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