ILLITE CRYSTALLINITY IN PELITIC AND MARLY ROCKS FROM THE NORTHERN APENNINES (SOUTHERN TUSCANY AND UMBRIA, ITALY)

Illite crystallinity (IC) was determined in 850 samples of pelitic and marly shale of the Alpine tectonic units belonging to the Northern Apennines all over southern Tuscany and western Umbria. The IC values fall in the range °DELTA2THETA = 0.18-0.74 (high-grade diagenesis to epimetamorphic grade), significantly shifted towards lower values than those found in northern Tuscany for the same units. Values falling in the range °DELTA2THETA - 0.58 - 0.95 were found near granite outcrops or geothermal areas affected by strong hydrothermal alteration. The following ranges of °DELTA2THETA were obtained: 1) Ophiolitic Complex: 0.41-0.67; 2) Cretaceous-Paleogene Flysch Complex: 0.280. 74; 3) Canetolo Complex: 0.26-0.57; 4) Tuscan Nappe: 0.24-0.68; 5) Cervarola-Falterona Unit: 0.43-0.65, with values of 0.32 and 0.37 near overthrust surfaces; 6) Umbrian Series 0.43-0.60; 7) Tuscan Metamorphic Units: 0.18-0.39. The regional distribution pattern does not show clear vertical and horizontal zonations as those observed in northern Tuscany and Liguria. The high values and narrower ranges of IC parameter in southern Tuscany and Umbria as compared with those of northern Tuscany and Liguria are attributed to a post-tectonic increase of the geothermal gradient produced by lithospheric and crustal thinning, mantle swelling and shallow emplacement of acidic magmas below the western side of the Northern Apennines.