Dora Maira coesite-bearing unit (Dora Maira ultra‐high‐pressure unit; Brossasco-Isasca Unit; Dora Maira Massif), Cuneo Province, Piedmont, Italy
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|Name(s) in local language(s):||Unità a coesite del Dora Maira (Unità di ultra-alta pressione del Dora Maira; Unità di Brossasco-Isasca; Massiccio Dora Maira), Provincia di Cuneo, Piemonte, Italia|
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 exposed in southern Dora Maira Massif between the lower Po Valley and the lower Varaita Valley. This unit, which is about 10x4x1 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 Cretaceous time. These rocks are now covered only by a thin fraction of the synmetamorphic overburden.
The whiteschists 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, 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 Pratolungo, Varaita Valley.
IMPORTANT NOTE: The use of the only 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.
22 valid minerals. 2 (TL) - type locality of valid minerals.
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- Hermann, J. (2003) Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif. Lithos 70: 163-182.