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

Abstract

The East Carpathian volcanic arc is the youngest region of calc-alkaline magmatic activity in Eastern Europe. A general age progression of the onset and cessation of magmatic activity occurs along the East Carpathian arc 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 arc that is thought to have taken place during the Miocene (9–5 Ma). Migration of magmatic activity from NW to SE along the arc may be explained by a corresponding migration of the magma-generating zone at mantle depths. Major and trace element characteristics of the erupted products are typical of subduction-related magmas and suggest an input of fluids from a dehydrating subducting slab into their mantle source region. Subduction of a narrow oceanic basin is considered to be the most probable cause 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 along the slab, causing termination of volcanism as the slab sank out of the magma-generation zone. Breakoff of the slab occurred at progressively shallower levels, southward along the arc, causing the volume of erupted arc magmas to diminish. Some unusual geological features at the southern end of the volcanic arc (e.g. contemporaneous eruption of alkaline and calc-alkaline magmas; extreme enrichment in K and other large ion lithophile elements in the arc magmas) may be accounted for by asthenospheric mantle upwelling into the void left behind by the broken slab and increased efficiency of dehydration of the remnants of the slab under the higher thermal regime.