Getermobarometry of the late Proterozoic to Paleozoic Barrovian metamorphic sequence in the Dacia megaunit: case study Eastern Serbia
Authors: Borojević Šoštarić Sibila , Anzulović Ana 
Published: February 2023

The Dacia megaunit in the Eastern part of Serbia comprises Getic and Supragetic nappe systems and corresponds to E-W striking Balkan Mountains (Sredna Gora and East Balkan units; sensu Schmid et al., 2020). The area of our study is located between Danube River to the East and Mlava to the West (Homoljske Mts., a part of Balkan Mts.) and consist of low to medium grade metamorphic rocks of Late Proterozoic to Early Paleozoic ages.Two different metamorphic units were sampled:(1) northern, low-grade metamorphic sequence is characterized by numerous types of chlorite sheets containing chlorite, epidote, muscovite, actinolite, hornblende and garnets together with quartz, albite and secondary calcite and fine-grained illite. Accessory minerals are titanite, rutile, ilmenite and apatite. &#160;&#160;The sampled schists were recognized as belonging to low and lower part of medium grade Barrovian metamorphic assemblages, characterized by zonal distribution of the index&#8211;minerals: chlorite, epidote, biotite, amphibole and garnet.(2) southern, medium-grade metamorphic sequence is characterized by different amphibolite rocks, with amphiboles (28-60 vol.%) ranging from tchermakite and magnesiohornblende to actinolite. Additionally, these rocks contain 17 &#8211; 40 vol.% of oligoclase, 5-22 vol% of quartz; 5 &#8211; 13 vol% chlorite (ripidolite), 0,4 &#8211; 13 vol% of Al-Fe epidote and 0,1-0,7 vol% of andradite garnet.Multielement diagrams normalized to N-MORB of low-grade metamorphic sequence show enrichment of LILE relative to HFSE with negative Nb and positive K, U and Pb anomalies, while medium-grade metamorphic sequence shows a disturbed pattern with LILE >> HFSE, positive Pb anomaly and in some cases U, Th, while Nb, Ti and Sr are negative. Both sequences show significant crustal influence.Medim-grade metamorphic sequence originate from an igneous precursor (andesite-subalkaline basalt protolith). Using Zr-Ti plot after Pearce, these rocks belong to volcanic arc basalts and within plate tholeiites. According to Meschede (1986) Zr/4-2Nb-Y and Wood (1980) Th-Hf/3-Ta plots, they display normal to enriched MORB characteristics similar to basalts from volcanic arc setting.Geothermobarometric calculations were made for garnet-amphibole-plagioclase assemblage from medium-grade metamorphic sequence using values of titanium in amphibole and aluminum in chlorites. Obtained temperature range between 600 and 750 &#176;C while pressure range between 7 and 9 Kb, corresponding to the recognized amphibolite facies of medium grade metamorphism. A direction of increase of pressure and temperature conditions within the prograde metamorphic sequence towards the south is proposed.References:Schmid SM, F&#252;genschuh B, Kounov A, Ma&#355;enco L, Nievergelt P, Oberh&#228;nsli R, Pleuger J, Schefer S, Schuster R, Tomljenovi&#263; B, Ustaszewski K, van Hinsbergen DJJ (2020) Tectonic units of the Alpine collision zone between Eastern Alps and western Turkey. Gondwana Res 78:308&#8211;374.Meschede, M. (1986) A Method of Discrimination between Different Types of Mid-Ocean Ridge Basalts and Continental Tholeiites with the Nb-Zr-Y Diagram. Chemical Geology, 56, 207-218.Wood, D.A. (1980) The Application of a Th-Hf-Ta Diagram to Problems of Tectonomagmatic Classification and to Establishing the Nature of Crustal Contamination of Basaltic Lavas of the British Tertiary Volcanic Province. Earth and Planetary Science Letters, 50, 11-30.