ILLITE CRYSTALLINITY IN PELITIC ROCKS FROM THE EXTERNAL AND NAPPE ZONES OF THE HERCYNIAN CHAIN OF SARDINIA (ITALY)

Illite crystallinity was determined in more than 250 pelitic rocks from the Nappe and External zones of the Hercynian chain of Sardinia. The regional distribution of illite crystallinity values indicates a clear Hercynian metamorphic zonation from anchizone to epizone moving northwards from the External to thr Nappe zone. In the Nappe zone iliite crystallinity values also reveal a roughly metamorphic zonation from the lower to the upper tectonic units of the pile of nappes as well as from the front to the root zone in the Gerrei Unit and in the Metamorphic Complex og Barbagia Unit. In the External zone contrasting illite crystallinity values were detected at the Sardic phase unconformity between the Cabitza Formation ( 0.25+/-0.01 ° delta2teta) and the overlyning "Puddinga" (0.39+/-0.03 °delta2teta). The status of illite crystallinity in the Cabitza Formation seems to record pre-Hercynian metamorphic effects.