Tectono-Metamorphic evolution of the anthracite - bearing Basin of La Thuile (External Brian^onnais Zone)

This paper deals with the microstmctural and petrographic study of the anthracitiferous basin of La Thuile (Val d'Aosta). Three tectonic units are distinguished from lower to upper structural level: 1) the Versoyen Zone (Br^hes de Tarentaise); 2) the Petit St. Bernard Zone; 3) the Houillere Zone. The Houillere Zone (External Briangonnais Zone) represents the lower and the more external tectonic unit belonging to the Briangonnais nappe system. The anthracitiferous basin of La Thuile consists of graphitic schists with minor intercalations of coarse metaconglomerates. These metasediments show two ductile deformation phases. The first tectonic foliation (S^) has developed under high pressure-low temperature conditions (phengite+chlorite+paragonite+pyrophyllite±chloritoid±mtile). The axes are NE-SW oriented and the axial plane cleavage plunges towards SE. A second pervasive tectonic foliation (S2) has developed under low P-low T metamorphic conditions. Therefore metamorphic evidence for a low thermal gradient (ca. 10° C/km), reflecting a subductive tectonic environment, has been detected in the Northern sector of the Houillere Zone. Hence, the tectono-metamorphic history inferred for the anthracitiferous basin of La Thuile, is consistent with the low geothermal gradient displayed by the adjoining tectonic units. Finally, comparing the inferred tectono-metamorphic evolution with other tectonic units of the Briangonnais nappe system a subductive Alpine evolution for both the Internal and the External sectors of the Briangonnais Zone is suggested.