Post-collisional magmatism associated with Variscan orogeny in the Danubian Domain (Romanian Southern Carpathians): the Motru Dyke Swarm

The metamorphic basement of the Danubian nappe system in the Romanian Southern Carpathians is comprised of Neoproterozoic high-grade metamorphics and granitoid intrusions, underlying low-grade Ordovician to early Carboniferous formations. This entire sequence is intruded by late Variscan age (Carboniferous to early Permian) post-collisional granitoid plutons. The Motru Dyke Swarm (MDS), a geographically-extensive system of intermediate to felsic dykes crosscuts the whole Danubian basement. Zircon U/Pb results on some MDS dikes indicate a largely inherited zircon population with an age distribution pointing to Pan-African origins and a peri-Amazonian provenance for MDS crustal sources. However, in the field the MDS dykes cross-cut late Variscan post-collisional plutons, supporting the assertion that these dykes are much younger, with Upper Paleozoic, most likely Carboniferous (Variscan) ages. This study is aimed at providing a better understanding of the petrogenesis, emplacement age, and evolution of the MDS using zircon U/Pb dating, geochemical, isotope, and petrographic data. The dykes record heterogeneous petrographic and geochemical (major and trace elements, as well as radiogenic isotope) signatures. MDS dykes are mainly basaltic-andesite, andesite, trachyandesite, and dacite in composition and petrography with porphyritic basaltic andesites being the most common. The absence of olivine and the presence of brownish amphibole, biotite- and quartz-mantled zircons, as well as mm- to cm-size enclaves of meta-sediments are the most distinctive petrologic features of the MDS. Published[1] major elements compositions define a complete differentiation series, ranging from basaltic andesites to rhyolites (50-72 wt% SiO2) with medium-K to shoshonitic calc-alkaline characteristics. The dykes are dominantly metaluminous (average ASI of 0.90). Considerable heterogeneity is preserved in their trace element compositions: ΣREE ranges from 41.18 to 378, Eu/Eu* from 0.79 to 1.14, Nb/U from 2.89 to 13.15, and Ce/Pb from 1.5 to 44.4. On the other hand, some ratios (i.e. U/Th, Nb/Ta, and Zr/Hf) remain comparatively constant which has led some to infer oceanic arc origins. Radiogenic isotope signatures (maximum value for 87Sr/86Sr of 0.7426, while ɛNd ranges from 2.6 to -16.1) do not confirm this model, indicating instead a heterogeneous source of mixed mantle and crustal components. Trace element and radiogenic isotope modeling broadly support, source mixing among sub-crustal enriched sources and crustal protoliths. The heterogeneities indicated for MDS sources suggest that the latest stages of Variscan magmatism in the Danubian domain was characterized by the development of small volume melts derived from diverse magmatic sources in both the crust and mantle. The occurrence of dykes with mantle-like signatures may argue for the occurrence of lower crustal delamination as a thermal trigger for MDS magmatism Acknowledgements: study financed through grant Idei PN-II-ID-PCE-2011-3-0100. [1] Femenias, O., T. Berza, M. Tatu, H. Diot & D. Demaiffe (2008). Nature and Significance of a Cambro-Ordovician high-K, Calc-alkaline Sub-volcanic Suite: The Late- to Post-orogenic Motru Dyke Swarm (Southern Carpathians, Romania) . Int J Earth Sci. 97: 479-496.