Laser step–heating ⁴⁰Ar/³⁹Ar analyses of biotites from meta–granites in the UHP Brossasco–Isasca Unit of Dora–Maira Massif, Italy

Abstract

Two undeformed metagranite samples were collected from the UHP Brossasco–Isasca Unit (BIU) of Dora–Maira Massif, Italy to carry out the laser step–heating ⁴⁰Ar/³⁹Ar analyses of individual biotite crystals. The metagranites occur as undeformed domains (m– to ten of meters in size) within strongly deformed augen–gneiss. They still preserve their medium– to coarse– grained igneous texture and are composed mainly of K–feldspar, plagioclase pseudomorph, quartz, and biotite that preserve their original igneous shape but are either re–equilibrated or replaced by new phases.

 Three biotite crystals from the first sample have similar age spectra showing 400 to 300 Ma, except for the first fraction (500–1500 Ma). On the contrary, the age spectra of five biotite crystals from the second sample are significantly different; these biotites show saddle shape ⁴⁰Ar/³⁹Ar age spectra, except for one crystal. The oldest fractions have ages (800 to 1300 Ma) three to four times older than that of the granite protolith (which is late Permian). This extremely high intensity of excess argon could be due to an ‘Excess–Argon Wave’ (EAW) phenomenon, occurred during the quick exhumation of the BIU, combined with the extremely short ductile deformation history. The observed variation of the biotite age spectra may reflect the different trapping processes of EAW and/or localized source of EAW.