Structurally controlled growth of fibrous amphibole in tectonized metagabbro: constraints on asbestos concentrations in non-serpentinized rocks

Asbestos nucleation and concentration in rocks are mostly associated with mechanisms of fibre formation, combined with the water-dependent mineralogical alteration produced during serpentinisation of ultramafic masses. Very little is known about the structural settings and tectonic histories that influence and control asbestos occurrence in non-serpentinised rocks, which are diffusely embedded within tectonised ophiolitic suites. Focussing on a case history provided by a tectonised metagabbro from the Ligurian Alps (northern Italy), a multiscale structural-petrographical approach is used to investigate the relationships between rock fabric and fibrous amphibole growth within the metagabbro. Meso- to micro-structural observations are used to document the role of structurally controlled fluid-rock interactions in localising the fibrous amphibole growth during ductile-to-brittle shearing (mylonitic foliation to shear veins). A qualitative structural scenario is here provided for illustrating the growth of asbestos amphiboles in shear veins during the progression of shear deformation towards semi-brittle rheological conditions. The mechanisms of structurally controlled growth of fibrous amphibole in non-serpentinised rocks imply an examination of the tectonic boundary conditions that are at the origin of the asbestos concentration in ophiolitic rocks involved in orogenic belt construction.