Reconstruction of multi-phase fluid flow history and tectonic evolution in a Variscan granite (Velence Mts., Hungary)

Abstract : The subject of this work is the analysis of the relationship between fracture systems on different scales in a relatively homogeneous granite body and the temporal and spatial relationship of hydrothermal fluid percolation of different ages which interacted with the granite body. The selected area is the Velence Mountains in Western Hungary, which was affected by both the Variscan and Alpean orogeny. Owing to their special geotectonical position, the Velence Mts. and its region was involved in the whole Alpine cycle from the rift phase until the collision. Selection of this area for studies is also supported by the occurrence of sufficient amount of large outcrops. Also, earlier studies by MOLNÁR et al. (1995); MOLNÁR (1996, 2004) proved that the granite was affected by several high temperature fluid flow events and that these fluid migration events are easily distinguishable by means of fluid inclusion studies. Understanding of fracture evolution in the frame of the tectonic history of the granite required field observations on fractures and tectonic phenomenon's and characterization of their correlation with microfracture properties as well as more accurate knowledge of the age of alteration resulted from the fluid/rock interaction. Geochemistry, mineralogy and physical conditions of the fracture forming hydrothermal systems were also necessary to be determined. Therefore studies on clay mineralogy of altered zones, K-Ar radiometric age dating, as well as lead and sulphur isotope analysis have also been carried out in addition to the fluid inclusion studies. In the knowledge of these general characteristics, it was possible to describe and compare the macroscopic and microscopic features related to the main hydrothermal processes (Variscan and Alpean). On the basis of regional knowledge gained by studies mentioned, a small representative area was selected for detailed investigation. This work was also supported by the fractal analysis of the mineralised veins, and the correlation of the fractal properties with other mineralogical characters and structural parameters. Zsolt Benkó, PhD theses Reconstruction of multi-phase fluid flow history... Geometrical and statistical analysis of the fracture systems in different magnitudes also supported evaluation of the paleo-porosity of the granite. An important question of the work was - in addition to the how and when the fractures formed - how the hydrothermal and other old fractures influenced the formation of the current fracture network of the granite. The results of various analyses altogether facilitated the determination of age and geologic connections of some hydrothermal formations. Results also made it possible to correlate the tectonic evolution of the Velence Mts. in regional context. Beyond the new local and regional geological knowledge, the work also conveys new methodological approach for studies of fracture systems. This knowledge may be useful in the economic geology, reservoir geology, and hydrogeology or even in environmental geology.
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Zsolt Benkó. Reconstruction of multi-phase fluid flow history and tectonic evolution in a Variscan granite (Velence Mts., Hungary). Earth Sciences. Université Henri Poincaré - Nancy 1, 2008. English. ⟨NNT : 2008NAN10116⟩. ⟨tel-01748468⟩

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