Melange di Tamier–Zött

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Darstellung und Status

Farbe CMYK
(0%,0%,20%,4%)
Farbe RGB
R: 245 G: 245 B: 195
Rang
lithostratigraphische Formation
Gebrauch
Element ist in Gebrauch
Status
informeller Begriff
Diskussion des Status

Nomenklatur

Deutsch
Tamier–Zött-Melange
Français
Mélange de Tamier–Zött
Italiano
Melange di Tamier–Zött
English
Tamier–Zött Melange
Herkunft des Namens

Tamierpass (TI) et Lago del Zött (TI)

Historische Varianten

Tamier metabasites (Masson 2002), Tamier–Zött wildflysch with blocks of gneiss and lamprophyres (Matasci et al. 2011 p.266)

Geografie

Geographische Verbreitung
Lepontin: Tamierpass, Robièi.

Referenzen

Erstdefinition
Matasci Battista, Epard Jean-Luc, Masson Henri (2011) : The Teggiolo zone: a key to the Helvetic–Penninic connection (stratigraphy and tectonics in the Val Bavona, Ticino, Central Alps). Swiss J. Geosci. 104, 257–283

p.266: 3.5.3 The Tamier–Zött wildflysch with blocks of gneiss and lamprophyres

This wildflysch forms the upper half of the Robièi formation in the sector from the Tamierpass (pt 2772 on the Swiss-Italian border; 679.100/139.220) to Robièi. In the Tamier area the first blocks of gneiss appear immediately above the wildflysch with blocks of marble (Carreras and Jequier 2002). Usually these two types of blocks are not mixed. This distinct distribution suggests two distinct events. The nature of the gneiss is variable, always different from the Antigorio as well as from the Lebendun gneiss. These gneisses can be either ortho or para, are often migmatitic and contain commonly a few small garnets. Sometimes they can also include thin brownish layers with a small amount of calcite. Bussien et al. (2011) provided zircon U–Pb ages on an orthogneiss block from the Tamierpass (285.9 ± 7.2 Ma) and on a paragneiss block from Zo¨tt (from 2,400 to 483 Ma). The size of the gneiss blocks progressively increases upwards in a way very similar to the marble blocks. At the base they are commonly dm- to m-large and the calcschist matrix can be locally enriched in coarse quartzic and feldspathic sand, tending to become arkosic, or in very small gneissic pebbles. At the top, in the cliffs below the Basodino peak, the blocks are really enormous and tightly packed in a rare micaschist matrix (Carreras and Jequier 2002). It is understandable that the accumulation of these huge blocks has been confused with the Lebendun gneiss that immediately overlies it and consequently was included into the Lebendun nappe on the sheet Basodino (Burckhardt and Günthert 1957). In the Basodino cliffs the Lebendun thrust is indeed situated 100–200 m higher than figured on this map. Gneiss blocks of all sizes are also numerous in the calcschists at Robièi (Fig. 6) and on the crest E of the Lago del Zött (1 km SW Robièi) (Della Torre 1995). However, here the Teggiolo zone is folded with the Lebendun nappe in a complex way and the geometric relations of the different wildflysch and gneiss types are more difficult to decipher. A remarkable peculiarity of the gneiss blocks in the Tamier and Zött areas is that they frequently contain dykes of basic magmatic rocks (Carreras and Jequier 2002). These dykes are black or dark green, dm- to sometimes m-thick, and frequently folded and boudinaged. Above the Tamierpass, where the accumulation of large blocks of migmatitic gneisses has been confused with the conglomeratic Lebendun gneiss, some of the boudinaged dykes have been noted on the Basodino map as pebbles of amphibolite in the conglomerate (Burckhardt and Günthert 1957). At Zött some m-large blocks in the calcschist are entirely made of the same basic rocks (Della Torre 1995). These rocks are anomalously rich in mafic minerals and are conveniently designed as lamprophyres. Their mineralogy and geochemistry has been investigated by Bussien et al. (2008). They mainly contain in various proportions biotite, hornblende, plagioclase, K-feldspar and epidote, sometimes accompanied by quartz or by large poekilitic crystals of clinopyroxene. Common accessories are apatite, titanite, chlorite, actinolite and rutile. Calcite is abundant at Zött: it is secondary but it is not clear if it results from exchange with post-magmatic fluids or with the calcschist matrix during Alpine metamorphism. At the Tamierpass these lamprophyres show an ultrapotassic trend (up to 6% K2O). They can also be very basic (SiO2 down to 46%) and high in Cr and Ni, which may result from local concentrations of amphibole and pyroxene in the center of the dykes. When these rocks were first mentioned by Masson (2002) the confusions between the different sorts of gneisses in the Basodino area (see above) were not yet cleared up and induced a suggestion that these dykes could be Mesozoic. Today it is obvious that this is not the case. Moreover, zircons from these lamprophyres have been dated by in situ methods (SHRIMP) and provided U–Pb ages of 284.8 ± 1.7 Ma at the Tamierpass (Swiss side) and 290.0 ± 1.3 Ma at Zött (Bussien et al. 2008). Two additional lamprophyre samples from the Tamierpass (Italian side) have been recently dated by LA-ICP-MS at 286.5 ± 5.3 and 287.8 ± 6.5 Ma (Bussien et al. 2011). The gneiss blocks, so abundant from the Tamierpass to Robièi, suddenly disappear at this point. Only the blocks of marble continue to exist in the Robièi formation farther SE on the left bank of the Bavona down to the surroundings of Campo. The gneiss blocks also seem to disappear in Italy 1.5 km SW of the Tamierpass. At Alpe Tamia the Lebendun thrust directly surmounts the wildflysch with blocks of marble.

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