Post-Collisional Alkaline Granites

Alkaline (A-type and highly fractionated felsic I-type) granites that may have formed in post-collisional plate tectonic environments are present in the Alaskan Cordillera, the New England and Lachlan Fold Belts of Australia, and the Arabian-Nubian Shield. Their emplacement is associated with diffuse extension and/or strike-slip shear movements that follow the proposed collisional events by ~25 to 75 Ma. The post-collisional alkaline granites are compositionally most similar to within-plate (anorogenic) alkaline granites, but also show affinities with volcanic arc and syn-collisional granites. They may easily be mistaken for anorogenic granites in complexly deformed or poorly exposed terranes. Most of the alkaline granites probably originate by middle or lower crustal anatexis, but the origin of the necessarily high melting temperatures remains unresolved. Their source rocks are just as likely to be previously unmelted mafic meta-tonalites as they are to be restite from which I-type granitic melt had been extracted earlier.