Continental margin arc magmatism in the Carpathians was related to the south-westwards subduction of the north-eastern European plate beneath the Pannonian Block. The Carpatho-Pannonian region straddles the African-European suture and contains a complex inter-related series of small crustal blocks and microplates. In order to accommodate the large scale movement of these crustal blocks, a narrow oceanic basin was subducted along the Carpathian rim beneath continental crust (30-40 km thick) of the Tisza-Dacia microplate. The East Carpathians of Romania represent the youngest and most voluminous segment of this juvenile arc and were active from the late Miocene to the Pleistocene (~12 to <0.5 Ma; Pécskay,1995).
Calc-alkaline lavas are predominantly evolved
(MgO <5%; Ni <50 ppm) and have trace element and isotopic signatures which suggest that widespread crustal contamination occurred (d18Ocpx =5.2-8.5; 87Sr/86Sr 0.7041-0.7110; 143Nd/144Nd =0.512902-0.512391; 206Pb/204Pb=18.43-19.04). Crustal assimilation processes were complex with some of the most evolved magmas being the least contaminated. Only a few volcanic centres display concomitant AFC trends (Mason et al., in press). Variations in isotopic and trace element signatures along the strike of the arc have been linked to variations in the composition of parental magmas and enriched crustal contaminants. Processes responsible for the production of primary arc geochemical signatures are difficult to resolve, with both subduction zone source enrichment and lower crustal assimilation strong possibilities to explain enrichments above expected mantle-wedge compositions.
The youngest segment of the arc, the Southern Harghita Mountains, was active as subduction processes waned in the Plio-Pleistocene (Szákacs et al., 1993). Contemporaneous alkaline volcanism occurred 40km behind the arc axis in the Persani Mountains. The volcanic arc in this region cuts away from the continental metamorphic basement of the Tisza-Dacia microplate and magmas were erupted through palaeo-accretionary wedge sediments. High-K to shoshonitic calc-alkaline magmas were enriched in LILE and HFSE with particularly high Sr/La and Ba/La towards the final stages of magmatic activity. Elevated incompatible element concentrations have been linked to smaller degrees of partial melting of a deeper source (Szákacs et al., 1993); however an additional role for increased fluid contribution from the subducted slab is also envisaged. Magmas were contaminated with up to 30% paleo-accretionary wedge material
in shallow magma chambers as opposed to metamorphic basement elsewhere in the arc. Pb-isotopic compositions are distinctive in the youngest part of the arc and Sr-Nd isotope systematics suggest a mantle source close in composition to that which gave rise the the alkaline magmas. Variations in the source composition, volume of erupted product, and tectonics in the overriding plate may have been controlled by the delamination or breakoff of the subducted lithosphere as subduction ceased. Closely related alkaline magams were erupted without significant extension in the back-arc region and a slab window model (Hole et al., 1991) to facilitate asthenospheric upwelling is suggested.
Hole, M. J., Rogers, G., Saunders, A. D. & Storey, M., Geology 19, 657-660 (1991).
Mason, P. R. D., Downes, H., Thirlwall, M., Seghedi, I., Szakacs, A., Lowry, D. & Mattey D., J. Petrology in press.
Pécskay, Z., Edelstein, O., Seghedi, I., Szákacs, A., Kovacs, M., Crihan, M. & Bernad, A., Acta Vulcanologica 7, (1995).
Szákacs, A., Seghedi, I., & Pécskay, Z., Rev. Roum. Géologie 37, 21-36 (1993).