Geology of the Monte Rosa massif: historical review and personal comments

This paper reviews two centuries of geological work on the Monte Rosa massi f, Western Alps, from de SAUSSURE's exploration and systematic mapping by G ERLACH and the Italian Geological Service, to BEARTH's fundamental contribu tion, subsequent developments on surface to subsurface geology, and debated paleostructural restorations, The Monte Rosa massif is modelled within the inner Penninic Monte Rosa nappe and minor ophiolites of the structurally c omposite overlying Piedmont zone. The Monte Rosa nappe consists of basement rocks and minor cover metasediments. The basement is derived from a Late C arboniferous and/or Permian granitic batholith and roofing high-grade parag neisses which, during the Lower-Middle Eocene, underwent to different exten ts a subduction-related eclogitic imprint and Late Eocene-Early Oligocene g reenschist-amphibolite facies reworking. The Dent Blanche-Piedmont-Monte Ro sa nappe stack cooled to nearly 250 degreesC 33-34 Ma ago and was then tran sected by postmetamorphic dykes. The Monte Rosa nappe also includes some narrow tectonic belts called the Fu rgg zone, characterized by abundant mafic boudins. Its origin and compositi on are debated. On the northern side of the massif, the Furgg zone (s.s.) i s sandwiched between the Monte Rosa basement and the Portjengrat-Stockknube l units. It consists of micaschists. leucocratic gneisses and marbles with mafic boudins, and has been referred to as: (i) a Permian-Carboniferous cov er of the Monte Rosa basement, with interfingerings of Mesozoic metasedimen ts and boudinaged ophiolites from the Zermatt-Saas and/or Antrona unite (ii ) a Mesozoic volcano-sedimentary cover sequence; (iii) a tectonic melange b etween Monte Rosa rocks and Piedmont ophiolites (iv) an inner tail of the o uter Penninic Valais oceanic suture. On the Italian southern side, the Furg g zone occurs at various structural levels, either on top of the polymetamo rphic basement or deeply inside it, allowing the Monte Rosa nappe to be sub divided into several subnappes. It consists of eclogitic to retrogressed ma fic boudins and minor marbles which are embodied within high-P garnet micas chists and their greenschist-facies derivatives (Alpine imprint), both orig inating from pre-granitic protoliths (without traces of Mesozoic metasedime nts), Therefore, the southern Furgg zone is an older lithotectonic complex of the Monte Rosa basement, strongly sheared and metamorphosed during Alpin e subduction and exhumation, and in places eventually accreted by minor oph iolitic slices along the contact with the overlying Zermatt-Saas nappe. Rec ent zircon dating suggests the occurrence of pre-Variscan protoliths also w ithin the northern Furgg zone. Contrasting paleostructural reconstructions are reviewed, and it is concluded that the Monte Rosa continental crust was originally located in the inner sector of the European passive margin, bet ween the Brianconnais domain and the Piedmont ocean.