SUBDUCTION OF CONTINENTAL BASEMENT: SCALES OF BRITTLE DEFORMATION, (RE-)HYDRATION, AND DENSITY FILTERING

Granulites, regardless of their composition, show essentially brittle behavior during subduction, down to 100 km or more. However, tectonic strain may induce fractures and enhance permeability to fluids, thus promoting reequilibration, with dramatic effects on rock properties. But how?

The Sesia Zone (Western Alps) is best known for its spectacular eclogites, but the dominant rocks resulting from Cretaceous subduction of continental basement are metasediments, notably eclogitic micaschists. Sparse relics of Permian granulites occur in these, especially in garnet and zircon. Many samples show several rims of calcic garnet surrounding a HT-core (low in Ca). Cores are riddled with a fossil fracture network (µm-wide), now filled which Ca-garnet. Resorption features suggest at least 3 stages of growth coupled with successive partial dissolution of earlier garnet. Resorption and growth are evident also in zircon.

Textures indicate that the hairline cracks resulted from strain associated with seismic failure, and Ca-garnet formed due to the invasion of reactive HP-fluids.

Thermodynamic modeling of the replacement process shows that hydrous assemblages (involving phengite and allanite) formed at these stages. Subunits within Sesia show a somewhat different conditions, the range being 1.6 – 1.9 GPa, 600 - 650 °C. These were attained between ~73 and 56 Ma (allanite and zircon LA-ICP-MS dating).

Density modeling indicates that all dry basement rocks remain positively buoyant relative to mantle peridotite, as do predominantly felsic rocks, even if converted to eclogitic assemblages. Some of the latter can resurface in orogens, while granulites of all compositions are prone to being accreted to continental roots. Tectonic bodies containing >70% mafic rocks will tend to sink into the mantle, provided they were pervasively eclogitized. Beneath the Western Alps, seismic tomography reveals a subduction graveyard at 500-600 km depth. Some of the lighter fragments may thus be found in the Sesia Zone, representing a less refractory filtrate of the hydrated Permian basement.

Potentially, the effectiveness of density filtering should be considered when modeling continental growth and recycling.