The Patagonian Batholith forms the core of the southern Andes for about 1900 km. We investigated the ages of magmatic evolution of this major granitic batholith with SHRIMP, or sensitive high mass-resolution ion microprobe. Zircons from four samples were dated near 45degreesS along a profile from the core of the batholith in Chile to pre-Andean Patagonia in Argentina, yielding ages of 191 Ma +/- 3, 107 Ma +/- 2, 96.3 Ma +/- 1.3, and 81.7 Ma +/- 1.3. An unexpected result is the presence of Precambrian zircon xenocrysts in the pre-Andean batholith apophyses, concentrated near 3.40-3.00, 2.75-2.50, and 2.15-1.95 Ga. This demonstrates the presence of rocks derived from a Paleoproterozoic-Archean basement, which contaminated the gabbro and two granitoids. (C) 2002 Elsevier Science Ltd. All rights reserved.