engleski

Pre-Variscan Barrovian metamorphism in the eastern part of the Slavonian Mountains, Tisia Unit (NE Croatia) : Application of quantitative phase diagrams and monazite age dating

Petrological investigations, quantitative phase diagram modelling and monazite dating were carried out on mediumgrade metamorphic rocks from the Kutjevačka Rijeka transect in the Slavonian Mts., Tisia Unit (NE Croatia). Micaschists contain complex zoned garnets with Mn-rich cores and Carich rims. Mn decreases steadily from core to rim, but there is an abrubt increase in Ca between core and rim. This complex zoning was not observed in garnets from intercalated paragneisses and amphibolites. Quantitative phase diagrams and garnet composition isopleths using bulk rock compositions revealed that the garnet cores formed at 584-592oC and 6.4-7.8 kbar. For establishing the PT conditions responsible for garnet rim formation the composition of the garnet cores was removed from the bulk data and a new effective bulk composition was established. Using quantitative phase diagrams, mineral isopleths and thermobarometric methods we calculated peak PT conditions of 600-660 oC and 11-12 kbar for the garnet rim and the matrix assemblage of biotite, muscovite, plagioclase and quartz. Staurolite mentioned in the literature was not observed in this study and the application of quantitative phase diagrams contoured for H2O mode isopleths supports our opinion that during the retrograde PT path the rock did not pass staurolite-bearing fields. PT conditions for the intercalated paragneisses and amphibolites are the same as for the micaschists. The Th, U and Pb contents of yttrium-rich accessory monazites indicate a pre-Variscan (428 ± ; ; ; 25 and 444 ± ; ; ; 19 Ma) age for the medium-grade metamorphism. These data are 70- 100 Ma older than previously published mica Ar-Ar and K-Ar ages from the study area.

Pre-Variscan; metamorphism; Barrovian; Slavonian Mountains; Tisia; Croatia; quantitative phase diagram; monazite; age dating

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