The geometric relationship between porphyroblasts and foliations is widely used to interpret the tectonic history of metamorphic rocks and orogenic belts. Such interpretation makes implicit assumptions about the mechanisms and kinetics of nucleation and growth. Within the complete Barrovian sequence in the Sikkim Himalaya, the discovery of some texturally unusual metapelitic rocks provide new insights into the nucleation behavior of porphyroblasts. We have combined information on the regional disposition of isograds, field relations, compositional zoning of garnet, geothermometry, bulk chemistry and element mobility to demonstrate that nucleation mechanism in such rocks can be determined by mass transport controlled chemical overstepping (the shift in free energy of a system away from equilibrium caused by a change in bulk chemistry). We show that such chemical overstepping may cause simultaneously crystallizing porphyroblasts from neighbouring domains to nucleate and grow into very different microstructures that may be interpreted as syn-tectonic in some cases and post-tectonic in others if only textural criteria are used. Thus, a robust interpretation of porphyroblasts as syn- or post-tectonic requires that the influence of chemistry on the nucleation and growth behavior be considered along with textural criteria. Such considerations can considerably simplify interpretation of the metamorphic history in this part of the Himalaya.

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