Dating and geochemistry of zircon and apatite from rhyolite at the UNESCO geosite Rupnica (Mt. Papuk, northern Croatia) and the relationship to the Sava Zone

Abstract

The Rupnica geosite, a key locality of the UNESCO-protected Papuk Geopark in northern Croatia, is well-known for an excellent exposure of columnar jointing in volcanic rock. This rock is defined as an albite rhyolite that comprises almost pure albite phenocrysts within a fine-grained matrix composed of microphenocrysts of albite, quartz and devitrified volcanic glass. Primary accessory minerals are clinopyroxene, apatite, zircon and magnetite. Haematite, apatite and anatase were found as inclusions in zircon. The albite rhyolite is characterized by a highly siliceous, peraluminous, oxidized (ferroan), dry, alkali-calcic to alkalic composition, with low CaO, MgO, and MnO contents and high FeOT/(FeOT+MgO) ratios. Normalized trace element contents display positive anomalies of K, Pb, and Zr as well as negative anomalies of Nb, P, Ti, Ba and Eu, together with an enrichment of light rare-earth elements (REE) relative to heavy REE. Zircon from the rhyolite of Rupnica is characterized by ratios of Th/U=1.13 and Zr/Hf=55 and contents of HfO2=1.04 wt. % typical for an early-stage igneous zircon crystallized from a dry high-temperature magma in a deep magma chamber. Apatite REE patterns show enrichment of light REE over heavy REE and a pronounced Eu anomaly, typical for apatite from granitoids formed in an oxidizing environment. The magma is of A-type and was generated at high temperatures at 800–900 °C by partial melting of lower- to mid-crustal rocks. The age of the albite rhyolite of Rupnica is Late Cretaceous at 80.8±1.8 (2σ) Ma, according to U-Pb dating of zircon, coeval with geochemically similar igneous rocks of Mt. Požeška Gora and Mt. Kozara within the Sava Zone.