Upper Palaeozoic collisional movements along the Ibero-Armorican arc were accommodated on oblique strike-slip ductile shear zone systems. In north-central Portugal, these shear zones are coincident with long linear belts of high temperature, low pressure metamorphism and S-type granitic plutons. A model is presented in which crustal thickening subsequent to collision was followed by anatexis at depths of 12–15 km; this was achieved by a combination of (i) increased heat flow, (ii) lowering of melting temperatures caused by fluxing of hydrous fluids, and (iii) a heterogeneous assemblage of greywackes and pelites which encouraged local high ductility contrasts. Shear zones are proposed as the most important influence in controlling the site of development of such belts; anatexis of metasedimentary crust and emplacement of the resulting S-type plutons within these zones took place without an appeal to crustal rifting or an asthenospheric heat source.

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