Variscan tectonic evolution in the Central Alps : a working hypothesis Autor(en): Mercolli, Ivan / Oberhänsli, Roland 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: 06.10.2021 Persistenter Link: http://doi.org/10.5169/seals-52084 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, 491-500, 1988 Variscan tectonic evolution in the Central Alps: a working hypothesis by Ivan Mercolli1 and Roland Oberhänsli1 Abstract The Upper Paleozoic magmatic rock associations outcroping in four different tectonic units of the Central Alps (Aar massif, Lower Austroalpine, Austroalpine and Southern Alps) can be devided into three series: 1)calcalkaline volcanic and plutonic rocks, representing the more conspicuous member of this magmatism, 2) alkaline rocks, mainly plutonics and 3) G-type rhyolite. The correlation of the magmatic activities with their proper tectonic regimes as well as some sedimentological and tectonical observations provide an interpretation of the Variscan evolution in terms of plate tectonics. The proposed model presumes that the four studied areas were located at the southern margin of the European continent above the subducting Proto Tethys oceanic plate. The heterochronous development of the magmatism in the four different areas is interpreted as to be caused by the relative movement of the "Central Alps" continental block with respect to the subduction zone. This movement is related to the lateral acretion of continental material in front of the "Central Alps" block. The Mesozoic rifting is finally responsible for the separation of Aar massif, Lower Austroalpine, Austroalpine and Southern Alps in a way that could satisfy the Alpine paleogeography. Keywords .magmatic activity, plate tectonics, Variscan evolution, Central Alps. Introduction king hypothesis for the Variscan plate tectonic evolution of this area. Since the works of Nicholas (1972), Bur- Observations on Upper Austroalpine ret (1972) and Laurent (1973), use of the plate magmatic rocks from four distinct tectonic units tectonic approach to interprète the evolution of (Aar Massif, Lower Austroalpine, Austroalpine the Variscan orogeny in Central Europe is and Southern Alps), together with their common (for review see Bard et al. 1980, Ziegler, relationships to regional Variscan metamor- 1982-1986, Lorenz and Nicholls, 1984). phism and deformation, have been chosen to Those studies are mainly based on the classical develop a tectonic model. Variscan regions which were not affected by the Alpine orogeny (central and southern Germany, Massif Central, Spain, Sardinia, Lithological and structural frame of the Upper Corsica). Despite the fact that the Paleozoic basement Paleozoic magmatism is an important part of the Alpine chain, in all paleogeographic reconstructions The studied units, except the Southern Alps, references to the Central Alps are scarce or absent. have undergone different degrees of Alpine The aim of this contribution is to combine metamorphism and deformation. As our attention the results of several studies carried out at the is concentrated on the Variscan problem, University of Bern on Paleozoic sequences the terms metamorphism and deformation from the Central Alps and to propose a wor¬ always refer to pre-Alpine (mainly Variscan) ' Mineralogisch-Petrographisches Institut, Universität Bern, Baltzerstrasse 1, CH-3012 Bern. 492 MERCOLLI, I. AND R. OBERHÄNSLI orogenic events. Conversely we are 1. The plutonism can be divided into two series: constrained to utilize the Alpine terminology of the a) A calc-alkaline series tectonic units (i.e. Lower Austroalpine) to Diorite, small tonalitic intrusions, large achieve a maximum of clearness in our exposition. granodioritic and granitic bodies represent It would perhaps be more correct to use the members of the magmatic those terms with the prefix "proto" (proto- sequence and are present, with variable lower Austroalpine), because the relative position proportions of the different lithologies, in ofthe different units before the early Meso- all of the studied tectonic units. Only the zoic rifting and subsequent Alpine over- Silvretta nappe lacks Variscan plutonics. thrusting is yet to be established. b) An alkaline series This series (monzonite, syenite and alkali granites) well developed in the Lower Austroalpine (Julier Bernina nappe), is present in the Southern Alps as part of the Baveno granitoids (Bonin, 1987) and is absent in the Silvretta nappe. The syenitic rocks of the Aar massif seem to be older than Carboniferous and are therefore not included in this description. 2. The volcanism can be subdivided in: a) A calc-alkaline series. High-Al basalts, andésites, dacites and rhyolites are the typical members of this series. The different lithologies are well documented in the Southern Alps (Lugano area) and in the lower Austroalpine (Err nappe). In the Aar massif mainly rhyolite and related volcanoclastic sediments are preserved. b) Large masses of monotonous rhyolitic ig- nimbrites Fig. 1 Geographic and tectonic location of the four These rocks seem to fit some of the criteria studied areas: for G-type rhyolites (Izett, 1981). 1. Aar massif - 2. Lower Austroalpine (Err and Julia- Typical outcrops can be found in the Bernina 3. nappes) - Austroalpine (Silvretta nappe) - Southern Alps and in the Silvretta nappe. 4. Southern Alps (Lugano area). The rhyolite of the Sandpass formation (Oberhänsli et al., 1988) could be assigned to this group. Fig. 1 shows the location of the four areas c) Rhyolitic ignimbrites with an alkaline studied in detail: tendency. 1. Northeastern Aar massif (Schaltegger, These occur as a small body in the Julier 1984; Oberhänsli, 1986, 1987; Schenker, Bernina nappe. 1986, 1987; Schenker and Albrecht, 1987; 3. Mafic dikes (dolerites and lamprophyres), Böhm, 1986, 1988). cutting the units mentioned above, are the 2. Lower Austroalpine, Err and Julier-Bernina latest magmatic products of the Variscan cycle. nappes (Bühler, 1983; Rageth, 1984; Mer- colli, 1985). Compositional change due to postmag- 3. Austroalpine, Silvretta nappe (Flisch, 1987; matic alterations and Alpine metamorphism, Zaugg, 1987; Graf, 1987). prevent in many cases a clear definition of the 4. Southern Alps, Lugano area (Buletti, 1985; primary magmatic relations. Therefore, it is Stille and Buletti, 1987; Wenger, 1987). sometimes difficult to classify beyond a doubt From these four areas it was possible to the lithologies in the previous scheme. deduce some of the common characteristics of the The main magmatic activity can so be Upper Paleozoic magmatism. related to subduction processes, with the calcal- VARISCAN EVOLUTION IN THE CENTRAL ALPS 493 kaline series (volcanics and plutonics) developing 2. The volcanics lay directly on the basement in an active continental margin while G- and are unconformably overlayed by the type rhyolite and the alkaline series reflect a Mesozoic sediments. cratonic environment. 3. In the Julier Bernina nappe the alkaline plutonics (syenites and alkali granites) are clearly intrusive into the calcalkaline series (diorites, granodiorites and granites) and Basic geological relations of the magmatic rocks therefore younger (Bühler, 1983 and Ra- in the different areas geth, 1984). 4. The slightly alkaline ignimbrites of AAR MASSIF rhyolitic the Julier Bernina nappe (Diavolezza region) can be related the The following informations, in light of a only geochemically to alkaline paleotectonic reconstruction, can be derived plutonics (Rageth, 1984). In composition from the calcalkali- from the Upper Paleozoic magmatic rocks and they differ clearly related sediments. rhyolites of the Err nappe. 1.The volcanic and volcanoclastic rocks lie generally directly on the polymetamorphic SILVRETTA NAPPE basement. Later tectonic events often affected the contacts. In the Silvretta nappe, relicts of Variscan 2. The subduction related volcanics of (mainly magmatism are scarce. Plutonic rocks are rhyolitic composition) and the associated absent and the volcanic rocks are restricted to the molasse type sediments have been wedged frontal