Age-metallicity relations of Galactic GCs from the literature. Left panels: sample of ages and metallicities from Marín-Franch et al. (2009). Right panel: sample of ages and metallicities from VandenBerg et al. (2013). Following the classification given by Massari et al. (2019), the different colours and symbols in each plot indicate the different galaxy progenitors of the clusters. The clusters of Gaia-Sausage Enceladus (G-E), Sequoia (Seq), Helmi stream (H99), Sagittarius (Sag), low-energy (L-E), and high-energy (H-E) are shown by solid circles, while tentative associations are shown by empty circles. Clusters classified as in situ by Massari et al. (2019; M-D and M-B groups) are shown with blue dots. Errors on ages are also reported. Bottom panels: Fits to the AMRs of accreted GCs in the G-E, H99, Sag, and L-E groups. For each group, the mean fit to the data as a function of metallicity is shown together with the corresponding standard deviation. Metallicities and ages are as provided by Marín-Franch et al. (2009) and were used for the fits shown in the left panel. In the right panel, we made use of the ages and metallicities reported by VandenBerg et al. (2013). The in situ clusters are also shown for comparison (blue points). The two clusters identified by a purple cross in the bottom-left panel are NGC 6441 and NGC 6121, which have been excluded from the fit, as motivated in the text. Error bars are not reported in these two panels to avoid having overly complex figures, but they have been taken into account in the fitting procedure (see text).