Structurally controlled growth of fibrous amphibole in tectonized metagabbro: constraints on asbestos concentrations in nonserpentinized rocks

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