The geology - Orto botanico 'Pellegrini - Ansaldi'

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The geology of the Apuan Alps

The Apuan Alps represent the most important “tectonic window” of the Apennine chain; for the erosion, rock outcrops with a low metamorphism that as a whole constitute the Apuan Metamorphic Complex. This complex is surrounded on three sides by the non-native tectonic units of the Tuscan Aquifer and of the Ligurian units, and it is composed of two main units (Unit of Apuan Alps and Unit of Massa).
From a geological point of view, the interest area is characterised by the presence of rocks outcrops that belong both to the Paleozoic base of the Unit of the Apuan Alps (pf, porphyroid and schist porphyritic) and to its Mesozoic and Tertiary succession (gr, grezzoni).
The section shows the complexity of the geological and structural order of the area.
The Apuan marble basin, unique in the world for its commodities dimensions and features, is a formation with a continuing trend with an extension of about 10 for 20 km and a thickness that in some points is more than 400 m. Its presence derives from the sedimentation of substantial calcareous deposits, in some areas particularly pure ones, for millions of years exposed to the high pressures and temperature caused by the superimposition of tectonic units (with thicknesses of various kilometres). In these conditions, the calcareous primitives were transformed to saccharoidal and ceroid marbles of extraordinary physical, chemical and aesthetic qualities.
Inside the marble basin, different varieties of cut stones can be found, like the statuary, the bardiglio, the ordinary, the venato, the arabescato, the cipolin and more less known ones but not less beautiful and appreciated.
The stratigraphic column draw attention to the thicknesses and the stratigraphic relations among the formations of the Unit of the Apuan Alps, in an ideal reconstruction of the central Apuane. The thickness of the base that constitutes the lower part of the stratigraphic column includes the three formations that constitute the Unit of the Apuan Alps, especially the phyllites, the porphyroid and schist porphyritic and the Orthoceras dolomite. Above are represented the rocks of Mesozoic and Tertiary succession, with particular attention to the great thickness of both the grezzoni and the marbles, from which formation derive different commodities varieties extracted from the marble basin.
The grezzoni are dolomite with limited metamorphic microstructural modifications, that can have different aspect according to their position in the formation. And so at the base there are more metamorphic breaches than dolomite elements, while in the middle part there are more stratified dark grey dolomite, in the upper part there are more dolomite with a yellowish alteration patina with traces of phyllite along the layer joints. Since they are carbonate rocks, there are usually karstification phenomena.
The formation of porphyroid is composed of metavolcanic rocks of rhyolitic composition, with quartz and feldspar phenocrysts in quartz-muscovitic matrix, and it derives from volcanic protoliths of rhyolitic composition, that can be associated with metavolcanic rocks of intermediate or basic composition.
L'ORTO BOTANICO PIETRO PELLEGRINI, Un giardino alpino nel Parco Regionale delle Alpi Apuane
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