Metamorphism of glaucophane-schist rocks at Fruška Gora complex, northern Vardar zone, Serbia: Glaucophane-riebeckite-pumpellyite-actinolite-epidote-chlorite schists with zonal Na-amphiboles

Abstract

The paper presents authors’ data on mineral assemblages in rocks of olistostrome melange in the northern part of the Vardar zone in the Fruska Gora Range, Serbia, Balkan Peninsula, which were affected by metamorphism to low grade of the glaucophane-schist facies in relation to Late Jurassic-Early Cretaceous subduction. The olistostrome is dominated by phengite-chlorite-quartz metasandstones and glaucophane-bearing metabasalts, with the latter rocks containing an equilibrium association of zonal riebeckite-glaucophane amphibole, chlorite, pumpellyite, actinolite, and epidote. The Na-amphibole is commonly zonal and has riebeckite cores and glaucophane outermost zones. The metasandstones contain phengite rich in Si (3.45 f.u.) and Mg (0.40 f.u.), which can be formed only under high pressures. The composition of the phengite and the position of glaucophane-pumpellyite-bearing mineral assemblages from the Fruska Gora Range in the P-T grid for low-temperature metabasites indicate that the culmination of subduction-related metamorphism occurred at T − 340–350°C, P = 4−8 kbar, which corresponded to a depth of 14–29 km. Comparison with certain typical glaucophane-bearing complexes shows that the Rbk → Gln zoning of the Na-amphiboles develops in the course of prograde metamorphism. The zoning of Na and Na-Ca amphiboles of the riebeckite-glaucophane-winchite-barroisite series is proved to be, along with zonal garnet crystals, a sensitive indicator of prograde high-pressure transformations in metabasites in the low-temperature region and can be utilized to distinguish between metamorphic zones and subfacies in glaucophane schist complexes.