Effect of metasomatism on the electrical resistivity of the lithospheric mantle-An integrated research using magnetotelluric sounding and xenoliths beneath the Nograd-Goemoer Volcanic Field

摘要

Long period magnetotelluric (MT) data were collected at 14 locations along a similar to 50 km long NNW-SSE profile in the Nograd-Godmodr Volcanic Field (NGVF), which is one of the five mantle xenolith bearing Neogene alkali basalt locations in the Carpathian-Pannonian region. As a result, a low resistivity anomaly (10 Omega m) was observed approximately at 30-60 km depth beneath the central part of the NGVF, indicating the presence of a conductive body beneath the Moho. This is the same area where upper mantle xenoliths with wehrlitic modal composition were collected from six quarries. The wehrlites were formed as a result of mantle metasomatism involving the peridotite wall rock and a mafic melt. The spatial coincidence of the geophysical anomaly and the petrographicgeochemical alteration in the upper mantle suggests their probable relationship. To test this assumption, we estimated the electrical resistivity of the wehrlites. The outcome reveals lower electrical resistivity for wehrlites (similar to 132 Omega m) compared to the non-metasomatized lherzolites (similar to 273 Omega m) from the same localities. However, it is still higher than the values acquired with long period MT soundings. Thus, further modelling was implemented in order to test the possible role of melt. The models revealed that even similar to 2-3 vol.% of interconnected melt is enough to lower the electrical resistivity below 1 Omega m in the wehrlites. The interconnected glass phase found in the wehrlites may be the evidence for this later solidified melt. All these suggest that melts may still be present in small amounts beneath the cooling NGVF. The intensive melt upwelling in the central part of the NGVF resulting in a wehrlitized mantle portion with low electrical resistivity, as well as the extensive basalt flows on the surface, can be explained by deep deformation zones, which provide excellent migration pathways for melts through the entire lithosphere.