Metamorphism of the westernmost Triassic metasedimentary rocks in the Sakar unit, Sakar–Strandja zone, Bulgaria

Abstract

: Metasedimentary sequence from the westernmost Sakar Unit (Klokotnitsa Village area), Sakar–Strandja Zone, comprises a variety of lithologies with Triassic protolith age. In the study area, the Sakar Unit is affected by low-grade metamorphism in the frame of the Maritsa dextral strike-slip shear zone and separates two first-order units of the Balkan orogenic system – the Rhodope massif and the Srednogorie Zone. We present the petrography, mineral chemical data, and thermodynamic modelling of metasediments for a better understanding of the protoliths origin and metamorphic evolution. The mineral assemblages in Triassic metasediments suggest sedimentary protoliths containing quartz, clay, and carbonate minerals (sandstones, clays, and limestones). The detrital minerals (quartz, albite, orthoclase and accessory phases – zircon, monazite, apatite) reveal potential granite source provenance. The dominance of phengite–chlorite association correlates with low temperature and water-saturated conditions. The Perple_X pseudosection, combined with modelled mineral assemblages and mineral chemistry isopleths of muscovite and chlorite, correspond to metamorphism in the range 275–330 °C and 3–4 kbar. The results correspond with a chlorite geothermometer. The thermodynamic modelling corroborates the petrographic observations and proves greenschist metamorphic conditions that affected the Triassic sedimentary cover of the westernmost Sakar Unit.