The metamorphic grade of the Tuscan Nappe in the La Spezia region of the Northern Apennines (NW Italy) is investigated in shales and marls through mineral assemblages and "crystallinity" of illite and chlorite. In this area the Tuscan Nappe is involved in a regional-scale fold (up to 15 km long and 4-5 km wide), affected by different conditions of strain in the right way-up limb compared to the inverted limb or hinge zone. This geological setting allows exploration of the relationship between tectonic strain and assemblages/properties of clay minerals. Illite and chlorite "crystallinity" data point to middle anchizonal or to upper anchizonal conditions for the Tuscan Nappe, depending on whether the CIS or the Kübler's scale is applied; the metamorphic grade estimated using Kübler's scale seems more consistent with the paleo-thermal and structural evidence. The most notable trend observed in the assemblages and properties of phyllosilicates from the different structural positions of the mega-fold is the disappearance of both illite/smectite mixed-layer minerals and intermediate Na/Kmicas in the higher-strain zones; these phases are likely substituted by larger crystallites of discrete illite and paragonite. The differences in the "crystallinity" of illite and chlorite main populations over the different structural positions are, on the contrary, very small (on average 0.02° as Δ2θ), most of them being within error limits. However such differences are systematic: there is a constant trend exhibiting mica and chlorite peaks slightly narrower in the most deformed zones than in the least deformed ones.
Very low-grade metamorphism in the Tuscan Nappe, Northern Apennines, Italy: relationships between deformation and metamorphic indicators in the La Spezia mega-fold
MONTOMOLI, CHIARA;
2003-01-01
Abstract
The metamorphic grade of the Tuscan Nappe in the La Spezia region of the Northern Apennines (NW Italy) is investigated in shales and marls through mineral assemblages and "crystallinity" of illite and chlorite. In this area the Tuscan Nappe is involved in a regional-scale fold (up to 15 km long and 4-5 km wide), affected by different conditions of strain in the right way-up limb compared to the inverted limb or hinge zone. This geological setting allows exploration of the relationship between tectonic strain and assemblages/properties of clay minerals. Illite and chlorite "crystallinity" data point to middle anchizonal or to upper anchizonal conditions for the Tuscan Nappe, depending on whether the CIS or the Kübler's scale is applied; the metamorphic grade estimated using Kübler's scale seems more consistent with the paleo-thermal and structural evidence. The most notable trend observed in the assemblages and properties of phyllosilicates from the different structural positions of the mega-fold is the disappearance of both illite/smectite mixed-layer minerals and intermediate Na/Kmicas in the higher-strain zones; these phases are likely substituted by larger crystallites of discrete illite and paragonite. The differences in the "crystallinity" of illite and chlorite main populations over the different structural positions are, on the contrary, very small (on average 0.02° as Δ2θ), most of them being within error limits. However such differences are systematic: there is a constant trend exhibiting mica and chlorite peaks slightly narrower in the most deformed zones than in the least deformed ones.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
