The Age of the Cretaceous Subvolcanic Bodies from South Transdanubia (Hungary), Based on Palaeomagnetic Measurements

Abstract

The palaeomagnetic results obtained from a collection of new samples taken from South Transdanubia partly
confirmed earlier interpretations, but also presented the opportunity for further development of the earlier picture. In this
process new support was given to the following statements:
—Palaeomagnetic directions for the “Lower Cretaceous” subvolcanic bodies are much more consistent with no tilt
correction than with it. Hence, they are obviously younger than the mid-Cretaceous folding. The Mórágy dykes cut
crystalline rocks and their directions are ab ovo without any correction. These directions are consistent and fit well the
declination trend of the Mecsek rocks.
—On the basis of geological observations from the Villány Hills the folding took place after the Albian. Therefore,
the subvolcanic activity could not have taken place in the Early Cretaceous. Regional considerations fix this magmatism
at the beginning of the Late Cretaceous. Palaeomagnetic directions from the Lower Cretaceous effusives, after tilt
correction, correspond to those from the sediments; thus they are older than the folding. Since the effusive and
subvolcanic activities are separated by the mid-Cretaceous folding, they cannot be the products of the same magmatism.
—Palaeomagnetic directions of the Mecsek and Mórágy subvolcanic bodies imply two significant rotations. The
older of these moved in an anti-clockwise direction and started after the Albian. The younger rotation was clockwise and
commenced after the intrusion of the Miocene Komló Andesite. The declinations which are observable in the presentday position are resultants of the two opposite rotations.
In the earlier interpretation, the older rotation was conceived as a simple, significant, ≈90° rotation. The increased
number of data made it possible to further develop this concept, given that the available data set points to a gradual change
from ≈0° till ≈90°. In other words, most of the subvolcanic bodies were intruded during the rotation towards the west, not
after it. This is because their present-day declination is less than the maximum eastern declinaton (≈90°), which is
displayed by the Komló Andesite. The smeared distribution of the declinations makes it possible to determine the
succession of the intrusive bodies: the oldest of these are the ones with the minimum declination; the youngest are
represented by those with the maximum eastern declination.
The newly-obtained palaeomagnetic direction from the Máriagyűd (Villány Hills) dyke does not show any evidence
of rotation. This direction is clearly younger than the folding. This circumstance constrains the relation between the
Máriagyűd dyke and the Mecsek and Mórágy subvolcanic bodies — i.e. the former is either older than the Mecsek and
Mórágy rocks or synchronous with the oldest of them.
From the above results the following palaeotectonic conclusions can be drawn:
—After the mid-Cretaceous folding, South Transdanubia (and probably of the whole of the Tisza Unit) rotated
towards the west relative to Europe (supposedly in Late Cretaceous). This rotation took place during the subvolcanic
activity in the Mecsek–Mórágy area.
—The magmatism which fixed the rotation should be incorporated into the palaeotectonic syntheses. In the last two
decades the rotation has been accepted in the framework of these syntheses but without taking into account the Late
Cretaceous alkali rift magmatism which actually dates this rotation.
—The post-subduction origin of the lamprophyric magmatism of the Villány Hills should be integrated into one
extensional palaeotectonic model. An explanation should be found for the existence of two different types of the upper
mantle in two magmatic areas. These areas are situated at a distance (30 km) from each other; this is several times smaller
than the supposed depth of the magma generation.