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The Xolapa Complex of southern Mexico is composed of mid-crustal arc-related gneisses of poorly resolved ages, intruded by undeformed Cenozoic calc-alkaline plutons. Twelve undeformed and deformed tonalitic/granodioritic samples from three transects across the Sierra Madre del Sur (Acapulco, Puerto Escondido, and Puerto Angel) were chosen for U-Pb zircon analysis. The measurements were performed on single crystals of zircons, using a multiple-collector laser-ablation inductively coupled plasma–mass spectrometer (MC-LA-ICP-MS). About 20–30 crystals were measured from each sample. Three gneisses and migmatites from the eastern transect (Puerto Angel), located 30–42 km from the coast, yielded Grenville-aged zircons (970–1280 Ma), suggesting that the samples represent Oaxacan basement, not deformed Xolapa Complex. The central transect (Puerto Escondido) yielded Oligocene ages …