In the mid-late Tertiary (27-6 m.y. ago), volcanism diminished, then ceased in the main Andean Cordillera in Chile between 29¿ and 30¿30'S. Over the same time span the subduction angle shallowed under this region, which today lies in the modern shallowly dipping, nonvolcanic segment of the south central Andes (28¿-33¿S). The trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crystal thickening that accompany this major tectonic transition. Volcanic changes include, in addition to a decreased rate of volcanism, a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks, a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks, and an increase in crustal assimilation. These volcanic changes also correlate in time with deformational events that help to account for crustal thickening and the space-time pattern of deformation and volcanism in the foreland region. Although the model stands alone, support for the tectonic setting and magmatic source region of these Tertiary volcanic rocks can be found in comparisons with recent volcanic rocks erupting over more steeply dipping modern Andean segments. The older Don¿ Ana Formation (27-18 m.y.) is similar to volcanic rocks in the Southern Volcanic Zone, while the younger Cerro de las Tortolas (14-10 m.y.) and Vallecito (6 m.y.) formations are similar to volcanic rocks over regions of thicker crust in the Central Volcanic Zone and the northern part of the Southern Volcanic Zone. ¿American Geophysical Union 1987