THE NEOPROTEROZOIC PAN-AFRICAN BASEMENT FROM THE ALPINE LOWER DANUBIAN NAPPE SYSTEM (SOUTH CARPATHIANS, ROMANIA)

The South Carpathians, which were thrust to the Moesian platform in th e Alpine orogeny (Late Cretaceous to Tertiary), include the Danubian n appe system. The Danubian pre-Alpine basement comprises two Variscan n appes, each one made up of partially retrogressed amphibolite facies r ocks intruded by granitoids and capped by an Ordovician-Devonian volca no-sedimentary cover. No lithological correlation can be established b etween the pre-Ordovician basements of these two units. The metamorphi c basement from the first Variscan nappe, the Dragsan Group, is compos ed of banded amphibolites with some augen and aluminous gneisses intru ded by granodioritic to tonalitic plutons. The banded structure of the amphibolites, together with their geochemistry, suggests a volcano-se dimentary sequence. Zircon U-Pb data on an intercalated augen gneiss h ave given an age of 777 +/- 3 Ma for the emplacement of the protolith of this gneiss. Nd model ages (T-DM) for the amphibolites range from 7 17 Ma to 817 Ma. At 777 Ma, epsilon(Nd) values cluster is at +8.3 to 9.8 and Sr initial ratios range between 0.7007 and 0.7023, indicating an oceanic origin without continental crust contamination. Major and t race elements from the Dragsan amphibolites consistently display an is land are signature, with three differentiation trends evolving from an early tholeiitic trend to a more differentiated low-K calc-alkaline o ne. The Dragsan terrane is similar to the early Pan-African juvenile t erranes of the Sahara. The basement of the second Variscan nappe, the Lainici-paus Group, is made up of metasedimentary rocks (mainly quartz ites, marbles and graphitic mica gneisses) cut by early leucogranitic dykes, medium-K calc-alkaline and alkali-calcic (mainly granitic) plut ons, and late medium-g porphyry diorite dykes. This magmatism can be b racketed between 588 Ma and 567 Ma (U-Pb zircon ages). The best preser ved pluton (Tismana, 567 Ma old), displays a composite alkali-calcic ( very high-g calc-alkaline) magmatic sequence, ranging from gabbro-dior ite to monzogranite, including an ultramafic pod. Ages and geochemical signatures resemble the Saharan late Pan-African granitoids. The exis tence of Late Precambrian partly juvenile terrains is thus confirmed w ithin the basement of South Carpathians, which renders then a segment of the European Alpine belt that can be successfully compared to the P an-African Trans-Saharan belt.