Balogh, Kadosa

Pécskay, Zoltán

Árva-Sós, Erzsébet

English

The systematic K/Ar chronologic study of post-Sarmatian alkali basaltic rocks in Hungary started in 1978. Since then about 250 determinations were carried out on samples representing the majority of occurrences. This work enabled us to establish a numerical time scale for the evolution of basaltic volcanic activity and to estimate the absolute ages of biostratigraphic units. In order to discover the possible disagreement of radiometric and geologic ages, which are caused by incomplete degassing, presence of xenolites and radiogenic argon loss, the isochron methods were used on cogenetic rocks and/or on fractions of different magnetic susceptibility and/or density of a single piece of basalt. The oldest basalts erupted in the Lower Pannonian* in the Danube—Tisza Interfluve Region. Their ages fall in the range of 8.1—10.4 Ma. The indicated age for the Lower-Upper Pannonian boundary is a bit younger than 8 Ma but due to the absence of basalts in the lower part of Upper Pannonian, this age estimation is uncertain. In Transdanubia, in the Balaton Highland, Bakony Mts. and Little Plain the oldest basaltic rocks are tuffs in the Pa' (Congeria ungulacaprae) level, these are unsuitable for dating. The oldest eruptive basalts are 5.5—6.0 Ma old, these are in a stratigraphically undefined position. Most of the basalts are younger than 5 Ma and volcanism terminated about 3 Ma ago. The age of boundary between the Pal (Congeria balatonica) and Pa? (Unio wetzleri) levels changes in space. The deposition of Pal sediments started 4.5 Ma ago (or even earlier) in certain areas and in . other places the end of the Pa| level is younger than 4 Ma. At the village of Bár (southeastern part of Transdanubia) jumillite overlies early Pleistocene red clay. Its age is 2.17±0.17 Ma. This shows that deposition of sediments, classed traditionally as Pleistocene in Hungary, started prior to 1.8 Ma. In North-Hungary, around town Salgótarján, basalts are 2.0—2.5 Ma old. Since they lie on eroded Oligocene and Miocene surfaces, their K/Ar data can not be related to the age of PliocenePleistocene boundary

K/Ar dating of post-Sarmatian alkali basaltic rocks in Hungary

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University of Szeged