metamorphic evolution of the pre-hercynian basement of the schwarzwald (federal republic of germany)
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Teczonqhysics, 157 (1989) 117-121
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
117
Metamorphic evolution of the pre-Hercynian basement of the Schwarzwald (Federal Republic of Germany)
R. STENGER, K. BAATZ, H. KLEIN and W. WIMMENAUER
Mineralogisch-Petrographisches Institut der Universitiit, Albertstrasse 23b, D-7800 Freiburg im Breisgau (F. R. G.)
(Received September 14,1987; revised version accepted February 1,198s)
Abstract
Stenger, R., Baa@ K., Klein, H. and Wimmenauer, W., 1989. Metamorphic evolution of the pre-Hercynian basement
of the Schwarzwald (Federal Republic of Germany). In: R. Meissner and D. Gebauer (Editors), The Evolution of
the European Continental Crust: Deep Drilling, Geophysics, Geology and Geochemistry. Tectonophysics, 157:
117-121.
The high-grade gneisses and migmatites of the Central Schwarzwald display a polyphase metamorphic history with
a dominating LP-HT metamorphism. Numerous small intercalations of eclogites (or eclogitic amphibolites), ultra-
mafics (both garnet- and spinel-bearing peridotites and pyroxenites), and granulitic gneisses together with miner4 relics
in the high-grade gneisses are indicative of HP and MP metamorphic events predating the LP-HT metamorphism.
The widespread occurrences of these rocks, their petrological evolution and their relationship to their gneissic ‘country
rocks strongly indicate that the metamorphic history of at least some of the gneiss complex is identical to that of the
HP and M P intercalations. Late Hercynian low-grade metamorphism is related to retrograde processes associated with
the uplift history and with brittle deformation of the basement.
Introduction
The Schwarzwald is part of the internal Moldanubian zone of the Hercynian fold belt in Europe. Its crystalline basement consists of high- grade pre-Hercynian gneisses and migmatites in- truded by various Hercynian granitoids, mostly of the S-type. Major thrust zones with deformed Palaeozoic sediments which are related to Hercynian collision and accretion tectonics Eisbacher et al., 1987) separate three crustal blocks with clear differences both in petrological and geophysical features (Fig. 1 and Table 1): the Saxothuringian Block in the north, and the Central Gneiss Complex (CGC) and the Southern Gneiss and Granite Complex (SGGC) in the south.
The metamorphic basement of the CGC com- prises high-grade metapsammitic and metapelitic gneisses, the original sediments of which were
0040-1951/89/$03.50 0 1989 Elsevier Science Publishers B.V.
Precambrian in age. Intercalations of acid and basic metavolcanics, metaquartzites, lime silicate rocks and rare marbles have been used to define distinct lithological units (Wimmenauer, 1980, 1984). Orthogneisses of granodioritic and tona- litic-trondlrjemitic composition are arranged con- formably with the large-scale tectonic style of the metasedimentary rocks. U-Pb data on zircons indicate the age of intrusion of the magmatic protoliths (520 f 15 Ma) (Todt and Busch, 1981).
Metamorphic conditions
The present facies of the gneisses is predomi- nantly that of a LP-HT metamorphism with widespread indications of partial anatexis. How- ever, numerous small bodies of eclogites (or eclo- gitic amphibolites), ultramafics (peridotites and pyroxenites) and mylonitic acid granulites indicate
118
Fig. 1. Simplified geological map of the Schwarzwald, with
regional distribution of HP and MP rocks. SAX-Saxo-
thuringian zone; CGC -Central Schwarzwald Gneiss Com-
plex; SGGC-gneiss and granite complex of the Southern
Schwarzwald.
a far more complex metamorphic history with HP and MP events.
Eclogites
The geological and petrological relationships of the eclogites and eclogitic amphibolites were in- vestigated by Klein and Wimmenauer (1984). Well over 100 small occurrences are widely dispersed in the gneisses and migmatites, most prominently at
the southeastern margin of the CGC (Fig. 1). Note that the eclogites are absent in the Southern Schwarzwald.
The primary mineral assemblage of the eclo- gites is omphacitic c~~nopyroxene + garnet f kyanite + quartz + rutile. Later transformation processes resulted in symplectite, corona, and kelyphite reaction structures:
Omphacite -+ diopside + plagioclase (symplectite) Garnet + quartz -+ orthopyroxene + plagioclase
(corona) Garnet -+ plagioclase + amp~bole (kel~~te) Kyanite + corundum + spine1 + plagioclase or
quartz
Most of these processes are not isochemical. They are interpreted as retrograde with respect to pres- sure, but probably prograde with respect to tem- perature.
According to Klein and Wimmenauer (1984) and additional new data, the eclogitic stage is characterized by temperatures between 600” and
TABLE I
Geological, petrological and geophysical differences between
the Central and the Southern Schwarzwald. Geophysical data
after Liischen et al. (1987)
Metapsammitic and metapelitic gneisses and migmatites
Central Schwarzwald Southern Schwarzwald
Tonahtic-trondhjemitic
orthogneisses prominent
Na-rich leptynites along
the southern margin
Eclogites and eclogitic
amphibolites
Garnet-spine1 peridotites
Metagabbros very rare
Pronounced low-velocity zone
Pronounced lamination of the
lower crust
Low-velocity zone fading out
Weak lamination of the
lower crust
Higher values of Bouguer Lower values of Bouguer
gravity gravity
Absent
K-rich leptynites (meta-
volcanics)
No eclogites
Spine1 peridotites only
Metagabbros and meta-
anorthosites (layered
intrusion)
119
700” C and by pressures of more than 1.2 GPa.
For the kyanite-bearing types, pressures of more
than 1.5 GPa can be estimated (Jd,, in ompha-
cite; Holland, 1979) provided that water is the
fluid phase acting during eclogitization. This is
indicated by the presence of quartz veins with
omphacite, kyanite, rutile and HP mica pseudo-
morphs within some eclogite bodies (Wimmenauer
and Stenger, 1987, this issue). A “granulitic” stage
around 1.0 GPa is calculated for the reaction of
garnet and quartz forming coronas of orthopyrox-
ene and plagioclase. The further retrograde evolu-
tion of the eclogites is equivalent to that of the
surrounding gneisses and migmatites leading to
amphibolite and greenschist facies mineral para-
geneses in the final stage.
Some exposures show alternating layers of
eclogitic metabasites and acid rocks, the latter
containing the same mineral relics or reaction
structures as the eclogites. Symplectite reaction
structures in rocks with acid or intermediate com-
position and even in the otherwise “normal” LP
paragneisses indicate the breakdown of a pyrox-
ene phase rich in the jadeite component, and of
phengitic or paragonitic HP micas (Wimmenauer
and Stenger, this issue). The reaction jadeite +
quartz = albite has been calibrated by Holland
(1980). Assuming Jd,, in clinopyroxene instead of
pure jadeite would reduce the pressure by about
0.15 GPa (at T = 680 o C) resulting in a calculated
minimum pressure of about 1.6 GPa for the acid
eclogite facies rocks.
Ultramafics
The distribution of the ultramafic rocks in the
Schwarzwald is shown in Fig. 1. They occur in the
gneiss and migmatite areas as loose blocks or in
very small outcrops without any clear contacts
with their country rocks. Ultramafic rocks are
often associated with acid and metasedimentary
gneisses of granulitic appearance, and in some
cases also with metabasitic rocks. In cores of the
shallow Kunklerwald drillhole, a completely
carbonatized ultrabasite (with picotite) shows
blackwall alteration at the contact with grant&tic
gneisses. In these cores the typical association
together with eclogites and granulites is observed.
The ultramafic rocks can be divided into
strongly serpentinized mantle peridotites both
garnet- and spinel-bearing, and ultramafic cumu-
late rocks (pyroxenites, websterites and wehrlites).
The primary mineral assemblage of the mantle
peridotites with olivine + clinopyroxene + ortho-
pyroxene * garnet f spine1 f amphibole is pre-
served only in relics. Retrograde mineral reactions
result in the formation of the following para-
geneses:
Olivine + serpentine, magnetite
Clinopyroxene, orthopyroxene + talc, amphibole,
serpentine; saponite
Garnet + spine1 + amphibole + plagioclase
Garnet + orthopyroxene + spine1 + amphibole
In a clinopyroxenite from Todtmoos (Southern
Schwarzwald), reaction rims of garnet between
plagioclase and clinopyroxene and between spine1
and clinopyroxene are observed. It is presumed
that these reactions indicate a prograde develop-
ment with respect to pressure.
Chemical analyses (including REE patterns) of
the mantle peridotites point to alpinotype lherzo-
lites of slightly depleted character, but preclude an
ophiolitic origin (Burgath et al., 1987). Micro-
probe analyses on relict mineral phases and ther-
mobarometric evaluation indicate pressures of
2.0-2.5 GPa and temperatures of approximately
900° C for the metamorphic equilibration of the
garnet-spine1 peridotites.
Granulites
A MP metamorphic stage is further evident in
supracrustal metasedimentary and acid (metavol-
canic) rocks with relics of granulitic mineral as-
semblages. They are best preserved in HT shear
belts with strong ductile deformation, but relict
garnet-kyanite parageneses can also be found
elsewhere in the LP paragneisses. The macro-
scopic and microscopic appearance of these
granulitic rocks shows their pronounced syn-
kinematic character (mylonitic banding, dynamic
recrystallisation of quartz and feldspar, and augen
structure). It is therefore apparent that they be-
long to the “group I granulites” of Pin and Vielzeuf
120
(1983). Note that basic grant&es are absent in the
CGC.
The granulitic mineral assemblage, with garnet,
kyanite, mesoperthitic and antiperthitic feldspars,
quartz and rutile, is largely overprinted by the
LP-HT metamorphism. Some of the typical
mineral reactions producing sillimanite, biotite and
cordierite include for instance:
Garnet + orthoclase + H,O + biotite + sillimanite
+ quartz
Biotite + sillimanite + quartz + cordierite
+ orthoclase
Garnet + kyanite + cordierite + spine1
Kyanite + corundum + quartz ( + plagioclase)
Kyanite + corundum + spine1 ( + plagioclase)
Kyanite + sillimanite
The metasedimentary and acid granulites do
not show any significant depletion in LIL ele-
ments, in contrast to most granulites of old
cratonic areas. Pressure-temperature data are not
easily obtained because of the strong retrogression
of the rocks. Thermometric and barometric analy-
sis indicates pressures between 0.7 and 1.0 GPa
and a wide temperature range between 650” and
750” C. The relict occurrence of mesoperthitic
feldspar (bulk composition Or,,, AbsO) and kyanite
points to temperatures even higher than 750° C
(P = 0.8-1.0 GPa) (Seek, 1971; Parson, 1978).
Conclusions
The data discussed in the previous sections are
collected in Fig. 2. The HP-HT event is rep-
resented by garnet-spine1 peridotites and by
eclogites and their acid country rocks as well as
mineral relics and reaction structures in some of
the LP gneisses. The MP-HT stage is recorded
by the granulitic stage of the eclogite transforma-
tion and by blastomylonitic acid granulites and
granulitic mineral relics in the gneisses. The
LP-HT metamorphism dominating in the gneisses
and migmatites has overprinted the preceding HP
and MP events. Finally, the LP-LT retrograde
evolution of late or post-Hercynian age is con-
nected with the uplift history and with brittle
deformation of the basement.
HP- HT- metamorphism
(eclogitic stage)
MP-HT-metamorphism
(granulitic stage)
Ll
loo 200 300 400 500 600 700 oc
Fig. 2. Metamorphic stages in the basement of the Central
Schwarzwald.
The distribution pattern of the HP and M P
rocks (Fig. l), especially the eclogites, their petro-
logical evolution, and the relationships with their
gneissic country rocks all very strongly indicate
that the metamorphic history of at least some of
the gneiss basement is identical to that of the HP
intercalations.
Contrasting equilibration conditions (and
therefore contrasting mineral assemblages) in
mafic and non-mafic rocks could be explained by
a mechanism proposed by Heinrich (1982, 1986)
who emphasizes the importance of kinetic factors
(different overprinting rates during unloading by a
combined dehydration-hydration process).
According to Koons and Thompson (1985). retro-
gression of rocks of non-mafic composition during
uplift occurs more readily than retrogression of
associated mafic rocks “due to both equilibrium
and non-equilibrium processes”. Additionally, one
must consider the effect of the LP-HT metamor-
phism with its very high fluid activity as recorded
in the fluid inclusion in the basement rocks (Behr
et al., 1987).
On the other hand, there are parts of the gneiss
basement which are lacking in any HP or M P
intercalations or related phenomena (for instance,
some of the “varied” units with amphibolites and
leucocratic gneisses as defined by Wimmenauer
(1980. 1984), or the granodioritic and
tonalitic-trondhjemitic orthogneisses in the CGC).
121
P
anatexites
T
Fig. 3. Schematic P-T paths of the heterogenetic components
of the pre-Hercynian basement in the Central Schwarzwald.
These rocks should have had a more simple meta-
morphic history. As a consequence, the simplified
sketch in Fig. 3 shows two different P-T loops
which join in the LP-HT metamorphism field.
The prograde history of these P-T paths is still
hypothetical.
The coming together of rock units of com-
pletely different metamorphic histories seems pos-
sible during continental collision and accretion,
processes which were very effective in the
European Hercynian belt (Lorenz and Nicholls
(1984) and Matte (1986) among others). For the
Central Schwarzwald a model similar to that de-
veloped for the Massif Central could be consid-
ered (Burg et al., 1984), involving a HP nappe
system of deep crustal provenance which was
brought together with an autochthonous complex
from a higher crustal level by deep-reaching thrust
processes. These units were welded by a subse-
quent LP-HT metamorphism.
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