In the segment 38¿--39¿S of the southern South American volcanic belt, the subalkaline central-vent volcanoes that form the late Quaternary volcanic front of the Andean orogenic arc are located in the Main Andean Cordillera 30--80 km west of the centers that formed the Pliocene/early Pleistocene volcanic front. The Pliocene/early Pleistocene volcanic front was located along a precordilleran uplift that diverges southeast from the Main Andean Cordillera. During the Pliocene the centers along this uplift erupted subalkaline magmas similar to those currently being erupted along the late Quaternary front. In the early Pleistocene the centers at the southeast end of this uplift began to erupt alkaline magmas. By the late Pleistocene, magmatic activity in these centers had diminished, and the volcanic front had migrated westward to its current location, but alkaline back arc basaltic volcanism continued in the late Quaternary within the valleys flanking the precordilleran uplift on which the Pliocene/early Pleistocene arc had been situated. The early Pleistocene alkaline orogenic arc lavas and the late Quaternary alkaline back arc basalts have La/Yb higher than, but Ba/La and 87Sr/86Sr similar to, the subalkaline magmas erutped from centers of both the Pliocene and late Quaternary volcanic fronts. Their TiO2 and La/Nb are intermediate between subalkaline magmas erupted from both the Pliocen and late Quaternary volcanic fronts and alkaline basalts of the Patagonian plateau lavas south of 39¿S which have intraplate oceanic island petrochemical characteristics. Compared with the subalkaline lavas of the current volcanic front, Quaternary alkaline arc and back arc lavas between 38¿ and 39¿S formed by lower degrees of partial melting of a similar mantle source. This mantle had been modified by source region contamination processes related to dehydration of subducted oceanic lithosphere such as occur below the current volcanic front in the southern Andes. ¿ American Geophysical Union 1989