U–Pb zircon geochronology and anomalous Sr–Nd–Hf isotope systematics of late orogenic andesites: Pieniny Klippen Belt, Western Carpathians, South Poland

Abstract

Late orogenic andesites and basaltic andesites in the Pieniny Klippen Belt (PKB) in the Western Carpathians display atypical isotope systematics, strongly departing from those observed among other Neogene–Quaternary calc-alkaline rocks in the Carpathian–Pannonian region. Low εNd values (−10.6 to −5.2) and low εHf values (−19.6 to −5.2) are accompanied by unusually low 87Sr/86Sr ratios (0.7052 to 0.7059). Major, trace element, isotope and geochronological data presented in this study indicate two magma sources evolving with low Rb/Sr ratio from which the PKB calc-alkaline rocks originated. Basaltic andesites showing less enriched signature and wider range of isotope compositions (εNd=−7.7 to −5.2, εHf=−11.3 to −5.2 and 87Sr/86Sr=0.7052–0.7059) are interpreted as having originated from the partial melts derived from an ancient metasomatized, sub-continental lithospheric mantle which during emplacement were contaminated by the upper crustal material. Andesites, whose precise timing of emplacement was constrained by in situ LA ICP-MS U–Pb zircon dating at 11.2±0.2 to 12.1±0.3Ma, show more enriched signature and narrow range of Nd and Hf isotope compositions (εNd=−10.6 to −10.1, εHf=−19.6 to −18.3) accompanied by low 87Sr/86Sr (0.70531–0.70541). U–Pb dating of the inherited domains in zircons revealed the presence of late Paleozoic, Precambrian and Archean age components which support crustal origin of the PKB andesites. In situ Hf isotope analyses of zircons suggest that andesites could have been derived from enriched (initial εHf=−13 to −10) Permian igneous domains in the lower crust. Generation of the calc-alkaline magmas in the upper lithospheric mantle and in the thickened crust was triggered by decompression or crustal delamination following Middle Miocene collision of the Alcapa block with southern margin of the European platform. The PKB fault zone and its splays acted as conduits which enabled ascent of small volumes of andesitic magmas into the upper crust of the Western Carpathians.