Development of columnar jointing in albite rhyolite in a rapidly cooling volcanic environment (Rupnica, Papuk Geopark, Croatia)

摘要

The Rupnica locality became the first protected geosite in Croatia in 1948 owing to a well-exposed phenomenon of columnar jointing developed in albite rhyolite. According to the geochemical signature, melt that originated at mid-crustal depths (10-15 km) reached a shallow emplacement level as a high-temperature lava. Non-destructive statistical analysis conducted at the geosite revealed that relatively narrow (29.4 ± 6.3 cm) predominantly four- and five-sided columns, tightly packed striae (4.8 ± 2.1 cm) and small-amplitude wavy sets of columns (wavelength of 32.5 ± 6.7 cm) dominate the exposed part of the Rupnica. A number of features quantified through this statistical approach, together with petrography, geochemistry and the geology of the geosite's surroundings, suggest the rapid cooling of a (sub)surface acidic lava body and the behaviour of the evolving volcanic system, developing columnar jointing under the combined influence of large-scale constitutional supercooling and thermal contraction processes.