Definition of porphyroblast in US English:



  • A larger recrystallized grain occurring in a finer groundmass in a metamorphic rock.

    • ‘Evidence of contact metamorphism of the wall-rock includes andalusite porphyroblasts enclosed in a fabric related to Caledonian nappe emplacement.’
    • ‘In contrast, 2 km to the NE on the southern side of the Dharlang Nala, there are kyanite grade assemblages with a weaker foliation, flattened symmetrically around garnet porphyroblasts.’
    • ‘Cordierite in these two metamorphic belts forms rounded, opaque black to brown porphyroblasts typically less than 0.5 inch across.’
    • ‘Such inclusions may be aligned, preserving evidence of the fabric in the rock when the porphyroblast grew.’
    • ‘The foliation in the lowest unit is locally overgrown by porphyroblasts of andalusite (now commonly altered to finegrained phyllosilicates) and by biotite.’