EUROPROBE News 9
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Timing and tectonic setting
of Neogene magmatism in the Pannonian Basin
and Carpathian Arc
Downes, H. (Dept of Geology, Birkbeck
College, Malet St., London, UK)
Pecskay, Z. (Institute of Nuclear Research, Hungarian Academy of Sciences, Ben tér 18/C,
Debrecen, Hungary).
Introduction
Neogene magmatism in the PANCARDI region can be
broadly classified into two types (Szabó et al, 1992): (a) an earlier phase of volcanism
of calc-alkaline affinity, identical in its petrography and geochemistry to
subduction-related magmatic rocks from all over the world; (b) a generally later phase of
alkaline volcanism, which is extremely similar to the late Tertiary/Quaternary intraplate
activity found elsewhere in Europe (Wilson and Downes, 1990). In this report, we will
discuss the timing of this magmatism and attempt to relate it to different stages in the
tectonic evolution of the PANCARDI region. For a more detailed review of all aspects of
PANCARDI magmatism, a Special Issue of the journal of the Italian Volcanological
Association "Acta Vulcanologica" (volume 7) has recently been published on this
topic.
Areal distribution of PANCARDI magmatism
Fig. 1 shows the areal distribution of the
volcanics as compiled by Pecskay et al. (1995). A large volume of the earliest volcanics
is buried in the Pannonian and Transylvanian Basins, although some outcrops also occur
around the edges of the basins. Much of this material is highly siliceous acidic
pyroclastic tuffs and ignimbrites, which may have travelled far from their source areas,
and in general the precise source areas of these deposits is unknown.
The main outcropping calc-alkaline volcanoes are
dominantly intermediate (andesitic) in composition and form a generally arcuate chain in
the Western Carpathians, trending W-E from central Slovakia and northern Hungary into
eastern Slovakia (localities 11-21 on Fig. 1). In the Ukrainian Carpathians, the trend of
the chain becomes NW-SE and in the Eastern Carpathians of Romania, NNW-SSE (localities
26-30). An exception to this geographic trend is the calc-alkaline volcanism of the
Apuseni Mts. situated between the Transylvanian and Pannonian Basins. The alkaline
magmatism is equally widespread, occurring sporadically across the whole of the region
from the Graz Basin to the Persanyi Mts. of the Transylvanian Basin.
The main regions of alkaline activity are on the
northern margin of Lake Balaton (locality 2) and in the region of southern
Slovakia/northern Hungary near Nograd-Novohrad (locality 13). However, alkaline volcanics
are absent in eastern Slovakia, Ukraine and most of the Eastern Carpathians of Romania.
Alkaline magmatism consists mostly of small basaltic cinder-cones and lava flows. Many of
these primitive basalts carry mantle-derived xenoliths (Downes et al., 1992; Vaselli et
al., 1995) which give us an insight into the composition of the lithospheric mantle
beneath the region.
Timing and tectonic setting of magmatism
Detailed and accurate geochronology is crucial to
enable us to understand the relationship between magmatism and tectonic events. A
first-order observation is that the alkaline magmatism post-dates the calc-alkaline
eruptions throughout the area. Only at the very southern part of the Eastern Carpathians
are the two types of magmatism contemporaneous. This clearly indicates a strong
relationship between magmatism and tectonic activity, i.e. calc-alkaline magmatism is
related to collisional events, whereas the alkaline type relates to extensional events.
Previous work had indicated that the age of the
calc-alkaline volcanism became progressively younger from west to east around the Inner
Carpathian arc. Recently, Pecskay et al. (1995) compiled and reviewed all of the available
geochronological data, based largely on K-Ar age determinations (Fig. 2). Their remarkable
conclusion was that much of the age-trend was not valid and, instead, acid magmatism began
in several regions 19 Ma ago, followed by the formation of andesitic stratovolcanoes along
the Western Carapthian arc at ca. 16 Ma. The majority of these calc-alkaline volcanoes
were extinct by 10 Ma, an age which coincides with a major E-W compressional event in the
region, which Decker and Peresson (1995) suggest was related to the entry of continental
crust into the subduction zone (i.e. cessation of subduction of oceanic crust). Only in
the Eastern Carpathians (from the Gutii volcanic area to the south Harghita volcanoes) do
we see a significant age-progression along the chain from 10 Ma in the north to 0.2 Ma at
the southern end of the chain. The age-progression along the Calimani-Gurghiu- Harghita
segment of the East Carpathian chain is very striking.
The tectonic significance of this result is that,
if subduction is responsible for the calc-alkaline magmatism, then the subducted slab
first reached the required depth for magma generation of 100-120 km (the "magma
generation window") simultaneously beneath the whole of the Western Carpathians
between 19 and 16 Ma ago, and ceased to generate calc-alkaline magma after only 6-9 Ma had
passed. In contrast, the slab which subducted beneath the Eastern Carpathians must have
reached the magma generation window at a progressively later and later time.
Another striking feature revealed by Fig. 2 is that
the oldest extension-related alkaline magmas (black shading on Fig. 2) are about 9-11 Ma
old and were erupted just as the calc-alkaline magmatism was waning. From this we can
conclude that we have a "switch" from a tectonic regime dominated by collision
to one dominated by extension at this time. After this, alkali basalts occurred
sporadically both in time and place, until the eruption of the youngest flow (Nova Banya
in central Slovakia) which overlies a post-glacial river terrace. These magmas may be
related to a widespread upwelling of the asthenospheric mantle beneath Europe (Hoernle et
al., 1995). If this is the case, then they could not be erupted from the upwelling
asthenosphere until after the subducting slab had become detached and had sunk into the
asthenosphere. Such "slab window" alkali basalts have been observed in other
regions of the world when a subducting slab has been detached.
Current debates and unanswered questions
The general features outlined above are largely
accepted by the volcanological and petrological community working in the region. However,
some disagreements still exist. For example, Ukrainian and Romanian colleagues have
pointed out that tectonic events such as nappe emplacement and thrust faulting had ceased
in the Eastern Carpathians long before the onset of magmatism. Thus, they argue, the
calc-alkaline magmatism of the Eastern Carpathians cannot easily be related to subduction.
Our response is that it will take a finite time for a subducted slab of oceanic crust to
reach the temperature and depth of the magma-generation window; this could account for the
gap between the cessation of tectonism and the onset of magmatism. The time interval
between the beginning of subduction and the arrival of the subducting slab in the magma
generation window will be a function of the angle of subduction and the rate of
subduction. In fact, if subduction is very shallow, as occurs in some parts of the Andes,
magma generation may not occur at all. In the Eastern Carpathians we see a remarkably
narrow volcanic zone and fast progressive movement of volcanism, indicating that the
period of time which the slab spent in the magma generation window was short. This in turn
suggests that the piece of oceanic crust which was subducted was of small dimensions. Slab
detachment in the Eastern Carpathians may have followed the arrival of unsubductible
continental crust of the Tornquist zone or East-European Platform at the trench around 10
Ma ago (Decker and Peresson, 1995).
Another unsolved problem is the origin of the
widespread acidic magmatism which is the earliest magmatic feature of the region. Although
most workers suggest that the source volcanoes were situated to the N and NE of these
basins, in the Western Carpathian arc, the precise relationship between the
stratovolcanoes and the pyroclastic deposits is unknown, particuarly as the pyroclastics
appear to pre-date most of the stratovolcanoes. It has also been suggested by Slovak
colleagues that many of the widespread intermediate and acidic volcanics of the Western
Carpathians are not directly related to subduction, but may be more closely linked with
some other major tectonic process such as lithosphere delamination, in which hot
asthenosphere is brought into direct contact with the base of the crust, heating it and
causing melting. Such hypotheses can be tested using isotopic and other geochemical
methods on the earliest products of the acidic magmatism.
Conclusions
In the past few years, before and since EUROPROBE
became interested in the PANCARDI region, researchers from all of the PANCARDI countries
have been engaged in joint collaboration both among themselves and with colleagues from
the UK, Italy and the USA. Numerous projects on individual volcanic regions have been
undertaken and many publications have resulted. Now, however, we are at a point where we
have a large amount of geochronological, geochemical and volcanological data which must be
synthesised into a tectonomagmatic framework. We should be asking ourselves "what is
the tectonic significance of our results?" and "how do our results fit into the
larger-scale picture of tectonomagmatic processes in the PANCARDI region?" Only in
this way will we be able to move forward to achieve a greater understanding of the
relationship between magmatism and tectonics in this region.
Acknowledgements
Much of this work was sponsored by OTKA grant T7278
and the University of London Central Research Fund.
References
Decker, K. and Peresson, H., 1995. Late Miocene E-W compression in
the Alpine-Carpathian-Pannonian region linked to late subduction in the Eastern
Carpathians. Terra Abstracts, 7, 271.
Downes, H., Embey-Isztin, A. and Thirlwall, M.F., 1992. Petrology
and geochemistry of spinel peridotite xenoliths from the western Pannonian Basin
(Hungary): evidence for an association between enrichment and texture in the upper mantle.
Contrib. Mineral. Petrol., 109, 340-354.
Hoernle, K., Zhang, Y.-S. and Graham, D., 1995. Seismic and
geochemical evidence for large-scale mantle upwelling beneath the eastern Atlantic and
western and central Europe. Nature, 374, 34-39.
Pecskay, Z. et al., 1995. Space and time distribution of Neogene-
Quaternary volcanism in the Carpatho-Pannonian Region. Acta Vulcanologica, Special Issue
7, 15-28.
Szabó, Cs., Harangi, Sz. and Csontos, L., 1992. Review of Neogene
and Quaternary volcanism of the Carpathian-Pannonian Region. Tectonophysics, 208, 243-256.
Vaselli et al., 1995. Ultramafic xenoliths from Plio-Pleistocene
alkali basalts from the Eastern Transylvanian Basin: depleted mantle enriched by vein
metasomatism. J. Petrol., 38, 23-54.
Wilson, M. and Downes, H., 1990. Tertiary-Quaternary
Extension-Related Alkaline Magmatism in Western and Central Europe. J. Petrol., 32,
811-849.
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