Download Probable anticlockwise P-T evolution in extending crust: Hlinsko

1, metamorphic Ceol., 1996, 14, 49-60
Probable anticlockwise P-T evolution in extending crust:
Hlinsko region, Bohemian Massif
P. P I T R A ' , ' A N D M. G U I R A U D 2
7 '
Ustav petrologie a strukturnigeologie, Universita Karlova, Albertov 6, 128 43 Praha 2, the Czech Republic
'Laboratoire de Min&alogie, URA CNRS 736, Muse'um National d'Histoire Naturelle, 61 rue Buffon, 75 005 Paris, France
ABSTRACT
In the Hlinsko region (Variscan Bohemian Massif, Czech Republic) a major extensional shear zone
separates low-grade metasedimentary series (Hlinsko schists) and high-grade rocks of the Moldanubian
terrane (Svratka Crystalline Unit). During late-Variscan extension, a tonalite intruded syntectonically
into the normal ductile shear zone, and caused contact metamorphism of the overlying schists. Concurrent
syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists. In order to
decipher the detailed petrological evolution of the Hlinsko unit situated in the hangingwall of this tectonic
contact, a phase diagram approach and pctrogenetic grids, calculated with the thermocalc computer
program, were used.
The crystallization/deformation relationships and the paragenetic analysis of the Hlinsko schists define
a P-T path with an initial minor increase in pressure followed by cooling. Calculated pseudosections
constrain this anticlockwise P-T evolution to the upper part of the andalusite field between 0.36 and
0.40 G P a for temperatures ranging from 570 to 530 -C. A low uHZ0is required to explain the presence of
andalusite-biotite-bearing assemblages, and could be related to the presence of abundant graphite.
I n contrast, the footwall rocks of the Svratka Crystalline Unit record decompression from around
0.8 GPa at a relatively constant temperature, followed by cooling. Thus, the footwall and the hangingwall
units display opposite, but convergent P-Thistories. Decompression in the footwall rocks is related to a
rapid exhumation. We propose that the inverse, anticlockwise P-T path recorded in the hangingwall
pelites is related to the rapid, extension-controlled sedimentation of the overlying flysch series.
Key words: anticlockwise P-T path; Bohemian Massif; late-orogenic extension; low-P metamorphism;
Variscan
INTRODUCTION
Numerous studies have shown that the final stages of
the orogenic evolution of a collision belt are dominated
by extensional tectonics (Coney & Harms, 1984;
Dewey, 1988; Gaudemer et ul., 1988; Menard &
Molnar, 1988; Burg et ul., 1994b). Although the
petrology of rocks lying in the footwall of major
normal faults is relatively well documented, knowledge
concerning the evolution of the hangingwall units
is lacking.
In the Hlinsko region (Variscan Bohemian Massif,
Czech Republic) a major extensional shear zone is
observed between low-grade metasedimentary series
(Hlinsko schists) a n d high-grade rocks of the
Moldanubian terrane (Svratka Crystalline UnitSCU) that are situated in the hangingwall a n d in the
footwall, respectively. The extensional character of this
tectonic contact and of the related structures in both
units has been demonstrated (Pitra et ul., 1994). The
purpose of this paper is to present the detailed
Corresponrlence: Pave1 Pitra, Laboratoire de Mineralogie,
MNHN, 61 rue Buffon, 75 005 Paris, France
(email: [email protected]).
petrological evolution of the hangingwall Hlinsko unit
and to discuss its possible geodynamic significance
within the extensional context.
GEOLOGICAL SETTING
The Hlinsko Palaeozoic sediments form a N-Selongated, E-verging synform bounded by the
Nasavrky plutonic complex to the west and by the
high-grade metamorphics of the Svratka Crystalline
Unit to the south-east (Fig. 1). From the bottom to
the top, four units are identified (Vachtl, 1962): (1) a
pelite and metavolcanic sequence (Vitanov series)
supposedly of Upper Proterozoic age, (2) pelites and
greywackes of the Hlinsko series for which a n
Ordovician age has been proposed, (3) the Mrikotin
series that contains pelites, graphitic pelites and lydites,
and quartzite beds, a n d whose Silurian age has been
confirmed by graptolites (Wurm, 1927; Horny, 1956),
and (4) the Rychmburk series, which forms a thick
sequence of pelites and greywackes with conglomerate
intercalations. The age of these strata is not clear
(Proterozoic, Siluro-Devonian o r Carboniferous). The
latter series displays a flysch character with general
grain coarsening observed towards the NE, i.e. upwards
49
50
P . P I T R A & M. C U I R A U D
1PE
16"E
51"N
49"N
a,
C
0
e,
s
CF
0)
Ul
0
%
.-C
z
Greywackes
(Rychmburk)
Graphitic pelites
a
Greywackes
-pelites (Hlinsko)
Volcano-sedim.
rocks(Vitanov)
Basic rocks
Deformed
a
Acid
rocks
Normal ductile
shearzone
t/
Fig. 1. Schematic geological map of the Hlinsko region and its location within the Bohemian Massif. Encircled numbers refer to
samples in Table I .
in the sequence and when approaching the faulted
contact with the tonalite (Vachtl, 1950). Moldanubianderived clasts were found in this part of the series
within the conglomerates (Chab, 1973), as well as
numerous synsedimentary normal faults. Thus we
interpret this series as a synextensional basin of
Carboniferous age, which fits well with the age of
extension in the Bohemian Massif (Pitra et al., 1994).
The Svratka Crystalline Unit, probably a part of the
Moldanubian terrane, comprises paragneisses, orthogneisses and migmatites intercalated with minor mica
schist beds, lenticular bodies of amphibolites and
skarns parallel to the main foliation. A tonalite
laccolith has intruded syntectonically the contact with
the Hlinsko sediments, triggering HT-LP metamorphic
crystallization in the overlying schists (Pitra et a/.,
1994).
Three deformational events are regionally distinguished in the Hlinsko metasediments. The S 1
cleavage is marked by the alignment of muscovite,
quartz, ilmenite and possibly chlorite and chloritoid in
the lowermost pelites. Second-generation structures
are well developed, with S2 being a crenulation
cleavage of variable intensity. S2 average spacing
decreases toward the contact with the tonalite and the
base of the sedimentary pile. The SE-NW-striking
subhorizontal and synmetamorphic lineation L2 is
marked by the long axis of staurolite and chloritoid
blasts, and/or by their boudinage. S2 is folded by the
deformational event D3. The S3 crenulation cleavage
is associated with the axial plane of the regional
syncline and displays a strain gradient with an overall
westward increase in intensity from the hinge of the
Hlinsko syncline. It bears subvertical mineral stretching
lineations near the contact with the Nasavrky granodiorite. Only S3 is observed within the uppermost part
of the Rychmburk sedimentary series.
The late Nasavrky granodiorite intruded the western
part of the region producing limited contact metamorphism (Sachsel, 1933; Vachtl, 1962). Steep magmatic
foliation and lineation that typically occur at the
pluton margins are consistent with the D3 strain
gradient in the country rocks. The granodiorite has
been dated at 360-366 Ma (K/Ar, whole-rock; Smejkal,
1960, 1964). However, recent dating at 320+4Ma
(Rb/Sr, whole-rock; Scharbert, 1987) suggest a younger
age for the associated granites.
It is assumed that D1 represents the local, and
earliest stages of the D2 north-westward shear deformation. D3 is related to the emplacement of the
Nasavrky granodiorite and is responsible for the
regional Hlinsko syncline.
PETROLOGY OF THE H L I N S K O SCHISTS
Petrography and mineral chemistry
In the Silurian (Mrakotin) pelites the S1 foliation is
parallel to the sedimentary layering SO defined by
muscovite-rich and quartz-rich layers. The relative
A N T I C L O C K W I S E P-T PATH, B O H E M I A N M A S S I F
~~~
~
~
51
~
abundance of these layers varies between end-members
with an equal proportion of both types, and rocks
dominated by muscovite-rich layers (>90%). Pelites
become generally more muscovite-rich when approaching the contact with the tonalite. The thickness of the
layers is about 1 cm in rocks with preserved sedimentary layering and decreases to about 1 mm where S2
is intensely transposed near the contact with the
tonalite. Muscovite-rich layers are composed of finegrained muscovite (70%), andalusite and/or Fe-Mg
minerals (staurolite, biotite, garnet, cordierite, chlorite
up to 25%), ilmenite and quartz ( < 5 % ) . The quartzrich layers are composed of quartz ( S O % ) , muscovite
(40%), ilmenite, garnet and chlorite, with other Fe-Mg
minerals being less abundant.
All the Mrakotin pelites are relatively Fe- and
Al-rich. A significant difference exists in bulk rock
composition between the centre of the syncline where
more aluminous and less ferriferous lithologies occur
[AI/(Al + Fe + Mg) = 0.88, Fe/( Fe Mg) =0.60] and the
SE limb, near the tonalite, which is characterized by
more ferriferous and less aluminous lithologies
[(Al/(Al+Fe+Mg)=0.79, Fe/(Fe+Mg)=0.64].
The representative mineral chemistry of individual
phases is listed in Table 1.
+
Andalusite. Andalusite is zoned chiastolite and contains inclusions of biotite, garnet and euhedral staurolite (Fig. 2a). It encloses also oval-shaped aggregates
of chlorite, biotite, muscovite and quartz. Straight
inclusion trails of ilmenite and graphite suggest that
andalusite grew partly before S2 as intense crenulation
is present outside crystals. The andalusite is ferric with
the Fe,O, content spanning 0.3-0.8 wt% and does not
contain any other oxides in significant amount.
Staurolite. Staurolite is euhedral and is often optically
zoned. Crystals may be broken, with chlorite crystallizing between fragments. It includes crenulated S1 trails
indicating that staurolite is late- to post-S2 (Fig. 2b).
However, staurolite inclusions within peripheral parts
of andalusite indicate that staurolite grew along with
and later than andalusite during D2. No significant
chemical zonation has been observed and the chemical
variations are supposed to reflect only change in bulkrock composition. The staurolite is rich in ZnO
(0.1-1.3 wt%), TiO, (0.15-0.65 wt%) and MnO (0.350.60 wt%) and X,, ranges from 0.85 to 0.93.
Biotite. Biotite crystals have overgrown S1 and are
deformed by S2. Although displaying microscopically
the usual optical features, biotite has been systematically transformed to oxychlorite. In the AFM diagram
projected from muscovite, compositions plot between
the position of biotite and that of chlorite and X,,
increases from 0.59 to 0.70 as it shifts towards chlorite.
As the compositions also tend towards the staurolite
position, it is suggested that the continuous destabilization of biotite to chlorite involves staurolite (see
Fig. 4). In some thin sections, biotite zonation is
marked by centres rich in ilmenite inclusions and clear
rims. Textural equilibrium with andalusite, staurolite
and garnet is common.
Chloritoid. Chloritoid lies within, and locally across,
S2. Near the centre of the regional syncline, it forms
tiny elongated rods in chloritized biotite, possibly as a
Table 1. Chemical compositions of principal metamorphic minerals of the Hlinsko pelites. Chl,,, represents the chlorite
composition within the andalusite inclusion. Numbers in the first row refer to the location on Fig. 1.
Locallon
SIO,
TIO,
AI,O,
CrD,
M@
FeO
MnO
ZnO
CaO
Na,O
K D
Sum
SI
TI
A1
Cr
Mg
+
hl n
Zn
Ca
Na
K
OH
Sum
X
FC
2
I
Ms H42 Ms H82
2
St H42
St 248
I
Bt H82
27 06
0 28
55 42
0.11
1.28
13.53
0.34
1.31
0.00
0.00
0.00
99.33
2x31
0 29
55 40
0.02
0 84
13.44
0 33
0 74
0.00
0.00
0.00
99 37
34 06
0.86
20.01
0 00
5.99
23.62
0 12
0 00
0.00
0 16
7.03
91.85
35.65
I .07
20 95
0.02
7.93
20.61
0.12
0 00
0.00
0.34
8.14
94.83
35 97
1.06
21.80
0.00
5.93
18.46
0.07
0.00
0 09
0 08
5.14
88.60
35 81
0.71
23.81
0.00
5.04
20.21
0 12
0 00
0.00
0.00
3.34
89.04
24.50
0.00
40.95
0.00
1.83
24.85
0.57
000
000
no0
000
92.70
24 11
0.08
4045
0.00
2.41
2507
077
000
000
0.00
0.04
9293
37 07
0.00
20.81
0.00
1.33
29 62
10.46
0.00
136
0 00
0 00
100.65
37.44
0.03
21.18
0 00
135
31 99
8.21
0.00
1.17
0 05
0 04
101 46
36.74
0.00
21.49
0 07
170
33.68
4.61
0.05
1.60
0 02
0 03
100.05
23 47
0.08
23.11
0 00
10.16
31.32
0 50
0 00
0.00
0.00
0.00
88.64
23.14
0 09
23 99
0.00
10 75
29.84
0.19
0.00
0.00
0.00
0.27
88 21
24 14
0.03
23 58
0.00
13.23
27.56
031
0 00
0 00
n 00
0.00
88.85
0.55
53.16
0 26
0.00
0.00
43 43
1.83
0.00
0.04
0.00
0.13
100.00
45 64
0.14
37.37
0.00
0.29
1.16
0.02
0 00
0.00
195
8.48
95.05
46.97
o 04
37.1 I
0.00
0.32
0.88
0 01
0.00
0.00
I .40
9.20
95.93
7 77
0 06
1876
0 03
0 55
3 25
0.08
0 28
0 00
0.00
0 00
8.07
0 06
18.62
0 01
0 36
3 22
0 08
0.16
0.00
0 00
0 00
2.83
0.06
2.02
0.00
0.70
1.21
001
0 00
0.01
0.01
0.52
2.00
9.36
0.64
2.79
0.04
2 18
0.00
0.58
131
001
0 00
0.00
0.00
0.33
2 00
9 25
0.69
101
0.00
1.99
0.00
0.11
0.86
0.02
0.00
000
0.00
0.00
2.00
5 99
0.88
1.00
0.00
1 97
0.00
015
0.87
003
0.00
000
0.00
000
2.00
6.02
085
3.01
0 00
2.01
0.00
0 I6
2.15
0.56
0.00
0 10
0 01
0 00
2 98
0.00
2.05
0 00
021
2.29
0.32
0.00
0.14
0.00
0 00
8.00
0.93
8.00
0.93
7.99
0 92
2.53
001
2 94
0 00
1 64
2 83
0.05
0.00
0.00
0.00
0.00
8 00
17.99
0.63
2.49
001
3.04
0.00
1.72
2.68
0.02
0.00
0.00
0 00
0.04
8.00
18.00
0.61
2.54
0.00
2 92
0.00
2.08
2.43
0 03
0 00
0.00
0.00
0.00
8.00
I 8.00
0.54
001
I01
0 01
0.00
0.00
091
0 04
0.00
0.00
0.00
0.00
30.50
0 90
2.71
0 06
1.88
0.00
0.90
I31
0.01
0.00
0.00
0.05
0.79
2 00
9 71
0.59
3 01
0.00
1.99
0.00
0.16
2.01
0.72
0.00
0.12
0.00
0.00
30.68
0.86
2.71
0.05
1 88
0.00
0.71
0.57
001
0 00
0.00
0.03
0.71
2.00
9.67
0 69
3 02
0.01
2 91
0.00
0.03
0.06
0 00
0 00
0.00
0.25
0.72
2.00
9 00
0 69
3.07
0.00
2 86
0.00
0 03
0 05
0 00
0.00
0.00
0.18
0.77
2.00
8.97
061
3
3
3
3
I
3
3
3
4
2
4
Bt 248 G Bt 24811 Bt 248 P Cld H82 Cld 248 Grt 248c Grt 2481. Grt H2r. Chl H42 Chi H2
2
Chl,,,
3
Ilm 248
198
52
P . P I T R A t; M. C U I K A U D
ANTICLOCKWISE P-T
product of the destabilization of biotite (Fig. 2c).
However, although rarely, it can also be observed in
equilibrium with biotite near the boundary with the
tonalite (Fig. 2d). Textural equilibrium exists between
chloritoid, staurolite and chlorite. Locally, chloritoid
was retrogressed into chlorite (Fig. 2d).
It remains unclear whether small elongated plates of
chloritoid parallel to S1 represent a first generation,
or result from a preferential crystallization of chloritoid
conformable with the early planar structure. No
chemical difference is observed between the two
possible generations of chloritoid. Chloritoid contains
MnO (0.5-1.15 wt%), but no recalculated Fe,O, (on
the basis of stoichiometry). The X,,(Cld) (0.85-0.89)
is systematically smaller than that of coexisting
staurolite.
Gurnet. Small dioblastic, equidimensional Garnet is
common. It is an Mn-rich almandine (Alm73-63,
Prp7-5, Sps29-19, Grs4-1). The rimward decrease in
spessartine (from 0.29 to 0.19) is compensated by the
correlative increase in almandine and pyrope, with the
X,, remaining constant at about 0.92. There is no
significant difference in the range of garnet composition
whatever its structural position. Garnet is in textural
equilibrium with andalusite, staurolite, biotite and
chlorite.
Chlorite. Chlorite occurs mainly in the quartz-rich
layers. Tiny crystals are exceptional within S 1, larger
ones occur mainly in crystallization tails of andalusite,
staurolite and garnet. Chlorite is also present in the
oval-shaped inclusions within andalusite. Isolated
grains form at the expense of biotite and/or chloritoid.
Chlorite is ferrous with an X,., (0.60 up to 0.68)
systematically slightly higher than that of biotite in
the same thin section. However, chlorite occurring
within andalusite inclusions has an X,, (0.53-0.59)
lower than biotite and matrix chlorites.
Oval-shaped inclusions within andalusite are composed
of chlorite, muscovite, scarce biotite and quartz, and a
very fine-grained mineral mass similar to pinite
(Fig. 2a). This chlorite is more Mg-rich than chlorites
in the matrix, supporting growth by destabilization of
cordierite.
Crystallization/deformation relationships
We have seen that S1 is formed of muscovite, quartz,
ilmenite, chlorite(?) and chloritoid(?). Cordieritec?),
andalusite, biotite and garnet contain straight or only
very slightly curved S1 inclusion trails and are thus
post-S1. Staurolite contains inclusions of S1 crenulated
by S2 and is therefore syn- to post-S2. As andalusite.
biotite and garnet are in equilibrium with staurolite,
and andalusite contains some staurolite as inclusions
in the peripheral parts, we interpret these minerals as
having grown as soon as the early stages of S2
development. They are also in textural equilibrium
with S1 muscovite, ilmenite and quartz. Cordierite
(contained only as inclusions in andalusite) is the
oldest S2 mineral, followed by the crystallization of
andalusite, biotite and garnet. Staurolite containing
curved inclusion trails has grown longer than andalusite in which the S1 trails are straight. Accordingly, we
assume a sequential, yet continuous, metamorphic
crystallization (Fig. 3).
Chloritoid and chlorite grow at the expense of
biotite but are also contained within S2 shear-bands.
Staurolite, chloritoid and chlorite have locally overgrown S2, but pre-date D3 structures. Therefore we
postulate syn- to late-S2 peak metamorphism.
Svratka
Fe-rich
Al-rich
0
.
0
--
0
=-
111.
111.
53
White micu. White mica is fine-grained muscoviterich phengite (Si=3.02-3.08 for 11 oxygens) with an
Na/(Na+K) ranging from 0.17 to 0.31. It defines S1
and is crenulated by S2. Muscovite is in equilibrium
with all other mineral species.
Hlinsko
Grt
Crd
And
St
Bt
Cld
Chl
PATH, BOHEMIAN MASSIF
0
0
Grt
Crd
And
St
Bt
Cld
Chl
-El- I- -I --11.
Grt
KY
St
Rt
Bt
Sil
Chl
+ Ms, Qtz, Ilm, Cr, fluid
Fig. 3. Summary of the crystallization/deformation relationships within the hangingwall Hlinsko pelites and in the footwall Svratka
mica schists.
54
1'. P I T R A & M . C U I R A U D
MnKFMASHTC
Paragenetic analysis
A
In a first approximation mineral parageneses can be
described using the classical pelitic system K,O-FeOMg0-A1,0,-Si0,-H2O-TiO,
(KFMASHT). Yet, Mn
is abundant in garnet and in smaller but still significant
amounts in staurolite, chloritoid and to a lesser extent
also in other minerals and is therefore an independent
component which must be taken into account in
describing the phase relationships. Moreover, graphite
is present as inclusions and in graphitic shales
intercalated in the Silurian series. The interaction of
graphite with water introduces C-bearing fluids into
the hydrous fluid generated during metamorphism (e.g.
Ohmoto & Kerrick, 1977). Hence CO, (kCH,) is an
independent component, and the most appropriate
system to describe the observed parageneses is the
system MnKFMASHTC. In the eastern limb of the
Hlinsko syncline, garnet occurs in all the parageneses
and is considered to be in excess, along with quartz,
muscovite, ilmenite, graphite and an H,O-CO, fluid.
From the textural evidence, we deduce that the peak
paragenesis is andalusite-cordierite-biotite near the
centre of the syncline (followed by andalusite-staurolite-biotite) and staurolite-biotite at the base of the
sedimentary sequence. Although staurolite began to
grow in equilibrium with biotite, the latter is progressively chloritized, with the local appearance of chloritoid, and staurolite-chlorite is the typical association
of this stage. Chloritoid is abundant in the lower part
of the sequence and chloritoid-biotite-chlorite equilibrium may be exceptionally observed. This evolution is
documented in the sequence of compatibility diagrams
on Fig. 4.
The peak parageneses are consistent with one
another within the same compatibility diagram (Fig. 4)
and therefore may reflect the same P- T conditions.
Andalusite-cordierite-bearing assemblages occur in the
most aluminous lithologies, in the centre of the
syncline, whereas they are absent in the more ferriferous
and less aluminous lithologies that predominate at the
base: the observed difference in mineralogy would then
reflect the difference in whole rock compositions.
Petrogenetic grids and f - T conditions
Standard geothermometry was used to put rough
constraints on the temperature stability field of the
observed parageneses. The garnet-biotite thermometer
of Williams & Grambling (1990) was used because it
allows the high Mn content in garnet to be taken into
account. It yields reasonable temperatures ranging
from 510 to 560 ^C. However, the poor analyses of
biotite cast doubts on the reliability of these results.
Garnet-chlorite thermometry (Dickenson & Hewitt,
1986; Ghent et al., 1987; Grambling, 1990) yields
coherent temperatures between 490 and 545 "C. The
geothermometer of Pownceby et al. (1987) based on
the Fe-Mn exchange between garnet and ilmenite
= 0 95 A1203 + 0 05 MnO
= 0 95 FeO + 0 05 MnO
' = 0 95 MgO + 0 05 MnO
+~rt
+Ms
a)
+Qtz
+Ilm
+fluid
"L
A/F'+M'
I
Oxycilorite
And
c
1-
F'
F/F'+M'
Bt
Fig. 4. Compatibility diagrams representing two stages of the
paragenetic evolution of the Hlinsko pelites. Projections have
been calculated using the rim composition of garnet present in
the rock. The composition of cordierite is that predicted by the
therrnocalc program. The biotite bar depicts the theoretical
biotite composition, empty points stand for biotite analyses in
the Hlinsko schists. The shaded oval represents the cluster of
'biotite' (oxychlorite) compositions from one thin section.
ChlCrdrepresents the composition of chlorite analysed within
the andalusite inclusions. (a) Peak-S2; (b) late-S2.
provides temperatures around 550-570 "C. Thus, the
part of the P-T evolution including garnet can be
bracketed between 500 and 600 "C.
In order better to constrain the P-Tevolution of the
Hlinsko pelites the petrogenetic grid approach using
the thermocalc computer program (Powell &
Holland, 1988; version 2.2b2, dataset April 1992) was
chosen. This approach allows us (1) to confine the
P-T conditions and to quantify the P-T evolution
using textural criteria of equilibrium, the chemical
equilibria being possibly broken during the P-T
evolution, (2) to consider a system containing Mn in
metamorphic phases and (3) to calculate the phase
diagrams at aHZOimposed by the presence of graphite
in the schists. In fact, the phase diagram for the
metamorphic facies of interest strongly varies with
respect to aHZOand ignoring this parameter could
hamper the chosen approach.
ANTICLOCKWISE P-T
As shown by Connolly & Cesare (1993), the
composition of the graphite-saturated C-0-H fluid
produced by dehydration reactions is buffered by the
condition of a constant H/O ratio, and is also uniquely
determined at isothermal-isobaric conditions. Using
the diagram of Ohmoto & Kerrick (1977), the X,,,
ranges from 0.85 to 0.90 for the P-T area of interest
(0.3-0.4 GPa, 500-600 C). Connolly & Cesare (1993)
show that considering nonideality of mixing, these
values should be higher. They find, however, their own
values (0.90-0.95 for the same P-T field) are slightly
overestimated. Thus, we consider that uH2, = 0.9 is a
good approximation of fluid conditions reigning in the
Hlinsko Silurian sequence.
The petrogenetic grid for the system MnKFMASH
has been calculated for the P-T-uH,, area of interest
(Fig. 5 ) . Ideal mixing between mineral end-members
was assumed, except for staurolite, where a mixing
model based on Darken's Quadratic Formalism was
used (Vance & Holland, 1993). Contents of Zn and Ti
in staurolite are significant and should vary with the
P-T conditions of equilibrium. In the absence of the
Ti and Zn end-members it is not possible to calculate
the effect of these components. Nevertheless, we use a
constant value of Ti + Zn=0.2 (=50/0) for 4 atoms in
PATH, BOHEMIAN MASSIF
the M2 site and an activity corrected expression for
staurolite a(fst)= 0.954 x4, where x = [Fe/( Fe + Mg)],,.
Interpreting the observed mineral parageneses with
the calculated P-T grid (Fig. 5) leads to constraints on
the peak P-T conditions of 0.3-0.4GPa and
550-590 "C, based on the following reactions:
1 MnKFMASH reaction (Cld,Chl), since staurolitecordierite is not stable;
2 the lowermost stability of the biotite-andalusite is
restricted to more than 0.3 GPa by the KFMASH
reaction (Cld,Chl,St);
3 towards high pressures and temperatures, the limit
is formed by And=Sil and the MnKFMASH reaction (Cld,St).
In general, the differences in mineral parageneses
depend as much on the bulk rock composition as on
the P-T conditions of metamorphism. In order to
explain the observed differences and constrain the P-T
evolution of the Hlinsko pelites, P-T pseudosections
were drawn for the two dominating bulk rock
compositions: (a) more aluminous and less ferriferous
lithologies from the centre of the syncline and (b) a
more ferriferous and less aluminous lithology from the
SE limb, near the tonalite (Fig. 6).
The syntectonic crystallization of andalusite-stauro-
0.45
0.4
0.35
0.3
0.25
520
55
540
560
Fig. 5. Part of the P Tpetrogenetic grid for the KFMASH and MnKFMASH systems, calculated for a,,,=0.9.
56
P . P I T R A & M. C U I R A U D
P (GPa)
MnKFMASH pseudosection
+ Grt, Ms, Qtz, fluid ( a ~ 2 0= 0.9)
Ik rock composition:
I
I
I
I
560
5s0
I
520
540
P (GPd)
MnKFMASH pseudosection
+ Grt, Ms, Qtz, fluid (aH20 = 0 9)
0 sol
R
T(T)
b)
R
0
tst
Cld Chl
0 15
~~~
~
~~~
T y
_-
520
~1~
_.,
l---- ' ~~
540
!
560
-
580
T(T)
Fig. 6. P-Tpseudoscctions calculated for the two main
lithologies in the Hlinsko pelites yield complementary
information about the P~-Tevolution.(a) The Al-rich rocks
havc preserved the initial stages of the P-Tevoluiion, involving
a slight increase in pressure; ( b ) the Fe-rich lithology has well
recorded thc late stages. dominated by cooling, whereas the
incrcase in pressure is inferred from (a).
lite-biotite (preceded by andalusite-cordierite-biotite)
and staurolite-biotite are contemporaneous in rocks
with different compositions. As deduced from the
compatibility diagrams, these parageneses are consistent with each other under the same P-T conditions.
Comparing the two pseudosections, the peak P-Tmay
further be limited to the high-pressure part of the
andalusite field (say higher than 0.35 GPa) as cordierite
does not occur in the Fe-rich Al-poor rocks.
Moreover, a P-T evolution can be observed within
the narrow pressure interval. The sequence of mineral
assemblages observed in the aluminous rocks in the
centre of the syncline is And-Crd-Bt > And-~
St-Bt > And-St-Chl. The crystallization of andalusite-staurolite-chlorite from andalusite-staurolitebiotite implies a path involving cooling from about
570 C to 550-530 C (stability of andalusiteestaurolite-chlorite). The observed crystallization of andalusite-staurolite-biotite replacing andalusite-cordieritebiotite precludes isobaric cooling only and requires a
slight initial increase of pressure from about 0.36 GPa
to 0.4 G P a according to the pseudosection on Fig. 6.
The paragenetic sequence in the less aluminous and
more ferriferous rocks, St-Bt > St-Chl(-Cld) > CldChl, records cooling at pressures higher than 0.35 GPa,
as no trace of cordierite has been found in this rock
type. The cooling is recorded to temperatures under
530 'C, in the chloritoid-chlorite assemblage stability
field.
The two different bulk rock compositions yield
complementary information about the P-T evolution.
The Al-rich rocks have better preserved the initial
stages involving the slight increase in pressure. The
late stages comprise cooling to low temperatures, and
are better recorded in the more ferriferous rocks,
abundant near the rim of the syncline. This may be
related to the more intense fluid circulation near the
major shear zone allowing diffusion processes to take
place at lower temperatures.
The X(Sps) values analysed in the garnets fit well
with the P-T stability fields of our parageneses.
However, calculated evolution of X(Sps) does not
match the observed one. X ( Sps) should increase along
the P-Tpath shown in Fig. 6, whereas it decreases from
the garnet core to the rims. On the other hand, the
small garnets do not display any synkinematic features,
which suggests rather fast nucleation and growth.
Consequently, the Mn zoning in garnet may not reflect
a change in P-T conditions but rather diffusion processes in a reservoir with limited Mn content. This is
consistent with the overall same composition range of
garnet whatever its structural position.
Our calculations confirm the observation of Pattison
(1989, 1991) that the presence of graphite is often
needed for rocks to display andalusite-bearing assemblages. In fact, lowering the uH20 shifts the reactions
involving aluminosilicate to lower temperatures and
into the andalusite stability field.
The topology of the calculated MnKFMASH
pseudosections is in excellent agreement with that of
Hudson (1980) drawn for the KFMASH system. It
shows that the major effect of adding Mn to the
chemical system is only the stabilization of a spessartine-rich garnet, the original KFMASH topology being
conserved.
DISCUSSION
An increase in pressure in the hangingwall Hlinsko
schists during the main metamorphic event is the most
interesting geological result. This increase is very slight
and probably lies within the uncertainties of the
calculated reactions. Moreover, the limits of stability
are also sensitive to the bulk rock composition.
Nevertheless, whatever the precise location of the
equilibria in the grid, the relative position of the
stability fields of the different parageneses remains
reliable. The cordierite-out reaction has a very flat
slope and therefore the disappearance of cordierite
ANTICLOCKWISE
corresponds to an increase in pressure within the
andalusite field. It would be theoretically possible for
cordierite to have crystallized within the sillimanite
field and to have reacted out during isobaric cooling.
However, no sillimanite is found in the rocks. Given
the abundance of aluminosilicate in the Al-rich rocks,
an early sillimanite would have been preserved.
Therefore, we consider that cordierite grew only within
the andalusite field.
Although no cordierite could be analysed with the
microprobe, there is strong evidence to indicate that
the chlorite-muscovite fbiotite inclusions within andalusite represent former cordierite:
1 inclusions have a characteristic oval shape and
optical attributes typical for pseudomorphs after
cordierite;
2 chlorite is more Mg-rich in the inclusions than in
the matrix (see Chl,,, in Table 1) and the calculated
equilibrium compositions of the phases involved in the
reaction Crd + Bt + And + H,O = Grt + Chl + Ms + Qtz
at T= 560 ‘ C and P=O.39 GPa correspond to the
compositions we measured in the inclusions.
Therefore, it is assumed that the increase in pressure
is realistic.
In order to explain this P-T evolution, we have to
consider the Hlinsko schists within their geodynamical
context. The Hlinsko syncline lies between the underlying, NW-dipping high-grade rocks of the Svratka
Crystalline Unit on the south-east and the granodiorite
of the Nasavrky complex that forms its western,
vertical to reverse limb (Fig. 1). Its uppermost part is
formed by a nearly unmetamorphosed flysch-type
(Rychmburk) sequence.
Svratka Crystalline Unit
In the Svratka Crystalline Unit, mica schists are
intercalated within migmatites and migmatitic paragneisses that form nearly 80% of the unit. Previous
studies have suggested that the Svratka metamorphics
exhibit early planar and linear fabrics associated with
high- to medium-pressure metamorphic conditions
(Nemec, 1968; Pertoldova, 1986). They are reworked,
showing a muscovite-sillimanite-biotite-bearing foliation associated with NW-dipping shear bands.
Semibrittle NW-dipping shear zones post-date both
structures and indicate the continuation of the deformation at low temperatures.
To the East, the SCU is directly thrust over the
Moravian nappe complex, where Devonian rocks are
affected by Variscan thrusting and metamorphism. The
Barrovian metamorphic zonation in both the Moravian
nappes and the Svratka Crystalline Unit show apparent
continuity of the thermal structure, i.e. identical dP/dT
profile and synmetamorphic prograde mineral growth
and kinematics (Schulmann et al., 1991). Thus, the
early metamorphism of the overlying SCU is directly
linked with the evolution in its footwall and should be
Variscan in age.
P-r P A T H , B O H E M I A NM A S S I F 57
Nemec (1968) deduced approximate peak metamorphic conditions of about 450-590 “C for around 1 GPa
from mica schists of the central part. Pertoldova (1986)
has studied the petrology of a skarn body in the SE
part of the unit and has calculated peak P-Tconditions
of 600 “C for 0.6-0.9 GPa. We have investigated several
mica schist samples from the NW part of the SCU in
order to verify these results.
Mica schists range in composition from rare light
quartz- and muscovite-rich mica schists to more
common darker biotite- and sillimanite-rich ones. In
the ‘dark’ mica schists the main foliation comprises an
intercalation of layers rich in biotite (XFe = 0.57-0.7)
intergrown with sillimanite and muscovite, and layers
rich in quartz and plagioclase (XAn 10-13). This
foliation wraps around scarce crystals of garnet
(Alm83-78, Prp8-12, Sps3-10, Grs3-5), tourmaline or
large plagioclase. Garnet is small (1-2 mm) and often
forms atoll-like porphyroblasts that include quartz,
plagioclase, muscovite and biotite crystals.
In the light mica schists, mica-rich and quartz-rich
layers define the main foliation. Mica-rich layers are
dominated by muscovite intergrown with biotite and
sillimanite and contain minor quartz, staurolite, kyanite, chlorite and ilmenite. Porphyroblasts of staurolite
and kyanite are generally parallel to, but also deformed
by, the foliation that wraps around large poikiloblastic
garnets. Rutile and ilmenite are common inclusions
but only ilmenite is observed in the matrix. Garnet
forms large subeuhedral porphyroblasts (up to 3 mm)
and contains inclusions of quartz, rutile (up to 0.1 mm),
less abundant ilmenite, small muscovite and aluminosilicate. Optical zonation, characterized by a core rich in
inclusions of large quartz grains, suggests two stages
of garnet growth. Elongate sigmoidal trails of quartz
inclusions within garnet cores demonstrate its synkinematic crystallization. The same structural position
of staurolite (up to 3 mm, Fe/( Fe + Mg) = 0.85-0.90),
kyanite and garnet (Alm80-87, Prp8-14, Sps4-1,
Grs8-1) with respect to the main foliation suggest
their contemporaneous growth during a first metamorphic event. The main foliation (muscovite-biotitesillimanite) wraps around large garnets, meaning it
developed later; yet, biotite (XFe= 0.57-0.64) is in
textural equilibrium with the minerals of the first
paragenesis. The last, retrogressive stage is documented
by the alteration of garnet, staurolite. kyanite and
biotite to chlorite (XFe= 0.60-0.64) and muscovite.
We deduce from the textural evidence that the first
recorded paragenesis is garnet-kyanite-staurolite-biotite, followed by the crystallization of garnet-sillimanite-biotite (Fig. 3). The presence of different Al,SiO,
polymorphs in the two successive parageneses shows
that they have equilibrated at significantly different
P-T conditions. Applying standard geothermometers
yields unreasonably scattered data, resulting probably
from insufficient equilibration of phases coexisting in
textural equilibrium. However, garnet-sillimanitebiotite is stable within the domain limited by the
58
P. P l T R A & M. C U I R A U D
KFMASH reactions Sil+ Grt = Bt + St and Sil+ Bt =
Crd+Grt (+Ms, Qtz. H,O). Taking into account the
widespread occurrence of migmatites in the region,
and the fact that muscovite is stable in the mica schists,
we can confine the temperature at around 600-650 T.
According to the KFMASH petrogenetic grid of Xu
et al. (1994), calculated using the same version of
thermocalc,we can thereafter bracket the pressure
between 0.4 and 0.6 GPa.
Garnet-kyanite-staurolite-biotite is a univariant
paragenesis in the KFMASH system. However, significant amounts of Ca in garnet suggest that garnet could
be stabilized by this additional component. The
GRAIL barometer (Bohlen et ul., 1983) was used
because garnet contains rutile, ilmenite and aluminosilicate a s inclusions and because it is largely independent
of temperature estimates. It yields pressures between
0.8 and 0.9 GPa. which implies temperatures from 610
to 660 C (KFMASH petrogenetic grid of Xu et al.,
1994), consistent with the lack of evidence of melting
at this stage.
The two metamorphic events recorded within the
SCU occurred at nearly the same temperature but at
clearly different pressures. The late crystallization of
chlorite suggests temperatures lower than 570 -C at
the end of metamorphism. Thus, the rocks of the SCU
record a decompression of at least 0.2 G P a at a
relatively stable peak temperature, followed by cooling.
Such an evolution is known in the footwalls of
extensional terranes (e.g. Chauvet et al., 1992; Rey
et ul., 1992). It is generally explained as the consequence
of the rapid exhumation of rocks underlying major
extensional faults, and thus the minor increase in
pressure seen in the Hlinsko schists is not related to
the history of thc SCU.
Extensional tectonics within the Hlinsko-Svratka
region took placc during the first period of
late Variscan extension (Pitra et ul., 1994) that began
in the inner part of the belt whilst convergence was
still active (Burg et ul., 1994a). Therefore, we may
expect that there was no major time gap between the
first, crustal-thickening-related metamorphic event and
the extension-related decompression of the Svratka
Crystalline Unit. Numerical modelling shows that only
fast exhumation rates ( > 2 km Myr-') provide an
explanation for a stage of isothermal decompression
(e.g. Rey et d..1992). Similar exhumation rates,
reaching 3-6 km Myr-', were measured in recent
extending orogens (Dallmeyer et ul., 1986; Davis, 1988;
Hacker el ul., 1990; Lonergan & Mange-Rajetzky,
1994). Thus, a decompression of 0.2-0.4 GPa. corresponding to an uplift of 6-12 km, may be accomplished
within 1L4 Myr.
Nasavrky granodiorite
The Nasavrky granodiorite intrusion produced a
limited contact metamorphism of all series of the
Hlinsko syncline (including the otherwise unmetamor-
phosed Rychmburk sequence) and a deformation that
clearly post-dates structures related to the peak
metamorphism. Hence, its intrusion cannot explain the
increase in pressure recorded in the Hlinsko schists.
Rychmburk flysch basin
The geological meaning of this pressure increase has
therefore to be sought in the syntectonic sedimentation
of the over 2-km-thick (Vachtl, 1962) flysch-type
Rychmburk sequence. Sedimentary filling of flysch
character with conglomerate beds that concentrate
near the boundary fault is common in continental halfgrabens (e.g. Allen & Allen, 1990). It is recognized in
the Rychmburk sequence (Vachtl, 1950), which also
appears to have been deposited in a half-graben during
normal faulting. High subsidence rates are commonly
observed in half-grabens (e.g. Allen & Allen, 1990;
Gordon & Heller, 1993), as well as related rapid
unroofing rates of their footwalls. We have to consider
a fast sedimentation associated with a decreasing heat
supply from the cooling tonalite and Svratka migmatites in order to account for the increase in pressure at
constant or even slightly decreasing temperature.
Schlische & Olsen (1990) argue, on the basis of a onedimensional numerical model, that a half-graben may
P
l.o[
(GPa)
Svratka
(Mold an u bian)
I
I
I
500
I
I
I
I
I
600
I
I
I
I
I
700
Fig. 7. Schematic P Tpaths for the Hlinsko pelites and the
mica schists of the Svratka Crystalline Unit accompanied by a
cartoon representing the tectonic evolution of both units. A
crustal-scale normal ductile shear zone enhanced the
emplacement of the syntectonic pluton that heated the
overlying schists. The footwall Svratka rocks were exhumed,
whereas the Hlinsko schists in the hangingwall were buried
under a thick syntectonic basin, which would explain the
recorded increase in pressure during the peak metamorphism.
A N T I C L O C K W I S E P-T P A T H , BOHEMIAN MASSIF
reach a cumulative thickness of more than 2 km in its
deepest part within 3 Myr. Their calculations are
consistent with field observations in the Mesozoic rift
basins of eastern North America (Schlische & Olsen,
1990) and the late Variscan Saar-Nahe basin (Henk,
1993). These values are compatible with the exhumation rates recorded in extensional footwalls. This
provides a plausible link between the metamorphic
and sedimentary histories of the Hlinsko region and a
constraint on the estimation of the metamorphism
duration.
CONCLUSIONS
The Hlinsko area is characterized by a ductile normal
fault that separates the footwall Svratka gneisses from
the pelites of the Hlinsko syncline in the hangingwall.
The main metamorphic crystallization in both units
(staurolite-biotite i andalusite in the Hlinsko schists,
garnet-sillimanite-biotite in the SCU) is related to
concordant extensional movements and is consequently
simultaneous. However, the footwall a n d the hangingwall units display opposite, but convergent, P-T
histories (Fig. 7 ) . Decompression in the footwall rocks
is related to rapid exhumation. We propose that the
inverse, anticlockwise, P-T path recorded in the
hangingwall pelites is related to the rapid, extensioncontrolled sedimentation of the overlying (Rychmburk)
flysch series. Although, to our knowledge, such a
P-T-t path has not yet been demonstrated in similar
extensional terranes, it may be common, as syntectonic
basins and fast subsidence are usually observed in the
hangingwall of large extensional faults.
ACKNOWLEDGEMENTS
L. Latouche and R. Black helped to improve the
manuscript. We are especially grateful to J.-P. Burg
for a complete critical review. K. Schulmann and B.
Hensen are sincerely thanked for useful suggestions.
We also thank P. O'Brien, J. Selverstone and a n
anonymous reviewer for their constructive comments.
P.P. benefited from a scholarship of the French
Ministry for Foreign Affairs.
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