Magmatic constraints on geodynamic models of subduction in the East Carpathians, Romania

The East Carpathian volcanic are is the youngest region of calc-alkaline ma gmatic activity in Eastern Europe. A general age progression of the onset a nd cessation of magmatic activity occurs along the East Carpathian are from older volcanic structures (ca. 12 Ma) in the NW to the youngest (<1 Ma) in the SE. Magmatism continued into the Plio-Pleistocene, significantly later than the end of basin closure and the onset of continental collision along the Inner Carpathian are that is thought to have taken place during the Mi ocene (9-5 Ma). Migration of magmatic activity from NW to SE along the are may be explained by a corresponding migration of the magma-generating zone at mantle depths. Major and trace element characteristics of the erupted pr oducts are typical of subduction-related magmas and suggest an input of flu ids from a dehydrating subducting slab into their mantle source region. Sub duction of a narrow oceanic basin is considered to be the most probable cau se of the East Carpathian magmatism and its migration. As thick continental crust began to enter the northern part of the trench at around 9 Ma, slab breakoff began although subduction of the detached slab continued at depth. As breakoff progressed from north to south, a rupture or tear propagated a long the slab, causing termination of volcanism as the slab sank out of the magma-generation zone. Breakoff of the slab occurred at progressively shal lower levels, southward along the are, causing the volume of erupted are ma gmas to diminish. Some unusual geological features at the southern end of t he volcanic are (e.g. contemporaneous eruption of alkaline and calc-alkalin e magmas; extreme enrichment in K and other large ion lithophile elements i n the are magmas) may be accounted for by asthenospheric mantle upwelling i nto the void left behind by the broken slab and increased efficiency of deh ydration of the remnants of the slab under the higher thermal regime. (C) 1 998 Elsevier Science B.V. All rights reserved.