TIMING OF HIGH-PRESSURE METAMORPHISM IN THE GETIC-SUPRAGETIC BASEMENT NAPPES OF THE SOUTH-CARPATHIAN MOUNTAINS FOLD-THRUST BELT

The south Carpathian orogen is an Alpine fold-thrust belt comprised of basement nappes separated by late Paleozoic and Mesozoic cover sequences. The main nappe complexes are: (1) the Danubian, containing a metamorphosed basement intruded by late Proterozoic granitoids, (2) the Severin nappe, a fragment of Jurassic oceanic crust, and (3) the Getic-Supragetic unit (GS), consisting of incorporated basement nappes displaying medium to high pressure metamorphic assemblages whose age and origin is controversial. The GS was thrust over the Danubian during the late Cretaceous as a result of the closure of the Severin oceanic basin. Recent geochronologic data suggest that the GS basement nappes might represent an out of place terrane of the late Paleozoic Variscan belt of southern Europe, a Himalayan-type orogen that resulted from the collision between Laurussia and Gondwana. Published data is limited to a couple of zircon 207Pb/206Pb analyses on syn-kinematic pegmatites [1] which yield ages of 333-338 Ma, and six hornblende 40Ar-39Ar measurements [2] which yield ages of 310-320 Ma. In order to constrain the timing of peak metamorphism in the area, we determined mineral Sm-Nd isochrons on three garnet-bearing crustal rocks (one garnet amphibolite and two eclogites), that have experienced peak metamorphic temperatures of ~650-750 0C and pressures of ~11-13 kbar. The calculated ages are 341±8, 344±7, and 358±10 Ma, confirming a Variscan age for the metamorphism of GS. A fourth mineral-whole rock Sm-Nd age of 316±4 Ma was obtained on a mantle-derived garnet peridotite from Foltea, a high temperature peridotite massif emplaced into the GS. The available data suggest that the pegmatites and migmatites within the GS formed subsequent to the peak metamorphism. The Sm-Nd results combined with published 40Ar-39Ar ages suggest cooling rates of about 10 C/Ma. Unroofing of the basement rocks and deposition of Carboniferous-Permian sediments took place during extension between 320-300 Ma [2]. The emplacement of the asthenosphere-derived peridotite in the GS took place during the late Variscan orogenic extension, possibly as a result of the delamination of mantle lithosphere in the area. Ref: [1] Ledru, P. et al. (1997), Rom. J. Mineral., 78; [2] Dallmeyer, R.D., et al. (1998), Tectonophysics, 290, 111-135.