Geochronology of magmatic and hydrothermal micas from the Larderello geothermal field, Italy

Granites cored in the Larderello geothermal field have exceptionally low solidus temperatures (< 600 degreesC) due to high Li, B, F concentrations. Micas from these granites can be discriminated into magmatic and hydrothermal generations based on textural observations and concordant microchemical and Ar isotopic data. Hydrothermal micas have characteristic high Cl/K ratios and correspondingly young step ages. Two granites in different parts of the field have distinct magmatic ages of ca. 2.5 and ca. 1.3 Ma, each followed by hydrothermal circulation up to I Ma. later.