The Pre-Alpine Evolution of the Continental Crust of the Central Alps — An Overview

Abstract

As for the European Variscides, the continental crust of the Central Alps is part of Gondwana. Geochronologically, this is manifested by a characteristic sequence of geological events presently found in the Brasilian shield or in Africa. However, in contrast to the latter shield areas, the Central Alpine and Variscan crusts consist, in their present erosion level, almost exclusively of recycled Gondwana crust, being geochronologically distinct from continental crust of the other supercontinent, Laurasia. Major Precambrian crust-forming events, as detected by ion-probe dating of detrital zircons from a paragneiss of the Gotthard Massif and a recent river sand from the Po delta, date back to 3.43 Ga and are concentrated around 2.6, 2.1, 1.0 and 0.65 Ga. Precambrian igneous rocks are, as notorious for the European Variscides, very rare in the Central Alps and geochronologically difficult to date. Often they are mafic rocks extracted from a suboceanic mantle, as for example 870-Ma-old gabbros of the Gotthard Massif, metamorphosed to eclogites in the Ordovician (468 Ma). Similar rocks are known from the Berisal complex (Simplon area), the Siviez-Mischabel unit (Valais) and from the southern part of the Penninic nappes. However, their ‘ages’ of ca. 1 Ga still need to be substantiated by further geochronological work, which is also the case for intermediate and felsic orthogneisses of the Upper Austro-Alpine Silvretta nappe.

There are a number of mantle melting events detected in mafic and ultramafic rocks of the Gotthard Massif, the southern steep belt (Lepontine area) and the Ivrea zone, which are probably related to oceanization and continental rift processes. They range from 3.17, 2.67, 2.45, 1.72, 1.27 Ga to Pan-African ages (650 and 670 Ma).

The post-Pan-African and pre-Ordovician evolution probably starts with rifting and oceanization processes in the course of which many precursors of the widespread metasediments were deposited. For the Central and Southern Alps this is documented by ages of ca. 600 Ma for the youngest detrital zircons of paragneisses metamorphosed first during the Ordovician. Rb-Sr whole-rock systematics of further paragneisses corroborate this finding. However, opposite to other parts of the European Variscides, Cambrian or Cambro-Ordovician formation of oceanic mafic rocks has been so far rarely dated unambiguously.

Ordovician orogenic activities are widely documented in the Central Alps. They include subduction zone HP-metamorphism at ca. 470 Ma (e. g. Gotthard Massif), regional amphibolite facies tectono-metamorphism and widespread granitic magmatism in the range of ca. 440–460 Ma followed by rapid uplift (conglomerates!). Evidence for Ordovician back-arc opening with formation of N-type MORBs exists for the external Aiguilles Rouge Massif.

Similar to the post-Pan-African evolution, postOrdovician-Silurian opening phases are difficult to detect and have only been described so far for the external Belledonne Massif. The Carboniferous evolution with rarely detected HP metamorphism is documented, however, by numerous ages. These are usually associated with regional amphibolite facies metamorphism and mainly granitic subduction and collision zone magmatism.

Post-Carboniferous rifting events are best documented in the southern Alpine Ivrea zone, probably because of the unique exposure of rocks from the lower continental crust. They are documented by magmatic/metamorphic activities which probably were caused by successive underplating of mainly mafic magmas continuing at least into the Middle Jurrassic.