Petrography of the Gula Group in Hessdalen, Southeastern Trondheim region, with special reference to the paragonitization of andalusite pseudomorphs.

Abstract

Al-rich pelites from Hessdalen, SE Trondheim region, Central Norway, belonging to the Cambrian Gula Group, are described petrographically. The schists carry abundant porphyroblasts of staurolite, garnet, kyanite and pseudomorphs after andalusite. X-ray diffraction analysis has shown that the white mica of the pseudomorphic mineral assemblage is paragonite with a little muscovite. It is proposed that the andalusite pseudomorphs are relics from a contact metamorphic aureole associated with pretectonic to early syntectonic emplacement of deep-seated, sub-surface gabbroic bodies, indicated by geophysical evidence. The subsequent high-pressure metamorphism of lower almandine-amphibolite facies, tantatively correlated with an early fold phase F1, inverted the andalusite porphyroblasts to aggregates of kyanite. The formation of paragonite occurred during a retrogressive metamorphic phase, as a pseudomorphosis of the kyanite in the altered andalusite porphyroblasts.