Kyanite bearing early Alpine metapsammite in the Larderello Geothermal Region (Italy) and its implication to Alpine metamorphism and Triassic paleogeography.

Detailed petrological studies on cuttings and core samples from the west-central Italian Larderello wells reveal the existence of two types of the early Alpine clastic sediments known as the Verrucano group. The first (Verrucano LR) is characterized by metamorphic partial recrystallization and preservation of sedimentary features along with a metamorphic grade reaching the stability field of pyrophyllite. This resembles the classical Verrucano of the Monti Pisani area. The second variety (Verrucano HR) is defined by strong recrystallization, overprints of sedimentary features and a metamorphic grade reaching the kyanite and chloritoid stability fields, and resembles the Verrucano of the Massa unit. Based on the mineralogical data, the two varieties represent parts of a single prograde metamorphic complex. The two Verrucano types often alternate due to tectonic displacements during the Tortonian compressional phase. The decreasing frequency of marble levels from the Verrucano HR towards LR with the metamorphic grade of HR comparable to Massa unit indicate that Verrucano sediments in the Larderello region were deposited in different parts of a single internal (western) Triassic basin. Transitional types found between the evaporitic levels of the Triassic Burano formation and the clastic members of the Verrucano LR are also mentioned in the paper. Further, it is shown that two main Alpine schistosities and traces of weak, late crenulation cleavage are present and that the climax of metamorphism (appearance of kyanite) was reached after the conclusion of the first phase of folding.