GeolCarp_Vol54_No5_299

PERMIAN-TRIASSIC EVOLUTION OF THE GREAT HUNGARIAN PLAIN 299

GEOLOGICA CARPATHICA, 54, 5, BRATISLAVA, OCTOBER 2003

299315

GEOCHRONOLOGICAL CONSTRAINTS OF THE VARISCAN,

PERMIAN-TRIASSIC AND EO-ALPINE (CRETACEOUS) EVOLUTION

OF THE GREAT HUNGARIAN PLAIN BASEMENT

GYÖNGYI LELKES-FELVÁRI

, WOLFGANG FRANK

and RALF SCHUSTER

Natural History Museum, Ludovika tér 2, H-1083 Budapest, Hungary; felvari@zoo.zoo.nhmus.hu

Institute of Geology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria; wolfgang.frank@univie.ac.at

Geological Survey of Austria, Rasumofskygasse 23, A-1031 Vienna, Austria; rschuster@cc.geolba.ac.at

(Manuscript received September 11, 2002; accepted in revised form March 11, 2003)

Abstract: Core samples of metamorphic basement rocks from the Great Hungarian Plain (Tisza Megaunit) were studied

by petrographic and geochronological methods (Ar-Ar, Rb-Sr, Sm-Nd). On the basis of microtextural features of Al

SiO

polymorphs in metapelites a regional distribution pattern was found, which correlates with geochronological age groups.

This pattern confirms the earlier established tectonic subdivision of the basement of the Great Hungarian Plain, except

for the Algyõ basement-high, which has to be considered to represent a separate unit. A muscovite concentrate from a

granite sample of the Mecsek Subunit yielded a Variscan Ar-Ar age of 299 Ma. The medium-grade metapelites of the

Villány-Bihar Subunit are characterized by kyanite porphyroblast and sillimanite. Typical Ar-Ar muscovite ages are ca.

310 Ma and prove a Variscan cooling age of the metamorphic assemblages. In the NE part of the unit ages in the range

of 202266 Ma indicate a later thermal overprint. The staurolite and andalusite-bearing gneisses of the Békés-Codru

Subunit yielded Variscan cooling ages of ca. 320 Ma. In contrast, the rocks from the Algyõ basement-high experienced

their first metamorphic imprint in Early Permian time. Based on a Sm-Nd garnet isochron the high-temperature/low-

pressure assemblages, including andalusite+K-feldspar±sillimanite formed ca. 275 Ma. An amphibolite facies, eo-

Alpine (Cretaceous) overprint in the stability field of kyanite+staurolite+garnet is proved by Ar-Ar muscovite ages in

the range of 8295 Ma. It was followed locally by an Early Tertiary deformation. Considering the lithologies and the

metamorphic and structural evolution, the Algyõ-high shows many similarities to the Saualm-Koralm Complex of the

Austroalpine unit. Like the latter and the Baia de Aries nappe complex in the Apuseni Mountains, it obviously represents

an eo-Alpine thrust sheet.

Key words: Tisza Megaunit, Hungary, basement, Variscan, Permian and eo-Alpine metamorphism, Ar-Ar, Rb-Sr and

Sm-Nd geochronology.

Introduction

Numerous studies are dealing with the paleogeographical evo-

lution of the ALCAPA region (internal Eastern Alps-Car-

pathians-Pannonian Basin: Neubauer 1992). Traditionally the

pre-Alpine arrangement of the tectonic units is based on the

facies evolution of the Mesozoic cover series (e.g. Haas et al.

1995). However recent petrological and geochronological

studies result in detailed knowledge about most of the crystal-

line basement units, giving additional arguments for various

tectonic reconstructions. These studies have shown, that be-

sides a widespread Variscan metamorphic imprint, an eo-Al-

pine and also a Permo-Triassic imprint are important over

large areas (Árkai 1991; Lelkes-Felvári et al. 1996, 2001;

Thöni 1999; Hoinkes et al. 1999; Plaienka et al. 1999; Haas

et al. 2001; Schuster et al. 2001).

The Pannonian Basin is located in the centre of the ALCA-

PA region. The main tectonic elements, forming the basement

of this Tertiary basin are the Tisza Megaunit in the SE and the

Pelsó Megaunit to the NW. They are separated by the mid-

Hungarian line a major tectonic element of regional impor-

tance with a complex history (Csontos & Nagymarosy 1998)

(Fig. 1). Contradictory opinions exist about the Permo-Meso-

zoic location of the Tisza Megaunit. In the palinspastic recon-

struction by Stampfli & Mosar (1999) the Tisza Megaunit is

located between the Southalpine and the Austroalpine units.

The European affinity of its Permo-Mesozoic cover series was

recognized by Géczy (1973) and Haas et al. (1995). Accord-

ing to Buda (1992) and Buda et al. (1999) the granitoids of

Mecsek Mts can be related to the Moldanubian Zone of the

Central European Variscides.

Klötzli et al. (in print) assume a position of the Tisza Mega-

unit east of the Austroalpine units of the Eastern Alps, in the

vicinity of the Bohemian Massif and the Carpathians. One

reason for the contrasting opinions is the incomplete knowl-

edge about the basement of the Pannonian Basin, which is

mostly covered by Neogene sediments. Especially the area of

the Great Hungarian Plain (GHP), located east of the river

Danube is exclusively known from boreholes (Fig. 2).

In this paper petrological and geochronological data are

given for the metamorphic rocks of the Tisza Megaunit, in the

area of the Great Hungarian Plain. Geochronological age data

prove a Variscan metamorphic imprint and give arguments for

an eo-Alpine and also a Permo-Triassic imprint in some areas.

Geological setting

The basement of the GHP consists of metamorphic rocks,

Permian and Mesozoic sediments and volcanic rocks. The

PERMIAN-TRIASSIC EVOLUTION OF THE GREAT HUNGARIAN PLAIN 301

ditions and radiometric ages in the range of 270330 Ma. 3)

Alpine ages of 7564 Ma were interpreted as contact effects of

Late Cretaceous magmatic bodies. However, the detailed cor-

relation of geochronological data with metamorphic evolution

and formation of mineral assemblages is still poorly under-

stood.

Eo-Alpine regional metamorphism in the basement of the

GHP was first reported by Árkai et al. (1998). It caused a very

low to low-grade prograde metamorphism in the Permo-

Mesozoic rocks beneath overthrust, polymetamorphic

Variscan basement rocks, and tectonic slices along the main

Alpine thrust zones. The K-Ar ages of fine-grained micas sep-

arated from basement rocks also show Alpine retrogression

within strongly tectonized metamorphic slices (Árkai et al.

2000).

Analytical techniques

Amphibole and garnet used for isotope determinations were

hand-picked under a binocular microscope. The coarsest mus-

covite and biotite (up to several mm) from the samples were sepa-

rated on a vibrating table and by grinding in alcohol, and a frac-

tion of 0.10.2 mm was analysed. The age dating was performed

in the Laboratory of Geochronology, University of Vienna.

To remove surface contaminations mineral concentrates

used for Sm-Nd and Rb-Sr analyses were leached in 2.5 M

HCl before decomposition for 5 minutes at about 50 °C.

Chemical sample digestion and element separation followed

the procedure outlined by Thöni & Jagoutz (1992). Overall

blank contributions are

≤

0.2 ng for Nd and Sm, and

≤

2 ng for

Rb and Sr. Nd and Sm concentrations were determined by iso-

tope dilution, using a mixed

147

Sm-

150

Nd spike, and run as

metals on a Finnigan



MAT 262 multicollector mass spec-

trometer. Nd was ionized using a Re double filament. Within-

run isotope fractionation was corrected for

146

Nd/

144

Nd =

0.7219. All errors quoted in Tables 1 and 2 correspond to 2

of the block mean (1 block = 10 isotope ratios). The

143

Nd/

144

ratio for the La Jolla international standard during the course

of this investigation was 0.511900±6 (9 runs). Errors for the

147

Sm/

144

Nd ratio are ±1%, or smaller, based on iterative sam-

ple analysis and spike recalibration. Sr and Rb concentrations

were determined using a VG



Micromass M 30 and Ta fila-

ments. Through the course of this study the value for the NBS

987 Sr standard was 0.71011±6 (12 runs). Maximum errors

for

Rb/

Sr ratios are estimated to be ±1 %.

For

Ar-

Ar age determinations the mineral concentrates

were irradiated at the 9MW ASTRA reactor at the Austrian

Research Centre Seibersdorf or at the Institute of Isotopes

Budapest and analysed using standard procedures with a VG-

Fig. 2. Geological sub-surface map of the Great Hungarian Plain. The main structural elements and the locations of the investigated logs are shown.

302 LELKES-FELVÁRI, FRANK and SCHUSTER

Sample

Log

Depth

Subunit

Lithology

Index minerals

age

[m]

Method

IA [Ma] PA [Ma] TGA [Ma]

Cegléd-1/13

Ce-3

Mecsek

granite

K-Fs+Pl+Ms+Qtz Ar-Ar Ms

299.0±3.9

295.3±4.5

Mezõsas-2

Sas-2

22742280

VBU

retrograde eclogite

Grt

---------

Öcsöd-3/1

Öcs-3

VBU

cataclastic micaschist

---------

Öcsöd-3/2

Öcs-3

VBU

cataclastic micaschist

Ar-Ar Ms

317.1±3.4

314.2±3.5

Szarvas-8/4

Szarvas-8 34533460

VBU

micaschist

Ky+Grt+St

Ar-Ar Bt

293.0±1.9

280.0±3.2

Szarvas-8/4

Szarvas-8 34533460

VBU

micaschist

Ky+Grt+St

Ar-Ar Ms

316.8±3.1

316.2±3.5

Szeghalom-15/4

Sz-15

VBU

micaschist

Ky+Sil

---------

Szeghalom-176/4

Sz-176

VBU

biotite-amphibolite

Ar-Ar Hb

309.7±6.0

309.4±7.3

Biharkeresztes-16/1

Bike-16

15211523

VBU

micaschist

Sil

Ar-Ar Ms

308.0±3.2

306.5±3.3

Ártánd-1/10

Ar-1

2824

VBU

micaschist

Ky+Sil

Ar-Ar Ms

------------- 297.0±2.3

Álmosd-1/10

Àlm-1

2824

VBU

micaschist

Grt+St

Ar-Ar Ms

202.0±1.7

194.2±1.7

Földes-6/2

Föl-6

22962300

VBU

biotite-amphibolite

Bt+Hb+Pl

Rb-Sr Bt

200±2

Földes-6/2

Föl-6

22962300

VBU

biotite-amphibolite

Bt+Hb+Pl

Ar-Ar Bt

222.7±1.7

215.6±1.9

Földes-12/3

Föl-12

19241932

VBU

micaschist

St-Grt

Ar-Ar Ms

266.4±3.3

264.8±3.4

Ruzsa-7/8

Ruzsa-7

2859

BCU

gneiss

And+St

Ar-Ar Ms

321.5±3.7

317.0±4.1

Ruzsa-D-1/3

Ruzsa-D-1 15701571

BCU

gneiss

And+St

Ar-Ar Ms

318.0±3.6

314.0±3.7

Sarkadkeresztúr-ÉNY-2/4 Sark-ÉNY-2 40704073

BCU

gneiss

And+St

Ar-Ar Ms

305.3±2.0

303.3±2.3

Mezõgyán-1/5

Mgyán-1

BCU

gneiss

And+St

Ar-Ar Ms

319.2±4.7

315.9±5.2

Ásotthalom-11/2

As-11

10601066

BCU

gneiss

Ar-Ar Ms

319.8±2.7

318.2±2.9

Kelebia-3/9

Kel-3

11221126

BCU

gneiss

Ar-Ar Ms

321.2±2.1

291.5±2.1

Pusztaföldvár-222/6

Pf-222

BCU

mylonitic gneiss

Ar-Ar Ms

309.1±2.7

305.5±3.2

Szeged-11/6

Szeged-11

BCU

diaphthoritic mylonite

Ar-Ar Ms

317.4±3.4

301.2±3.7

Szeged-11/6

Szeged-11

BCU

diaphthoritic mylonite

Ar-Ar Ms+Chl

301.0±3.1

282.0±3.6

Dorozsma-7/10

Do-7

Algyõ-high

mylonitic micaschist

Grt+Bt+Ms+Gr

Sm-Nd Grt

273±7

Dorozsma-38/2/1

Do-38

Algyõ-high

mylonitic micaschist

Grt1+Pl+Bt+K-Fs ---------

Dorozsma-54/10/1/1

Do-54

Algyõ-high

mylonitic micaschist

Grt1+Pl+Bt+K-Fs ---------

Algyõ-52/5

Algyõ-52

25692572

Algyõ-high

micaschist

Ar-Ar Ms

------------- 58.4±1.3

Újszentiván-2/9

Uszi-2

33953398

Algyõ-high

micaschist

Ar-Ar Ms

85.5±1.2

84.1±1.3

Ferencszállás-K-6/3

F-K-6

23452351

Algyõ-high

micaschist

Ar-Ar Ms

90.2±1.0

88.9±1.2

Ferencszállás-13/18

F-13

25702572

Algyõ-high

micaschist

Ar-Ar Ms

82.4±1.4

81.3±1.5

Ferencszállás-K-3/5

F-K-3

Algyõ-high

micaschist

Ar-Ar Ms

82.9±1.8

81.3±1.9

Kiszombor-7/4

Zomb-7

Algyõ-high

micaschist

Ar-Ar Ms

------------- 95.2±1.8

Forráskút-12/6

Fkút-12

31363139

Algyõ-high

micaschist

Ar-Ar Ms

------------- 70.2±1.4

Üllés-31/7/1

Ü-31

2959

Algyõ-high

micaschist

Ar-Ar Ms

87.7±1.6

87.1±1.8

Probe

[ppm]

147

Sm/

144

143

Nd/

144

±2sm

Do-7/10 L

70.2

22.9

0.1967

0.512036

0.000011

Do-7/10 Fs

36.6

6.01

0.0993

0.511884

0.000005

Do-7/10 Grt2

9.97

7.55

0.4578

0.511245

0.000014

Do-7/10 Grt1

10.9

7.45

0.4148

0.512446

0.000012

Sample

[ppm]

Rb/

Sr/

±2sm

Földes-6/2 whole rock

54.7

299.0

0.5296

0.70657

0.00009

Földes-6/2 Bt

183.7

17.61

30.440

0.79156

0.00006

5400 Fisons Isotopes



mass spectrometer. Age calculation

was done after corrections for mass discrimination and radio-

active decay using the formulas given in Dalrymple et al.

(1984). The J-values are determined with internal laboratory

standards, calibrated by international standards including

muscovite Bern 4M (Burghele 1987) and amphibole MMhb-1

(Samson & Alexander 1987). The errors given on the calculat-

ed age of an individual step include only the 1

error of the

analytical data. The error of the plateau and total gas ages in-

cludes an additional error of ±0.4 % on the J-value, based on

repeated measurements of the standard.

The samples used for isotope analyses are described in the

Appendix.

Petrographic description

Crystalline rocks of the logs were investigated by petro-

graphic and geochronological methods. During the first phase

of this research more than a thousand thin sections were in-

vestigated from all the tectonic units mentioned above to get

an overall picture of the main characters of the metamorphic

rocks. It turned out that the microtextural features of Al

SiO

polymorphs in metapelites are useful to group the rocks into

regional units, which can be correlated with the geochrono-

logical age groups.

In the following section, we describe characteristic features

of the investigated samples grouped according to the classical

tectonic subdivision of the GHP of the quoted authors. Sam-

Table 1: Sample localities, characteristic index minerals and age data of the investigated samples.

Table 2: Sm-Nd and Rb-Sr isotopic data from the Algyõ base-

ment-high and the Villány-Bihar Subunit.

Fig. 3. Lithologies of the Villány-Bihar and Békés-Codru Subunits.

A Retrogressed eclogite, Mezõsas-2/22742280 m. B Kyan-

ite-bearing micaschist, Szarvas-8/4. C Staurolite-garnet-bearing

micaschist, Szarvas-8/4. D Cataclastic, staurolite-bearing mic-

aschist, Öcsöd-3/1. E Kyanite-sillimanite-bearing gneiss, Szegh-

alom-15/4. F Staurolite-garnet-bearing micaschist, Álmosd-1/10.

G Mylonitic garnet micaschist, Földes-12/3. H Staurolite-an-

dalusite-bearing micaschist, Ruzsa-7/8. Photos AE and H: plane-

polarized light; F and G: crossed-polarized light. Lettering in the in-

dex of minerals refer to the proposed age of the mineral:

V=Variscan, P=Permo-Triassic, A=eo-Alpine.

▲

PERMIAN-TRIASSIC EVOLUTION OF THE GREAT HUNGARIAN PLAIN 305

ple locations and additional information are given in Table 1

and Figs. 58.

Mecsek Subunit

Metamorphic rocks of this subunit crop out outside the

GHP, to the west of the river Danube in Transdanubia, in the

Mórágy Hill. Starting from there a long, SW to NE directed

granitoid range, flanked by gneisses and micaschists is known

in the subsurface basement of the GHP. Al

SiO

modifica-

tions were not described from this area until now.

Villány-Bihar Subunit

The VBU is a composite subunit with internal thrusts,

where cataclastic crystalline rocks were thrust over differently

metamorphosed Mesozoic sedimentary and magmatic rock se-

quences (Pap 1990; Árkai et al. 1998). Sillimanite is wide-

spread among Al

SiO

polymorphs, and kyanite porphyro-

blasts occur in some lithologies. Retrogressed eclogites were

described from several logs along a zone exceeding 50 km,

parallel to the confining lineaments (M. Tóth 1995, 1997). A

new occurrence of retrogressed eclogite was identified in the

log Mezõsas-2. At present it is the easternmost occurrence in

this narrow eclogite belt.

Even if the high-pressure minerals are almost completely

destroyed by later overprint, the eclogitic origin is document-

ed by the microfabrics (Fig. 3A). The eclogite facies assem-

blage included garnet

+omphacite±hornblende±phengite+

quartz+apatite+rutile. Garnets are up to 1 mm in diameter. In

many cases they exhibit typical atoll-shapes with cores re-

placed by symplectites of biotite and plagioclase. The latter are

interpreted as remnants of a Ca-rich, high-pressure garnet

which has been partly or fully replaced during an amphibolite

facies overprint, whereas the rims (garnet

) remained stable.

Omphacite is not preserved. However the typical symplectite

textures of diopside and plagioclase replacing omphacite are

very common. Greenish hornblende crystals up to 1 mm are of-

ten surrounded by epidote and/or carbonate. During a green-

schist facies overprint diopside has been replaced by fine-

grained, faint green mineral aggregates including chlorite.

Fig. 4. Lithologies of the Algyõ basement-high. A Mylonitic

garnet micaschist, Dorozsma-38/2/1: Garnet porphyroblast (G

)

with large inclusions of biotite and plagioclase within a fine-grained

mylonitic matrix (S

) composed of biotite+plagioclase+quartz+

chlorite. B Mylonitic kyanite-biotite schist, Dorozsma-38/2/1:

fine-grained kyanite pseudomorph after andalusite in a fine-grained

biotite-rich matrix. The kyanite aggregate is replaced by sericite

along the edges. CD Staurolite kyanite-bearing micaschist,

Újszentiván-2/9. E Garnet-staurolite-bearing S-C mylonite, For-

ráskút-12/6. FG Mylonitic garnet-K-feldspar-bearing mic-

aschist, Dorozsma-7/10. H Garnet-bearing micaschist with

graphite pigment: Ferencszállás-K-6/3. The photos AF and H:

plane polarized light; G: crossed-polarized light. Lettering in the in-

dex of minerals refer to the proposed age of the mineral:

V=Variscan, P=PermianTriassic, A=eo-Alpine.

Fig. 5.

Ar-

Ar age data from the Mecsek- and Villány-Bihar Subunit. Muscovites, biotite and hornblende yield ages in the range of 317±4

to 293±2 Ma. They represent Variscan cooling ages. The younger age from Álmosd is interpreted as a rejuvenated Variscan age.

▲

306 LELKES-FELVÁRI, FRANK and SCHUSTER

Essential rock-types of VBU are fine-grained biotite-pla-

gioclase gneisses interlayered with coarse, K-feldspar-bearing

augengneisses, amphibolites and biotite-amphibole gneisses.

Subordinate micaschists contain kyanite porphyroblasts

(Fig. 3B) and staurolite (Fig. 3C,D). Garnet is a common

component in all lithotypes. Several pre- syn- and post-tecton-

ic types can be distinguished. Small garnet inclusions in pla-

gioclase are typical. Kyanite is present as up to some mm in

size, and sometimes deformed porphyroblasts. It is often re-

placed by sericite or coarse-grained muscovite. In one case

kyanite replacement by sillimanite has been observed (pers.

comm. by Zachar; Fig. 3E). Sillimanite is generally associated

with biotite flakes, occurring along borders of quartz rods and

as microveins cutting earlier structures. Andalusite was found

in one log forming a coarse-grained andalusite-quartz-vein.

In the NE part of the VBU Al

SiO

polymorphs are not

known. In the area of Álmosd coarse-grained micaschists and

amphibolites are present (see also Árkai 1987). The mic-

aschists are characterized by a mica-rich matrix and assem-

blages containing staurolite and garnet (Fig. 3F). Garnet

forms round grains, often present as inclusions within stauro-

lite. Garnet is also present as atoll-shaped crystals with cores

replaced by biotite and quartz. Late staurolite idioblasts con-

tain intrafolial folds marked by graphite pigment. Postkine-

matic biotite, muscovite and chlorite idioblasts measuring up

to 8 mm crosscut the schistosity planes. Quartz-rich bands

show static recrystallization with fine mica flakes and stauro-

lite poikiloblasts with quartz inclusions. Near to Földes mylo-

nitic micaschists (Fig. 3G), marbles and carbonate-mic-

aschists occur together with amphibolites.

Békés-Codru Subunit

There is a distinct contrast compared to the northern subunit

as no solitary kyanite porphyroblasts occur here, but instead

andalusite porphyroblasts are common in a northernmost nar-

row zone. In other areas Al

SiO

polymorphs were not found.

The coarse-grained andalusite-bearing gneisses contain au-

gens of feldspars and coarse muscovite flakes embedded in a

finer-grained mica matrix, often showing S-C structure. The

main foliation (S

) is defined by muscovite, biotite and idio-

blasts of ilmenite. Coarse mica flakes contain porphyroblasts

of garnet, staurolite and plagioclase. The S

foliation occurs as

relics of crenulations outlined by white mica within plagio-

clase. Garnet makes up solitary idioblasts in the matrix and is

included in staurolite. It is partially replaced by coarse biotite

flakes and/or chlorite. Staurolite idioblasts are marginally re-

placed along fractures by sericite and chlorite. Large biotite

flakes are strongly kinked. Plagioclase occurs as augens, some

of them with strong zoning and as porphyroblasts partially

overgrowing also the main schistosity S

. Andalusite porphy-

roblasts invade all pre-existing textures. They are partially re-

placed by sericite (Fig. 3H).

Algyõ basement-high

In the surroundings of the village of Algyõ a structural

basement-high has long been known. In this area lithologies

are varied with different types of gneisses, micaschists, am-

phibolites, chlorite schists, quartzites, pure and impure mar-

bles. Most of the rocks exhibit clear indications for a poly-

metamorphic history, with the whole range of typical defor-

mational microtextures. Minor texturally prograde rocks with-

out later overprint also occur.

Banded, mylonitic gneisses represent a very characteristic

lithology. They are fine-grained and dark-coloured, due to

high biotite content and graphitic pigment. In between there

are light-coloured layers of quartz and feldspar up to several

cm thick. Pre-mylonitic garnet porphyroblasts up to 2 cm in

diameter are frequent. Typically they contain coarse-grained

inclusions of biotite and plagioclase (Fig. 4A), indicating a

pre-mylonitic coarse-grained microtexture of the rocks. Por-

phyroblasts of K-feldspar and relics of staurolite occur in

some cases. There are also patches of fine-grained kyanite

(Fig. 4B). Szederkényi (1984) interpreted them as relics of

pre-existing andalusite and our investigations support this

idea. However they also partly represent former staurolite or

sillimanite. Staurolite and K-feldspars attain more than 1 cm

in diameter (Fig. 4C,F).

An overprint in the stability field of kyanite+staurolite+

garnet occurred contemporaneously to a ductile deformation

of the rocks. Deformation caused the development of a mylo-

nitic foliation S

, defined by fine-grained biotite, plagioclase

and quartz (Fig. 4E), as well as by newly formed muscovite.

The pre-existing garnet, staurolite and K-feldspar crystals act-

ed as porphyroclasts with respect to S

. In many cases the gar-

net porphyroblasts were broken and overgrown by a younger

garnet generation. The latter is rich in inclusions and also

present as tiny, anhedral grains in the matrix (Fig. 4F,G). The

K-feldspars were ductilely deformed and are preserved as au-

gens with subgrains along the crystal edges (Fig. 4F,G). Stau-

rolite was partly or fully replaced by fine-grained idioblasts

of kyanite and staurolite (Fig. 4C,D). The kyanite pseudo-

morphs are elongated within the mylonitic foliation. A green-

schist facies retrogression of various degrees caused chlori-

tization of garnet and the formation of chlorite and sericite in

the matrix.

In logs from Ferencszállás graphite-rich micaschists occur

with a fine-grained, mica-rich matrix overgrown by garnet

porphyroblasts and large biotite flakes (Fig. 4H).

Geochronological results

The

Ar-

Ar analytical data and age spectra are presented

in Figs. 58, the Rb-Sr and Sm-Nd analytical data in Table 2

and Figs. 6 and 9. All geochronological analytical data are

available from the authors.

Mecsek Subunit

Muscovite from a granitoid (sample Cegléd-3/13) yielded

an Ar-Ar plateau age of 299±4 Ma (Fig. 5). This age is in

agreement with other Variscan ages from this subunit (Klötzli

et al. in print).

Villány-Bihar Subunit

From the eclogite zone an amphibole separate from an am-

phibolite (Szeghalom-16/4) has been dated. It yields a plateau

PERMIAN-TRIASSIC EVOLUTION OF THE GREAT HUNGARIAN PLAIN 307

Fig. 6. Ar-Ar and Rb-Sr age data from Földes in the NE-part of the Villány-Bihar Subunit. The ages in the range of 266±3 Ma to

200±2 Ma are interpreted as rejuvenated Variscan ages.

type Ar release pattern with an age of ca. 310±6 Ma and an

oldest age domain of 323 Ma. The low temperature steps

show a staircase pattern with minimum ca. 250 Ma and an ex-

cess Ar content in the first two steps (Fig. 5).

Sillimanite and kyanite-porphyroblast bearing rocks sup-

plied plateau-type Ar-Ar muscovite ages in the range of

297±2317±3 Ma (Ártánd-1/10, Szarvas-8/4, Biharkeresztes-

16/1), whereas an age of 293±2 Ma were obtained on a biotite

(Szarvas-8/4). Also a muscovite concentrate, separated from a

cataclastic micaschist from a tectonic slice near to the north-

ern border of the unit, yielded a plateau-type age spectra with

an age of 317±4 Ma (Öcsöd-3/2; Fig. 5). These data are in the

range of typical Variscan cooling ages.

From the NE part of this subunit younger Ar-Ar ages have

been determined (Fig. 6). Muscovite and biotite from Földes

yield plateau type Ar-spectra of 266±4 Ma (Földes-12/3) and

223±2 Ma (Földes-6/2). Rb-Sr dating of biotite from a biotite-

bearing amphibolite (Földes-6/2) yields 200±2 Ma (Fig. 6).

Although there is no large spread in the

Rb/

Sr ratio, the

age is well defined, but it is either a typical Variscan or an Al-

pine age value. From a staurolite-garnet micaschist from Ál-

mosd-1/10 a saddle-shaped Ar-Ar muscovite age pattern of

ca. 202±2 Ma was obtained with a distinct rejuvenation at the

outermost rim.

Békés-Codru Subunit

Plateau type Ar-Ar muscovite ages from the main part of

this subunit are in the range of 305±3 Ma to 322±4 Ma, mea-

sured from coarse-grained andalusite-bearing gneisses (Ru-

zsa-7/8, Ruzsa-D-1/3, Sarkadkeresztúr-ÉNY-2/4, Mezõ-

gyán-1/5; Fig. 7). Gneisses without andalusite from this subunit

also fit this range (Ásotthalom-11/2, Kelebia-3/9).

Additionally, a log from Pusztaföldvár (P-222/6) with a

narrow but rather intense ductile deformation zone, exhibiting

postkinematic growth of white micas, was investigated. Mus-

covite yields a plateau-type age pattern of 309±3 Ma with no

significant rejuvenation in the low temperature steps. From a

diaphtoritic mylonite (Szeged-11/6) two separates of fine-

grained (0.10.2 mm) white mica, contaminated by a small

amount of chlorite were measured. The purer sample yielded an

Ar-Ar age of 317±3 Ma, whereas the other gave 301±3 Ma.

Algyõ basement-high

In this tectonic unit the age of the oldest mineral assem-

blage was determined by Sm-Nd analyses from garnet and ky-

anite-bearing mylonites (Dorozsma-7/10). The isochron, cal-

culated from a porphyroclastic garnet separate (Grt

) and a

feldspar concentrate (Fs) yielded a Permian age of 273±7 Ma

(Fig. 9). Additional analyses of a leached garnet (Grt

) and the

leachate (L) do not fit exactly to the isochron. However, all

ages that can be calculated between the four data points are in

the range of 287 to 242 Ma and indicate a Permian age.

The cooling age of the following, amphibolite-grade over-

print was determined by Ar/Ar analyses on muscovites: six

yielded plateau or plateau-type ages. Four of them fall into the

narrow range of 82±188±2 Ma (Ferencszállás-K-3/5, Fer-

encszállás-13/18, Üllés-31/7/1, Újszentiván-2/9), and two

spectra exceed 90 Ma and exhibit slightly disturbed and sad-

dle-shaped patterns (Ferencszállás-K-6/3, Kiszombor-7/4).

Compared with the other samples, they have the same crystal-

lization and deformation history. The reason for the higher

ages is uncertain. Limited rejuvenations of ca. 70 Ma in low-

temperature steps are present in four of the samples. Two S-C

mylonites, attaining biotite-grade at Forráskút-12/6 and chlo-

rite-grade at Algyõ-52/5 supplied saddle-shaped age pattern

of the relict micas with distinctly lower total gas ages of 58±1

and 70±1, proving a distinct rejuvenation after the Late-Creta-

ceous cooling. The youngest ages from the low-temperature

steps are 53 and 42 Ma respectively, which indicate an Early

Tertiary deformation (Fig. 8).

PERMIAN-TRIASSIC EVOLUTION OF THE GREAT HUNGARIAN PLAIN 309

Discussions

In this chapter the distribution of the Al

SiO

polymorphs

and the new geochronological age data is discussed, with re-

spect to the established tectonic subdivision of the basement

of the GHP. After that the metamorphic histories of the base-

ment blocks are compared to each other.

Metamorphic evolution of the Villány-Bihar Subunit

The main stages of the metamorphic evolution in the VBU

are an eclogite facies imprint preserved only within metabasic

rocks, and a subsequent amphibolite- and greenschist facies

overprint. The eclogite facies assemblage included garnet

omphacite±hornblende

±phengite±kyanite±zoisite+quartz

+apatite+rutile+ore. The eclogites exhibit several stages of

retrograde overprints, causing the formation of different types

of symplectites. During an amphibolite facies overprint most

of the high-pressure minerals were destroyed and an assem-

blage including garnet

+diopside+plagioclase+biotite+ horn-

blende

+ore developed. Greenschist facies retrogression is

displayed by chloritization of diopside and the crystallization

of epidote and carbonate.

M. Tóth (1997) proposed two distinct metamorphic cycles

for the PTt evolution of the eclogitic rocks. In this model

the first event caused the eclogite facies imprint, with pres-

sures up to 1012 kbar and 600650

C. For this event a Cale-

donian age was suggested (Szederkényi 1996). It was fol-

lowed by exhumation to greenschist facies crustal levels. The

subsequent amphibolite facies event and the following green-

schist facies retrogression have been proposed to be Variscan

in age. However, there are no geochronological age data

available, giving the age of the eclogite facies event. From our

petrographic investigations the eclogite facies assemblage and

the overprinting amphibolite facies assemblage can be placed

within the frame of a single clockwise PTt evolution path.

The investigated metapelites of the VBU show clear evi-

dence of a medium- to high-pressure amphibolite facies meta-

morphic event. In the metapelites assemblages of garnet+ ky-

anite+ staurolite are followed by the formation of sillimanite

and later greenschist facies retrogression. This clockwise P

Tt path can be correlated with the post-eclogite facies evolu-

tion of the eclogites. Cooling ages measured with several

methods on different lithologies yielded ca. 308±3317±3 Ma

and point to a Variscan age of this tectonothermal event. As

there are no indications for an older metamorphic assemblage

in the country rocks, and as we could not find indications for a

two stage metamorphic evolution in the eclogites we favour

an early Variscan age of the eclogite facies event.

Several samples yielded Ar-Ar muscovite ages younger

than typical Variscan cooling ages in the NE part of this sub-

unit. A plateau type age of 297±2 Ma from Ártánd is only

slightly younger, whereas coarse muscovite fishes from a

mylonitic micaschist without kyanite from Földes yield a re-

markably younger age (266±3 Ma). Important for the interpre-

tation of this area is the Rb-Sr biotite age of 223±2 Ma from

the same locality (see below). Also a muscovite from the tex-

turally undisturbed amphibolite facies metamorphic rocks of

Álmosd yielded a saddle-shaped Ar-Ar spectra of 202±2 Ma

and a distinct rejuvenation in the low-temperature steps.

These younger ages of the NE part of the VBU might reflect

an independent Permian event, or a rejuvenation of Variscan

ages during a later, Cretaceous overprint.

One argument for a PermianTriassic age comes from the

geochronological age data: to open and fully reset the Ar-Ar

isotopic system in muscovite, temperatures of at least up to

420

C are necessary. At these temperatures a total reset of the

Rb-Sr biotite ages can be expected. Therefore the Rb-Sr bi-

otite age of ca. 220 Ma supports a Triassic cooling of the

rocks. Ar-Ar muscovite ages in the range of 280 to 190 Ma to-

gether with Rb-Sr biotite ages of 225 to 150 Ma have been

found in several parts of the Austroalpine crystalline base-

ment unit, where the Cretaceous overprint did not exceed low-

ermost greenschist facies conditions. These data are interpret-

ed as reflecting cooling from a high to a lower geothermal

gradient, during a Permo-Triassic extensional event (Schuster

et al. 2001). Our data from the VBU are definitely too scarce

to justify such an interpretation. However, similar data were

recently published from the Szeghalom structural basement-

high (Balogh in M. Tóth et al. 2000). A series of ten K-Ar

data on amphibole concentrates yielded three age groups of

315, 260 and about 220 Ma.

On the other hand several facts argue for an eo-Alpine over-

print of the rocks: the existence of a Cretaceous event along

the northern border of the VBU is well documented by Árkai

et al. (1998). Fine-grained (<2

m) mica from Mesozoic sedi-

ments situated beneath overthrust complexes, as well as from

polymetamorphic rocks of the overthrust tectonic slices yield-

ed Cretaceous ages in the range of 64 to 99 Ma. Very low- to

low-grade metamorphic conditions were reached during my-

lonitic and cataclastic deformation of the rocks. In our data

this Cretaceous overprint is hardly visible, because we inves-

tigated the coarse white mica flakes belonging to the main

Variscan mineral assemblages. For example the cataclastic

gneisses from log Öcsöd represent crystalline rocks thrust

over Cretaceous metabasites. Its coarse-grained muscovite

yielded an age of 317±3 Ma, but there is also a fine-grained

sericite present in the sample, which has not been investigated

during this study.

Fig. 9. Sm-Nd garnet isochron from the Algyõ basement-high. The

age is interpreted as the formation age of the high-temperature/

low-pressure assemblage of the rocks.

310 LELKES-FELVÁRI, FRANK and SCHUSTER

Another argument is that the Triassic facies evolution of the

VBU is characterized by a clastic Upper Triassic (Keuper)

succession. Such a sedimentary environment is not expected

when a Permian lithospheric thinning associated with pro-

grade metamorphism took place (Schuster et al. 2001).

However, at present we have to leave it open, whether the

younger ages in the NE part of the VBU are due to rejuvena-

tion caused by an eo-Alpine overprint or a prolonged cooling

history during PermianTriassic times. Further investigations

are in progress to solve this question.

Metamorphic evolution of the Békés-Codru Subunit

Assemblages containing andalusite and staurolite indicate

low-pressure amphibolite facies conditions along the northern

border of the BCU from Ruzsa in the SW to Mezõgyán in the

NE for the dominant metamorphic imprint. Muscovite from

these coarse-grained assemblages yielded cooling ages in the

range of 305322 Ma. The same range was also obtained from

rock types without andalusite. A mica-rich fraction from a

mylonitic micaschist coming from Szeged, from the flank of

the Algyõ basement-high also fits this range, even if the Ar-

pattern shows distinct rejuvenation in the low-temperature

steps.

Metamorphic evolution of the Algyõ basement-high

From the basement-high in the surroundings of Algyõ a se-

ries of samples with characteristic features have been investi-

gated, which are not known from other locations in the Tisza

Megaunit. On the basis of the microfabrics at least two meta-

morphic events can be distinguished in most of the samples:

in the samples from Dorozsma: a first, coarse-grained assem-

blage including garnet

+staurolite

+plagioclase+K-feldspar

+quartz±muscovite developed. Relics of kyanite aggregates

render possible the former presence of andalusite and silli-

manite. The occurrence of andalusite, sillimanite and K-feld-

spar clearly indicate high temperature/low pressure character-

istics and high-amphibolite to granulite facies conditions.

Considering the Sm-Nd garnet isochron age this association

formed in Permian time at ca. 275 Ma.

Several previous papers deal with the P-T conditions of the

first metamorphic imprint in the Algyõ basement-high.

Szederkényi (1984) found 570630

C at about 2 kbar in am-

phibolites from Algyõ. In contrast Horváth & Árkai (2002)

determined 520560

C and 810 kbar from Ferencszállás.

The latter conditions are far outside the stability field of an-

dalusite and sillimanite. A possibility to explain this misfit is

to suppose that the Algyõ basement-high itself is a composite

Fig. 10. Sub-surface map of the Great Hungarian Plain, showing the distribution of the index minerals and the geochronological age data.

Kyanite porphyroblasts and sillimanite is restricted to the VBU, while andalusite porphyroblasts are widespread in the BCU. Kyanite and

andalusite porphyroblast-bearing rocks of the VBU and BCU supplied exclusively Variscan ages. In the NE part of the VBU, in the area

of Földes and Álmosd ages younger than typical Variscan cooling ages were found. The Algyõ-high is characterized by ubiquitous kyan-

ite aggregates, a Permian age of garnet porphyroclasts and middle Cretaceous to Tertiary Ar-Ar muscovite ages.

PERMIAN-TRIASSIC EVOLUTION OF THE GREAT HUNGARIAN PLAIN 311

unit. Partly it consists of rocks with a first Variscan assem-

blage of medium- to high-pressure characteristics, whereas

other parts got their first imprint during a Permo-Triassic

high-temperature/low-pressure event. At first sight this inter-

pretation might seem to be unlikely, but in the Austroalpine

unit such a situation has been observed (Schuster et al. 2001)

(see also discussion below).

The second event caused the formation of the main assem-

blage garnet

+kyanite±staurolite

+plagioclase ± muscovite+

quartz and a contemporaneous mylonitization of the rocks.

With respect to the equilibrium assemblage kyanite+stauro-

lite+garnet, medium-pressure amphibolite facies event can be

assumed. Recently conditions of 650680

C at 59 kbar

have been published for this overprinting event (Horváth &

Árkai 2002). At present the exact timing of the peak of this

metamorphic imprint is unknown. However, all investigated

samples yielded Ar-Ar muscovite cooling ages in the range of

82 to 95 Ma, without any older relics. Similar ages by Balogh

& Pécskay (2001) were also reported. These data indicate an

eo-Alpine age of the imprint. After the metamorphic peak a

greenschist facies overprint occurred. It caused sericitization

and chloritization during ongoing deformation. The intensity

of this overprint is variable in the different samples. It might

explain the younger Ar-Ar muscovite ages of 58 to 70 Ma.

Algyõ basement-high and Austroalpine Saualm-Koralm

Complex

On the basis of the new results the rocks from the Algyõ-

high exhibit a completely different evolutionary history com-

pared to other basement rocks of the Tisza Megaunit. As men-

tioned in Lelkes et al. (2001, 2002) the very special

lithologies of the Algyõ-high resemble the metamorphic suc-

cession of the Saualm-Koralm Complex and the Strallegg

Complex of the Austroalpine domain in the Eastern Alps. The

latter forms eo-Alpine thrust sheets, which hold high tectonic

positions in the eo-Alpine nappe stack (Weissenbach 1975;

Frank 1987; Krohe 1987; Schuster et al. 2001). They were af-

fected by Permo-Triassic lithospheric extension (Habler &

Thöni 2000; Schuster et al. 2001), and an intense eo-Alpine

tectonometamorphic overprint (Frank 1987; Thöni & Miller

1996). Lithospheric extension caused a high-temperature/low-

pressure metamorphic imprint in the stability field of an-

dalusite and sillimanite and the formation of gabbroic intru-

sions and numerous anatectic pegmatites. The Cretaceous

overprint reached eclogite and following amphibolite facies

conditions and the pre-existing andalusite and sillimanite

were replaced by kyanite aggregates. This was accompanied

by mylonitization during exhumation processes including N-

to NW-directed thrusting.

The microtextural features of the Algyõ basement-high are

very similar to those of the Saualm-Koralm Complex and give

additional argument for the metamorphic history discussed

above.

Contrasting metamorphic evolutions

In the past, several authors proposed pre-Variscan meta-

morphic evolution for parts of the crystalline basement in the

GHP. However, at present either geochronological age data or

undoubted microtextural observations prove such interpreta-

tions.

The oldest available age data in the range of 300 Ma to 330

Ma have been measured on hornblende and muscovite by the

K-Ar and Ar-Ar method. As they derive from medium-grade

rocks, they have to be interpreted as metamorphic cooling

ages. They indicate a Variscan age of the dominant tectono-

thermal imprint. At present nothing is known about the exact

timing of the eclogite facies event, or about the timing of the

metamorphic peak in the VBU or BCU.

For the understanding of the Variscan geodynamic evolu-

tion of the area, it would be important to know if the an-

dalusite in the BCU developed contemporaneously to the

pressure peak and the kyanite formation, or synchronously to

the temperature peak and the sillimanite formation in the

VBU. In the first case a completely different tectonic setting is

evident, whereas in the second case only different metamor-

phic conditions during the exhumation of the rocks can be as-

sumed. However, as kyanite porphyroblasts overgrown by sil-

limanite and andalusite occur in different basement blocks,

and no other Al

SiO

polymorph have been found in the an-

dalusite-bearing lithologies, there is no microtextural evi-

dence about the relative age relationship of these minerals in

the units of the GHP. This is in contrast to the SW part of the

Tisza Megaunit in Transdanubia: in the Mecsek Subunit both

kyanite and andalusite are known from the same rock and an-

dalusite is a late crystallization phase overgrowing kyanite

(Lelkes-Felvári et al. 1989; Török 1990).

A key point in understanding the contrasting histories of the

Algyõ basement-high with respect to the surrounding BCU,

are the ages of the andalusite-bearing mineral assemblages. In

the BCU andalusite is well preserved and like the coexisting

garnet, it is Variscan in age. On the other hand, in the Algyõ-

high andalusite is totally transformed into kyanite aggregates

during the eo-Alpine tectonothermal event; one would con-

clude that lithologies from the BCU were the precursor rocks

of the Algyõ-high. The important observation, that the first

generation of garnet in the Algyõ basement is Permian in age

although only demonstrated up to now in a single sample

is not compatible with this assumption. The Permian gar-

net age is likely also because of the remarkable analogies of

the Algyõ-high and the Austroalpine Saualm-Koralm Com-

plex.

There is another remarkable difference in the tectonic evo-

lution of the different crustal blocks in the GHP. In the VBU

Lower Permian rhyolites are only described from one restrict-

ed area, and tectonic contacts of the Mesozoic sediments to

the crystalline basement rocks are proved in several places. As

the rocks cooled down at about 310 Ma, they stayed in a shal-

low crustal level since that time, and we do not know when

they reached the surface. In contrast, in the BCU Lower Per-

mian rhyolites and Lower Triassic redbeds are overlying the

basement with a sedimentary contact (Majoros 1998). For this

reason the basement rocks formed the surface in late Paleozo-

ic times. As the Algyõ-high experienced a Permian metamor-

phic imprint, located in middle crustal levels at that time,

consequently, there are no Permian-Triassic sediments in strat-

igraphic contact.

312 LELKES-FELVÁRI, FRANK and SCHUSTER

Tectonic relationships

According to Tari et al. (1999) the Algyõ basement-high

represents a metamorphic dome, which formed as a core com-

plex during Miocene extensional processes. In this model the

lower part of a Variscan crustal section experienced a thermal

overprint in Cretaceous times and was exhumed by a Tertiary

fault system. On the basis of the new data such an interpreta-

tion is unlikely with respect to the contrasting evolution of the

surrounding BCU. As mentioned before, the rocks of the Al-

gyõ-high are not the result of eo-Alpine, overprinted litholo-

gies of the BCU. Furthermore, they experienced not only a

Cretaceous thermal overprint, but also mylonitic deformation.

Therefore the proposed scenario would only be possible when

the BCU would have been thrust onto a basement wedge with

the characteristics of the Algyõ-high. This must have hap-

pened after the eo-Alpine tectonothermal event, and prior to

the formation of the core complex.

Therefore we interpret the Algyõ sequence as a tectonic

outlier of a Cretaceous metamorphic nappe system. A similar

interpretation has been already proposed by Dimitrescu

(1995), on the basis of lithological correlation. He compared

the sequence of the Algyõ basement with the Biharia Unit,

considered of that time the uppermost unit of the Biharia

nappe system in the Apuseni Mountains (Balintoni 1994).

Both units are characterized by carbonates, completely lack-

ing in the surrounding basement. Cretaceous Ar-Ar ages were

reported from the uppermost nappe of the Apuseni Mountains

(Baia de Arieº nappe-complex, Soroiu et al. 1969; Dallmeyer

et al. 1994, 1996, 1999). The exact timing of the emplacement

of this nappe, resting on top of a Variscan metamorphic base-

ment is unknown. As the Variscan basement of the BCU

shows a weak, low-temperature hydrothermal alteration, we

can exclude emplacement during the eo-Alpine tectonometa-

morphic event. The emplacement of the Algyõ-high is defi-

nitely younger than the main cooling of the rocks, which is

dated at ca. 80 Ma. The Ar-Ar age spectra of some micas

show indications for a younger thermal overprint causing mi-

nor rejuvenation up to 40 Ma. A possible solution would be

that the final emplacement of this unit occurred in the Early

Tertiary.

Conclusions

The observed distribution of the Al

SiO

polymorphs and

the new geochronological age data confirm the established

tectonic subdivision of the basement of the GHP, except for

the Algyõ basement-high, which has to be excluded from the

VBU and considered as a separate unit.

The distribution of the Al

SiO

polymorphs shows a clear

regional distribution: kyanite porphyroblasts together with sil-

limanite are restricted to the VBU, while andalusite porphyro-

blasts are widespread in the BCU. The Algyõ-high is charac-

terized by ubiquitous kyanite aggregates, which are

interpreted as pseudomorphs after pre-existing andalusite and

minor sillimanite and staurolite.

A granite of the Mecsek Subunit yielded an Ar-Ar musco-

vite age of ca. 300 Ma. Variscan cooling ages of ca. 310 and

320 Ma were found in the VBU and BCU, indicating

Variscan formation ages for the observed assemblages. Only

in the NE part of the VBU (Földes and Álmosd) were ages

younger than typical Variscan cooling ages measured. They

indicate a Permo-Triassic, or an eo-Alpine thermal overprint.

In the Algyõ basement-high a first Permo-Triassic HT/LP

event, with peak conditions at about 275 Ma is proved at least

for some lithologies. The Ar-Ar mica ages from this unit are

exclusively younger than middle Cretaceous. They demon-

strate the age of cooling after the amphibolite facies imprint

and subsequent deformational overprint of the rocks. Consid-

ering the metamorphic and structural evolution, the Algyõ-

high shows many similarities to the Saualm-Koralm Complex

of the Austroalpine. Similarly to the Baia de Arieº nappe-sys-

tem in the Apuseni Mountains, they might represent an eo-Al-

pine thrust sheet. It took place on top of the BCU after the

cooling of the rocks, in latest Cretaceous or Tertiary times.

The observed zonal distribution argues for distinct meta-

morphic histories of the different tectonic elements of the Tis-

za Megaunit. This implies that this terrane is not as homoge-

neous in a tectonic sense, as it has been presented in parts of

the literature.

Acknowledgments: The Hungarian Oil and Gas Plc. is

thanked for providing core samples for investigations. T.

Szederkényi generously offered the thin section collection of

Szeged University for the microscopic survey of the base-

ment. Research was supported by the Grants 23940 and

37243 of the Hungarian Scientific Research Fund (OTKA),

by FARGON and by the Austrian Science Fund FWF, Project

P14525-GEO. The authors thank M. Jelenc for help with the

Sm-Nd isotope analyses. Discussion with B. Cserepes-Meszé-

na and A. Nusszer is greatly appreciated. J. Zachar is thanked

for providing photos. Constructive reviews by P. Árkai, M.

Janák and F. Neubauer is acknowledged.

Appendix

Thin section description of the samples investigated by geochrono-

logical methods. Mineral names are abbreviated according to Kretz

(1983).

Mecsek Subunit

Cegléd-1/13: Medium-grained granite with mineral assemblage of

Kfs+Pl+Qtz+Ms+Bt+Zrn. Ms is well preserved and up to 7 mm in size.

Kfs is altered. Bt is replaced by Chl+Cc+ore.

Villány-Bihar Subunit

Öcsöd-3/2: Cataclastic St micaschist. The coarse-grained matrix is

composed of Bt+Ms+Pl+Qtz. St forms idioblasts up to 3 mm partly re-

placed by sericite.

Szarvas and Ártánd area: Ky bearing, fine- to medium-grained Qtz-

micaschists, Pl gneisses (Szarvas-8/4) and Mc gneisses (Ártánd-1/10).

The main foliation (S

) is defined by micas and elongated Qtz rods. Rel-

ics of S

are outlined in Pl microaugens by inclusions of Qtz, St, Grt and

Bt. Pl is rarely zoned (Ártánd). Postkinematic Ky idioblasts are partially

replaced by Ms and sericite. St idioblasts contain Bt inclusions and are

partially replaced by sericite. Sil fibres are associated to Bt. Grt makes

up small idioblasts and atoll-like crystals with cores filled up by

Ms+Bt+Sil.

Biharkeresztes-16/1: Coarse-grained Sil-Pl gneiss. Mica-rich layers

PERMIAN-TRIASSIC EVOLUTION OF THE GREAT HUNGARIAN PLAIN 313

contain deformed Bt flakes and subordinate Ms. Abundant Sil is associ-

ated with Bt. Pl xenoblasts are replaced by sericite. Spn idioblasts are

evenly distributed. Late carbonate veins crosscut the rock.

Földes-6/2: Fine-grained mylonitic Bt-amphibole gneiss. Fine lami-

nae composed mainly of acicular Am, some Bt and ore alternate with

fine-grained Qtz-rich layers and coarser, Bt-rich layers containing also

Cal granoblasts.

Földes-12/3: Fine-grained mylonitic Bt-Ms gneiss with Pl-microau-

gens attaining 3.5 mm. Recrystallized polycrystalline Ms-fishes are em-

bedded in a Qtz matrix with very fine-grained Bt, some Ms and some

small Pl granoblasts. Grt idioblasts (0.75 mm) are included in Ms and

Pl, bigger crystals occur in the matrix.

Álmosd-1/9: Micaschist. Within a fine-grained Mu-rich matrix Grt

(4 mm) and St (9 mm) porphyroblasts occur. The S

of Grt idioblasts are

strongly discordant to the S

of the matrix. The cores of some atoll

shaped Grt crystals are filled with coarse Bt+Qtz. Small, round Grt

(2 mm) are included in St. Late St idioblasts contain intrafolial folds

marked by Gr. Bt idioblasts up to 8 mm crosscut the schistosity planes.

Qtz-rich bands show static recrystallization, containing fine mica flakes

and St poikiloblasts.

Szeghalom-176/4: Bt-amphibole gneiss, composed of nematoblasts

of Am (up to 2 mm), Bt flakes and granoblastic Qtz and Pl with relict

zoning. Grt up to 2.5 mm is partially or completely replaced by Pl, with

Grt relics as small islands. Pl is partially replaced by sericite. Bt up to

4 mm is almost completely replaced by Chl and Spn.

Békés-Codru Subunit

Ruzsa-7/8, Ruzsa-D-1/3, Sarkadkeresztúr-ÉNY-2/4, Mezõgyán-1/5:

Coarse-grained And-bearing gneisses, often showing S-C structures. Fs

augens and coarse Ms flakes are embedded in a finer-grained mica ma-

trix rich in Bt. The main foliation (S

) is defined by the micas, contain-

ing porphyroblasts of Grt, St and Pl. Relic of a crenulated cleavage S

outlined by Ms inclusions within Pl. Grt makes up solitary idioblasts in

the matrix or small inclusions in St. It is partially replaced by coarse Bt

flakes and/or Chl. St is marginally replaced by sericite and Chl. Bt

flakes are strongly kinked. Zoned Pl augens, occur also as late porphy-

roblasts including partially also S

. And porphyroblasts invade all pre-

existing textures. Idioblasts of Ilm are included in the porphyroblasts

and the matrix.

Kelebia-3/9: Pl gneiss with micro-augen texture. Elongated, poly-

crystalline augens of Pl (up to 3.5 mm) contain idioblasts of Grt

(0.25 mm), green Bt, Ms and Ap inclusions. Bt flakes (2 mm) are dis-

persed, small flakes of Ms occur in Qtz-rich domains and are intergrown

with Bt in mica-rich domains.

Ásotthalom-5/6: Garnet-bearing Ms-Bt gneiss. Fine-grained mica-

rich layers contain deformed Bt flakes (up to 4 mm), partially altered to

Chl, Ms flakes crowded with small, postkinematic Ms. Grt up to 5 mm

is replaced by Bt. Subhedral grains of Pl (3.5 mm) are surrounded by Bt.

Polygonal Qtz-rich layers contain small Ms flakes.

Pusztaföldvár-222/6: Coarse-grained garnet-bearing micaschist with

strong Ms recrystallization. Bt is completely replaced by Chl and Spn

aggregates. Grt (2.5 mm) is replaced by Chl. Within a narrow ductile

shear zone Ms suffered grain size reduction and Chl and Cal appear as a

new mineral phase.

Szeged-15/5: Mylonitic micaschist with quartz ribbons. In the Ms-

rich, fine-grained matrix some Chl is present making up postkinematic

flakes and lenses along opacitic seams. Elongated porhyroclasts of Grt

(up to 1 cm) are replaced by carbonate and Chl. Pl xenoblasts contain

Ep aggregates.

Algyõ basement-high

Algyõ-52/5: Mylonitic S-C quartz-micaschist. Qtz and mica-rich

bands alternate. It is cut by thin penetrative shear-plains with grain-size

reduction. Mica-rich bands are composed of deformed, recrystallized

Ms and Bt fishes set in a finer-grained matrix of decussate flakes of Ms

and Bt. Grt is broken, elongated and partially replaced by

Ms+Bt+Chl+ore. Pl forms polycrystalline augens and lenses with Bt in-

clusions and recrystallized mosaic structures.

Ferencszállás-K-3/5: Micaschist, gently folded with anastomosing

very fine, brittle, chloritic shear planes. In the matrix composed of de-

cussate flakes of Ms and finer-grained Bt round, broken Grt porphyro-

clasts occur. They are up to 3 mm and partially replaced by Bt and Chl.

Ferencszállás-K-6/3: Banded, fine-grained, slightly folded mic-

aschist. Late idioblasts of Grt (up to 2.5 mm) include folded trails of Gr,

slightly discordant to S

. Further they make up atolls with Qtz and Bt-

rich cores. Chl flakes (1.5 mm) occur in the pressure shadows of Grt.

Qtz-rich bands show static recrystallization.

Ferencszállás-13/18: Banded Ms-Bt paragneiss with mylonitic folia-

tion. Mica-rich bands composed of decussate Bt+Ms flakes alternate

with Qtz-rich bands with granoblastic textures. Pl contains relics of ear-

lier folds enhanced by Bt flakes. Prekinematic porphyroblasts of Grt are

up to 4 mm and show inclusions of Bt and Rt.

Üllés-31/7: Crenulated micaschist. Mica-rich bands of decussate

flakes of Bt+Ms alternate with Qtz+Pl-rich bands. Two generations of

St can be distinguished: St

attaining several mm occur as isolated relics

with uniform optical orientation within aggregates of Ky±St. They of-

ten have dark, fluid-inclusion-crowded rims. Small idioblasts of St

oc-

cur in the mica-rich, Gr-bearing matrix. Some are intergrown with Ky,

containing inclusion-rich cores. Grt

is broken and partly replaced by

Chl. Inclusion-rich cores are surrounded by clear rims composed of sev-

eral adjoining tiny crystals. Late shear plains S

marked by Bt crosscut

the mylonitic foliation S

Újszentiván-2/9: Plagioclase gneiss. Relic microfolded schistosity

), outlined by Gr pigment is enclosed in Pl and St. Anhedral grains of

Pl (5 mm) and porphyroblasts of Grt and St are set in a matrix, com-

posed of postkinematic flakes of Ms and Bt. St relics (up to one cm) are

surrounded by Ky aggregates and sericite. Grt

(6 mm) contains Ilm and

Qtz inclusions. Broken parts surrounded by strings of Grt

also make up

glomeroblasts.

Kiszombor-7/4: Crenulated micaschist. Ms and Bt with strong postki-

nematic crystallization make up mica-rich layers. Qtz-rich granoblastic

bands bear some anhedral Pl. Broken and round Grt crystals up to

2.5 mm in size are partially replaced by Chl and Bt.

Forráskút-12/6: Mylonitic micaschist with S-C structure. Coarse,

polycrystalline Ms fishes and small augens of Pl are embedded in fine-

grained Qtz-rich granoblastic matrix with a penetrative schistosity, con-

taining very fine-grained Bt flakes. Small idioblasts of St (single crys-

tals or crystal groups, 0.250.5 mm) are cut by shear plains. Euhedral

rims of Grt idioblasts (up to 0.8 mm) contain inclusions from the mylo-

nitic matrix.

Dorozsma-7/10: Fine-grained, strongly foliated mylonitic micaschist

to gneiss. Porphyroclasts of Grt

, Kfs and Pl are embedded in a fine-

grained matrix composed of Ky+Bt+Qtz. Grt

shows inclusions of

coarse-grained Bt and Pl. Kfs forms up to one centimetre large crystals

with recrystallization at the crystal edges. The second Grt generation

(Grt

) is rich in tiny inclusions, and forms rims around pre-existing Grt

and small idioblasts in Qtz-rich domains.

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