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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