The External Massifs, relics of Variscan basement in the Alps

[AUFS TZE] The External Massif s, relics of Variscan Basement in the Alps

By JORGEN F. yON RAUM~R, Fribourg*)

With 15 figures and I table

Zusammenfassung

Neue chemische und strukturelle Daten lassen die komplexe metamorphe Entwicklung im Bereich des l~ristallinen Grundgebirges des helvetischen Bereiehs (Zentral- und West- alpen) besser verstehen.

Die Relikte des vorkarbonischen Altkristallins - die sogenannten Herzynischen Massive oder Externmassive - k6nnen wahrscheinlich in einen ~ilteren Kristallinanteil und eine jiingere Bedeckung unterteilt werden, und beide unterlagen von kaledonischer bis zu variszischer Zeit der folgenden Kette metamorpher Ereignisse:

1) Wfihrend eines spfitkaledonischen Hochdruckereignisses wurden disthen-fiihrende Hochdruck-Mineralparagenesen ausgebildet. Dabei wurden ungef~ihr Bedingungen von 8-10 Kb/550 ~ erreicht. Es ist wahrscheinlich, dal3 zur gleichen Zeit Variszischer Deckenbau entstand. Infol- gedessen ist das Hochdruckereignis wohl auf eine Krustenverdickung zuriickzuffihren (Kollision? oder duktile flache fJberschiebungen?).

2) In Abh~ingigkeit vom erreichten Temperaturmaximum stellte sich in der Folgezeit, wahrscheinlich friihvariszisch, mindestens eins der drei folgenden Entwicklungssta- dien ein: a) Mineralparagenesen mit Biotit - Sillimanit + Granat b) Mineralparagenesen mit Cordierit - Sillimanit - Kalifeldspat c) Granitoide Erstschmelzen mit Cordierit Es wurden generell ungef~ihr 4-5 Kb/600-650 ~ erreicht.

3) W~ihrend der regionalen Abkiihlungsgeschichte bildeten sich Zerrklfifte mit Quarz - Kalifeldspat - Andalusit, denen in einem noch sp~iteren Stadium Kluftbel~ige yon Muskowit - Andalusit folgten.

Auf Grund dieser generellen Entwicklung miissen die Externen Massive als Relikte einer kontinentalen Kruste aufgefaBt werden, deren metamorphe Entwicklung vollkommen mit derjenigen des mitteleurop/iischen Variszikums iibereinstimmt.

Summary Chemical and structural data give new insight into the complex metamorphic evolution

of crystalline basement of the Helvetic Realm (Central and Western Alps). The relics of pre-Carboniferous basement - the so called External Massifs - may be sub-

divided into an older basement unit with a younger cover, which together suffered the following metamorphic evolution from late Caledonian until Variscan times:

1) A late Caledonian high pressure grade metamorphic event with formation of kyanite- bearing mineral assemblages. Conditions of 8-10 Kb/550 ~ may have been attained. It is not excluded, that the high pressure evolution has been accompanied by

*) Author's address: Prof. Dr. J. F. VON RAUMER, Institut fiir Mineralogie und Petrogra- phie der Universitfit - P&olles, Ch-1700 Fribourg/Switzerland.

The present paper profited largely by the new research program on the petrology and comparative geochemistry of the main lithologies in the External Massifs (Swiss National Science Foundation project Nr. 2.516-0.82).

1 Geologische Rundschau, 73, 1, 1 -31, Stuttgart 1984 1

JI]RGEN F. VON RAUMER

formation of late Caledonian nappe structures. (Crustal thickening due to collision? or ductile thrusts?)

2) Depending on the maximum temperature attained the later geothermal evolution, probably of early Variscan age, led to at least one of the following stages: a) mineral assemblages of biotite-sillimanite + garnet b) mineral assemblages of sillimanite-cordierite-K-feldspar c) Second regional anatexis with formation of cordierite-bearing granitoids Condit ions of about 4-5 Kb/600-650 ~ have been attained.

3) During the late cooling history alpine type tension gaps of Variscan age with quartz- K-feldspar-andalusite were formed. Latest joints were covered by muscovite-andalusite.

These results confirm that the External Massifs have to be interpreted as pieces of a former continental crust with a metamorphic evolution known from the mid-European Varis- can crystalline basement.

R~sum~

De nouvelles donn~es chimiques et structurales permettent une meilleure comprehension de l '6volution m6tamorphique du socle cristallin dans le domaine Helv6tique (Alpes Cen- trales et Occidentales).

Les unit6s du socle, d'gtge ant6-Carbonifere, - les Massifs Externes ou Massifs Hercyni- ens - permettent une subdivision probable dans un socle plus ancien avec une couverture plus r6cente, qui ensemble montrent l '6volution m&amorphique depuis le Cal6donien jusqu 'au Varisque pr6coce suivante:

1) Un 6venement tardi-cal6donien de haute pression avec formation d'assemblages /t disth~ne. Les conditions de 8-10 Kb/550 ~ ont 6t6 atteintes. La formation de nappes tardical6doniennes est probable. (Epaississement crustal resultant d 'une collission?, de chevauchements ductils?)

2) L'6volution thermique post6rieure, probablement d'~ge Varisque pr6coce, m~ne sui- vant le maximum de temperature atteint vers un des stages suivants: a) formation d'assemblages ~ biotite - sillimanite + grenat b) formation d'assemblages ~ cordierite - sillimanite - feldspath potassique c) I16me anatexie avec mobilisation de granitoides ~ cordierite. Les conditions de 4-5 Kb/600-650 ~ ont ~t~ atteintes.

3) Pendant le refroidissement tardif se forment des fissures remplies d 'andalousite - quartz - feldspath potassique. Des diaclases encore plus tardives sont tapiss6es d 'andalousi te et muscovite.

Ces resultats confirment que les Massivs Externes font entierement part d 'une crofite continentale avec une 6volution m&amorphique telle que l 'on connait dans le socle cristallin Varisque du reste de l 'Europe.

I(paTKoe conep~caane

Ha OCUOBaHH HOBblX cTpyrTypHbIX rl XHMHqeCKHX ~aHHblX y~aeTc~ npoc~e~fTb CJIO~HyrO HCTOpHIo Fiopo~ re~baeTcKoro pernoHa - IleHxpa~ble H 3aHa~Hble AYibnbl.

PenHKTb~ ~aoKap6oHOB~,~X ~peBHHX rlopo~, T. H. repttHHCKHe, HSIH BHeLUH!ae MaCCHBbl, MO>K- HO Bepo~ITHO, HojIpa3~2e:l~ITb Ha 60:~ee ~peBrtHfi KpHcTan~IHHOBbI~ MaTepHa~ rI Ha 60.aee FIO3~2HHe noKpOBbI, IIpHqeM 06a HOllBepFYlHCb 3a nepno;I BpeMeHH OT Ka:~e6oncKoro /ao Ba- pnccKoro ropoo6pa30Bawe~bHOrO npoHecca c~e~ylottlrIM HpoueccaM MeWaMOpqbH3Ma:

2

The External Massifs, relics of Variscan Basement in the Alps

l) Bo BpeMs lqO3~HeKaJIe~OHCKOrO nepHo~Ia, xapaKTepH3y~o~erocg 6027blMHM ~laB~eHHeM, oSpa3oBaaHcb napareneabi MnnepaaoB BblCOrOro ~IaBaeHnn, co~lepxamHe 2IHCTeH. Ilpn 3TOM yC33OBHn ~IaBJIeuHn n TeMnepaTypbi COCTaBanaH npHMepuo 8 - 10 K6ap H 550 ~

BecbMa BepO~THO, qTO K 3YOMy BpeMeHH BO3HI3KaeT I4 BapHccKH!] IIoKpoB, npHqeM B ~IaH- HOM pa~oHe IIpoHcxo~IHT yTO~nleHHe 3eMHO-q KOpbI, qTO IIpHBO~IHT H K 3HaqHTe~bHOMy yBe- :m~eHriro 2aBYleH!4~I (KOTITIH3HH? H.IIH naOCKrte na~nrtrn?).

2) B 3aBrlCrIMOCTI40T ~OCTHrHyTOrO xeMnepaxypnoro MaKc~MyMa B nocaeay~omee, Be- pottxnee Bcero, B paHHeaapnccKoe BpeM~l, Hlvtea~i MeCTO, nO-KpanHeH Mepe, Tp!4 noc3e)IoBa- Te~bHO CMeSfltomHec~ CTaimn pa3BHTnn:

a) naparese3bi MnsepaaoB C 614OTI/ITOM, CHa3IHMaHHTOM, rpauaTOM; 6) naparese3bt Mnuepa30B C Kop~IHep~TOM, C~33HMaHHTOM, KaflI/IeBbIM nOaeBbIM tuna-

TOM, C) o6pa3OBaHHe rpanHTor/o~O6HblX lIOpOA C Kop)2HepHTOM. ~TO ~portcxo~HT rlpH yCYlO-

BHaX 4 -- 5 ~Sap n 600 - 650 ~ 3) Bo BpeM~ perrIoHa~bHoro OX:Ia~K~IenB~ o6pa3ytoTC~ TpellarIHbI pacT~KeH~, 3ano:~neH-

Hble KBapUeM, Ka2IHeBblM HOYleBblM 1LIHaTOM I/I aH~Ia:~y3nTOM, 3a KOTOpblMH c~eAoBa:I~ Ha 6O- aee HO3~HHX CTa~H~X eme n MyCKOBHT -- aH~Ia~y3HT. Ha OCHOBaHHH TaKoro o~IIIero pa3BH- T~ c:Ie~yex pacCMaTpHBaTb BHeI~HHe MaCCHBbl, KaK peoTrI~Tb~ MaTep~KOBO.~ KOpbI, MeTa- MOpqbHOe pa3BHT~e KOTOpbIX HO~IHOCTblO COOTBeTcTByeT ~0 BpeMeHH I~pHMepHo TaKOBOMy cpe~HeeBpolIe~cxoro BapHccKoro CO6bff~a.

1.1 I n t r o d u c t i o n

The Alpine chain comprises two main groups of elements, those inherent to true alpine history and those representing relics of ante-alpine evolution.

These relics reveal that the crustal parts now occupied by alpine features took part also in a very long history of ante-alpine evolution, which in the north of the Alpine domain is accepted to be mostly of Variscan and Prevariscan age. The main structural units of Variscan basement in Central Europe were documented by STILLE (1951) based on the ideas of SuEss (1888) and in continuat ion of the structural concept of KOSSMAT (1927). These structural interpretations ended at the northern limit of "Neoeuropa" , although scientific work in the alpine domain had been in progress for almost 200 years.

The pieces of the "puzzle" of Variscan elements in the alpine domain, which from North to South are gradually transformed by higher alpine metamorphism, remained that time still an obstacle for the general interpretation of an ante- alpine history of evolution. Nevertheless the External Massifs (Argentera - Haut Dauphin6 - Belledonne-Grandes Rousses - Aiguilles Rouges-Mont Blanc - Aar- Gotthard) are good examples of Variscan relics in the Helvetic realm. All are sur- rounded by a mesozoic cover lying unconformably on the older units.

It is the aim of this paper to furnish new information on the metamorphic evolution of the Aiguil les-Rouges-Massif and to propose a synopsis in the light of the new data, for the main geologic evolution in the five External Massifs.

Jr0RGEN F. yon RAUMER

\ \ \ Aa r -

\ ~ ' ~ G o t t h a r d

Gen~ve Q Mont B lanc -

\ " Aig. Rouges

Bel ledonn e

�9 ]'or in o

Haul Douph ine

4

• Arg ent erc~

Fig. 1. General situation of the External Massifs in the Helvetic Realm,


The author (YON RAUMER 1976, 1981) has already discussed the surprising similarity in lithology and geologic evolution of the five External Massifs, and the reader is referred to these publications for references, as in the present paper only the most important literature will be cited.

1.2 G e n e r a l G e o l o g y

All five basement outcrops of the Helvetic Realm, the so called External Mas- sifs, have some general lines of evolution in common (YON RAUMZR 1981). They are composed of a rock series comprising mostly detrital sediments (metagreywackes, metapelites), with only a few metacarbonate horizons appearing as marbles of reduced thickness. Metabasic rocks are much reduced in thickness. They comprise normal plagioclase bearing amphibolites as well as garnet amphibolites, but amphibolites of eclogitic origine as well as true eclogites are known. The normal amphibolites are accompanied by K-feldspar bearing biotite-amphi- bole-rocks, which ressemble, in chemical composition, the "Durbachite"-series described in the Black Forest region. More important are a thick series of acidic fine grained gneisses and K-feldspar augengneisses, which seem to represent a subvolcanic to volcanic series. This means that a volcano-detritic series appears to be dominant in most of the massifs. Notwithstanding amphibolitic rocks tend to become more important in the Haut Dauphin6 and Aar-Massifs, and eclogitic amphibolites are relatively abundant in some regions of the Argentera-Massif.

It still remains difficult to establish a lithostratigraphic column, as all the rock units attained an amphibolite facies grade of metamorphism with formation of anatexites. All these rock units were crosscut by large granitoid bodies, which also suffered transformations of amphibolite facies grade.

The problem supposes if different cycles of metamorphic rocks are observed. These questions arose for the Aiguitles Rouges (VON RAUMER 1981, 1983), and have also been discussed for the Haut Dauphin6 (LE FORT 1973), where a monometamorphic series in the western part is opposed to a more complex central part. LE FORT (1973) thought that alternatively both could represent different tectono-metamorphic levels of the same age.

In general an older crystalline basement of amphibolite facies grade could have been covered by younger sediments, and both together show a new overprint of amphibolite facies grade, resulting in an older polymetamorphic basement covered by a younger monometamorphic series.

Besides these more general characteristics concerning the Prevariscan and Var- iscan evolution all five massifs have been involved in a complicated pattern of sedimentary, tectonic and metamorphic evolution.

It is worthwhile to mention the occurrence of metapelitic series in the Grandes Rousses area, which yielded a Cambrian age by microfossils (GIORGI 1979, GIORGI et aL 1979).

A new kind of evolution begins with the occurrence of a series of mostly detrital sediments, which yielded an Upper Visean age (BELLIERE • STREEL 1980), in the western part of Aiguilles Rouges, whilst GIBERGY (1968) supposed a Visean age for similar sediments with fossil traces for the Haut Dauphin& The sediments represent an interbedding of shales, greywackes and volcanic material, and similar sediments have been described from Belledonne. Continental detrital

JURGEN F. YON RAUMER

sediments are also described by FRANKS (1966, 1968) from the Aar-massif. The question arises, whether these series of greenschist facies grade represent a true, lower metamorphic grade cover on the Variscan crystalline basement, or if they are the lower grade metamorphic equivalents of units, which in the domain of the massifs already appear in the amphibolite facies grade. If the latter were true, we must admit a mecanism bringing lower grade metamorphic rocks into the neighbourhood of their higher metamorphic grade equivalents, for example, differences in erosion levels or the formation of nappes.

True graben fillings are represented by the coarse clastic sediments including some coal measures of Westphalian D - Stephanian A age (JONGMANNS 1968). They show, that in late Carboniferous times, great differences of relief, probably due to high uplift rate favoured a high erosion rate. These late fillings with Perm- ian and Carboniferous detrital sediments are bound to intramontanous graben systems which reveal the destruction of the former quite uniform crystalline basement. After the observations of MALARODA (1975) and YON RAUMER (1976) sedimentation is highly influenced by the tectonic events, and this influence may be ovserved even in sediments of Cretaceous age from Argentera (MALARODA 1975).

After a short period of denudation the massifs were mostly covered by triassic sediments, but already the rifting history (BERNOULH 1983) announcing the alpine events, which were to largely transform the whole domain, had begun. Rifting was accompanied by tilting (LEMOINE 1983), arid some of the main mor- phological features visible today are the result of tectonic events of Jurassic age.

N S

Fig. 2. N-S-cross section through Mont Blanc and Aiguilles Rouges (Dots: Permo-Car- boniferous detrital sediments); a) Geotectonic situation of Mont Blanc and Aiguilles Rouges under the pile of alpine nappes (after HOMEWOOD et al. 1980); b) Triassic surface in the Aiguilles Rouges area after the alpine deformation as drawn by BADOUX (1972). Updoming of the crystalline basement with formation of shearing planes. The horizontal shearing zones and the tension gaps correspond to observed features discussed in the text

and in Fig. 3.


1.3 A l p i n e T r a n s f o r m a t i o n s

BADOUX (1972) showed the contrastbetween the rigid behaviour of crystalline basement and the plastic transport of the overlying nappes, where after RAMSAY (1981) the high plasticity is limited to certain narrow zones. The new cross-sections (Fig. 2) (HoMEWOOD et al. 1980) allow to understand better the tectonic situation of the External Massifs, which under a pile of nappes behaved like huge wedges more or less involved in the alpine structures. Like in the more eastern cross-section (FREv et al. 1980) they received metamorphic transformations depending on pressure, temperature and fluid-transport (MULLIS 1979). Spe- cially the criteria of temperature - the plasticity of the crystalline compounds (VOLL 1976, 1980, 1983) - influenced the behaviour during alpine tectonics. The alpine transformations increase when going from North to South, and some examples from Aiguilles Rouges and Mont Blanc shall show this increasing influence of alpine metamorphism.

L

I . �9 .

\ - " , . . ~ r /

�9 �9 �9 o ~ %

r

NW~lr~ / ISE Fig. 3. Pole diagram of shearing planes developed in metapelitic rocks of Emosson area (Aiguilles Rouges). Lower hemisphere of Schmidt projection. Triangles: nearly horizontal shearing planes with direction of transport (linears - marked triangles) towards NW. Dots: Corresponding second set of nearly vertical shearing planes with corresponding linears (marked dots). Lower inset: Corresponding deformation plan in a NW-SE-cross section

(without rotation).

J~]RGEN F. VON RAUMER

The Aiguilles Rouges suffered the transformations of very low grade metamorphism with formation of pumpellyite, prehnite and laumontite in amphibolites (YON RAUMER 1974) and stilpnomelane in nearly undeformed Permian rhyolites. Granitoid orthogneisses yielded chlorite-albite mineral assemblages, when approaching higher levels to the overlying nappes. Quartz shows first stages of undulation and low angle boundary crystallization (polygonization). Depending on lithology conjugated shearing systems with corresponding tension gaps developed in different sizes. In the slightly deformed Triassic cover the tension gaps are 10 cm long. Orthogneisses developed a general, closely spaced fracture cleavage with tiny chlorite filled tension gaps, and micaschists show two sets of large shearing planes with growth of fiber quartz crystals parallel to the shearing planes (Fig. 3). All three types of brittle shear respresent probably o n e answer to the same alpine deformation in the neighbourhood of the overlying nappes. The starting polygoniziation of quartz and the general occurrence of mineral assemblages of lowest metamorphic grade lead to the assumption that in the domain of the Aiguilles Rouges about 275 ~ may have been attained during alpine metamorphism.

4a 4b 4c

Fig. 4. Examples of alpine metamorphism from the Mont-Blanc-granite. a) Polygonal recrystallization of quartz (specimen MB 204). Bar corresponds to 0,5 mm. b) Primary biotite replaced by new green biotite and sphene. Sphene is concentrated on cleavage planes of the former biotite (specimen MB 117). Size of the whole biotite-flake 1 mm. c) Brittle behaviour of plagioclase during alpine deformation. Formation of small tension gaps filled with quartz (black) and biotite. Both grew parallel to the direction of greatest elongation

(Fig. 5). Bar corresponds to i mm.

Much stronger are the transformations in the Mont Blanc domain, where all the rock series attained greenschist facies grade (YON RAUMER 1967, 1974), and the Mont Blanc granite received its typical "protogine" aspect. This granite may serve as an example for all types of transformations, which very often depend on the primary minerals present. Quartz displays a typical equigranular recrystallization (Fig. 4a), which after the criteria of VOLL (1976, 1980) should represent a recrystallization temperature of about 350-400 ~ Primary brown biotite has been replaced by a mosaic of new green biotite (Fig. 4b), and the cleavage of the primary biotite may be still visible by the formation of sphene parallel to the former cleavage plane. Stilpnomelane is present in most of the thin sections (YON RAUMEP, 1969), and its crystallization is limited to the border zones between bio-


Fig. 5. Detail from Fig. 4c. Tension gaps lower part. Growth of fiber quartz paraltel to the opening direction (specimen MTB 307). Picture corresponds to 1 mm.

rite and K-feldspar, or it may appear as small fibers in fractures of K-feldspar. The normally acid plagioclase shows little transformations to white mica and epidote minerals. K-feldspar shows first traces of chessboard albite replacement, but otherwise it displays like plaglioclase only a brittle behaviour, where cleavage planes and tension gaps are filled by quartz (Fig. 4c, 5). Conforming to these observations are the results of fluid inclusions, in quartzes from alpine tension gaps, where POTY (1969) and POTY et al. (1974) found conditions of 400 ~ and 2,5 Kb for conditions of alpine metamorphism. Tectonic transformations have in general a still more brittle character, and in the higher levels of the granite shear planes covered by quartz and epidote reveal the same direction of transport (VON RAUMER 1967) as has been observed in the Aiguilles Rouges (Fig. 3).

Besides these general observations made in the Mont Blanc granite some quartz rich rocks display locally steeply dipping stretching lineations due to their higher plasticity. VON RAUMER (1971) mentionned the higher plasticity of K-feldspar augengneisses, where large K-feldspar phenocrysts display a stretching of about 300% due to their brittle behaviour in a highly plastic matrix (Fig. 6). Permian metarhyolites display a stretching of about 1000V0, but the fine grained matrix may have been stretched to even higher values. These few data suffice to characterize the general influence of alpine metamorphism.

In their actual morphology both massifs follow general tectonic lines which become visible in ERTS I Photographs (GUILLEMOT et al. 1973), where present day isostatic movements are balanced.

JI3RGEN F. VON RAUMER

Fig. 6. Brittle behaviour of K-feldspar phenocrysts in a ductile matrix of granitoid composition. K-feldspar-augengneisses near Petoudes (Mont Blanc Massif). Black: original K-

feldspar, white: filling by quartz and albite.

2. New petrological data from Aiguilles Rouges Massif

2.1 M i n e r a l a s s e m b l a g e s a n d m e t a m o r p h i c e v o l u t i o n

Recent research on the petrological and structural evolution of a metapelitic series in the Aiguilles Rouges massif reveal a multiphase evolution of Prevaris- can and Variscan rocks (YON RAUM~R 1983). The geological map shows the wide distribution of metapelites and metagreywackes together with granitoid orthogneisses. Metabasic and quartzitic layers as well as marbles are preserved as bou- din structures with a complicated internal structural pattern.

All rocks display minerals of amphibolite facies grade, and besides the relics and retrograde late transformations the following mineral paragenesis are observed:

metapelites + metagreywackes: Musc + Qu + Plag + Bi + Sill + Gar metabasic rocks: Hbl + Plag ___ Bi _ Gar - Qu ___ Di metacarbonates: Calcite + Diopside + Plag _ Graphite __+ Dol Granitoid gneisses: K-feldspar + Qu + Plag + Bi ___ Gar + Musc

Ky Sill

Fig. 7. AFM-diagramm of possible mineral-assemblages of relic associations with kyanite and actually observed associations in the sillimanite zone. Dots: field of distribution of analyzed metapelitic rocks from Emosson-area (Aiguilles Rouges) after YON RAUMER

(1983).

10


Very locally occurring anatectic structures show that temperatures of about 630-650 ~ were attained. The presence of relics shows that the mineral associations mentionned above represent only a final stage of a more complex metamorphic evolution. It can be partially reconstructed by the observations made on the composition of the metapelitic series. These clearly display two compositions, one poor in CaO (1.0-1.9 wt.9/0 CaO) with plagioclase (An 26) and very few relictic garnets and a second one with higher CaO (2-4 wt.~0 CaO) containing plagioclase (An 46) and a great number of garnets.

No difference can be seen between either composition when plotted in an AFM-projection (Fig. 7) where all analyzed samples (YON RAUMER et al. 1983) concentrate in a field limited by the Mg/Mg + Fe - values of 0,36-0,52 with a slight variance of A1203.

Fig. 8. Zoned garnet from metapelites from Emosson-area (Aiguilles Rouges). The rim area corresponds to a late evolution in the sillimanite-zone (also represented by new small gar-

nets) Specimen AR 1124, size of garnet 6 ram.

Besides the observed mineral association of the sillimanite zone (Gu~DOTTI 1970, 1974) relics of staurolite and kyanite have been found. Also garnets show a multiple history, which can be observed by different stages of pseudomorphous replacement and by chemical zonation (Fig. 8, 9).

The observed stages of replacement are - garnet relics with plagioclase and few biotites. - small garnet relics with biotite and few plagioclases. Biotite is partially

replaced by fibrolite. - pseudomorphs of biotite with much fibrolite.

11

JURGEN F. YON RAUMER

Fig. 9. Contemporaneous growth of sitlimanite and plagioclase in the sillimanite zone. Lower dark part: biotite. Upper grey part: plagioclase with some quartz inclusions. At the limit of biotite and plagioclase growth of fibrous sillimanite (clear) into the plagioclase.

Specimen AR 1303, size of picture 3 ram.

Very similar structures have been discussed by GUtDOTTI (1974) and YARDLEY (1977).

The chemical zonation of garnets also reveals a more complex evolution (VON RAUMER et al. 1983). Large garnets (about 5 mm dia) show a multiple history, which comprises from core to rim three different stages:

core Mn, Ca-rich, poor in Fe, Mg main zone compared with the core shows a loss of Mn, Ca and gain of Mg, Fe

with rising Mg/Mg + Fe-values towards the rim rim not allways represented. Diminishing Fe, Mg- and rising Ca, Mn-

values This chemical evolution is only partially seen in smaller garnets, which only

represent part of the whole evolution, and some small garnets only display the composition which is typical for the rims of the large garnets. Those small garnets are supposed to represent the latest stage of growth. In accordance with KRETZ (1973) growth of garnets started at different moments during the metamorphic evolution, where the large garnets represent, with their core an early evolution, whereas their rims and the small garnets show only the late stage of metamorphic evolution.

It is interesting to observe the inclusions of staurolite in garnet, which seem to occur at a certain value of Mg/Fe of the garnet host independently of its growth history. This value of Mg/Fe of the garnet host coincides with the general change

12


30

20

Fe I0 20 30 - 40 Mg

30

20

Fe I0 20 30 " 40 Mg

b

c

Fig. 10. Chemical composition of garnets during metamorphism in the Ca-Fe-Mg- and the Mn-Fe-Mg-triangles. Areas between the dotted arrows comprise the main chemical evolution trends from core to rim. Triangles: composition of garnet relics coexisting with plagioclase. Square: composition of garnet grown in blastomylontic structures. Bars in the upper triangle: minimum composition of Ca in garnets. Bar marked with St (lower triangle): Staurolite appearing in garnet host, garnet composition approximately Mg/ Mg + Fe = 1,1; a) sceletal garnet grown in blastomylonite structure (composition corresponds to square); b) pseudomorphs of garnet :relics with plagioclase, quartz and biotite after older garnet (composition of garnet corresponds to triangles); c) strongly zoned garnet (similar to Fig. 8) with chemical compositions corresponding to the entire evolution

path with a core rich in Ca and Mn.

13

JORGEN F. y o n RAUMER

of composition of garnet from the Mn-Ca-richer core to the Mg-Fe-richer main zone (Fig. 10). This critical composition of Mg/Fe of garnet also coincides with a minimum composition in CaO for those garnets having early high values of CaO. It is interesting to note, that a similar minimum composition of CaO is found in those garnet relics which together with plagioclase form the early stages of pseudomorphous structures after idiomorphic garnets. The chemical evolution in the main zone towards higher values of Mg/Fe seems to represent a "prograde" metamorphic evolution of the host rocks. As the Ca-minimum composition coincides with the formation of pseudomorphs, it can be assumed, after the equation of GHENT (1976) that the grossularite component of garnet has been transformed to anorthite plagioclase in the presence of kyanite. As this reaction is more pressure sensitive due to its high reaction volume it can be assumed, that the following "prograde" history starts with decreasing pressure and probably increasing temperature. This means that the host rock started with earlier mineral associations representing higher pressure conditions before attaining the sillimanite zone. Together with muscovite, plagioclase and quartz the following AFM-paragenesis might have existed:

- kyanite - staurolite - biotite - staurolite - garnet - biotite - kyanite - biotite The metamorphic evolution followed in consequence the kyanite - sillimanite

type of metamorphism. The earlier evolution leading to the staurolite - kyanite mineral association has

not been seen. Nontheless it is supposed, that an early stage of metamorphism was represented by minerals of greenschist facies grade. When plotted in the ACF-, ANaKF- and AFM-diagramms after HOSCnEK (1967), the analyzed samples do not fulfill the conditions to form chloritoide. In consequence the mineral association of greenschist facies grade could have had the composition of muscovite-paragonite-chlorite and quartz.

2.2 L a t e s t a g e e v o l u t i o n

The metapelites attained, as indicated above, the sillimanite zone, and locally anatectic first melts appear, but it is surprising that no cordierite has been found. Only REINHARD (in COLLET et al. 1952) mentionned in his original notes one specimen containing the mineral paragenesis of M u s c - Plag - Qu - Bi - Sill - Cord. This paragenesis corresponds probably to rocks with a higher Mg/ M g + F e - value. Besides this very local observation, cordierite, more or less transformed to pinite, occurs very often in granitoid rocks, which discordantly cut the rock series mentionned above. They correspond to melts formed at deeper levels and are the signs of the highest temperatures attained in this region.

After the peak of metamorphism had been reached only a few transformations occurred, which in general did not change the mineral composition in a spectacular way, when speaking about Variscan events, whereas alpine retrograde transformations have already been mentionned under Chapter 1.3.

The most important late stage mineral is andalusite. As a rock forming mineral it may be found only in a very few places, and is mostly decomposed to white mica and chlorite. Most andalusite occurs as idiomorphic crystals found in

1 4

The External Massifs, relics of Variscan Basemen t in the Alps

\

20cm tl i( + \ / \ /

Fig. 11. Tension gaps of late stage evolution in the metapelitic series from Emosson (Aiguilles Rouges) filled with quartz, K-feldspar and andalusite (dark), the latter often

replaced by chlorite, muscovite and paragonite.

alpine type tension gaps of Variscan age, where it coexists with quartz and K- feldspar (Fig. 11). An even later paragenesis of andalusite with white mica is found on late joints. Both types of occurrences appear to represent a late cooling history after the peak of metamorphism.

2.3 E v o l u t i o n in t h e P - T - s p a c e

The polyphase evolution of minerals enhances the application of geother- mometers and barometers. The relative high values of Mn in the cores of large garnets indicate greenschist facies grade before attaining the staurolite isograd, and they might even represent conditions of high pressure (CARON et al. 1981). The formation of staurolite from muscovite and chlorite occurs after HOSCHEK (1967) at about 550 ~ The wide distribution of sillimanite indicates minimum temperatures between 510 ~ (HOLDAWAY 1971) and 625 ~ (RICHARDSON et al. 1978). Locally occurring anatectic melts in rocks of greywacke composition show that maximum temperatures for that region and the given outcrop level were about 650 ~ A late stage transformation of biotite to chlorite enhances the application of the geothermometer after FERRY & SPEAR (1978) to evaluate the temperatures for the mineral association biotite - garnet - sillimanite.

Also the pressure determination depends on valuable relics and, as already mentionned above, the early pseudomorphs of plagioclase, biotite and garnet relics after idiomorphic garnet should furnish an indication of pressure after the geobarometer of GHENT (1076). Taking into account the presence of kyanite together with quartz and garnet, the distribution of Ca between garnet and coexisting plagioctase indicate pressures between 8-10 Kb (YON RAt;MER 1983), when temperatures of 500-550 ~ are accepted for the paragenesis of garnet - biotite - kyanite - quartz. Although such "numbers" have to be applied carefully, it is interesting to note same recent pressure determinations on eclogites from Lac Cornu (Aiguilles Rouges). LIEGO~S & DUC~ESNE (1981) calculated for the garnet - pyroxene composition minimum conditions of formation of 11 Kb and 780 ~ indicating an event in the deeper crust, with low geothermal gradients (15- 20 ~ In consequence the kyanite - garnet mineral association of the metapelites could represent a high pressure metamorphic event before the mineral

15

JORGEN F. VON RAUMER

association of sillimanite - biotite was established. As the formation of pseudomorphs after garnet is bound to a decreasing pressure, the P-T-path probably followed a line which is characterized by decreasing pressure and perhaps rising temperature. The observation of sillimanite overgrowth in kyanite clearly indicates, that the evolution path crossed the stability limit of kyanite and entered the stability field of sillimanite. At deeper levels formation of granitoid melts with cordierite was possible. These appear as discordant granitoid bodies, and their outer limits are given by structurally controlled tectonic elements (VON RAUMER 1976, 1981).

As the late stage events show the formation of andalusite, the evolution path must have descended to lower pressures and temperatures. The coexistence of andalusite together with quartz and K-feldspar is limited to tension gaps, where according to MtJLLIS (1975, 1976) pressures lower than 1,5 Kb comparative to the host rock may be found, in comparison with the experiences from true alpine tension gaps.

In conclusion the metapelitic series from Aiguilles Rouges show an entire evolution from probable greenschist facies grade to a kyanite - sillimanite type of metamorphism with a late stage of andalusite formation. This metamorphic evolution seems to represent, for the rock series concerned, a more monometamorphic cycle, and there is some evidence, that an older metamorphic series has been overprinted by the same type of metamorphism.

The probably older series from Aiguilles Rouges are mainly composed of granitoid gneisses and migmatites, where some relics of amphibolites, quartzites and metacarbonates appear. These gneisses suffered a regional blastomylonitization, and BELLIERE (1958) described the different phenomena under the "gneiss de Chrsrrys". Independently of rock composition K-feldspar, biotite and plagio-

-~1o5

1

§ ,

# , -+-

J"-" 0 1 K m p

J

Fig. 12. Large scale F3-fold of Variscan age in the metapelitic series of Emosson area (Aiguilles Rouges). Horizontal bars: granitoid orthogneisses; black: amphibolites; dots:

quartzites.

16


clase display a regional granulation and recrystallization to quartz-ribbons. BEL- LIERE (1954) discovered kyanite in these rocks, and this new research gives evidence that kyanite has been an early phase related to these tectonic transformations.

2.4 S t r u c t u r a l e v o l u t i o n

The geometrical reconstruction of deformational structures has to take account of the NW dipping triassic nonconformity, which originally had an horizonzal situation. Therefore all structures have to be rotated to recompense the NE-SW striking transgression surface dipping 40 ~ towards NW.

The metamorphic evolution is accompanied by a sequence of deformational structures. The most obvious of these is a regional schistosity of N 20 E marked by the cleavage planes of biotite. It corresponds to a steeply dipping axial plane of large scale folds of several km which can be followed by the mapping of orthogneiss bodies (Fig. 12). The corresponding fold axes (F3) , which after rotation display a vertical pattern, show different aspects depending on the lithology of the deformed rocks (Fig. 13). Orthogneisses and amphibolites display well rounded folds, whereas micaschists have a more complex "V"-shape pattern.

Older structures are preserved as isoclinal small scale intrafolial folds, which are recognizable from deformed quartz segregations. They certainly represent at least two sets of different folds. The large boudins of amphibolites and quartzites display complex internal isoclinal pattern, which are cut by the main regional schistosity. As well the marble bodies show a very complex internal structure, where different sets of folds may be recognized. This fold style is easily explained by the high plasticity of the deformed marbles during high grade metamorphism; and therefore it is difficult, to compare the structures in marbles with the structures observed elswhere.

More obvious are younger F:structures which correspond to NE-SW striking, horizontal fold axes of mostly well rounded shape (Fig. 13). When comparing metamorphic history and sequence of deformational structures, the metamorphic history from formation of kyanite until the crystallization of biotite - sillimanite paragenesis is limited by the large scale F:folds and the later, more regular F 4- folds (Fig. 14). The vertical axial planes of F4-folds appear to serve as a structural element for intrusion of cordierite bearing granitoids.

It is interesting to notice that the blastomylonite structures described above have been folded by the F:folds. They correspond probably to plastic shear zones which developed during large scale folds, as kinds of fold nappes of smaller scale or as zones of extensive thinning in very elongated fold limbs. An interesting observation is that the blastomylonites have strong linear structures with spindles of small grained plagioclase like stretching fibers. They coincide with folds, which could represent sheath folds parallel to the stretching direction in a.

As F4-folds have deformed all older structures remains a question, whether the F:axial planes formed an originally flat lying structure, or if they belonged to an almost vertical system of large scale isoclinal folds. For the moment the first version is preferred.

The vertical fold pattern of F3-folds ("Schlingenbau") in consequence is the result of F4-deformation and late stage updoming of a former more horizontal

2 Geologische Rundschau, Bd. 73 17'

gL

so!:t~s o!l!l~d'e~ooJ ~til u! splo J l~!lOjUj~u! ~B pu~ t::l (~ 'ritz OZ ol spuodszJJo3 sfi~le :t~ztt ":~ttuI~ auolN pue so~'nol:l SZIl!n~!v tuoaj suo~'l'e~zuzg plo3 luz~zjjt.p .Io szcIK~l plo~I 's "g'~.cl

B ~ LJ

-'~s ~..-~, i ~ r~,l ,~~ ~-~

I

I jfl I

ill i

q

3~.~ " -~


Fig. 14. Structural diagrams (lower hemisphere of Schmidt projection), from Emosson area (Aiguilles Rouges) representing F 3 and F4 fold-generations. Small dots: F3-fold axes in the eastern Emosson area. Opened circles: Corresponding axial planes of F3-folds in the eastern Emosson area. Black squares: F3-fold axes from western Emosson area. Opened squares: Corresponding axial planes to the F3-folds of western Emosson area. Great dot: Projection of F3-fold axes of Eastern Emosson after rotation. Black triangles: F4-folds from Emosson and Salanfe areas. White triangles: Corresponding axial planes to the F4-folds.

pa t tern of axial planes and thrust planes. The two structures described above limit the main metamorphic history of

Aiguilles Rouges. A series of younger structures is well known, and some of these structures cannot yet be at tr ibuted to any certain per iod of evolution. They locally introduce quite complex interference patterns, but in general the more impor tan t older structures are still preserved.

2.5 C o n c l u d i n g r e m a r k s f o r t h e A i g u i l l e s R o u g e s

Combin ing all the new data and observations we have been able to summarize a t ime table of events, where different steps of evolution can be recognized:

a) A rock series of p robab le Precambrian to Paleozoic age has been deposi ted on an unknown basement. These sediments of reduced thickness comprise mainly greywackes and peli t ic mater ial with some small amount of inter- leaved carbonaceous material . Associated with these sediments are a series of magmat ic rocks which comprise basic and acidic lavas with their corresponding tuffs. Local ly K-fe ldspar augengneisses appear .

with some feldspar-rich layers (black) and double folded quartz veins (dotted). Emosson area (Aiguilles Rouges). b) Highly plastic behaviour (F 1 and F2 fold generations) from Salanfe area (Aiguilles Rouges). c) F3 and older folds in metapelitic rocks from Emosson- area (Aiguilles Rouges). d) F3-folds with different style depending on lithology. Heavy con- tours correspond to massive amphibolites. Emosson area (Aiguilles Rouges). e) F3-fold with complex earlier fold pattern in metapelitic rocks from Emosson (Aiguilles Rouges). f) Opened F3-fold in metapelites with quartzitic layers and quartz veins (black), with earlier intrafolial folds. Vieux Emosson (Aiguilles Rouges). g) F 4 cylindrical fold in different gneisses near the contact of the Vallorcine granite. Van d'en Bas (Aiguilles Rouges). h) F,-

fold with axial plane filled by granitoid material. Bovine area (Mont Blanc).

19

JfJRGEN F. VON RAUMER

These rocks have been cut by discordant granitoid bodies. It still remains a question, whether we must try to differentiate an older basement from a younger cover, both having suffered the same late metamorphic history. The faint traces supporting this idea have still to be supported by clear evi- dences.

b) The whole series suffered a regional metamorphic transformation, which follows the kyanite - sillimanite metamorphic trend with a late stage of andalusite formation. The relics of the older, high pressure paragenesis, kyanite - garnet + staurolite have been observed in metapelites and metagreywackes as well as in blastomylonitic zones. Large scale F3-folds accompany this high pressure event, which probably attained conditions of 10 Kb and 550 ~ The metamorphic evolution lead to the paragenesis of biotite - sillimanite + garnet, and the corresponding structures display horizontal fold axes running N E - S W with vertical axial planes. This evolution path is presumed to descend to lower pressures, as garnets decomposed through a reaction significant for high reaction volumes. This later stage of metamorphism probably fulfilled P - T conditions of about 5 Kb and 630-650 ~ This rising temperature when compared with the former high pressure paragenesis, can be well established, as locally the solidus for formation of granitoid melts has been attained.

c) These locally appearing melts are the indication of the temperature peak of metamorphism. Independently the mobilization of granitoid melts with cordierite took place at deeper levels (higher temperatures) and intruded as discordant granitoid bodies structurally controlled by main tectonic lines. Older, more diffuse granodiorites with cordierite (cordierite bearing grano- diorite from Fully area) are followed by very large dikes of cordierite bearing granites of Vallorcine type. These granitoids represent the final stage of this younger metamorphic evolution, which ended probably in early Varis- can times.

d) A late cooling history is documented by the formation of tension gaps containing andalusite - K-feldspar and quartz, and even younger joints covered by andatusite and muscovite.

e) The intrusion of younger granites of probably Upper Carboniferous age and the deposition of a variety of sediments since Upper Carboniferous time in intramontanous sedimentary troughs are the signs of the tectonic break-down of the former homogeneous crystalline basement. The alpine history brought still some final transformations.

3. The External Massifs in the light of the new data

The observations made for the Aiguilles Rouges give new ideas to compare all five massifs in a general way. Their parallelity of lithology and metamorphic evolution has already been discussed (VON RAUMER 1976, 1981). A sequence of metamorphic events such as has been found in the Aiguilles Rouges cannot be an isolated feature but should appear in a much larger area.

The M o n t B 1 a n c area, independently of its geographic situation before alpine events, has largely suffered from alpine transformations, which were the

20


subject of chapter 1.3. Nontheless the many occurrences of garnet amphibolites or transformed eclogitic amphibolites lead to the conclusion, that in this area a similar sequence of metamorphic events has occurred. YON RAUM~R (1971, 1976) mentionned the large distribution of migmatites containing metasedimentary relics, which generally display steeply dipping fold axes of large scale folds of Var- iscan age. As well the occurrences of cordierite bearing granitoids are well known.

From A r g e n t e r a BOGDANOFF (1970, 1973, 1979), BOGDANOFF & PRUNAC (1976) and PRUNAC (1975) report for the western parts an older metamorphic event with anatectic features and formation of sillimanite. Eclogitic relics are well known. A younger imprint of Barrovian type of metamorphism produced regionally distributed kyanite. Locally a second anatexis is admitted. BORTOLAMI & SACCr~I (1968) showed for the eastern part, how older relics of staurolite - kyanite + garnet are replaced by a new mineral paragenesis of biotite - plagioclase - quartz - sillimanite - cordierite - K-feldspar. VON RAUMER (1976) mentionned the narrow relationship between formation of blastic plagioclase, the occurrence of cordierite bearing granitoids and the regional anatexis II, which can be seen in the direct thermal evolution from the cordierite - sillimanite paragenesis and transforms also older blastomylonites.

Much detail comes from the H a u t D a u p h i n 6 area (PECHER 1970, PECHER & VIALON 1970, LE FORT & PECHER 1971, LE FORT 1973). An older metamorphic series with an older generation of migmatites has been transformed to blastomylonites yielding kyanite, sillimanite and garnet, found in the central area, whilst in the western part in a monometamorphic series of metasediments with kyanite - staurolite - garnet occurs. The older series of the central part later underwent a new transformation giving cordierite bearing granitoids.

PECHER & VIALON (1970) give conditions of 6,5-8,6 Kb and 600-650 ~ for formation of kyanite, sillimanite and garnet in the blastomylonites.

For the B e l l e d o n n e - G r a n d e s R o u s s e s regions ToB~ (1959) and KALSBEEI((1962) described different localities with paragenesis of kyanite - staurolite - garnet. CARME (1970 a, b; 1971; 1973) referred to polymetamorphic micaschists and gneisses containing biotite, garnet, kyanite and staurolite with a growing stage of anatexis, and underlain by cordierite bearing granodiorites. GROS (1974) described a blastomylonitic series with kyanite - staurolite - garnet from the northern part. This observation needs to be enlarged, as cordierite bearing mobilizates have been found in the same region. GASQUET (1979) observed garnet - cordierite - biotite - (sillimanite) and early stages of first melts, which are followed by the Sept Laux granite. For the given paragenesis he postulated conditions of 635 _ 30 ~ and pressures of 4-6 Kb.

The domain of the A a r - a n d G o t t h a r d massifs is highly transformed by alpine metamorphism. Nontheless ARNOLD (1970 a, b) established a sequence of cristallization in the Gotthard massif. Precambrian or Lower Paleozoic rock series suffered an early metamorphism of amphibolite facies grade and anatexis, before being transformed under granulite facies conditions. Crosscutting granitoids of Caledonian age suffered together with their country rocks a blastomylonitic transformation of Late Caledonian or Early Variscan age and furnished the mineral paragenesis of kyanite - sillimanite - garnet - biotite - plagioclase. In the Aar-massif a metamorphic series of amphibolite facies grade has been

21

JI]RGEN F. VON RAUMER

transformed by an anatectic event which yields widely distributed granitoids with cordierite (RuTISHAUSER 1973 a, b, c; RUTISHAUSER et al. 1978).

When summarizing these different observations from the five External Massifs it is possible to distinguish between the monometamorphic evolution of a series of cover rocks and the transformation of an older crystalline basement. The latter was composed of a heterogeneous series of rocks which attained anatexis and were crosscut by discordant granitoid bodies. Eclogites and granulite facies series may occur. This older basement could represent a series of Precambrian to Lower Paleozoic age. During transformation of the monometamorphic series this older basement has been transformed into polymetamorphic rocks. Very typical are blastomylonite zones, which in the Haut Dauphin~ (VIALON ~; PECHER 1970, "Lineament de Pillatte"), Belledonne (GRos 1974) and Aiguilles Rouges (BELLI- ERE 1954, 1958) yielded kyanite - sillimanite - garnet. Very similar paragenesis are observed in Mont Blanc and Argentera (YON RAUMER 1976), and ARNOLD (1970 a, b) gave a detailed description of the regional recrystallization from the Gotthard massif.

Both, older basement and cover, were involved in the new metamorphic overprint starting with high pressure assemblages and receding to low pressure mineral compositions (Tab. 1):

Table 1. Crystalline basement in the Helvetic Realm. Late Caledonian to early Variscan metamorphic evolution

Argentera

W Eclogite Sill. Kyanite Staur.Gr.

Haut Dauphine Belledonne

Kyanite Staur.Gr. Bi-Sill- Cord - KF Grdr. + Cord

W E

Kyanite Kyanite Staur.Gr. Gr.

Sill

Kyanite Staur.Gr. Sill

Aig.R.-M.B.

Eclogite

Kyanite Staur.Gr. Bi-Sill

Aar Gotthard

Grdr. + Cord

Grdr.+Cord Grdr.+Cord

Granulites

Kyanite Gr. Sill

Grdr. + Cord

During the tectono-metamorphic cycle granitoid rocks appear at different moments. These may be subdivided into three main groups:

a) Orthogneisses of granitoid composition having suffered the high pressure metamorphic event - acid, coarse to fine grained layered gneisses, sometimes with K-feldspar

phenocrysts. They could represent a volcanic to subvolcanic series. - Coarse grained augengneisses with K-feldspar phenocrysts, which may

represent granite bodies of perhaps Paleozoic age. b) The groupe of cordierite bearing granitoids subdivide into different species

according to their age of intrusion, but all contain different quantities of mostly transformed cordierite.

22

400


500 600 700 , I , I i I ,

o C

10

II'

kb

\ \

60*., ,. ~ \ . . . . ".'~ m \\

. . . . .

s

/ / Ky

,iLl

' ' " 1 , ,

\ \

o : :!::!:ii~! !!.:~ i ! ! :.ii!::

" - . I N

F -

�9 . /

" ' , - - ] c o - -

�9 . /~" I

10

20

3O

km

i t I ,

Fig. 15. Metamorphic evolution of crystalline basement in the Helvetic Realm (Western and Central Alps) from Caledonian until Variscan time. A12SiO5 - triple - point after HOL- DAWAY (1971). Region of first melts after THOMPSON (1982). Staur: limits for staurolite-stability, at lower temperature after RICHARDSON (1968), at higher temperature after PIGAGE & GRZENWOOD (1982). Triangle: Pressure determination on eclogites from Aiguilles Rouges (LIEGOIS & DUCHESNE 1981). Square: Pressure determination on coexisting garnet - plagioclase-couples in metapelitic rocks from Aiguilles Rouges (YON RAUMER 1983). Roman numbers and corresponding areas: Stages of evolution in the P-T-diagramm corresponding to table 1 and the final discussion. I. Early, probably Caledonian high pressure metamorphism with kyanite-bearing mineral assemblages. Ih Early Variscan evolution with the following stages depending on temperatures attained: a) assemblages with biotite - sillimanite; b) assemblages with cordierite - sillimanite - K-feldspar; c) formation of cordierite-bearing granitoids. III: Variscan tension gaps filled with quartz - K-feldspar - andalusite. IV: Late stage joints covered by muscovite-andalusite.

2 3

c)

JfJRGEN F. VON RAUMER

The oldest members of this group appear as diffuse gneisses, where highly transformed, anatectic material appears in the close neighbourhood to more homogeneous looking granodioritic rocks with cordierite. All tran- sitions in mineral facies between the country rock and the granitoid body are found. Younger members of this group show crosscutting features in relation to their surrounding country rocks, which exhibit, in general, a lower mineral facies grade than the granitoids. Most of the cordierite bearing granitoids from all the massifs have according to my own unpublished data high Al-contents and high corundum nor- mative CIPW values. They seem to represent a striking role in the tectonometamorphic evolution of the Variscan massifs. Most spectacular are the youngest granites, which mostly have Late Varis- can ages and appear as coarse grained discordant granitic bodies with abundant K-feldspar phenocrysts. According to their CIPW norm they have a certain tendency towards alcaline composition in their evolution.

4. Final discussion and conclusion

Younger sediments together with their underlying basement received a metamorphic overprint, which produced a polymetamorphic basement with a monometamorphic cover. This evolution can be recognized in all five massifs, and the sequence of events may be classified in the following way:

a) An older, already metamorphic crystalline basement with discordant granite bodies suffered a new metamorphic overprint. Well defined blastomylonite zones formed during this event yielding kyanite - sillimanite - garnet.

b) Younger sediments covering this basement were transformed to a monometamorphic series, which during an early phase yielded the mineral assemblage kyanite - staurolite - garnet. During a later phase mineral assemblages of biotite - sillimanite _+ garnet, in general became stable. Depending on the geothermal gradient and the temperature attained locally, muscovite and quartz reacted to form sillimanite and K-feldspar. The mineral assemblage of biotite - cordierite - sillimanite - K-feldspar can also be observed. The final stage of this evolution is the formation of granitoid melts (Anatexis II) with cordierite, which are typical for all five massifs.

The tectonic events accompanying this metamorphic evolution have been established for Argentera (BOGDANOFF 1979), where a close relationship between the crystallization of kyanite, the formation of blastomylonite zones and the evolution of nappe like folds has been described. Structural research from Aiguilles Rouges (YON RAUMER, in prep.) shows, that the blastomylonite zones can be related to the highly thinned fold limbs of large scale Fffolds. They may be interpreted also as actually vertical, plastic shear zones, which can represent Late Caledonian or Early Variscan tectonic elements. An indirect datation was made by ARNOLD (1970 a, b), who determined a Late Caledonian age for the formation of "Streifengneis"-granitoids, and in consequence the formation of blastomylonites could have occurred in Late Caledonian or Early Variscan times. As the

24


granite of Sept Laux (Belledonne) has a radiometric age of 322 m.y (DEMEULZ- MZESTZR 1982), the whole process of second anatexis (cordierite bearing granitoids) should be older, and an age of about 350 m.y. seems to be reasonable for this event.

This Late Caledonian to Early Variscan evolution shows early mineral assemblages of relative high pressures, which coincide with tectonic elements indicating a more horizontal tectonic transport, horizontal axial planes and shearing zones. The stretching fibers and sheath folds mentionned from the Aiguilles Rouges should reveal the sense of transport in the shearing planes. In consequence a period of crustal thickening may have occurred in this first stage of metamorphic evolution. The high pressure minerals have been replaced by the mineral assemblage of biotite - sillimanite _ garnet. As the replacement of garnet by plagioclase indicates high reaction volumes, the general tendency of evolution has been towards lower pressures. The final evolution depends on the actual erosion level.

As already shown, besides the mineral assemblage of biotite - sillimanite + garnet there may occur, as in Argentera, transitional assemblages of biotite - sillimanite - cordierite - K-feldspar, before the appearance of first melts of cordierite bearing granitoids. This continuous evolution exposed in Argentera, is interrupted in the other massifs, where biotite - sillimanite _ garnet assemblages are crosscut by cordierite bearing granitoids. In consequence their erosion level cannot be as advanced as in Argentera.

A structural control between the cordierite bearing granitoids and their host rock is evident. The granitoids appear as first melts in a well established fracture cleavage and enter parallel to the F4-axial planes. Also the blastomylonite zones seem to influence the shape of the granitoid bodies. The latter seem to represent an important time mark in the evolution of the External Massifs from Caledo- nian to Early Variscan evolution (VON RAUMER 1976, 1981). Their mecanism of formation may be explained by different processes. They could represent wet melts, where Al-rich micas favoured according to HOSCHEK (1974) the formation of Cordierite, or they could equally well, represent dry melts in the sense of THOMPSON & TRACY (1979) and THOMPSON (1982). For both mecanisms of formation the optimal P-T-conditions would have been attained.

The question arises, how the sequence from a kyanite to sillimanite type of metamorphism and the formation of cordierite bearing granitoid melts can be explained. Obviously the evolution in time lead to higher geothermal gradients (Fig. 15), and after RICHARDSON (1970) the kyanite - sillimanite type metamorphism seems to represent a normal geothermal gradient.

I f the early high pressure phase can be related to a crustal thickening process, how can the following pressure release be produced. The simplest way seems to be an unloading process due to regional uplift and contemporaneous erosion to restablish isostatic equilibrium. ENGLAND & RICHARDSON (1977) and WZLLS (1979) established models where the temperature increase is strongly related to erosion of overthrust continental crust, and these models would explain how first melts can be produced by the change of geothermal gradients during uplift and erosion. The observation of cordierite bearing granitoids in the backland of sub- ducted crust seems to be a current event, as observed in a smaller scale in southern Brittany (COGN~ 1976, 1977), and also well established from the Himalaya

25


region (LE FORT et al. 1980). According to THOMPSON (1981) only the crustal thickening by magmatic accretion (WELLS 1980) seems to furnish enough heat to follow the kyanite - sillimanite type of metamorphism and to enable the formation of granitoid melts. In consequence the explication of the general process has to combine both, the formation of high pressure mineral assemblages and the late formation of granitoids. We may accept a crustal thickening process to pro- duce high pressure assemblages, but there is no indication, whether collision or large scale overthrusts were the general motor.

If one assumes a combined model of crustal thickening and magmatic accretion, an unloading process descending from 10 Kb towards 5 Kb (unloading rate 1 Kb/10 m.y.) would favour outcrop structures which are encountered in the External Massifs. Admitting an age of about 350 m.y. for the cordierite bearing granitoids, the crustal thickening process could have occurred in Late Caledo- nian times.

This sequence of events would complement the views of ZWART (1969). Hercy- nian metamorphism has in general a low-temperature character and is related to a great number of granites, but this aspect is only a f i n a 1 one, which has to be related to an initial stage of Late Caledonian high pressure events. This would also explain the observation of DORNSIEPEN (1978), as to why hercynian low pressure series appear just in the domain of Ordovician metamorphism.

It is interesting to look at parallel events of tectonometamorphic evolution from the non-alpine domain. VON RAUMER (1981) has discussed the surprising parallelism of events in Southern Brittany. BURO & MATTE (1978) and MATTE & BURG (1981) observe a similar sequence of events from the Massif Central. Early, horizontal shearing horizons are related to high pressure mineral assemblages. Low pressure mineral assemblages and cordierite bearing granitoids appear in a late stage. There, crustal thickening processes appear as nappe systems transported from the North towards the South. After BURG & MATTE (1978) these events are older for the external parts of the Massif Central and become younger towards the South. In consequence, the formation of Hercynian nappes in the Alpine domain cannot be fixed definitivly in time. Each of the External Massifs follows approximately, the same P-T-evolution, but the evolution in time may have been different depending on its situation in the regional structure. In any case, traces of large scale Variscan nappes have to be accepted as pieces in the "puzzle" of the Variscan basement in the alpine domain. In consequence the large picture of Variscan nappes drawn recently by TOLLMANN (1982) should be extended to include the Alps, and the small pieces of Variscan basement, show, that they took part in the Variscan evolution of the mid European continent. Finally it should be recalled that the External Massifs, more or less allochtho- nous after the alpine events, are the relics of a Variscan continental crust and represent the Late Carboniferous until Late Permian horsts, where, similar to the New England gneiss domes, updomed Variscan nappes have been preserved.

Acknowledgements

I am grateful to S. Bogdanoff (Paris), A. Ploquin (Nancy) and G. Vivier (Grenoble) for the helpful suggestions and discussions and to R. Oliver (Grenoble) spending much time for an acceptable final english version.

26

The External Massifs, relics of Variscan Basemen t in the Alps

R e f e r e n c e s

ARNOLD, A.: On the history of the Gotthard Massif (Central Alps, Switzerland). Eclogae Geol. Helv. 63, 29-30, 1970.

- : Die Gesteine der Region Nalps-Curnera im nord6stlichen Gotthardmassiv, ihre Metamorphose und ihre Kalksilikatfels-Einschl/isse. Beitr. Geol. Karte der Schweiz, N. F., 138, 1-128, 1970.

BADOUX, H.: Tectonique de la nappe de Morcles entre Rh6ne et Lizerne. Beitr. Geol. Karte der Schweiz, N. F. 143, 1-78, 1972.

BELLIERE, J.: Sur la pr6sence des silicates d'alumine (sillimanite, andalousite, disth6ne) dans le massif des Aiguilles Rouges (Haute Savoie). C. R. Acad. Sci. 1395-1397, 1954.

- : Contribution fi l'&ude p6trog~n&ique des schistes cristallins du massif des Aiguilles Rouges (Haute Savoie). Ann. Soc. G6ol. Belgique 81, M~moire, 1-198, 1958.

- & SXREEL, M.: Roches d'gtge vis6en sup~rieur dans le massif des Aiguilles Rouges (Haute Savoie). C. R. Acad. Sci. Paris, 290 D, 1341-1343, 1980.

BERNOULLI, D.: Die fr/ihe Geschichte des Atlantik-Tethys-Systems: Kinematik und Kon- tinentalrand-Entwicklung. Vortrag und Zusammenfassung 73. Jahrestagung der Geolo- gischen Vereinigung in Berchtesgaden, 23.-26. Febr. 1983.

BOGDANOEr, S.: Quelques pr+cisions sur la structure et le m6tamorphisme du Massif de l'Argentera au Nord de St.-Etienne-de-Tin6e (Alpes Maritimes). C. R. Acad. Sci. 270 D, 2893-2896, 1970.

- : Pli pennique et dbformations superpos6es dans la partie nord-ouest du massif m~tamorphique de l'Argentera-Mercantour (Alpes Maritimes). R. A. S. T. Paris, 1973.

- : Analyse structurale dans la pattie occidentale de l'Argentera-Mercantour (Alpes Mari- times) Th+se Paris-Sud, 1-316, 1980.

- & PRUNAC, M.: Tectonique des gneiss et migmatites du massif de l'Argentera (Alpes Maritimes), France et Italie. 4 ~ A. S. T. Paris 1976.

BORTOLAMI, G. & SACCHI, R.: Osservazioni geologico-petrografiche sui medi valloni de S. Anna e Rio Freddo (Massicio cristallino dell'Argentera). Mem. Soc. Geol. It. VII, 37-64, 1968.

BURG, J. P. & MATTE, P. M.: A cross section through the French Massif Central and the scope of its Variscan geodynamic evolution. Z. dt. geol. Ges. 129, 429-460, 1978.

CARME, F.: Age briov6rien probable de la majeure pal'tie des s6ries suppos6es D6vono-Dinantiennes et existence d'un cycle orog6nique ant6-hercynien, sans doute cadomien, dans la chaine de Belledonne (Alpes Frangaises ). C. R. Acad. Sci., 271 D, 631-633, 1971.

- : Pr6cisions sur le m6tamorphisme majeur des schistes cristallins de la chaine de Belle- donne; essai de zon6ographie et distribution verticale des zones au niveau de la coupe de la Romanche. C. R. Acad. Sci., 277 D, 2133-2136, 1973.

- : Pr6cisions nouvelles sur l'ampleur et le style de la tectonique tangentielle hercynienne dans la chaine de Belledonne. C. R. Acad. Sci., 277 D, 2309-2312, 1973.

- : Sur une paragen~se tardive ~ disthene, sillimanite prismatique et zoisite dans les anatexites ~ pinite des environs d'Allemont (ChMne de Belledonne); essai d'interpr6tation p&rog~n~tique. C. R. Acad. Sci., 278 D, 565-568, 1974.

CARON, J. M., KIENAST, J. R. & TRIBOULZ/, C.: High-pressure - low-temperaturemetamor- phism and polyphase alpine deformation at Sant'Andrea di Cotone (Eastern Corsica, France). Tectonophysics 78, 419-451, 1981.

COGN~, J.: Les grandes lignes structurales du Massif Armoricain. In: Kossmat-Symposium, Nova Acta Leopoldina N. F. 224, 177-192, 1976.

- : La chaine hercynienne ouest-europ6enne correspond-elle /l un orog6ne de collision? Propositions pour une interpr6tation g6odynamique globale. Coll. Int. CNRS 268, Ecologie et g6ologie de l'Himalaya, 111-129, 1977.

COLLET, L.W., OULIANOEF, N. & REINHARD, M.: Note explicative de la feuille 24 de l'Atlas. Blatt Finhaut. Geologischer Atlas der Schweiz, 1 : 25 000, 1952.

27

JURGEN F. VON RAUMER

DEMEULEMEESrER, P.: Contribution fi l'6tude radiom~trique g l'argon et au strontium des massifs cristallins externes (Alpes Frangaises ). Th6se 3eme cycle, Grenoble, 1982.

DORNSIEPEN, U.F.: Ein Uberblick fiber die europ/iischen Variszisden. Z. dt. Geol. Ges. 129, 521-542, 1978.

ENOLAND, P. C. & RICHARDSON, S. W.: The influence of erosion upon the mineral facies of rocks from different metamorphic environments. J. Geol. Soc. 134, 201-213, 1977.

FERRY, J. M. & SPEAR, F. S.: Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Contrib. Mineral. Petrol. 66, 113-117, 1978.

FRANKS, G. D.: The development of the limnic Upper Carboniferous of the eastern Aar massif. Eclogae Geol. Helv. 59, 943-950, 1966.

- : A study of Upper Paleozoic sediments and volcanics in the northern part of the eastern Aar massif. Eclogae Geol. Helv. 61, 49-140, 1968.

FREY, M., BUCHER, K., FRANK, E. & MULLIS, J.: Alpine metamorphism along the geotra- verse Basel-Chiasso - a review. Eclogae Geol. Helv. 73, 527-546, 1980.

- , HUNZIKER, J. C., FRANK, W., BOCQUET, J., DALPIAZ, G. V., JAGER, E. & NIGGLI, E.: Alpine metamorphism of the Alps - A review. Schweiz. Min. Petr. Mitt. 54, 247-290, 1974.

GASQUET, D.: Etude p6trologique, g6ochimique et structurale des terrains cristallins de Belledonne et du Grand Chg~telard. Traverses par les gal6ries E. D. F. Arc - Isere, Alpes Frangaises. Th6se de 3eme cycle, Grenoble, 1-230, 1979.

GHENX, E.D.: Plagioctase-garnet-AlzSiOs-quartz: a potential geobarometer - geothermometer. Amer. Mineral. 61, 710-714, 1976.

GIBERGY, P.: D6couverte de ~gr6s g trous~) renfermant des d6bris d'organisme dans les schistes noirs de Valbonnais (s~rie crystallophyllienne dans les Massifs .Cristallins Externes des Alpes Fran~aises). C. R. Acad. Sci. 267 D, 1251-1254, 1968.

Gloa~I, L.: Contribution fi l'6tude des terrains cristallins du massif des Grandes Rousses, Is6re, France. Th6se doct. sp6c. Univ. Grenoble, 1-185, 1979.

- , GIRAUD, P. & VACHARD, D.: Sllr la pr6sence de micro-organismes d'gtge Cambrien dans les schistes cristallins du versant occidental du Massif cristallin externe des Grandes Rousses. (Alpes Occidentales). C. R. Acad. Sci., 288 D, 1079-1082, 1979.

GRos, Y.: Etude p6trologique et structurale du Beaufortin (Nord de Belledonne). Th6se de 3eme cycle, Grenoble, 1-114, 1974.

GUIDOTTI, C. V.: The mineralogy and petrology of the transition from the lower to upper sillimanite zone in the Oquossoc area, Maine. J. Petrol. 11,277-336, 1970.

- : Transition from staurolite to sillimanite zone, Rangeley Quadrangle, Maine. Geol. Soc. Amer. Bull. 85, 475-490, 1974.

GUILLEMOT, J., GUY, M. & LOBJOIT, M.: Un systeme coh6rent d'alignements structuraux commun aux Alpes et aux Pyr~n+es mis en 6vidence par le satellite ERTS I. C.R. Acad. Sci., 277 D, 481-484, 1973.

HOLDAWAY, M . J . : Stability of andalusite and the aluminium silicate phase diagram. Amer. J. Sci. 271, 97-131, 1971.

HOMEWOOD, P., Gosso, G., ESCHER, A. & MILNES, A.: Cretaceous and Tertiary evolution along the Besangon-Biella traverse (Western Alps). Eclogae Geol. Helv. 73, 635-649, 1980.

HOSCHEK, G.: Untersuchungen zum Stabilit~itsbereich von Chloritoid und Staurolith. Contr. Min. Petr. 14, 123-162, 1967.

- : Experimentelle Untersuchungen zum Schmelzverhalten von Biotit in Metamorphiten. Fortschr. Mineral. 52, Beih. 2, 26-27, 1974.

JONGMANNS, W.: Die Karbonflora der Schweiz. Beitr. Geol. Karte der Schweiz, N. F. 108, 1960.

KALSBEEK, F.: Petrology and structural geology of the Berlanche-Valloire area (Belle- donne-Massif, France). Diss. Leiden, 1-136, 1962.

2 8


KOSSMAT, F.: Gliederung des varistischen Gebirgsbaues. Abhandl. s/ichs, geol. Landesamt 1, 1-39, 1927.

KRZTZ, R.: Kinetics of the cristallization of garnet at two localities near Yellowknife. Canad. Mineralogist 12, 1-20, 1973.

LE FORT, P.: G6ologie du Haut Dauphin6 cristallin (Alpes Frangaises). Sciences de la Terre, M6m. 25, 1-375, 1973.

- & PECHER, A.: Pr6sentation d'un sch6ma structural du Haut Dauphin6 cristallin. C. R. Acad. Sci., 273 D, 3-5, 1971.

-, DEBON, F. & SONET, J.: The "Lesser Himalayan" cordierite granite belt, typology and age of the pluton of Manserah (Pakistan). Geol. Bull. Univ. Peshawar Vol. 13, 51-61, 1980.

LEMOINE, M.: Mesozoic evolution of the Western Alps: Tethyan Ocean and European pas- sive continental margin histories. Vortrag und Kurzfassung der 73. Jahrestagung der Geologischen Vereinigung in Berchtesgaden, 23. bis 26.2. 1983.

LI~GOIS, J. P. & DUCHESNE, J. C.: The Lac Cornu retrograded eclogites (Aiguilles Rouges Massif, Western Alps, France): evidence of crustal origine and metasomatic alteration. Lithos 14, 35-48, 1981.

MALARODA, R.: Osservazioni e considerazioni sulla tettonica del cristallino del massiccio del'Argentera (Alpi Marittime). Mem. Ist. Geol. Min. Univ. Padova 29, 1-20, 1975.

MATTE, Ph. & BURG, P.: Sutures, thrusts and nappes in the Varriscan arc of Northern Europe -: plate tectonic implications. In: Thrust and Nappe Tectonics, Geol. Soc. Of London, Spec. Publ. No. 9, 351-358, 1981.

MULLIS, J.: Growth conditions of quartz crystals from Val d'Illiez (Valais, Switzerland). Schweiz. Min. Petr. Mitt. 55, 419-429, 1975.

- : Das Wachstumsmilieu der Quarzkristalle vom Val d'Illiez (Wallis, Schweiz). Schweiz. Min. Petr. Mitt. 56, 219-268, 1976.

- : The system methane-water as a geologic thermometer and barometer from the external part of the Central Alps. Bull. Min6ral. 102, 526-536, 1979.

NIGGLI, E.; Metamorphic map of the Alps. Explanatory text. Subcomm. Cartogr. Meta- morphic belts of the World (Unesco, Paris), 181-242, 1978.

PECHER, A.: Etude p6trographique de la partie orientale du massif Ecrins-Pelvoux. Le socle ancien. Th6se 3eme cycle, Grenoble, 1-122, 1970.

- & VIALON, P.: Pr6sence de gneiss du ~ffaci6s granulite~ dans le noyau pr6cambrien du massif des Ecrins-Pelvoux. (Alpes du Dauphin6, France). C.R. Acad. Sci. 270 D, 666-668, 1971.

PIGAGE, L. C. & GREENWOOD, H. J.: Internally consistent estimates of pressure and temperature: the staurolite problem. Am. J. Sci. 282, 942-969, 1982.

POTY, B.: La croissance des cristaux de quartz dans les filons sur l'exemple du filon de La Gardette (Bourg d'Oisans) et des filons du Massif du Mont-Blanc. Sciences de la Terre, M6m. 17, 1-135, 1969.

-, STALDEg, H. A. & WE~SBROD, A. M.: Fluid inclusions studies in quartz from fissures of Western and Central Alps. Schweiz. Min. Petr. Mitt. 54, 717-752, 1974.

PRUNAC, M.: Analyse structurale dans le socle de l'Argentera-Mercantour. Th+se 3eme cycle, Orsay, 1-123, 1975.

RAMSAY, J. G.: Tectonics of the Helvetic Nappes. In: Thrust and Nappe Tectonics, The geological Society, 293-309, 1981.

VON RAU~4ER, J. F.: Kristallisation und Gefiigebildung im Mont-Blanc-Granit. Schweiz. Min. Petr. Mitt. 47, 499-580, 1967.

- : Stilpnomelan als alpinmetamorphes Produkt im Mont-Blanc-Granit. Contr. Mineral. Petrol. 21, 257-271, 1969.

- : Das Mont-Blanc-Massiv - Altkristallin im Bereich schwacher alpiner Metamorphose. Schweiz. Min. Petr. Mitt. 51, 193-225, 1971.

2 9


- : Zur Metamorphose amphibolitischer Gesteine im Altkristallin des Mont-Blanc- und des Aiguilles-Rouges-Massivs. Schweiz. Min. Petr. Mitt. 54, 471-488, 1974.

- : Variszikum in den Zentral- und Westalpen. Nova Acta Leopoldina N. F. 224, 119-144, 1976.

- : Le massif du Mont Blanc - socle pr6permien dans un cadre alpin. Bull. Soc. Frib. Sci. Nat. 65, 123-155, 1976.

- : Variscan events in the Alpine region. Geologie en Mijnbouw, 60, 76-80, 1981. - : Die Metapelite von Emosson (Aiguilles Rouges Massiv) als Beispiel spfitkaledonisch-

friihvariszischer Metamorphose im Altkristallin des Helvetischen Bereichs. Schweiz. Min. Petr. Mitt. 63, 2, 1983 (ira Druck).

- : Ein struktureller Vergleich praepermischer Altkristallinbereiche in den Aiguilles Rouges und im Mont-Blanc-Massiv (in prep.).

-, GALETTI, G. & PFEIFER, H. R. (1983): Zur Geochemie vorpermischer Metapelite im Be- reich von Emosson (Aiguilles Rouges Massiv) (in prep.).

-, SCHWANDER, H. & STERN, W.: Die chemische Zusammensetzung von AFM-Mineralen aus den vorpermischen Metapeliten des Aiguilles-Rouges-Massivs (in prep.).

RICHARDSON, S. W.: Staurolite stability in a part of the system Fe-AI-Si-O-H. J. Petrol. 9, 467-488, 1968.

- : The relation between a petrogenetic grid, facies series and the geothermal gradient in metamorphism. Fortsehr. Mineral. 47, 65-76, 1970.

-, GILBERT, M. C. & BELL, P. M.: Experimental determination of kyanite-andalusite and andalusite-sillimanite equilibria; the aluminium silicate triple point. Am. J. Sci. 267, 259-272, 1969.

RUTISHAUSER, H.: Die quantitative Erfassung von Migmatiten im AufschluBbereich. Schweiz. Min. Petr. Mitt. 53, 99-124, 1973.

- : Die historische Entwicklung der Ansicht fiber die Entstehung des Lauterbrunner Kri- stallins (Aarmassiv). Mitt. Natf. Ges. Bern, N. F. 30, 63-85, 1973.

- : Die Beziehungen zwischen dem Lauterbrunner Kristallin und dem Gastern-Granit. Schweiz. Min. Petr. Mitt. 53, 472-474, 1973.

- & HO~t, Th.: Der Kontakt zwischen Gastern-Granit und Lauterbrunner Kristallin im Gastern-Tal (Aarmassiv, Schweiz). Mitt. Natf. Ges. Bern N. F. 35, 1-53, t978.

ST~LLE, H.: Das mitteleuropfiische variszische Grundgebirge im Bilde des gesamteuro- p~iischen. Beih. Geol. Jahrb. 2, 1-138, 1951.

SUESS, E.: Das Antlitz der Erde, Wien 1988. THOMPSON, A. B.: Mineral reaction in pelitic rocks: I. Prediction of P-T-X (Fe-Mg) phase

relations. II. Calculation of some P-T-X (Fe-Mg) phase relations. Amer. J. Sci. 276, 401-424 and 425-454, 1976.

- : The pressure-temperature plane viewed by geophysicists and petrologists. Terra Co- gnita 1, 11-20, 1981.

- : Dehydration melting of pelitic rocks and the generation of H20-undersaturated granite liquids. Amer. J. Sci. 282, 1567-1595, 1982.

-, & TRACY, R.J.: Facies series melting and reactions in the system CaO + KA102 + NaA102 + A1203 + SiO2 + HzO. Contr. Min. Petr. 70, 429-438, 1979.

TOBI, A. C.: Petrographical and geological investigations in the Merdaret-Lac Crop region (Belledonne Massif, France). Leids Geol. Med. 24, 181-281, 1959.

TOLLMANN, A.: Grogr/iumiger variszischer Deckenbau im Moldanubikum und neue Gedanken zum Variszikum Europas. Geotektonische Forschungen 64, 1-91, 1982.

VOLL, G.: Recrystallization of quartz, biotite and feldspars from Erstfeld to the Leventina Nappe, Swiss Alps, and its geological significance. Schweiz. Min. Petr. Mitt. 56,641-647, 1976.

- : Deformation, crystallization and recrystallization. In Abstracts: The effect of deformation on rocks. International conference, G6ttingen, 9.-12. 4. 1980.

80


- : Deformation und Kristallisation wichtiger gesteinsbildender Minerale und ihre Aus- wirkung auf den GroBbau der Westalpen. Vortrag und Kurzfassung der 73. Jahresta- gung der Geologischen Vereinigung in Berchtesgaden, 23.-26.2. 1983.

WELLS, P.R.A.: P-T-conditions in the Moines of the Central Highlands, Scotland. Jour. Geol. Soc. 136, 6, 663-671, 1979.

- : Thermal models for the magmatic accretion and subsequent metamorphism of continental crust. Earth Planet. Sci. Lett. 46, 252-265, 1980.

YARDLEY, B. W. D.: The nature and significance of the mechanism of sillimanite growth in the Connemara schists, Ireland. Contr. Min. Petrol. 65, 53-58, 1977.

ZWART, H. J.: Metamorphic facies series in the european orogenic belts and their bearing on the cause of orogeny. Geol. Ass. Canada, Spec. Paper, 5, 7-16, 1969.

- & DORNSIEPEN, U. F.: The tectonic framework of Central- and Western Europe. Geol. Mijnbouw 57, 627-654, 1978.

, : The Variscan and pre-Variscan tectonic evolution of Central- and Western Europe: a tentative model. Int. Geol. Congress (Paris 1980) C6: Geologie de 1 'Europe, 226-232, 1980.

81

Documents

The External Massifs, relics of Variscan basement in the Alps