Bregaglia granite contact metamorphism zone, Sondrio Province, Lombardy, Italyi

Regional Level Types
Bregaglia granite contact metamorphism zone	Zone
Sondrio Province	Province
Lombardy	Region
Italy	Country

Complex system of silicate fracture-reaction-seal veins occurring in dolomite marble inclusions of the Bergell granite/quartz-diorite intrusion. In the Italian portion of the Bergell intrusion aureola, dolomite marbles are particularly abundant in the eastern sector, i.e. in the area Monte Rosso-Cima di Vezzeda (Sissone Valley, Malenco Valley) and in the area Valle Preda Rossa-Valle Scermandone (Masino Valley).


A temperature of about 620 ± 20 °C at a pressure of 2.5 ± 0.5 kbar has been estimated for the maximum temperature of contact metamorphism.
The typical contact metamorphic assemblage in the marbles is: dolomite + calcite + forsterite + clinohumite + spinel + phlogopite. During cooling and uplift of the Bergell area a regional fracture system developed as a result of significant post-intrusive deformation (Bucher-Nurminen, 1977). It cuts across the marbles and the intrusives. The extension fractures served as pathways for percolating hydrothermal fluids. The hydrothermal activity occurred in the temperature interval between 600 °C to 300 °C of the cooling path. The fluids were chemically not in equilibrium with the wall-rock dolomite and consequently produced a system of reaction veins. Some wall-rock alteration occurred also in the granitic and quartzdioritic intrusives (epidote, scapolite formation).
The vein formation mechanisme is complex and involves the following steps:
1) Brittle extension fractures in dolomite (due to shear stress);
2) Silica-saturated fluid (from cooling granite) invaded the fractures;
3) Fluid/rock desequilibrium during deformation induces irreversible mass-transfer;
4) Transition hydrostatic to lithostatic pressure regime during sealing increases silicate/calcite ratio;
5) Reopening due to increasing shear stress; repeated fracture-reaction-seal cycles.
The veins range from a few mm to a maximum of about 60 cm in thickness. The lateral extension is typically several metres, often across the entire marble inclusion in the intrusives. Each vein can be subdivided into a central zone where phlogopite-rich bands of the original marble are absent and a replacement zone where occasional bedding is preserved. Therefore, the veins grew by both, deposition of material in an opening extension fracture (central zone) and by replacement of dolomite marble. Mineralogically the different zones of the veins consist of calcite (always present) plus one or more of the following silicates: chondrodite, clinohumite, forsterite, tremolite, talc, diopside, and phlogopite. Other minerals found occasionally include: spinel, chlorite, quartz, dolomite, and antigorite.

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