THE METAMORPHIC EVOLUTION OF THE HERCYNIAN BASEMENT OF NORTH DOBROGEA: CONSTRAINTS FROM PETROLOGICAL STUDIES AND MONAZITE DATING

Part of the Hercynian belt fringing the southern margin of the East European Craton, the strongly foliated pelitic gneisses from Orliga Group, North Dobrogea Orogen, Romania, show the dominant mineral assemblage: biotite + muscovite + quartz + plagioclase ± garnet ± kyanite ± staurolite + tourmaline + zircon + monazite + apatite. X-ray composition maps of Ca, Fe, Mg and Mn in garnet, measured using the electron microprobe, show typical growth zoning in most grains. The peak temperature calculated based on phase equilibria is 680 oC at 8.5 kb, in agreement with thermo-barometric calculations.

Spot analyses performed on 10 matrix monazite grains in both low and high Ca or Y zones gave ages ranging between 324-275 Ma, with a few ages as young as 255 Ma, regardless of the compositional zoning. The monazite ages are consistent with the existing Ar-Ar ages indicating that a major thermal event occurred in the Late Carboniferous-Early Permian.

The ages and metamorphic conditions are quite distinct from the typical low-pressure, high-temperature Hercynian metamorphism recorded in other structural nappes. The present day association of distinctly different metamorphic types in different nappes requires late juxtaposition of different crustal levels, presumably by thrusting. The timing of this thrusting is not well constrained, but may be related to Cimmerian deformation.