Dora-Maira coesite-bearing unit, Cuneo Province, Piedmont, Italyi

Regional Level Types
Dora-Maira coesite-bearing unit	Rock Unit
Cuneo Province	Province
Piedmont	Region
Italy	Country

The Dora Maira coesite-bearing unit, also known as Dora‐Maira ultra‐high‐pressure (UHP) unit or Brossasco–Isasca Unit, is a ∼1‐km‐thick continental slice sandwiched between lower‐pressure rock units. It is exposed in the southern Dora Maira Massif between the lower Po Valley and the lower Varaita Valley.

This unit, which is about 10 x 4 x 1 km in size, consists of orthogneisses and subordinate meta-pelites, marbles and metabasites which underwent UHP metamorphism at P>35 kbar. Ultra-high-pressure metamorphism has created the very unusual pyrope-coesite assemblage in whiteschists and jadeite-coesite assemblage in granofels, emerged from a depth of 100 km during the Cretaceous. These rocks are now covered only by a thin fraction of the synmetamorphic overburden.

The whiteschist outcrops are famous for their (nearly) colorless, very pure pyrope (over 95% pyrope molecule) as crude crystals up to coconut size. Peculiar ultra-high-pressure phases like ellenbergerite, phosphoellenbergerite, magnesiodumortierite, and coesite are found as inclusions armoured in pyrope.

The main composition of the whiteschists is quartz, phengite, talc, kyanite, and pyrope. Some pyropes contain more than 50 vol.% of mineral inclusions, largely kyanite.

Coesite (or its pseudomorphs consisting of polycrystalline quartz aggregates) is found not only in the pyrope-bearing whiteschists but in a wide spectrum of lithologies. It can be found armoured in minerals such as garnet (pyrope and almandine rich), clinopyroxene (jadeite and omphacite), kyanite and zoisite. Coesite relics are often surrounded by peculiar radial cracks in the host mineral, resulting from the increase in volume connected to the coesite to quartz reaction.

Well-known individual localities for pyrope are: Case Parigi (Case Paris) and Case Ramello, Po Valley; Gilba Valley and Pratoluogo, Varaita Valley.

The Pinerolo unit with its metaconglomerates, that is included, is suggested as a good site for Alpine geology training.

IMPORTANT NOTE: The use of only the simplified indication "Dora Maira Massif" to designate the coesite-bearing terrain occurring in the southern sector of this massif, although quite diffuse in the literature and among mineral collectors, should be avoided and discouraged, because the Dora Maira Massif, consisting of different tectonic slices of continental basement intruded by large bodies of late- to post-Variscan granitoids, is the crystalline basement of the Cottian Alps.

It is an ellipsoid-shaped body of about 70×25 km, which extends from south to north from Maira Valley (Cuneo Province) to Susa Valley (Torino Province).
It includes a very large array of different lithologies and mineral localities, such as the quarries of "Pietra di Luserna" leucogranitic orthogneiss at Montoso and in the Luserna Valley, Monte Bracco, Rocca di Cavour inselberg, the talc mines of Germanasca Valley, the graphite mines in the area near Pinerolo, the gneiss quarries of Borgone di Susa and Bussoleno in Susa Valley, etc.

A wide sector of the Dora-Maira Massif, from Pellice (Grange Subiaschi) to the Sangone Valley (Grange Martinetto and Rolando), through Germanasca (Envie, Sapatlè, Pleinet, Comba la Fracia, Malzas, Fontane, Crosetto, Maniglia) and Chisone Valley (La Roussa), has been the centre of talc mining since XVII century, representing nearly the total Italian production.

Geochronology

Mineralization age: Eocene : 35.4 ± 1 Ma

Important note: This table is based only on rock and mineral ages recorded on mindat.org for this locality and is not necessarily a complete representation of the geochronology, but does give an indication of possible mineralization events relevant to this locality. As more age information is added this table may expand in the future. A break in the table simply indicates a lack of data entered here, not necessarily a break in the geologic sequence. Grey background entries are from different, related, localities.

Geologic Time				Rocks, Minerals and Events
Phanerozoic
	Cenozoic
		Paleogene
			Eocene	ⓘ Zircon 35.4 ± 1 Ma Masueria, Gilba Valley, Brossasco, Cuneo Province, Piedmont, Italy

Mindat Articles

Giant pyrope crystals in Ultra High Pressure (UHP) rocks of the Dora Maira Massif by Olav Revheim

Localities in this Region

Other Regions, Features and Areas that Intersect

References