Devolatilization-Generated Veins in the Lower Crust:
the Synmetamorphic Veining Model

B. Cesare Dept. of Mineralogy and Petrology, Univ. of Padova, Italy

J. A. D. Connolly IMP-ETH, Zürich, Switzerland

Thermodynamic, mechanical, and field constraints indicate that at lower crustal conditions (> 12 km) rocks must have near zero permeability, and pore fluids must be near lithostatic pressure. Consequently, during metamorphic devolatilization, fluid pressure will be controlled by the interdependent processes of: (i) reaction, (ii) porosity production, and (iii) fluid escape. Numerical modeling of the behavior of a pelite composition show that at the onset of amphibolite-facies conditions, the univariant chlorite breakdown reaction is able to generate significant fluid pressure anomalies. Even in the case of low heating rates such as those of regional metamorphism, fluid overpressure may ultimately cause rock failure (hydrofracturing), one-pass fluid flow out of the system, and formation of metamorphic veins by mineral deposition in the fractures. Such conclusions can be extended to most lower crustal rock types undergoing devolatilization, and in particular to deep-seated contact aureoles.

One of the possible end-member behaviors for the vein/host-rock system during devolatilization can be described by the non-metasomatic synmetamorphic veining model. In this model, the mechanisms of rock failure, fluid flow, mass-transfer and mineral growth within veins are considered as related to, and controlled by, the unique driving force of devolatilization reactions.

Rock failure and fluid behavior

The fast fluid release that occurs at univariant devolatilization reactions leads to the increase of fluid pressure (Pf) up to the failure condition for hydrofracturing:

Pf > s3 + t0.

The sudden drop of Pf caused by crack opening causes a fluid pressure gradient between the fracture and the host-rock, and fluid flow towards the fracture, until mechanical equilibrium is restored when the pressure of the vein-filling fluid approaches lithostatic value. After this initial, transient flow, the fluid is virtually stagnant. Because the fluid originates from devolatilization of adjacent rock, it is also in thermal and chemical equilibrium with the host-rock.

Mass-transfer and mineral growth

Strained crystals at vein walls provide favourable nucleation sites (besides the nuclei already present within the host-rock) for the product assemblage minerals. The chemical potential gradients existing between reactants and products of the devolatilization activate the transport of chemical species, continuously fed by dissolving reactants. Mass-transfer towards the vein occurs via intercrystalline diffusion in the intergranular fluid network. In the fluid-filled vein, mineral growth is faster, and grain-size coarser than in the host-rock, resulting in a pegmatitic texture.

Synmetamorphic veining only requires devolatilization of a low-permeability rock body, and results a pegmatitic vein whose mineralogy is equal to the assemblage product during devolatilization of the host-rock, and with orientation and phenomenology typical of hydrofracturing.The mechanism does not consider the effects of ductile deformation accompanying devolatilization: ductile deformation should only modify the mesoscopic structures of veins, without affecting the veining mechanism.

The model has been tested with the field example of the And-Bt-Qtz-bearing veins of the Vedrette di Ries contact aureole, where the vein assemblage is interpreted as the stable, high-T side of the discontinuous AFM dehydration reaction:

Grt + Chl + Ms + Gph = And + Bt + Qtz + GCOH fluid

Application of the synmetamorphic veining model allows the problems of mineral (e.g. Al2SiO5) "insolubility" to be overcome: regardless of its solubility, any phase produced during a devolatilization reaction can be effectively transported by intercrystalline diffusion in a fluid medium, and deposited in veins.