As proved by a hydrocarbon exploratory borehole, a pre-Alpine polymetamorphic complex was thrusted over a low-temperature prograde metamorphic Mesozoic sequence in a Neogene depression of the Pannonian Basin, eastern Hungary. The detectable first, prograde, medium thermal gradient amphibolite facies event was followed by a retrograde, medium thermal gradient amphibolite facies event was followed by a retrograde metamorphism connected to a mylonite formation in the overthrusted complex. The polymetamorphic history of this gneiss-mica schist-amphibolite complex has also been recorded by the changes in chemistry of amphibole, white K-mica, and plagioclase. The footwall Mesozoic parautochthon is built up by an upper sub-unit (dolomitic-fine clastic), a middle (calcareous-fine clastic) and a lower one (calcareous-basic to intermediate volcanoclastic). On the basis of phyllosilicate crystallinity and vitrinite reflectance data, the metamorphic grade of the upper part corresponds to the low-temperature part of the anchizone and that of the middle and lower parts to the epizone, having been very close to the boundary between the epizone and anchizone. The maturity parameters of the basin-filling Neogene formations refer only to diagenetic conditions, indicating non-equilibrium conditions. The Neogene (<5 Ma) burial has had no detectable effects on the metamorphic conditions of the basement units, which had already occupied their present tectonic position during their post-metamorphic uplift (cooling) in the Upper Cretaceous. The retrograde metamorphism in the polymetamorphic nappe and the low-temperature prograde metamorphism in the Mesozoic parautochthon proved to be of Eo-Alpine (Cretaceous) age, and was older than the overthrusting. The tectonic (shearing) strain connected to the overthrusting influenced the structural state of phyllosilicates in a rather irregular way, both in the hanging wall and the footwall units.
|Number of pages||32|
|Journal||Acta Geologica Hungarica|
|Publication status||Published - Jan 1 1998|
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