The Neoproterozoic Pan-African basement from the Alpine Lower Danubian nappe system (South Carpathians, Romania)

Abstract

The South Carpathians, which were thrust to the Moesian platform in the Alpine orogeny (Late Cretaceous to Tertiary), include the Danubian nappe system. The Danubian pre-Alpine basement comprises two Variscan nappes, each one made up of partially retrogressed amphibolite facies rocks intruded by granitoids and capped by an Ordovician-Devonian volcano-sedimentary cover. No lithological correlation can be established between the pre-Ordovician basements of these two units. The metamorphic basement from the first Variscan nappe, the Drăgşan Group, is composed of banded amphibolites with some augen and aluminous gneisses intruded by granodioritic to tonalitic plutons. The banded structure of the amphibolites, together with their geochemistry, suggests a volcano-sedimentary sequence. Zircon UPb data on an intercalated augen gneiss have 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 717 Ma to 817 Ma. At 777 Ma, ϵ 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 trace elements from the Drăgşan amphibolites consistently display an island arc signature, with three differentiation trends evolving from an early tholeiitic trend to a more differentiated low-K calc-alkaline one. The Drăgşan terrane is similar to the early Pan-African juvenile terranes of the Sahara. The basement of the second Variscan nappe, the Lainici-Păiuş Group, is made up of metasedimentary rocks (mainly quartzites, marbles and graphitic mica gneisses) cut by early leucogranitic dykes, medium-K calc-alkaline and alkali-calcic (mainly granitic) plutons, and late medium-K porphyry diorite dykes. This magmatism can be bracketed between 588 Ma and 567 Ma (UPb zircon ages). The best preserved pluton (Tismana, 567 Ma old), displays a composite alkali-calcic (very high-K calc-alkaline) magmatic sequence, ranging from gabbro-diorite to monzogranite, including an ultramafic pod. Ages and geochemical signatures resemble the Saharan late Pan-African granitoids. The existence of Late Precambrian partly juvenile terrains is thus confirmed within the basement of South Carpathians, which renders then a segment of the European Alpine belt that can be successfully compared to the Pan-African Trans-Saharan belt.