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Prior to the ad 1902 Plinian eruption of 8 km³ of dacite and subsequent growth of the >1 km³ Santiaguito dacite dome complex, Santa María volcano grew into an 8 km³ composite cone over ∼75 kyr in four phases (at 103–72, 72, 60–46, and 35–25 ka). The 1902 eruption occurred after an ∼25 kyr period of repose in growth of the composite cone. To provide context for processes that ultimately led to the 1902 eruption, we present geochemical and isotopic (Sr, Nd, Pb, U-series) data from lavas of the composite cone for which ages are constrained by ⁴⁰Ar/³⁹Ar dating. The four cone-building phases comprise basaltic to basaltic-andesite lava (51·4–56·1% SiO₂) whose major- and trace-element compositions suggest that crystallization was important in differentiation. Relative to other Central American arc volcanoes, these lavas also have large ²³⁸U excesses and high ²⁰⁷Pb/²⁰⁴Pb ratios that imply melting of …