«Gneiss minuti»
Darstellung und Status
- Rang
- lithostratigraphische Einheit
- Gebrauch
- Element ist nicht in Gebrauch
- Status
- lokaler Begriff (informell)
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- Historische Varianten
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Hornfelsgneise (Reinhard 1964), Gneiss minuti = Biotithornfelsgneise (Boriani et al. 1990)
Beschreibung
- Beschreibung
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variously clay-rich metamorphosed siltites and fine-grained metasandstones (Boriani et al. 1990)
Referenzen
- Neubearbeitung
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(1990) :
The evolution of the "Serie dei Laghi" (Strona-Ceneri and Scisti dei Laghi): the upper component of the Ivrea-Verbano crustal section; Southern Alps, North Italy and Ticino, Switzerland. Tectonophysics 182, 103-118
The Gneiss minuti are the highest structural and probably stratigraphìc part of the sequence (Baechlin, 1937; Boriani et aL 1977), They were derìved from terrigenous sediments whose grain-size was finer than that of the Ceneri-gneiss protolìth. In the typical exposures. these fine-grained. iron-grey rocks (greenìsh in the zones that underwent retrograde metamorphism) show a nearly Ìsotropic texture, though they can be split into centimetres-thick plates. Because of their granoblastic polygonal texture they were called "Hornfelsgneise" by Swiss authors (see Reinhard, 1964). Their mineraI assemblage is often similar to that of the Ceneri gneiss, except for the generaI lack of K-feldspar. Ca-silicate nodules are similar to those of Ceneri gneisses and equally common. Frequent intercalations of the two rock types occur in the transition zone wÌth the Ceneri gneiss. Giobbi Origoni et al. (1982/1983) Ìnterpreted these units as interfingering. later transposed by isoclìnal folds. According to these authors, the whole sequence of Strona-Ceneri, with its relative scarcity of pelitic with respect to arenitic and conglomeratic sediments suggests a shallow, epicontinental depositional environment.
A preliminary study of the typological characters of the zircon populations of the paragneisses of the Strona~Ceneri sequence (Caironi, 1986) has suggested that the source rocks of the sediments should be basic to intermediate intrusives for both Gneiss minuti and Ceneri gneiss.
A muscovite-rich paragneiss variety occurs in Val Veddasca ("Schieferiger Biotitplagioklasgneis ± Muskovit fiihrend"; Reinhard 1964) and in Val Cannobina (shown partly as "Gneiss biotitico-plagiodasici" and partly as "Gneiss minuti" on the Geological Map of the Verbania area by Boriani et al., 1977). It probably represents a more clay-rich facies of the Strona-Ceneri sediments, heteropic with the Ceneri gneiss. Ca-silicate nodules do not occur in this variety.
The protolith of Gneiss minuti did not undergo such a pervasive granitization as the protolith of Ceneri gneiss did. It was preferential\y injected "Iit par lit" by the granitic residues. as shown by the alternating meta-arenaceous and metagranitic composition of the banded varieties of Gneiss minuti; it was sometimes intruded by swarms of pegmatitic dykes, now appearing as thin metapegmatite lenses. Flattened ovoidal nodules. a few centimetres long. rich in micas, gamet and kyanite (and/or sillimanite) are present in the more metapelitic levels of Gneiss minuti near the metapegmatites. In some of these nodules. Baechlin (1937) found relics of chiasto\ite porphyroblasts. According to Bigioggero and Boriani (1975) the nodules represent AI-silicates that grew as a result of contact metamorphism induced by the intrusion of the pegmatites in the protolith of Gneiss minuti. The poikiloblasts were deformed and transformed into the typical AI-silicate nodules during the following regional metamorphism. If the interpretation of Bigioggero and Boriani (1975) is correct, these nodules should represent a rare example of polymetamorphism. in which the older recognizable event is a contact metamorphism and the younger one is a regional metamorphism. Formation of AI-silicate poikiloblasts during the emplacement of the Ordovicìan granites can be considered as evidence for the lack of a regional metamorphism in their country rocks and therefore as evidence for the presence of a single (Hercynian) regional metamorphism in the Strona-Ceneri sediments.