Petrogenesis and mantle source characteristics of Quaternary alkaline mafic lavas in the western Carpathian–Pannonian Region, Styria, Austria

Abstract

In the Styrian Basin, i.e. the westernmost part of the Carpathian–Pannonian Region (CPR), the Pliocene to Quaternary post-extensional phase was accompanied by eruption of alkaline mafic magmas, some of which carry mantle xenoliths. The rocks range from nephelinites (Stradnerkogel and Waltrafelsen) to (predominant) basanites and ne-basanites (Klöch and Steinberg). They have high Ce/Pb, Nb/U and Nb/La ratios reflecting asthenospheric mantle source characteristics with negligible crustal contamination, differentiation en route to the surface and/or interaction with the lithospheric mantle. The calculated depths of magma generation are >100km for the basanites and ne-basanites in comparison to 135km or more for the nephelinites, implying an origin in the garnet stability field. The temperatures of mantle melting for the basanites and ne-basanites are from ≈1400 to 1500°C.Modeling calculation using the most primitive Styrian sample (a basanite) gives a mantle potential temperature (Tp) of 1466°C similar to average Tp of upper mantle sources beneath MORB indicating that the Styrian magmas were generated from asthenospheric mantle sources at ambient temperatures that preclude plume activity beneath the study area.The nephelinites have elevated Zr/Hf (51–67) and La/YbN (29–31) ratios and negative K and Ti anomalies on the PM-normalized multi-element diagrams, similar to those of carbonatites. These characteristics suggest that their source had experienced enrichment by carbonatitic liquids; an inference supported by their estimated content of ~5% CO2. By contrast, the trace element signatures of the basanites and ne-basanites suggest that their asthenospheric source, which experienced higher degrees of melting than the nephelinites, was nearly unaffected by carbonatite metasomatism. From the overall similarity of the trace element distribution patterns and the narrow range of their SrNd isotopic ratios, all the rocks can be related to a similar (OIB-like) asthenospheric mantle source, approximating the European Asthenospheric Reservoir (EAR-type).