Geochemical and physical properties and evolution of the lithospheric mantle beneath the Nógrád-Gömör volcanic field (Northern Pannonian Basin, Central Europe)
thesisposted on 28.03.2022, 19:44 by Nóra Liptai
The Nógrád-Gömör Volcanic Field (NGVF), in the northern Pannonian Basinin Central Europe, is one of five key localities where the lithospheric mantle of the Carpathian-Pannonian region can be studied using peridotite xenoliths hosted in late Miocene to Pleistocene alkali basalts. This work uses geochemical data for minerals, whole-rocks and silicate melt-inclusions, along with crystal preferred orientations and 'water' contents in nominally anhydrous mantle minerals, to interpret the geochemical and physical evolution of this mantle domain and its role in the tectonic history of the region. Geochemical analyses revealed imprints of multiple metasomatic events, identified by Mg-numbers of olivine and REE concentration of pyroxenes, that affected a heterogeneously depleted lithospheric mantle. These events include (1) formation of Nb-poor amphibole by reaction with subduction-related volatile-bearing silicate melts/fluids, (2) metasomatism by a mafic melt of intraplate origin, resulting in enrichment of U-Th-(Nb-Ta)-LREE in clinopyroxene and formation of Nb-rich amphibole, and (3) the youngest melt-rock reaction, causing enrichment in Fe, Mn, Ti and LREE and overprinting older geochemical signatures. This latest melt was trapped as inclusions, and is similar in composition and origin (OIB-type basalt from low-degree partial melting of a garnet lherzolite) to the earlier metasomatizing melt and the host basalt. The xenoliths display varying physical properties depending on their depth of origin: shallower (lower-temperature) layers have fine-grained textures and more dispersed crystal preferred orientation, whereas deeper layers are coarse-grained and have stronger fabrics. The latest observable deformation of the lithospheric mantle of the NGVF occurred in a dominantly transpressional regime, following the Miocene extension of the Pannonian Basin. This regime was linked to the convergence of the Adria microplate and the European platform, and produced recent mantle structures in sublithospheric layers, inferred from the observed high seismic anisotropy. Deformation effects are overprinted by annealing in all xenoliths, resulting in decreasing intragranular deformation from north to south within the NGVF. Although there is no direct correlation between fabric properties and geochemistry, annealing is most probably linked to the presence of metasomatizing melts in the upper mantle prior to entrainment of the xenoliths in the host basalt. The latest metasomatic event is detectable in higher equilibration temperatures and Fe-enrichment of xenoliths in the central part, suggesting that the heating effect of this melt was still active during the time of sampling by the host magma. The host basalt appears to have influenced the amount of incorporated 'water' in the NGVF xenoliths as they are extremely dry compared with other xenoliths from the Carpathian-Pannonian region. Lack of correlation with geochemistry or physical properties, as well as the unusually wide range of H2O cpx/opx, indicates loss of 'water' during or after ascent. The highest 'water' content of xenoliths is found in the only locality with pyroclastic, rather than effusive eruption, consistent with post-eruption dehydration. A small part of the xenoliths, which may have retained their original 'water' content, indicate a depth-related concentration distribution.