Porphyroblast nucleation, growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development

Abstract

In regional metamorphic rocks, the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain-rate gradients across the boundaries between the partitioned zones. These generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer. Phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain, because of their uniquely layered crystal structure. However, most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients, and hence are susceptible to dissolution and solution transfer.As a consequence. zones of progrcssivc shearing become zones of dissolution of most minerals, and of concentration of phyllosilicates and graphite. Exceptions are mylonites, where strain-rates are commonly high enough for plastic deformation to dominate over diffusion riitcs and therefore over dissolution and solution transfer. Porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing, as they would be dissolved by the effects of shearing strain on their boundaries. However, they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones, which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth. Zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation, this includes a build up of stored strainenergy relative to zones of progressive shearing, in which dissolution is occuring.Porphyroblast growth is generally syndeformational. and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account.Porphyroblasts in a contact aureole do not grow statically either, as microfracturing, associated with emplacement, allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening.The criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning, reactivation of deforming foliations, and a general lack of rotation of porphyroblasts, with the spectacular exception of genuinely spiralling garnet porphyroblasts.

Low pressure metamorphism in the Orrs Island -Harpswell Neck area, Maine

“…Inclusions in porphyroblastic minerals preserve evidence for transposition of a near-horizontal S, to a near-vertical S2 by progressive crenulation cleavage development (Bell and Rubenach, 1983;Bell et al, 1986). Preservation of successively later stages of crenulation cleavage development in biotite, garnet, staurolite, and andalusite, and the post-kinematic nature of sillimanite (Dunn, 1988;Lang and Dunn, in preparation) indicate that metamorphism was truly progressive.…”

Porphyroblast rotation: eppur si muove*?

“…They state that spherical objects can also be stationary in non-coaxial progressive deformation if flow is partitioned around the object. If a rigid strain shadow is present adjacent to a porphyroblast, the strain shadow and porphyroblast may act mechanically as a single elongate rigid object (Bell, 1985;Bell et al, 1986). Such a combined porphyroblast-strain shadow aggregate will rotate at a very slow rate in non-coaxial flow in some orientations, leading to oblique SJS, patterns ( Fig.…”

“…, 1986;Bell &Johnson, 1989Steinhardt, 1989;Johnson, 1990a, b; Hayward, 1990) One of the main advances in the study of porphyroblasts in the last decade has been the discovery by Fyson (1975Fyson ( , 1980, Bell (1985) and Bell et al (1986) (1980). These structures are interpreted by Bell (1985Bell ( , 1986 to form by flow partitioning around a stationary porphyroblast, as indicated; (b) millipede structures can also form by passive deformation of a foliation around a rigid object in absence of flow partitioning of the type envisaged by Bell (1985Bell ( , 1986; three possible settings are shown, two of which are situations where the porphyroblast rotates with respect to the kinematic frame.…”

“…1986;Bell et al, 1986;Bell & Johnson, 1989Steinhardt, 1989;Johnson, 1990a, b;Hayward, 1990). These papers have made the useful contribution of calling attention to the fact that many apparently rotational fabrics can also be explained as non-rotational.…”

Occurrence and growth history of texturally sector- and sigmoidal-zoned garnet in the San Lo renzo Schists in the Sierra Nevada de Santa Marta massif (Co lombia)

“…In regional metamorphic rocks, the partitioning of deformation into progressive shearing and shortening components results in the fact that garnet porphyroblasts cannot nucleate and grow in zones of active progressive shearing, as they would be dissolved by the effects of shearing strain on their boundaries (e.g., Bell et al, 1986). Castellanos et al (2004Castellanos et al ( , 2010 report several examples of tectonic dissolution of garnet.…”