manuskript doručený: 2014-10-20

Manuskript revidovaný: 2015-02-16