Collision-related Early Paleozoic evolution of a crustal fragment from the northern Gondwana margin (Slavonian Mountains, Tisia Mega-Unit, Croatia): Reconstruction of the P–T path, timing and paleotectonic implications

Abstract

An orthogneiss from the oldest metamorphic complex at Mt. Papuk (Tisia Mega-Unit, Croatia) enables the quantification of the P–T evolution of Early Paleozoic rocks of the Panonian Basin basement in contrast to neighboring peri-Gondwanan terrains which are significantly overprinted by pre-Variscan, Variscan, and Alpine events. Two different groups of Ce-rich monazite within oval-shaped corona microstructures have been observed. Age dating of the corona cores yielded two populations with average ages of 528±7 (2σ) Ma and 465±7Ma, respectively. Furthermore, an Y-rich group, found inside garnet cores, was dated at 616±23Ma. Th-rich monazite included in garnet rims yielded an age of 491±6Ma. The youngest monazite group at 417±20Ma is located inside mica. The orthogneiss precursor was a calc-alkaline to high-K calc-alkaline igneous peraluminous crustal rock (diorite) from an active continental marginal setting. The calculated P–T pseudosection in the MnNCKFMASHTO system in combination with assemblage characteristics and mineral chemistry data provides good constraints on the P–T evolution: for stage I peak P–T conditions of 13kbar and 670°C were derived followed by stage II, which was characterized by moderate cooling accompanied by uplift to mid-crustal levels (5.2kbar and 610°C). Subsequently, the system cooled to 480°C at ~4.4kbar (stage III). Formation of titanite rims on ilmenite suggests further cooling to 4kbar and 400°C (stage IV). The clockwise P–T path implies exhumation from a tectonically thickened crustal setting (ca. 45km depth at a geothermal gradient of ~15°C/km) to mid-crustal levels (ca. 18km) followed by cooling at depths <14km. Crustal thickening was due to the collision of a continental plate (Gondwana) with a smaller plate, which was underthrust.