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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)
Authors:Olivier Féménias  Tudor Berza  Mihai Tatu  Hervé Diot  Daniel Demaiffe
Institution:(1) Laboratoire de Géochimie Isotopique et Géodynamique Chimique, DSTE, Université Libre de Bruxelles (U.L.B.) (CP 160/02), 50, av. Roosevelt, 1050 Bruxelles, Belgium;(2) Institutul Geologic al Romaniei, 1, Caransebeş str., Bucuresti, 78344, Romania;(3) “Sabba S. Ştefănescu“ Institute of Geodynamics of the Romanian Academy 19-21, Jean Louis Calderon str., Bucuresti 37, 70201, Romania;(4) Université de La Rochelle, av. M. Crépeau, 17042 La Rochelle cedex 1, France;(5) UMR CNRS 6112, UFR des Sciences et Techniques, BP 92208, 44322 Nantes Cedex 3, France
Abstract:The Motru Dyke Swarm intrudes the Precambrian Danubian basement of the Southern Carpathians (Romania). It is a marker of a sub-volcanic event that occurred during the early Palaeozoic (Cambrian to Ordovician). The geographical distribution of dykes on a ∼2,000 km2 area is heterogeneous; several areas of high dyke density have been the subject of a detailed petrological and geochemical study. Taken altogether, the 150 samples define a single complete magmatic series, from basaltic andesite to rhyolite. Whole-rock major element variations show a medium- to high-K, calc-alkaline magmatic suite. The compositional variations and the general decrease of trace element contents (both compatible and incompatible, including REEs) from basaltic andesite to rhyolite are consistent with 1) the fractionation of the observed phenocryst assemblages, Ca-amphibole (Ti-pargasite to magnesiohornblende) followed by intermediate plagioclase, clinopyroxene and accessory biotite and quartz and 2) the absence of lower and/or upper crustal contamination. Trace elements diagrams display typical arc patterns (LILE, Pb and LREE enrichment and relative depletion in Nb-Ta, Zr-Hf and Ti). The Th/U, Nb/Ta and Zr/Hf ratios are constant and close to the mantle values throughout the whole series, which argues that the parental magma was generated from a single and homogeneous enriched lithospheric mantle source. The field regional evidence implies that melting occurred during a late- to post-orogenic period of lithospheric extension, and thus took place quite lately after the cessation of Pan-African subduction.
Keywords:Dyke Swarm  Calc-alkaline  Late- to post-orogenic  Geochemistry  Romania
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