Geochemical evolution of the lithospheric mantle beneath the Styrian Basin (Western Pannonian Basin)

Abstract

The Styrian Basin is located at the westernmost part of the Carpathian-Pannonian region, in the transition zone between the Pannonian Basin and the Eastern Alps. The lithospheric mantle beneath the Styrian Basin (SB) was sampled by Plio-Pleistocene alkali basalts, which brought mantle xenoliths to the surface. Mantle xenoliths from the SB are mostly coarse granular, amphibole-bearing spinel lherzolites with microstructures indicating extensive annealing. Three geochemical events were recorded in the SB xenoliths. The initial, ancient partial melting event was followed by an old metasomatism, which formed lithological heterogeneities (e.g. websterite, dunite) occurring as veins, bands and layers. The most recent geochemical event recorded in the SB xenoliths is the migration of a hydrous alkaline melt originating from a nephelinitic melt source. This melt migrated from the asthenosphere, forming melt channels in the lithospheric mantle. Close to the channels, the metasomatic agent caused extensive amphibole and sparse phlogopite formation. This particular pargasite-phlogopite equilibrium mineral assemblage in the spinel facies was not described before in the literature based on our knowledge. The reaction caused significant enrichment in basaltic elements (such as Ti, Fe), K, light rare earth elements and incompatible trace elements such as Zr and Hf. Further from the channels, the metasomatic melt became enriched in volatiles (mainly H2O and CO2) and fluid mobile elements (e.g. U, Pb, Cl and P), but depleted in basaltic elements, K and LREE, which resulted in modal decrease of amphiboles. Previous studies suggested a dehydrating subducted slab beneath the Styrian Basin, however geochemical fingerprints of subducted slab derived fluids were recognized only in one xenolith. Mantle portions with different geochemical characteristics developed in the lithosphere laterally, however metasomatic fingerprints were erased later due to annealing and chemical re-equilibration at ambient mantle conditions. The Neogene evolution of the lithosphere overwrote the previous state of the upper mantle beneath the region, thus the signs of the most recent metasomatism by nephelinitic melts could have been only preserved due to this extensive annealing event.