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)

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 ã 2,000 km 2 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 (Tipargasite 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.

Dissolved Ba as discriminator between two adjacent karst catchments that are both subject to allogenic recharge (Sohodol valley, Vâlcan Mountains, Romania)

“…5b). For devising the diagrams, since most of the Sohodol watershed is developed on Lainici-Păiuş rock formations, within which three types of magmatic intrusions had been identi ed, use has been made of the whole rock chemical composition data presently available for those intrusions: namely, the analyses provided by Savu et al (1974) for leucogranitoid dykes and sills collected from the Jiu and Şuşiţa valleys, those provided by Savu et al (1971) for granites collected from the Suseni pluton (located immediately to the east of Sohodol watershed), and those provided by Féménias et al (2008) for Motru Dyke Swarm andesites and basaltic andesites (collected from the areas designated as Jiu Susita and North Bistrita). Unfortunately, no similar chemical analyses are also available for any of the Lainici-Păiuş metamorphic rocks.…”

“…The more recent, metasedimentary Lainici-Păiuş Group includes a lower "Carbonate-Graphitic Formation" underlying an upper "Quartzitic and Biotite Gneiss Formation" (Liégeois et al 1996). In addition, three types of magmatic intrusions were distinguished within the metamorphic formations: (i) dykes and bodies consisting of Late Precambrian leucogranitoids (Liégeois et al 1996); (ii) Late Precambrian granites and granodiorites building up large plutons (Savu et al, 1971;Berza, 1978); (iii) Early Paleozoic (Pre-Silurian) dykes and sills ("Motru Dyke Swarm") which include mainly andesites and basaltic andesites (Féménias et al, 2008). Overall, two thirds of the Lainici-Păiuş Group occurrence area was estimated to be occupied by granitoids (Berza 1978), yet a thorough cartographic representation of this setting was provided only for the large Precambrian plutons (Fig.…”

“…Yet in the area considered by the present study, Ba proved to be, out all analysed constituents, the only one being able to properly discriminate between two distinct karst catchments. Extended areas within the watersheds which provide allogenic recharge to those catchments are occupied by magmatic intrusions, whose chemical composition speci cities (reported in Savu et al 1971Savu et al , 1974Féménias et al 2008) were taken into account in an attempt to explain the recorded particular behaviour of Ba.…”