Dora Maira massif
| Dora Maira massif | ||
|---|---|---|
| location | Piedmont , Italy | |
| part of | Cottian Alps , Western Alps | |
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| Coordinates | 44 ° 39 ′ N , 7 ° 13 ′ E | |
| rock | Crystalline | |
| Age of the rock | Paleozoic | |
The Dora Maira massif is located in the inner area of the western Alpine arc in the Italian region of Piedmont and is about 45 km from the northeastern city of Turin (it. Torino). The massif represents a dome-shaped rock belt, which is composed of Paleozoic crystalline units and extends over 70 km from the Susa Valley (it. Val di Susa) in the north to the Maira (it. Valle Maira ) in the south.
Geological position and interpretation
The Dora Maira massif, together with the Monte Rosa and Gran Paradiso massifs, form the Pennine internal massifs of the Western Alps . These are attributed to the Briançonnais , which lay between the two Penninic ocean basins ( Valais and Piedmont-Liguria ). Some scientists also assume an origin from the southern European continental margin. The three crystalline massifs together form a structural window unit in the metaophiolithic sequences of the Schistes Lustrés ceiling, which is assigned to the upper Penninic (Piedmont zone). The corresponding overlying rocks of the Monviso and Rocciavrè massifs form the western border of the Dora Maira massif.
Internal structure
The Dora Maira massif represents a stacked sequence of three main tectonic units, which are represented by the Pinerolo, Venasca and Dronero units. These three units are separated from one another by powerful, diffuse-ductile deformation zones and are overprinted by more recent late Eocene processes of metamorphosis .
Pinerolo unit
The Pinerolo unit represents the lowest unit, which is composed of orthogneiss as well as overlying metacarbonates and graphite mica schists, possibly of carbonic age. This present orthogneisses and Carboniferous rocks are in more of Permian diorite - intrusions penetrate. The paragenesis: almandine + chloritoid + quartz + phengite + paragonite + rutile ± ilmenite in occurring metapelites is typical for the Pinerolo unit, the peak of the metamorphosis is given as ≤ 8–12 kbar and ≤ 500 ° C.
Venasca unit
This complex is inserted between the Pinerolo- and Dronero unit consists of polymetamorphic gneiss with eclogites and marbles together, and were of powerful Palaeozoic granite - intrusions by beat. The Venasca unit can also be subdivided into two sub-units, which are referred to as coesite - "hot" eclogite and "cold" eclogite unit.
Coesite "hot" eclogite unit
The coesite "hot" eclogite unit contains ultra-high pressure paragenesis represented by mighty quartz - kyanite - garnet - phengite slate and biotite phengite gneiss . Eclogites and marbles can occur in association with the aforementioned slate formation. In addition to the aforementioned rocks called to pyrope - quartzites in the form of Boudin's unlocked within the mentioned gneiss Massive be, these were possibly through processes of metasomatism from the granitic host rocks, also raises the possibility that it is in these rocks to meta evaporites act could. The maximum PT conditions, which were reached at the height of the prograde metamorphosis, are given for the pyrope quartzites with 30 kbar (corresponds to a depth of approx. 100 km) and 700 - 750 ° C. In comparison, the representative bedrock, the biotite-phengite-gneisses, show lower PT conditions, which are characterized as around 15 kbar and 630 ° C.
"Cold" eclogite unit
Above the coesite "hot" eclogite unit is the "cold" eclogite unit, which is lithologically very similar to the coesite-carrying basement. The only field criterion to differentiate these two units is the presence of chloritoid in the characteristic metapelitic schists of the "cold" eclogite unit. The associated metamorphic conditions are defined as around 15 kbar and 550 ° C.
Dronero unit
The Dronero unit is interpreted as a volcanic-sedimentary sequence that has a carbonic to permo- Triassic age. This unit is therefore composed of permocarbon quartz-chloritoid-anchorite-schist and Permotriassian phengite-rich quartzites. The permocarbon slates also contain kyanite or glaucophane locally , which thus indicate conditions that were formerly blue- slate facies with high pressures and low temperatures. Overall, the metamorphic PT conditions for the Dronero unit are given as around 10–12 kbar and 500 ° C.
Individual evidence
- ↑ Christian Chopin: Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some Consequences . In: Contributions to Mineralogy and Petrology . 86, No. 2, April 30, 1984, pp. 107-118. doi : 10.1007 / BF00381838 .
- ↑ Stefan M. Schmid, Fügenschuh, Bernhard; Kissling, Eduard; Schuster, Ralf: Tectonic map and overall architecture of the Alpine orogen . In: Eclogae Geologicae Helvetiae . 97, No. 1, April 30, 2004, pp. 93-117. doi : 10.1007 / s00015-004-1113-x .
- ^ Nikolaus Froitzheim: Origin of the Monte Rosa nappe in the Pennine Alps — A new working hypothesis . In: Geological Society of America Bulletin . 113, No. 5, January 1, 2001, p. 604. doi : 10.1130 / 0016-7606 (2001) 113 <0604: OOTMRN> 2.0.CO; 2 .
- ^ Dov Avigad: Exhumation of coesite-bearing rocks in the Dora Maira massif (western Alps, Italy) . In: Geology . 20, No. 10, January 1, 1992, p. 947. doi : 10.1130 / 0091-7613 (1992) 020 <0947: EOCBRI> 2.3.CO; 2 .
- ↑ H. -P. Schertl, Schreyer, W .; Chopin, C .: The pyrope-coesite rocks and their country rocks at Parigi, Dora Maira Massif, Western Alps: detailed petrography, mineral chemistry and PT-path . In: Contributions to Mineralogy and Petrology . 108, No. 1-2, June 30, 1991, pp. 1-21. doi : 10.1007 / BF00307322 .
- ↑ Hans-Peter Schertl, Schreyer, Werner: Geochemistry of coesite-bearing “pyrope quartzite” and related rocks from the Dora-Maira Massif, Western Alps . In: European Journal of Mineralogy . 20, No. 5, October 1, 2008, pp. 791-809. doi : 10.1127 / 0935-1221 / 2008 / 0020-1862 .
literature
- Christian Chopin, Michard, Andre: Geology and petrology of the coesite-bearing terrain, Dora Maira Massif, Western Alps . In: European Journal of Mineralogy . 3, April 1991, pp. 263-291.
- GV Dal Piaz, Lombardo, B .: Early Alpine eclogite metamorphism in the Penninic Monte Rosa-Gran Paradiso basement nappes of the northwestern Alps . In: Memoir - Geological Society of America . 164, 1986, pp. 249-265.
- HP Schertl, Schreyer, W .: Geochemistry and possible protoliths of coesit bearing pyrope quarzites and related rocks from Dora Maira Massif, Western Alps, Italy . In: Supplement to EOS Transactions, AGU . 77, 1996, p. 46.