Relation between xenoliths and magnetotelluric (MT) conductors in the Pannonian Basin (PB) - Short review

MT deep soundings in the PB detected Conductive Anomalies (CA) both in the crust and at the Lithosphere Asthenosphere Boundary (LAB) in the upper mantle. The shallow depth of the LAB (60-70 km) correlates well with the high heat flow in the PB. The origin of the crustal conductors is supposed in fluids and carbonaceous rocks (graphite?). In the Bakony-Balaton Highland and Nógrád-Gömör Volcanic Fields, Pliocene basalt volcanoes transported peridotite xenoliths to the surface. Behind textural analyses, thermobarometry is the geochemical tool for the deep lithospheric xenoliths to reconstruct their P-T paths and put them in the stratigraphic column of the lithosphere. On the P-T values of xenoliths geotherms – corresponding to the heat flow values of the PB – can be determined for different volcanic areas and a general geotherm too for the PB. The crossing of these geotherms with the adiabatic curves (AAC) of the lithosphere confirms the shallow position of the LAB according to different geochemists. Further aim of the study is to find relation between the carbonaceous materials and fluid inclusions in minerals of the xenoliths and the Transdanubian CA in the upper crust and the cause of abrupt decrease of the seismic activity in the middle crust in the volcanic areas

Determination of the Lithosphere-Asthenosphere Boundary (LAB) beneath the Nógrád-Gömör Volcanic Field by combined geophysical (magnetotellurics) and geochemical methods

Understanding the fundamental role of LAB is substantial for the investigation of the geodynamic evolution of the Earth. The LAB depths can be estimated by different geophysical methods (seismology, magnetotellurics), however these depths are controversial. It has been emphasized in the literature that combined geophysical and geochemical approach may lead to better understanding of these depths. The magnetotellurics (MT) is very powerful method because it indicates the sudden increase in conductivity at the LAB. The mantle xenoliths (small fragments of the lithospheric mantle) provide the information to reconstruct their P-T paths. In the Carpathian-Pannon region (CPR) five, well-studied occurrences of mantle xenoliths-bearing Plio-Pleistocene alkali basalts are known, which makes the CPR a very promising area for investigating the inconsistency in the LAB estimates. As a test area Nógrád-Gömör Volcanic Field (NGVF) has been chosen. The host basalt erupted at the NGVF collected mantle xenoliths from a small volume of the upper mantle in a depth of about 40-50 km. The major element geochemistry of the studied xenoliths indicates that most of them represent common lherzolitic mantle, whereas others show strong wehrlitisation process. This metasomatism is supposed to be caused by a migrating mafic melt agent, resulting in the transformation of a large portion of lherzolite to wehrlite beneath the NGVF, possibly just below the crust mantle boundary. In aim to detect the LAB at the research area and find the correlation with petrologic and geochemical results we carried out MT deep soundings. The campaign contained 12 long period MT stations with 3-5 km average spacing along 60 km long profile SSE to NNW direction. This presentation summarizes the preliminary results of the combined geophysical and geochemical approaches to determine the LAB depths

Fluid-Enhanced Annealing in the Subcontinental Lithospheric Mantle Beneath the Westernmost Margin of the Carpathian-Pannonian Extensional Basin System

Mantle xenoliths from the Styrian Basin Volcanic Field (Western Pannonian Basin, Austria) are mostly coarse granular amphibole-bearing spinel lherzolites with microstructures attesting for extensive annealing. Olivine and pyroxene CPO (crystal-preferred orientation) preserve nevertheless the record of coeval deformation during a preannealing tectonic event. Olivine shows transitional CPO symmetry from [010]-fiber to orthogonal type. In most samples with [010]-fiber olivine CPO symmetry, the [001] axes of the pyroxenes are also dispersed in the foliation plane. This CPO patterns are consistent with lithospheric deformation accommodated by dislocation creep in a transpressional tectonic regime. The lithospheric mantle deformed most probably during the transpressional phase after the Penninic slab breakoff in the Eastern Alps. The calculated seismic properties of the xenoliths indicate that a significant portion of shear wave splitting delay times in the Styrian Basin (0.5 s out of approximately 1.3 s) may originate in a highly annealed subcontinental lithospheric mantle. Hydroxyl content in olivine is correlated to the degree of annealing, with higher concentrations in the more annealed textures. Based on the correlation between microstructures and hydroxyl content in olivine, we propose that annealing was triggered by percolation of hydrous fluids/melts in the shallow subcontinental lithospheric mantle. A possible source of these fluids/melts is the dehydration of the subducted Penninic slab beneath the Styrian Basin. The studied xenoliths did not record the latest large-scale geodynamic events in the regionthe Miocene extension then tectonic inversion of the Pannonian Basin.We acknowledge the constructive criticism and helpful comments of Q‐K. Xia, an anonymous reviewer, and the Editor, John Geissman. We are grateful to F. Barou for his assistance during EBSD‐SEM analyses. L. E. Aradi is grateful to Bernardo Cesare, Levente Patkó, and Raúl Carampin for their help during the EPMA measurements. The FTIR analyses were carried out with the help of Judith Mihály and Csaba Németh. This research was partially granted by the Hungarian Science Foundation (OTKA, 78425 to Cs. Szabó). K. H.'s work was funded by the European Union Seventh Framework Programme Marie Curie postdoctoral fellowship (grant PIEFGA‐2012‐327226) and by the Juan de la Cierva Postdoctoral Fellowship (grant FPDI‐2013‐16253) of the Spanish Ministry of Economic and Competitiveness (MINECO). This project has been implemented with the support provided to I. J. Kovács from the National Research, Development and Innovation Fund of Hungary, financed under the K119740 funding scheme. The data used in this paper are listed in the references, tables, and supporting information. The raw EBSD and geochemical data are available from the corresponding author upon request. The FTIR spectra are available at the PULI (Pannonian Uniform Lithospheric Infrared spectral database) website (http://puli.mfgi.hu/). This is the 86 publication of the Lithosphere Fluid Research Lab (LRG)

Constraints on the thickness and seismic properties of the lithosphere in an extensional setting (Nógrád-Gömör Volcanic Field, Northern Pannonian Basin)

TheNógrád-GömörVolcanic Field (NGVF) is one of the five mantle xenolith bearing alkaline basalt locations in the Carpathian Pannonian Region. This allows us to constrain the structure and properties (e.g. composition, current deformation state, seismic anisotropy, electrical conductivity) of the upper mantle, including the lithosphere-asthenosphere boundary (LAB) using not only geophysical, but also petrologic and geochemical methods. For this pilot study, eight upper mantle xenoliths have been chosen from Bárna-Nagyk˝o, the southernmost location of the NGVF. The aim of this study is estimating the average seismic properties of the underlying mantle. Based on these estimations, the thickness of the anisotropic layer causing the observed average SKS delay time in the area was modelled considering five lineation and foliation end-member orientations. We conclude that a 142– 333km thick layer is required to explain the observed SKS anisotropy, assuming seismic properties calculated by averaging the properties of the eight xenoliths. It is larger than the thickness of the lithospheric mantle. Therefore, the majority of the delay time accumulates in the sublithospheric mantle. However, it is still in question whether a single anisotropic layer, represented by the studied xenoliths, is responsible for the observed SKS anisotropy,as it is assumed beneath the Bakony–Balaton Highland Volcanic Field (Kovács et al. 2012), or the sublithospheric mantle has different layers. In addition, the depths of the Moho and the LAB (25 ± 5, 65 ± 10 km, respectively) were estimated based on S receiver function analyses of data from three nearby permanent seismological stations