Collision with Gondwana or with Baltica? Ordovician magmatic arc volcanism in the Marmarosh Massif (Eastern Carpathians, Ukraine)

The pre-Alpine Marmarosh Massif is a tectonically complex unit of the crystalline basement within the Eastern Outer Carpathians. In the eastern (Ukrainian) segment of this massif, two units have been identified—the Bilyi Potok Nappe and the Dilove Nappe. Petrological investigations coupled with zircon U–Pb dating were performed on metavolcanic rocks (porphyroids) and their phyllite host rocks, sampled from three locations within the Dilove Nappe. The geochemical characteristics of the meta-rhyodacite porphyroids revealed a volcanic arc affinity of the protolith, with U–Pb zircon ages of 452.8 ± 1.5 Ma and 461.5 ± 1.6 Ma and zircon saturation temperatures in the range of 823–892 °C. The phyllite host rocks (meta-tuff) yield a U–Pb zircon maximum estimate for the eruption age at 584.7 ± 2.9 Ma. Peak amphibolite-facies metamorphism (M1) was estimated at the pressure of 600–900 MPa with a temperature range of 560–600 °C. Retrogression (M2), possibly related to Alpine nappe stacking and shearing, is assumed to have taken place at 200–250 MPa and 384–222 °C. The volcanic arc is interpreted as an early Caledonian arc that was subsequently accreted to the margin of Baltica during the closure of the easternmost Tornquist Ocean rather than Cenerian (early Paleozoic) orogenic events on the margin of East Gondwana.