Upper panel: theoretical AMR of the SMC (black line, Pagel & Tautvaisiene 1998), superimposed with the SMC GCs (Paper II, green circles). The white dots mark the position on the theoretical AMR of ages 11, 12 and 13 Gyr. Lower panel: average Si and Ca abundances as a function of [Fe/H] for SMC stars of this work observed with UVES (red squares) and MIKE (red triangles). The blue shaded area highlights the [Fe/H] region of the stars of our sample with −2.2 < [Fe/H] < −1.8 dex.