The Middle Miocene volcanic evolution of the Borzsony Mountains, North Hungary, is
presented, correlating new volcanological, petrological, geochemical, geophysical
and paleontological data and establishing a detailed stratigraphy on the basis of
additional K/Ar radiometric and paleomagnetic measurements. For the earliest volcanic
activity, previous biostratigraphy showing an Early Badenian age has been confirmed
and precisely defined by paleomagnetic investigations. The first-stage volcanic formations
(16.5-16.0 Ma), deposited in a shallow marine environment, include resedimented, syn-eruptive,
garnet-bearing dacitic volcaniclastics (originating mostly from small-scale ignimbrite
eruptions) and coeval, garnet-bearing dacitic lava domes, sometimes with their volcaniclastic
aprons. As the eruptions filled the marine basin, subaerial dacitic-andesitic volcaniclastics,
comprising minor ignimbrites and different types of debris-flow deposits were also
deposited. A part of the latter may have been related to the formation of two or three
medium-sized calderas. The second stage (16.0-14.5 Ma) was characterized by andesitic
lava dome activity terminated by a hydrothermal event. During the first half of this
stage, a ca. 30 degrees CCW rotation occurred. The third stage produced the most voluminous,
moderately explosive, andesitic - basaltic andesitic High Borzsony subaerial lava
dome complex erupting up to the Badenian/Sarmatian boundary (ca. 13.7 Ma). Correlation
of K/Ar geochronological and volcanological data shows that lava dome activity of
the second and third stage may have been coeval with marine sedimentation in the southern
Borzsony.