Fluid rock interactions as recorded by Cl-rich amphiboles from continental and oceanic crust of Italian orogenic belts

A number of samples of Cl-rich amphiboles coming from oceanic and sub-continental gabbro bodies has been studied in order to compare their microstructural and compositional peculiarities and to investigate the fluid-rock interactions in different geodynamic contexts. The development of a first group of amphiboles outcropping in the Northern Apennines was the result of a hydration event that has been ascribed to oceanic metamorphism. The second group was found in a slice of continental crust subducted during Alpine collision, in a subcontinental metagabbro from the Sesia-Lanzo Zone of Italian Western Alps. Their development has been ascribed to a hydrothermal event that took place after the exhumation of the metagabbro during pre-Alpine lithospheric extension. The Cl-amphiboles are either found in veins, as granoblastic aggregates in different microstructures or as rims of zoned amphiboles, where brown-amphibole cores (sometimes Ti-rich), and successive green amphibole, are rimmed by the Cl-rich amphibole. All amphiboles show edembergite to pargasite compositions up to glaucophane and crossites when reequilibrated under HP conditions, with a direct correlation between Fe and Na(A) vs. Cl content, and inverse correlation of Mg and Na(M4) vs. Cl. A comparison with other Cl-amphiboles that have been observed both in oceanic and continental settings, allow discussing the role played by Clrich fluids infiltration both in oceanic and continental crust, during lithospheric extension. The large variations in Si, AlIV, AlVI, Fe, Mg, K and Cl may be related to the combination of different factors, such as Cl-content and related cristal-chemical constraints, whole rock composition, PT conditions of reequilibration, the microdomains where the amphibole grows and the variable aHCl/fluid/aH2O/fluid ratio of the fluid in equilibrium with the amphiboles at various stages of the metamorphic evolution. Amphiboles that locally contain extremely high Cl contents (up to 4% wt) could have been in equilibrium with a locally enriched Cl-fluid. As suggested by the fact that the Cl content of amphibole into the veins is generally lower than in amphibole rims far from the veins, these equilibrium conditions probably were reached at places where the system was locally closed. In addition, hydration reaction consumed the H2O component of the fluid, leading to a re-equilibration of the crystallising amphibole with the remaining Cl-enriched fluid. Equilibration temperatures up to 350 °C can be attributed to the Northern Apennines amphiboles, and up to 550 °C to the ones from the Sesia-Lanzo Zone.