Indications of Variscan nappe tectonics in the Aar Massif

Autor(en): Oberhänsli, R. / Schenker, F. / Mercolli, I.

Objekttyp: Article

Zeitschrift: Schweizerische mineralogische und petrographische Mitteilungen = Bulletin suisse de minéralogie et pétrographie

Band (Jahr): 68 (1988)

Heft 3: Geodynamik des eurpäoschen Variszikums : Kaledonisch- Variszische Strukturen in den Alpen

PDF erstellt am: 05.10.2021

Persistenter Link: http://doi.org/10.5169/seals-52086

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http://www.e-periodica.ch SCHWEIZ. MINERAL. PETROGR. MITT. 68, 509-520, 1988

Indications of Variscan nappe tectonics in the Aar Massif

by R. Oberhänsli1, F. Schenker2and I. Mercolli1

Abstract

In the Aar Massif a reinvestigation of the so called "quartzporphyre" dikes, the widespread sericite schists, and the psammitic layers occurring together with these rocks, show their vast extension within the complex. Many of these rock sequences have been recognized to represent meta-sedimentary volcanoclastic series and some of them have been dated as Carboniferous (Westphalian D-Stephanian). The Upper Paleozoic igneous rocks, volcanic and plutonic, belong to a calcalkaline suite and therefore relate to subduction processes. The deformation and the fact that these volcanoclastic sequences show contact metamorphic overprint due to the intrusion of the Upper Paleozoic granites can be explained by a compressional synsedimentary tectonic regime. While the molasse type volcanoclastic series were deposited, deeper parts of the same intramontane basins hade already been wedged into the basement, where they were intruded by syngenetic granites. This gives ample evidence for Variscan faulting and nappe tectonics.

Keywords: Aar massif, volcano-sedimentary sequences, compressive tectonics, Variscan nappes, Variscan basement.

Introduction (1941); Hügly (1927); Huttenlocher (1921, 1933, 1947); Jenny (1973); Koenigsberger Up to now the Aar massif has been (1910); Labhart (1965, 1966, 1977); Liechtli interpreted as a section of Variscan basement which (1933); Pflugshaupt (1927); Schmidt (1886, has undergone Alpine metamorphism and 1891); Sigrist (1947); Stalder (1964); Staub deformation. Therefore attention was paid (1911); Steck (1966, 1968, 1984); Weber (1912, primarily to the deformation and recrystallization 1924); Wehrli (1896); Widmer (1949); Wyss history due to Alpine orogenesis. The Aar massif (1932); Zbinden (1949). However, a geologic has been subdivided into different Variscan and magmatologic interpretation as well as the batholiths and into several pre-Variscan po- relationship to the large granitic plutons had lymetamorphic basement complexes. From the not been worked out. margins of the massif fossiliferous strata have Today, modern petrogenetic theory allows been dated as Carboniferous (Rothpletz, an interpretation of the paleotectonic regime of 1880; Wehrli, 1925; Corsin, 1946; Jongmans, an area from its magmatic assemblages. With 1951,1960; Taylor, 1976). this concept in mind, new emphasis has been Volcanic and subvolcanic rocks as well as given to the volcanoclastic sequences volcano-sedimentary sequences had already (Schenker, 1980, 1986, 1987; Schenker und been recognized and mapped by many geologists: Abrecht, 1987; Böhm, 1986, 1988 this volume; Baer (1959); Baltzer (1988); Brückner Riesen, Gnos, Vögeli, Dollinger, work in (1943); Kajel (1973); Fellenberg (1893); progress), to plutonic rocks (Abrecht, 1975, Fischer (1905); Franks (1966, 1968a, b); Heim 1980; Kupfer, 1977; Oberhänsli, 1985, 1987a, (1878, 1891); Huber (1948); Hugi (1923); HÜGI b; Seemann, 1975; Schaltegger, 1984, 1986)

1 Mineralogisch petrographisches Institut, Universität Bern, Baltzerstr. 1, CH - 3012 Bern. 2 GEMAG AG, CH-6248 Alberswil. 510 OBERHÄNSLI, R., F. SCHENKER AND I. MERCOLLI and to the pre-Alpine deformation (Kammer, Surface character of the studied rock sequences 1985) in the eastern and central Aar massif. The aim of this study is to present a new Because of the metamorphic overprint the working hypothesis on nappe tectonics based original volcanoclastic nature of many sericite on a reinvestigation of the volcanic and volca- schists in the Aar massif is still a point of noclastic sequences in the Aar massif. A map debate, although an intrusive origin has been emphasizing the Upper Paleozoic volcanics suggested by former workers (cit. see p. Therefore, and related sediments and the hitherto emerging the arguments for the surface character of ideas of Paleozoic tectonic events are these rocks shall be summarized: presented. - interlayering of pelitic, psammitic and Three major observations emerge from conglomeratic rocks, often of epiclastic origin, more recent investigations. with intermediate to acidic meta-volcanics. preserved sedimentary contacts of meta- 1) Franks b) two - (1968a, recognized volcanics and meta-volcanoclastics with unconformable volcanoclastic sequences in the eroded basement gneisses. eastern Aar massif. The older volcanoclastic relics of pyroclastic textures, including shows a deformation and - sequence strong strata. and is lapilli bearing metamorphic overprint of probable horizons Lower Carboniferous This is - ignimbritic ("quartzporphyric age. sequence rocks") with preserved original structures like overlain by a younger volcanic sequence fiamme, glass shards, quartz shards (disintegrated, which has been dated with plant remains as lobate, concave-shaped Westphalian D-Stephanian quartz (Rothpletz, fragments). 1880; Jongmans, 1951, 1960) and is Since for a tectonic reconstruction the covered, with a marked unconformity, by surface character of the studied rocks is of parautochthonous Mesozoic rocks. fundamental importance, in the following, we 2) In contrast to the established opinion, strictly refer to sequences which undoubtfully Schenker (1986; Schenker und Abrecht, show surface characteristics. 1987) demonstrated that the major pluton of the Central Aar granite (281 ma Rb/Sr, Wütrich, 1965; Schaltegger, 1986) is Tectonic map of the Aar massif intrusive into, and therefore younger than some of the volcanoclastic sequences. This In the eastern and central parts of the Aar is indicated by contact metamorphic phenomena. massif volcanoclastic sequences are relatively well known and widespread. In the western part many of the so called "quartzporphyric 3) The majority of the Upper Paleozoic mag- dikes" have been identified as volcanoclastic matic rocks from the Aar massif belong to a sequences. They are shown in a simplified calcalkaline rock series (Schenker, 1980, tectonic (Fig. 1 ; based on previous investigations: 1986; Böhm, 1986,1988 this volume). map Oberhänsli, 1985, Schenker, 1986) These evidences led to the idea that the Aar together with major Alpine and Variscan massif was generated in a destructive continental lineaments (Kammer, 1985). margin, linked with subduction processes Within the gneissic basement an alignment and underwent compressional tectonics rather of amphibolites and serpentinites representing than horst and graben tectonics during the pre-Variscan (Caledonian?) protoliths (possibly Upper Paleozoic. An approach to this problem ophiolitic) is evident. The outlines of the is the réévaluation of the volcanoclastic Upper Paleozoic volcanoclastic and plutonic sequences, which represent Paleozoic surfaces. rocks (281 ma, Wüthrich, 1965; Schaltegger, In analogy to the Alpine Pennine nappe 1986) as well as the Mesozoic sediment wedges system, where even minor and extremely thinned are mostly parallel to the pre-Variscan directions. and deformed Mesozoic carbonaceous rocks Generally where outcrop conditions are are taken as Alpine nappe separators (over- favourable, the northern contacts of the thrust horizons), the Paleozoic volcanoclastic volcanoclastic sequences with the basement gneisses sediments were taken to subdivide structural are observed to be primary (Trift, Diechterg- units within the Aar massif. letscher; Fig. 1, Tr, Di). The southern contacts VARISCAN NAPPES IN THE AAR MASSIF 511

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S-- fcH *fl3 cd cd CA 2 00 *5 ^« °ê fté o 'S g" o Ë u .2 (2 5 'Ë - > S? bb ùs > 512 OBERHÄNSLI, R., F. SCHENKER AND I. MERCOLLI to the basement gneisses are always of tectonic lites. These meta-sediments contain lithic nature. The volcanoclastic sequences, therefore, components of the surrounding basement. Some of can be taken as indicators of Variscan the meta-pelites show contact metamorphic dislocation zones, represented in Fig. 1 as Variscan overprint and form "Knotenschiefer" and chi- thrusts. The deformation zones indicated astolite-schists. This metamorphism seems to as Alpine thrusts are marked by Mesozoic be related to the intrusion of Upper Paleozoic sediment wedges and can be followed as mylonitic granitoids, as delineated by crosscutting grano- zones into the basement gneisses. The main dioritic dikes (Böhm, 1986; 1988 this volume). Variscan and Alpine thrusts are oriented sub- Volcanoclastic rocks and welded tuffs are parallel and spatially follow each other closely. intercalated in these meta-sediments. This typical In the Furtwangsattel area (Fig. 1, F), however, volcanic sequence (conglomeratic beds, rhyo- the Variscan and Alpine trends diverge and the lites) occur repeatedly (Böhm, 1986; 1988 this Alpine deformation zone splits off into the volume). The chemical composition of the basement gneisses. In this area, definite distinction metamorphosed volcanic extrusive rocks is calc- between Variscan and Alpine structures is alkaline and varies from dacitic to rhyodacitic possible. (Böhm, 1986; 1988 this volume). Layering of In the north one major volcanoclastic the meta-sedimentary and meta-volcanic complex sequence, together with Mesozoic sediments, delineates a megascopic fold with a steeply roots in the basement gneisses. In its eastern plunging foldaxis, indicating Hercynian part this sequence is thrust northwards over the directions. parautochthonous Mesozoic cover to form the The younger Sandpass-Formation (Fig. 1, Alpine Windgällen fold (Fig. 1, Wi). A second Sp), described by Böhm (1986; 1988 this southerly lying volcano-sedimentary sequence volume) belongs to the Permian Verrucano-se- shows complex folding (Fig. 1, Ts) together quences and is composed of green and black with the enclosing basement gneisses. Along schists of volcanic origin. These rocks, in part this structure small dioritic intrusive bodies strongly mylonitized, contain pebbles of the occur. In the southeastern part the intrusive bodies crystalline basement gneisses and plutonic forming the Central Aar batholith obliterate rocks. The chemical composition of the the relationships between several other volcanic rocks is rhyolitic, with a slight enrichment volcanoclastic sequences cropping out at the eastern in potassium (Böhm, 1986; 1988 this (Fig. 1, Ab, Oa) and western (Fig. 1, Vg) ends volume) and therefore, differs from the older of the Aar massif. typically calc-alkaline volcanics. They could be termed G-type rhyolites after Izett (1981). This formation is conformably overlain by the base Volcanoclastic formations of parautochthonous Triassic rocks. Böhm considers the "Klein Tödi-Formation" of Franks The different volcanoclastic formations are (1968a) to be identical to the Sandpass-Formation. heterochronous and differ in their lithostrati- graphic compositions as well as their contact relationships. EASTERN AAR MASSIF

SOUTHEASTERN AAR MASSIF From the eastern part of the Aar massif Franks (1968a, b) described four formations, In the southeastern Aar massif, Böhm (1986; which are partly meta-volcanic. 1988 this volume) distinguishes two volcanoclastic Franks (1968b) defined the Bifertenfirn- sequences of differing age. The older Formation (Fig. 1, Bf) as a meta-sedimentary one, the Val Gliems Formation (Fig. 1, Vg) consists sequence of hornfelses derived from argillaceous of meta-sediments and meta-volcanoclas- siltstones and fine grained sandstones. tic rocks. The meta-sediments can be subdivided Banded hornfelses show poikiloblastic flakes into 1) banded psammitic schists, of muscovite and quartz phenocrysts. This derived from sandy turbiditic protoliths and 2) formation contains spotted slates ("Knotenschiefer"; black pelitic schists representing metamorphosed "Knoten" consist of muscovite, equivalents of marine or limnic argil- quartz and opaques), which are products of VARISCAN NAPPES IN THE AAR MASSIF 513

contact metamorphism due to the intrusion of tion is cut by the younger Aar granite and the the Tödigranite (Hügi, 1941). Dioritic dikes tuffaceous strata contain chlorite and biotite, crosscut the meta-sediments. Franks correlated possibly produced through contact metamorphism. these rocks with the Val Gliems-Forma- tion. However, he did not recognize the volcanic/volcanoclastic character of some of the layers neither in the Biferten- nor in the Val CENTRAL AAR MASSIF Gliems-Formation. The Biferten-Formation (Fig. 1, Bg) of From the central as well as the eastern Aar Franks (1968a) is divided into three members. massif Schenker (1986) has described several The lowermost Volcanic member contains a sequences of sedimentary and volcanic rocks. basal conglomerate with reworked metamor- The Windgällen-Formation (Fig. 1, Wi; phic rocks from the uncomformably underlying Fig. 2) after Schenker (1986) consists of a Bifertenfirn-Formation. Above the basal porphyric microgranite, which intrudes the conglomerate, epiclastic and rarely pyroclastic overlaying sequence of ignimbritic, calcalkaline material was deposited: e. g. volcanic breccias, rhyolites, pyroclastic tuffs and epiclastic tuffaceous sandstones, lithic tuffs, crystal tuffs sediments (lahars, pelitic schists). It is interpreted and ashes. In the epiclastic breccias andesitic, as subvolcanic equivalent to the rhyolites. Dogger dactitic and rhyodacitic components occur. sediments transgressively overlay the The volcanic member is overlain by the Estua- volcanoclastic sequence with a marked unconformity. rian member consisting of conglomerates, The volcanoclastic rocks are only weakly sandstones and mudstones. The latter containing metamorphosed and exhibit ample primary the plant bearing beds dated as Westphal- structures, such as cooling columns, fluidal ian D-Stephanian (Rothpletz, 1880; Jong- textures and flamme. This formation is thrust mans, 1960). At the top of the formation, the over the parautochthonous sediments in a Lacustrine member is composed of mudstones, mega fold and roots together with Mesozoic siltstones and sandstones. rocks in the basement gneisses of the Madera- The Klein Tödi-Formation (Fig. 1, Kt) nertal. underlies the Mesozoic rocks in the Sandpass area The Trift-Formation (Fig. 1, Tr; Fig. 3) can (Franks, 1968a) and contains coarse epiclastic be considered as an extension of the breccias with reworked basement gneisses, Windgällen-Formation (Schenker, 1986). The Mesozoic granites and granodiorites alternating with rocks are absent. In the area of the Klein lapilli tuffs and banded rhyolites. The volcanic Griessenhorn, (Furtwangsattel, Fig. 1, F) Var- sequence contains porphyritic microdiorites iscan and Alpine lineaments diverge. The and is cut by acid porphyric dikes. East of the Upper Paleozoic volcanoclastic sequence and Sandpass the formation changes its character the Variscan thrusts turn towards southwest. and pyroclastic rocks and bedded tuffs The Alpine thrust can be traced further predominate. In epiclastic sequences, metamorphosed westward. The Trift-Formation is composed of rocks similar to the Val Gliems-Forma- ignimbritic and epiclastic volcanic series as well tion or Bifertenfirn-Formation occur. as clastic, conglomeratic sediments. The meta- The Tscharren-Formation (Fig. 1, Ts) after rhyolites are strongly deformed but generally Franks (1968a) can be divided into three units. not mylonitized. In the north the meta-volcanic A tuffaceous member with dark vitric and lithic sequence overlies the basement with a sharp tuffs, a conglomeratic member and an ignim- contact, considered to be sedimentary, whereas britic member with light coloured acid tuffs the southern contact is marked by a thin my- and ignimbrites. The first unit consists of lonitic zone. The volcanic components show discontinuous beds of black carbonaceous dacite to rhyolitic compositions belonging to a mudstones and siltstones intercalated with banded calcalkaline suite. tuffs, crystal tuffs and lapilli tuffs. The The Diechtergletscher-Formation (Fig. 1, Di; conglomerates and breccias are composed of Fig. 4) can be divided into a pyroclastic and an fragments of the surrounding beds and represent epiclastic unit (Schenker, 1986). The epiclastic epiclastic deposits. The ignimbritic member unit contains meta-greywackes and fanglome- consists of rhyolitic flows, interbedded with ratic, argilleous meta-sandstones which are tuffs and sediments. In some places this forma¬ intercalated with pyroclastic meta-volcanics. The 514 OBERHÄNSLI, R., F. SCHENKER AND I. MERCOLLI

Windgällen formation oolithic marly limestone with chamositic and hematitic ooids; "Eisenoolith" DOGGER spathic limestone with debris consisting of 200m T: echinodermata, dolomite, quartz and volcanoclastics clayey-calcareous quartzsandstones with dolomitic and volcanoclastic components; unconformable transgression with paleosol

Windgällen formation: alternance of ignimbritic rhyolites and pyroclastic tuffs with interbedded epiclastic sediments CARBONI (lahars, tuffites argillites) FEROUS 0m Porphyrie microgranite, biotite-free; irregular contacts to the overlying ignimbritic rhyolites Fig. 2 Idealized section of the Windgallen formation. Location of the base of this section: 698.080/184.240/2710.

Trift formation basement: amphibolites and migmatites

220m tectonic contact: mylonites interbedding of meta conglomerates and meta-greywackes

meta pyroclastites: massive, fine grained, with flamme up to 20 cm, rich in phenocrysts

conglomeratic coarse grained meta-sediments (Trift hut, SAC) » • o O meta rhyolite: light green grey, porphyric C Q - o o alternance of fine to coarse grained laminated epiclastic meta sediments and foliated flamme rich meta pyroclastites

meta rhyolite: light grey, foliated with sharp contact at the base 0m gneiss: granitic, leucocratic, schistose, strongly chloritized and sericitized

migmatic gneisses: with calcsilicates, microcline pegmatites and ultramafic xenoliths, locally dioritic, granodioritic

Fig 3 Idealized section (profile) of the Trift formation modified after Schenker and Abrecht (1987). Location of the base of this section: 671.500/170.075/2300. VARISCAN NAPPES IN THE AAR MASSIF 515

Diechtergletscher formation basement: migmatic biotite-plagioclase gneisses with banded "Schollen" amphibolites, ultramafic lenses and microcline pegmatites, weakly deformed basaltic to andesitic dikes, contact to the Diechter formation partly mylonitic, in places primary, sedimentary (no paleosol) 200m Diechtergletscher formation: pyroclastic sequence: light brown grey foliated ignimbritic lapilli- and crystal tuffs, rhyolitic to rhyodacitic; K> phenocrystals: corroded quartz, hypidiomorphic V K-feldspar, seritized albite, biotite-opaque " pseudomorphs after amphibole or pyroxene; * » matrix: felsitic with relictic pseudofluidal textures v V 100m epiclastic sequence: dark green grey, strongly deformed (isoclinal folds) alternance of thin fanglomeratic, sandy, silty meta-sediments and pyroclastic meta-volcanics; fanglomerate components: 80% pyroclastites and intraclasts, 20% basement and microcline pegmatites; metamorphic parageneses: sericite-biotite-epidote, andalusite-white mica-biotite-K feldspar-albite, almandine-biotite-quartz Om Aar granite: weakly foliated hololeucocratic to leucocratic, fine to medium grained granite with porphyric portions; contact: primary magmatic, intrusive, subparallel to the foliation of the metasediments, a^2H2i^22^^2S====n===== Fig. 4 Idealized cross section (N-S) of the Diechtergletscher formation modified after Schenker and Abrecht (1987). 1. pre-Variscan amphibolite-facies metamorphism with subsequent anatexis; 2. syn-volcanic hydrothermal alteration; 3. syn-granitic contact metamorphism; 4. Alpine greenschist facies metamorphism; Location of the base of this section: 669.175/166.950/2690.

pyroclastic unit is built up by ignimbritic crystal Regional comparison tuffs and lapilli tuffs. Additionally shoshon- itic andésites forming subvolcanic bodies Despite the lithological similarities in the (?necks) and dikes are observed. Rocks of this formations described above, three types can be formation are strongly deformed and isocli- soundly distinguished based on lithology, age nally folded. The northern contacts to gneisses and structure: and amphibolites of the basement are partly 1) Units with volcanic rocks of intermediate to stratigraphie. The southern contact is intrusive: acid calc-alkaline compositions; their the Aar granite cutting the meta-sedimentary associated conglomerates contain basement unit produced stoping phenomena and developed detritus; intense isoclinal folding has affected a contact aureole (Schenker, 1986; these units. Schenker and Abrecht, 1987). The effects of 2) Units with volcanic rocks consisting of calc- contact metamorphism are recorded in pelitic alkaline ignimbritic lavas and tuffs; the layers by garnet-biotite as well as andalusite- epiclastic sediments contain few or no basement muscovite-biotite parageneses. material; these units may unconform- 516 OBERHÀNSLI, R., F. SCHENKER AND I. MERCOLLI

Tab. 1 Comparison and compilation of different volcano-sedimentary sequences in the Aar massif.

FRANKS SCHENKER BÖHM THIS WORK (1968) (1986) (1986)

Mesozoic

Permian \^"V V V Sandpass F. Sandpass F.

Bifertengrätli F. Windgällen F. Windgällen F. Tscharren F. Trift F. Sandalp Sandalp F. Bifertengrätli F. Windgällen F. Tscharren F. Westphalian D MADERANER GROUP Kleintödi F. Brunnital Oberaar 2 Ausserberg Tscharren F. Diechtergl. F. \/ \/ \/ Bifertenfirn F. Val\/Gliems F. Bifertenfirn F. pre Westphalian Val Gliems F. Gliems F. Val CAVADIRAS Diechtergl. F. GROUP Oberaar 1

ably top and underlay the former units and rhyolitic volcanoclastic rocks underlying Tri- the Mesozoic respectively. assic sediments in this area. Their stratigraphie 3) Units revealing no or a minor hiatus at the position and lithology show affinities to the Paleozoic-Mesozoic transition; volcanic Sandpass-Formation. From the Brunnital rocks are mildly alkali-rhyolitic (G-type). transsect several intercalations of volcanoclastic These units could correspond to the Permian sediments of the Tscharren type have been Verrucano spilite and keratophyre vol- identified by Riesen and Gnos (diploma work canics. in progress, Oberhänsli et al., 1987). Based on their lithological aspects, associations and deformations, the Upper Paleozoic formations can be grouped as follows (Tab. 1): CENTRAL AAR MASSIF We propose Cavadiras Group for the intensely folded, basement component bearing, From the Grimsel area near to the Oberaar intermediate to acid and to our opinion older power-dam (Fig. 1, Oa), two volcanic formations and Maderaner Group for the less sequences can be observed over a short distance deformed, mainly rhyolitic younger formations (Dollinger, diploma work in progress). One is devoid of basement components. isoclinally folded (Tab. 1, Oberaar 1), shows We shall compare further volcanic chlorite- and sericite-rich schists and can be sequences found elsewhere in the Aar massif to interpreted as corresponding to the Bifertenfirn/ the three types of formations discussed above. Val Gliems type. The other (Tab. 1, Oberaar 2), less deformed ignimbritic sequence is equivalent to the younger formations (Windgällen, EASTERN AAR MASSIF Tscharren).

North of Disentis Weber (1924) mapped WESTERN AARMASSIF "quartzporphyres". A closer investigation showed that they can be interpreted as volca- From the Lötschenpass area a volcanic noclastic sequences folded into the basement sequence intruded by the Gasterngranite has revealing affinities to the Windgällen- or been identified by Zaugg and Arnold (pers. Tscharren-Formation. Furthermore, we found comm). VARISCAN NAPPES IN THE AAR MASSIF 517

The "quartzporphyres" of Ausserberg pine compression, which incorporated the par- (Fig. 1, Ab) have already been compared to the autochthonous Mesozoic cover along deep Tscharren-Formation by Huttenlocher in reaching south vergent thrust planes. Thus the 1933; an observation which we can fully original vergence and inclination of the Variscan confirm. nappes can not yet be established soundly. In the Variscan belt in Central Europe which was not affected by later Alpine tectonic Paleotectonic frame events, nappe transport directions are to the north (Behr et al., 1984). We therefore infer Indications of a compressional tectonic similar transport directions for the Variscan setting during Variscan orogeny of the Central nappes in the Aar massif. Alps can be deduced from Franks work (1968a, b). Comparing the different lithostra- tigraphic volcanoclastic sequences from Lower Variscan nappe structures Carboniferous to Permian, he distinguished two pre-Alpine (pre-Mesozoic) unconformities To "individualize" Variscan nappes, we between Bifertenfirn-, Bifertengrätli-Forma- propose a speculative solution by combining tions and the Mesozoic sedimentary cover. the different supracrustal sequences as given in Further arguments for compressional tectonics Fig. 5. Three prominent zones of volcanoclastic are: sediments and pre-Alpine deformation zones 1) Thrust-faults (Fig. 1) which separate differ¬ can be correlated (1, 2, 3; Fig. 5). Several ent parts of the basement (Kammer, 1985). smaller lineaments are considered to represent 2) Strong isoclinal folding in the older pre- minor deformation zones (la, 2a, b, 3a, b, c; Westphalian formations and less intensive Fig. 5). In the central parts of the Aar massif folding in the uppermost Carboniferous the thrusts are crosscut by the Upper Paleozoic formations, indicating a continuous shortening plutons. While in the northern part of the Aar of the crustal segment connected with massif the Variscan structures can be igneous activity. distinguished from the Alpine structures, in the 3) Surface rocks must have been brought to a southern part the increasing intensity of Alpine considerable depth in order to be intruded deformation obliterates their distinction. and metamorphosed by the syngenetic Aar Despite the Alpine overprint, Variscan sutures granite. delineated in some parts by volcano-sedimentary 4) The calcalkaline chemistry of the igneous sequences, can be followed in the basement rocks suggests crustal thickening. gneisses. The old volcanic sequences today lie sub- The three major alignments (1, 2, 3; Fig. 5) vertically or show a slight southward vergence. subdivide four portions of the Aarmassiv. This position is doubtless due to the later Al¬ "Nappe 1", the northernmost portion consists

Nappe 1 f~l ] Nappe 2 t À Nappe 3 I \ I Nappe 4

Fig. 5 Interpretative correlation of Upper Paleozoic surface rocks to delineate four possible Variscan nappes. 518 OBERHÄNSLI, R., F. SCHENKER AND I. MERCOLLI

of the anatectic Lauterbrunnen-Innertkirchen The striking feature of parallelism of Kim- complex and the Gasterngranite (Fig. 1) and meric and Alpidic structures throughout the contains very little basement gneisses. Alpine orogenic belt from the Alps to Asia has "Nappe 2" contains the biotite-plagioclase been discussed by Sengör (1985). A similar gneisses and magmatites of the Erstfelder- pattern of parallelism of Caledonian, Variscan gneiss Zone, the Guttannen unit (Abrecht and and Alpine structures can be found within a Schaltegger, 1988) and the museovite chlorite small basement complex, the Aar massif. schists and gneisses in the northern part of the Lötschental. Lenses of calcsilicate felses, garnet amphibolites, banded amphibolites with Acknowledgements muscovite-chlorite layers are common (Hügi et al., 1985). We thank Ernst Niggli and Hans Anton Stalder for their ceaseless to restudy the Aar "Nappe 3" is composed of biotite-plagioclase encouragement massif. J. Abrecht, E. H. Brown and V. D. Dietrich gneisses of the Stampfhorn-Ofenhorn helped with critical comments. unit (Abrecht and Schaltegger, 1988) which contains conspicuous meta-ultramafic lenses and microcline pegmatites. Biotite-sericrite gneisses with intercalations of meta-pelitic References schists as well as chlorite schists and gneisses J. Die Granit-Kontaktzone und die associated Abrecht, (1975): are with dioritic amphibolites and migmatischen Gesteine im Gebiet des Gruben- amphibolite-rich migmatites (Schollenamphi- und Aerlengletschers (Haslital, Schweiz). Ph. D. bolite). thesis Univ. Bern, unpubl., 158 p. "Nappe 4" is essentially composed of the Abrecht, J. (1980): Zur Bildung migmatitischer aus dem Altkristallin des and Schollenamphibolite augengneisses, biotite gneisses sericite Aarmassivs. Geol. Rdsch. 69,695-725. schists of the southern Aarmassiv. Within this Abrecht, J. and Schaltegger, U. (1988): Aplitic in¬ section of the basement syenitic to monzonitic trusions in the Central Aar massif basement: rocks occur. All these rocks are strongly Geology, petrography and Rb/Sr data. Eclogae geol. Helv 81/1,277-239. foliated and show Alpine metamorphic overprint. Baltzer, A. (1888): Das Aarmassiv (mittlerer Teil) nebst einem Abschnitt des Gotthardmassivs; Beitr. Geol. Karte Schweiz 24/IV. Conclusions Behr, H., Engel, W., Franke, W., Giese, P. and Weber, K. (1984): The Variscan belt in Central Europe: main structures, geodynamic implications, During Upper Paleozoic the Aar massif open questions; Tectonophysics, 109, formed as a part of a Cordillera at the southern 15-40. margin of the proto-European continent (Mer- Böhm, C. (1986): Petrographische Untersuchungen im Gebiet Val Gliems-Val Russein, Graubünden; and 1988 this colli Oberhänsli, volume). In Diploma thesis, Univ. Bern, unpubl. the Aar massif, epiclastic sediments; coarse Böhm, C. (1988): Vulkanite im östlichen Aarmassiv grained alluvial deposits, lahars, and lacustrine (Val Russein, Graubünden). Schweiz, mineral, sediments are intercalated with intermediate to petrogr. Mitt., 3/68, 501-508. CoRSiN, P. (1946): Sur de la flore carbonifère du acid volcanics. The volcanoclastic l'âge sequences massif du Tödi (Suisse). C.r. Soc. géol. Fr., 14. were deposited into intramontane basins during Fellenberg, E. von, Mösch, C. und Schmidt, C. the Upper Paleozoic. These molasse type (1893): Geologische Beschreibung des westlichen basins were continuously deformed and over- Teils des Aarmassivs enthalten auf dem nördlichen der Rhone gelegenen Teil des Blattes thrusted basement sedimentary by wedges. Their XVIII der Dufourkarte. Beitr. geol. Karte infill was draged to depth, folded and Schweiz 21. subsequently intruded by granitic rocks. The calc- Fischer, O. (1905): Über einige Intrusivgesteine der Schieferzone des alkaline compositions of volcanic as well as am Nordrand zentralen Granites aus der Umgebung der Sustenhörner, rocks indicate a plutonic generation by Tscherm. mineral, petrogr. Mitt., 24,1-68. subduction related processes. This implies convergent Franks, G. (1966): The development of the limnic plate boundaries and therefore a compres- Upper Carboniferous in the eastern Aar massif. sional tectonic regime. The fact that supracrus- Ecolgae geol. Helv., 59,943-950. G. (1968a): A of Paleozoic sedi¬ tal rocks between diverse basement Franks, study upper occur ments and volcanics in the northern part of the blocks suggests major thrust faults and nappe eastern Aar massif. Eclogae geol. Helv., 61, structures. 49-140. VARISCAN NAPPES IN THE AAR MASSIF 519

Franks, G. (1968b): The pre-Westphalian (Hercyn- Karte Schweiz, NF 124. ian) metamorphism and structures of the Tödi Labhart, T. (1966): Mehrphasige alpine Tektonik Area (Aar massif)- Schweiz, mineral, petrogr. am Nordrand des Aarmassivs. Eclogae geol. Mitt., 48, 667-694. Helv., 59, 803-830. Heim, A. (1878): Untersuchungen über den Mechanismus Labhart, T. (1977): Aarmassiv und Gotthardmassiv. der Gebirgsbildung im Anschluss an die Gebr. Bornträger, Berlin-Stuttgart, Sammlung geolog. Monographie der Tödi-Windgällen- geol. Führer, 63, 173 p. Gruppe, Basel. Liechti, H. (1937): Recherches pétrographiques et Heim, A. (1891): Geologie der Hochalpen zwischen tectoniques dans la vallée de Göschenen. Reuss und Rhein; Beitr. geol. Karte Schweiz 25. Schweiz, mineral, petrogr. Mitt., 13,491-563. Huber, W. (1948): Petrographisch-mineralogische Mercolli, I. and Oberhänsli, R. (1988): Variscan Untersuchungen im südöstlichen Aarmassiv. tectonic evolution in the Central Alps: A working Schweiz, mineral, petrogr. Mitt., 28, 555-642. hypothesis. Schweiz, mineral, petrogr. Mitt., Hugi, E. (1923): Über einige Gesteins- und Mineral¬ 3/68,491-500. vorkommnisse der Wasserstollen des Kraftwerks Oberhänsli, R. (1985): Mineralogy and Geochemistry Amsteg (Uri). Schweiz, mineral, petrogr. Mitt., 3, of meta-lamprophyres from the Central Swiss 263-297. Alps; Habilitationsschrift, unpubl. Huttenlocher, H. (1921): Beiträge zur Pétrographie Oberhänsli, R. (1986): Geochemistry of meta-lam¬ und Geologie des westlichen Aarmassivs. Jb. phil. prophyres from the Central Swiss Alps; Schweiz, Fak. II, Univ. Bern, 1,43-51. mineral, peotrg. Mitt., 66, 315-342. Huttenlocher, H. (1933): Ergänzende Bemer¬ Oberhänsli, R. (1987): Mineralogy and Alpine kungen zur Geologie und Pétrographie des metamorphism of meta-lamprophyres from the Central südwestlichen Aarmassivs. Schweiz, mineral, Swiss Alps. Schweiz, mineral, petrogr. Mitt., petrogr. Mitt., 13, 281-291. 67,321-338. Huttenlocher, H. (1947): Über Verschiedenheiten Oberhänsli, R., Riesen, D., Gnos, E. and Mercolli, im Verlaufe magmatischer und metamorpher I. (1987): Bericht über die Exkursion der SMPG Prozesse, erläutert an Beispielen aus dem im Brunnital 6./7. Oktober 1987. Schweiz, Aarmassiv. Mitt. natf. Ges. Bern, N.F. 4, 85-116. mineral. petrogr. Mitt., 67, 399-403. Hügi, T. (1941): Zur Pétrographie des östlichen Aar¬ Pflugshaupt, P. (1927): Beiträge zur Pétrographie massivs und des Kristallins von Tamins. Schweiz, des östlichen Aarmassivs. Schweiz, mineral, mineral, petrogr. Mitt., 21, 1-120. petrogr. Mitt., 7, 321-378. Hügi, T., Ledermann, H., Schläppi, E., Collet, L., Rothpletz, A. (1880): Die Steinkohlenformation Frey, M. und Parejas, E. (1985): Blatt Lötschen- und deren Flora an der Ostseite des Tödi. tal Nr. 1268, 1:25 000. Geol. Atlas der Schweiz. Schweiz, paläont. Abh., 6. Hügly, W. (1927): Petrographisch-geologische Schaltegger, U. (1984): Geologie und Pétrographie Untersuchungen im östlichen Aarmassiv. Jb. phil. der Gneiszone von Erstfeld in der Umgebung des Fak. Bern, 7,91-97. Sustenpasses, Aarmassiv. Diploma thesis, Univ. Izett, G. (1981): Volcanic ash beds: recorders of Bern, unpubl. Upper Cenozoic silicic pyroclastic volcanism in Schaltegger, U. (1986): Geochemie und Rb-Sr-Sy- the Western United States. J. geophys. Res., 86, stematik der Aarmassiv-Granite zwischen Grim- 10200-10222. sel und Reusstal. Schweiz, mineral, petrogr. Mitt., Jenni, J. (1973): Die Vorkommen von Bleiglanz, 66,462-466. Kupferkies und Zinkblende des Bristenstocks Schenker, F. und Abrecht, J. (1987): Prä-aargraniti- (Kt. Uri). Beitr. geol. Karte Schweiz, geotechn. sche Anatexis, variszische Kontaktmetamorphose Serie, 53. und alpidische Regionalmetamorphose im Jongmans, W. (1951): Mitteilungen zur Karbonflora Oberhasli (zentrales Aarmassiv, Schweiz). der Schweiz. Eclogae geol. Helv., 43,95-104. Schweiz, "mineral, petrogr. Mitt., 67, 13—26. Jongmans, W. (1960): Die Karbonflora der Schweiz. Schenker, F. (1980): Geologische und mineralogi¬ Beitr. geol. Karte Schweiz, NF 108. sche Untersuchungen im Gebiet der Grossen und Kajel, L. (1973): Geologisch-mineralogische Unter¬ Kleinen Windgälle, Maderanertal, Kt. Uri; suchungen an Karbonvorkommen im westlichen Diploma thesis Univ. Bern, unpubl. und nördlichen Aarmassiv. Ph. D. thesis, Univ. Schenker, F. (1986): Spätpaläozoischer saurer Mag- Bern, unpubl. matismus und Beckenbildung im Aarmassiv unter Kammer, A. (1985): Bau und Struktur des nördlichen kompressiver Tektonik. Ph. D. thesis, Univ. Aarmassivs und seiner Sedimente zwischen dem Bern, unpubl. Sustenpass und Grindelwald. Ph. D. thesis, Univ. Schenker, F. (1987): Hinweise für kompressive Neuchätel, unpubl. Tektonik während der Ablagerung von oberpaläozoischen Koenigsberger, J. (1910):Erläuterungen zur geolo¬ Sedimenten und Vulkaniten im Aarmassiv. gischen und mineralogischen Karte des östlichen Bull. Ver. Schweiz. Petroleum-Geol. u. -Ing., Aarmassivs von Disentis bis zum Spanort. Speyer 54/124,45-57. Verlag, Freiburg i. Br. Schmidt, C. (1886): Geologisch-petrographische KÜpfer, T. (1977): Mineralogisch-petrographische Mitteilung über einige Porphyre der Centraialpen und geochemische Untersuchungen der Syenodio- und die in Verbindung mit denselben auftretenden rite und der Ganggesteine im Puntegliasgebiet. Gesteine. N. Jb. f. Min. Beil. Bd. 4. Ph. D. thesis, Univ. Bern, unpubl. Schmidt, C. (1891): Beiträge der im Gebiet von Blatt Labhart, T. (1965): Petrographische Untersu¬ XIV der geol. Karte der Schweiz in 1:100000 chungen am Südrand des Aarmassivs. Beitr. geol. auftretenden Gesteine. Anhang zu Beitr. geol. Karte 520 OBERHÄNSLI, R., F. SCHENKER AND I. MERCOLLI

Schweiz, 25. Weber, F. (1912): Petrographische Detailkarte des Seemann, U. (1975): Mineralogisch-petrographische Gebietes von Punteglias-La Gonda (Centrale In- und geochemische Untersuchung der granitischen trusivzone am Aarmassiv-Ostende) 1:20000. Gesteine der Val Punteglias. Schweiz, Beitr. geol. Karte Schweiz, Spez. Karte Nr. 102A. mineral. petrogr. Mitt., 55, 257-306. Weber, F. (1924): Geologische Karte des Tödi-Vor- Sengör, C. (1985): Die Alpiden und die Kimme- derrheintal-Gebietes: Ostende des Aarmassivs riden: Die verdoppelte Geschichte der Tethys. und der Wurzelregion der helvetischen Decken, Geol. Rundsch. 74,181-213. 1:50000. Beitr. geol. Karte Schweiz, Spez. Karte Sigrist, F. (1947): Beiträge zur Kenntnis der Pétro¬ Nr. 100A/B. graphie und der alpinen Zerrkluftlagerstätten des Wehrli, L. (1896): Das Dioritgebiet von Schlans bis östlichen Aarmassivs. Schweiz, mineral, petrogr. Disentis. Beitr. geol. Karte Schweiz, NF 6. Mitt., 27, 39-183. Wehrli, L. (1925): Das produktive Karbon der Stalder, H.A. (1964): Petrographische und minera¬ Schweizeralpen. Beitr. geol. Karte Schweiz, geo- logische Untersuchungen im Grimselgebiet. techn. Serie, NF 6. Schweiz, mineral, petrogr. Mitt., 187-398. Widmer, H. (1949): Zur Geologie der Tödigruppe. Staub, R. (1911): Geologische Beschreibung der Ph. D. thesis, Univ. Zürich, 98 p. Gebirge zwischen Schächental und Maderanertal. Wüthrich, H. (1965): Rb-Sr Altersbestimmungen Beitr. geol. Karte Schweiz, NF 32. am alpin überprägten Aarmassiv. Schweiz, mineral. Steck, A. (1966): Petrographische und tektonische petrogr. Mitt., 45, 875-971. Untersuchungen am Zentralen Aaregranit und Wyss, R. (1932): Petrographisch-geologische Unter¬ seinen altkristallinen Hüllgesteinen im westlichen suchungen westlich der Grimsel im Aarmassiv im Gebiet Belalp-Grisighorn. Finsteraarhorn-Lauteraarhorngebiet. Mitt. natf. Ges. Bern, Beitr. geol. Karte Schweiz, NF 130. 1-90. Steck, A. (1968): Die alpidischen Strukturen in den Zbinden, P. (1949): Geologisch-petrographische Zentralen Aaregraniten des westlichen Aarmassivs. Untersuchungen im Bereich südlicher Gneise des Eclogae geol. Helv., 61,19-48. Aarmassivs. Schweiz, mineral, petrogr. Mitt., 29, Steck, A. (1984): Structures de déformations Ter¬ 221-356. tiaires dans les Alpes centrales. Eclogae geol. Helv., 77, 55-100. Taylor, C. (1976): Tectonic studies at the SW-end of the Aar massif, Switzerland. Ph. D. thesis, Luton Manuscript received June 6 1988; revised manuscript College, GB, unpubl. accepted August 19, 1988.