The “green stones” from Ossola Valley (Piedmont, Northern Italy): historical uses, petrographic and geochemical investigations

摘要

The “green stones” (mostly serpentinites and talc-chlorite schists) from Ossola Valley had a great historical, artistic and archaeological importance. Used since the Bronze Age, they are characterized by easy workability and excellent thermal resistance and show many applications: pots for cooking, millstones, stoves and fireplaces. Artistic and ornamental uses are also documented in many churches and buildings in the Ossola Valley. In the past the “green stones” were quarried in many localities of the Ossola region, but nowadays the extraction occurs only in the Vigezzo Valley. The “green stones” derive both from ophiolitic (Antrona Zone) and ultramafic complexes within the Orselina-Moncucco-Isorno Zone, Monte Leone unit, Sesia-Lanzo Zone. Petrographic investigations evidenced mineralogical and textural heterogeneity and many varieties were distinguished. The serpentinites from the Antrona Zone have deep green colour, very fine grain size, marked foliation and non-pseudomorphic texture in the types from the Bognanco and Antrona area, whereas pseudomorphic texture were recognized in samples from the Vigezzo Valley. The Srp is associated with Mag, cataclastic Ol, Tr, Ttn, ± Ep in the Bognanco Valley; also with Tlc in the Vigezzo valley; with Mag, Ep and carbonates in the Antronapiana area. The varieties from the Orselina-Moncucco-Isorno Zone have green to brown colour, weak or no foliation and fine grain size. Tlc and Chl are the prevailing minerals and are associated with Tr and small amounts of Ol and Srp in the rock types from the Brevettola Valley; variable amounts of carbonates (Mgs and Dol) are present in the Anzuno Valley (Domodossola) rock types. The serpentinized peridotites that occur near Montecrestese have grey-greenish colour, breccia-like texture and medium to coarse grain size. Ol and Tlc prevail and are associated with Chl (which shows decussate texture), Srp and Mag. The varieties from the Vigezzo valley are serpentinized peridotites of the Sesia-Lanzo Zone. They are medium grained, massive rocks with spotted green colour; they present pseudomorphic texture and the mineralogical composition is: Srp, cataclastic Ol, Tr, Tlc, Chl +- Dol. Whole-rock geochemical analyses evidenced a pronounced variability in the SiO2, Al_2O_3, CaO and MgO contents. The green stones derived from ophiolitic complexes mainly show a slight depletion in LREE and a flat HREE pattern. On the contrary, those derived from other ultramafic complexes present heterogeneous patterns mainly due to LREE.