7 research outputs found
Determination of the Lithosphere-Asthenosphere Boundary (LAB) beneath the Nógråd-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 Nógråd-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
FluidâEnhanced Annealing in the Subcontinental Lithospheric Mantle Beneath the Westernmost Margin of the CarpathianâPannonian 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 themore 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 regionâthe Miocene extension then tectonic inversion of the Pannonian Basin