The early Paleozoic cumulate gabbroic rocks from the southwest part of the Tisza Mega-Unit (Mt. Papuk, NE Croatia): evidence of a Gondwana suture zone

The Mt. Papuk heteroadcumulate pyroxene–amphibole gabbronorites, which outcrops at the southern margin of the Tisza Mega-Unit, is suggested to stem from the deep oceanic crust formed by the in situ crystallisation in a supposed magma chamber. Amphibole oikocrystals are found to define a poikilitic texture of analysed rocks. A common enclosure in amphibole is the cumulus orthopyroxene, and rarely, the clinopyroxene and/or plagioclase and spinel. The chemical composition of related minerals and their crystallisation sequence suggest the sub-solidus crystallisation of gabbronorite in an open system at high temperatures and medium pressures. Parental magmas originated from the moderately depleted mantle source, which was metasomatised prior to melting. Early mineral fractionation gave rise to the assemblage consisted of spinel, pyroxene, plagioclase and intercumulus amphibole. The rocks’ bulk chemistry, mineral crystallisation sequence, pyroxene geochemistry and myriad of high Ca-plagioclase, which coexists with igneous Ca-amphibole are all in favour of the strong subduction influence typical for mafic intrusion formed above mantle wedge in the root of an island arc at depths of 10–21 km. Herein presented geochemical and isotopic data (⁴⁰Ar–³⁹Ar: 487.1 ± 4.3 Ma and Sm–Nd: 505 Ma) go along with the existence of an intra-oceanic arc related to geodynamic events that took place in the Prototethyan oceanic realm s.l. during middle Cambrian to earliest Ordovician. These events were likely correlated with the subduction of the Quaidam(?) back-arc ocean, or alternatively, with the subduction and closure of Prototethyan branches located between microcontinental fragments of Asia. Initially, the closure of back-arc oceans led to crust fragmentation and, then, addition of non-metamorphosed mafites into the obducted sequence further from the active continental margins of Gondwana and Laurassia at the time of the formation of Pangea in the late Palaeozoic.