Key Points Zircon and titanite from the HB3gr 40Ar/39Ar hornblende reference material were dated by ID-TIMS. Zircon U-Pb data indicate initial crystallization at 1090.10 ± 0.16 Ma, a date 1.7% older than the accepted K-Ar date (1072 ± 11 Ma) for HB3gr. Single grain titanite analyses ranged between 1082 ± 0.75 and 1086 ± 0.81 Ma, and with 40Ar/39Ar hornblende dates are interpreted to record pluton cooling at 30--50 °C Ma-1.