Deformation and metamorphism of the Gran Paradiso nappe, Western Alps, Italy

F.M. Brouwer, R.L.M. Vissers & R. Compagnoni*

Utrecht University, Netherlands
* Universit‡ di Torino, Italy

Poster presented at the GV-meeting Berne, Switzerland, February 1998 and the 4th Dutch Earth Scientific Conference (NAC IV), Veldhoven, Netherlands, May 1998.
Abstract published as: Terra Nostra (Schriften der Alfred-Wegener-Stiftung) 98/1, 41-42.

Abstract

The Gran Paradiso nappe in the Italian western Alps consists of a sequence of pre-Variscan metasediments and minor metabasic intercalations, intruded by late-Variscan granitoids. This composite of fine-grained gneisses (gneiss minuti) and intrusive granitic rocks underwent intense polyphase Alpine deformation and regional metamorphism.

The dominant structure is a distinct chlorite-bearing foliation, continuous across the contact of metasediments and orthogneisses, and developed at either low angles or parallel to metasediment-orthogneiss contacts. This mainphase structure involved intense shearing along distinct zones surrounding domains in which earlier fabrics are seen to have been folded and less intensely sheared. The mainphase structure clearly postdates the development of a demonstrably pre-Alpine differentiated layering in the gneiss minuti. It also overprints a generation of early-Alpine folds, the development of a locally chloritoid-bearing gneissic layering in the orthogneisses and chloritoid-bearing layers in phengite (or ãsilveryä) micaschists. Strong mainphase, EW oriented stretching led to transposition of old fabrics, stretching lineations and fold axes and, locally, to the development of sheath folds. The mainphase deformation was followed by deformation in small-scale ductile shear zones seen in all lithologies and brittle-ductile kinking in the fine-grained gneisses. A late stage of semi-brittle deformation led to the development of quartz and quartz-chlorite veins. The present structural analysis allows construction of a cross-section across a major part of the Gran Paradiso massif. It illustrates the roughly domal geometry of the massif and its smaller-scale structures.

The metamorphic history of the Gran Paradiso rocks is reflected in the metamorphic assemblages. Pre-Alpine contact metamorphism of the metasediments related to the intrusion of the late-Variscan granitoids is recorded by relic assemblages including pseudomorphs of kyanite after sillimanite, scarce corundum and green spinel, Fe-rich and markedly red-brown biotite and plagioclase (Callegari et al., 1969; Dal Piaz & Lombardo, 1986). Assemblages related to Alpine peak-pressure conditions are mainly seen in the metabasites and the silvery micaschists. They include relics of garnet-omphacite-glaucophane and talc-phengite ± chloritoid ± glaucophane assemblages, respectively. In most metabasite pods, however, this assemblage was replaced by epidote and barroisitic amphibole during retrogression. In the metasediments as well as in the augengneisses the stable assemblage records the greenschist facies overprint in the form of chlorite-albite-garnet-quartz assemblages developed during mainphase deformation. A late thermal pulse is recorded by a stage of garnet growth in equilibrium with new biotite, growth zoning in plagioclase, and the occurrence of hornblende surrounding actinolite in the metasediments.

This research is (in part) supported by The Netherlands Geosciences Foundation (GOA) with financial aid from the Netherlands Organisation for Scientific Research (NWO).

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