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 Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. 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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 Cd X3 oCA H :cd 00 •o c T3 :0 H cO T3a cdc 00 CO « :cd P£ g»< <2J ^ S S cd C? öR s Ü <P. 43 m CQ S1 4> O-Ä « 8 T3 •— C Û cd 7. «S CO <£ 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