Contrasting P–T–t paths from the basement of the Tisia Unit (Slavonian Mts., NE Croatia): Application of quantitative phase diagrams and monazite age dating

Medium-grade mica schists and intercalated paragneisses and amphibolites from the basement of the Tisia Unit, Slavonian Mountains., northeastern Croatia, contain complexly zoned garnets. At the Kutjevo locality, mica schists are characterised by garnets with Mn-rich cores and Ca-rich rims. Mn decreases steadily from core to rim and Ca increases abruptly. This is in contrast to the paragneisses and amphibolites which contain garnets with smoothly decreasing Ca from core to rim. Quantitative phase diagrams and garnet composition isopleths calculated from bulk rock analyses reveal that the Ca-poor garnet cores in the mica schists formed during an earlier event at 584–592 °C and 6.4–7.8 kbar. Ca-rich rims formed at conditions of 600–660 °C and 11–12 kbar — calculated using garnet isopleths and mineral thermobarometry. The paragneiss and amphibolite provide similar P–T information for the later peak event (ca. 650 °C, 10–12 kbar) but do not preserve a record of the earlier, lower P–T event and modelling shows that garnet was not stable at these conditions. Contrary to previous studies on this outcrop and rock type, no staurolite was observed and quantitative phase diagrams contoured for H2O mode isopleths indicate that the rock did not cross staurolite-bearing fields during the retrograde P–T path. Mica schists from the Krndija locality contain zoned polyphase garnets. Phase diagram calculations reveal that Ca-rich garnet cores formed between 520 and 630 °C and 7–8 kbar. Rims have a lower Ca content and formed at considerably reduced pressures together with andalusite and staurolite at ca 530–570 °C and 3–4 kbar. Since both localities were traditionally considered to be part of the same tectono-metamorphic unit, evidence presented here clearly shows that this cannot be the case. EMP monazite ages are Variscan (350 Ma) in the Krndija mica schists and around pre- or early Variscan (440 Ma) in the Kutjevo mica schists. We therefore propose a more complex internal structure and metamorphic history for this area than previously recognised suggesting a metamorphic evolution of the Slavonian Mountains that includes a pre-Variscan and a Variscan cycle.