Isotopic compositions of samples from six Quaternary volcanoes located in the northern and southern extremities of the Southern Volcanic Zone (SVZ, 33–46°S) of the Andes and from four centers in the Austral Volcanic Zone (AVZ, 49–54°S) range for ⁸⁷Sr/⁸⁶Sr from 0.70280 to 0.70591 and for ¹⁴³Nd/¹⁴⁴Nd from 0.51314 to 0.51255. The ranges are significantly greater than previously reported from the southern Andes but are different from the isotopic compositions of volcanoes in the central and northern Andes.

Basalts and basaltic andesites from three centers just north of the Chile Rise-Trench triple junction have ⁸⁷Sr/⁸⁶Sr,¹⁴³Nd/¹⁴⁴Nd,La/Yb,Ba/La, and Hf/Lu that lie within the relatively restricted ranges of the basic magmas erupted from the volcanic centers as far north as 35°S in the SVZ of the Andes. The trace element and Sr and Nd isotopic characteristics of these magmas may be explained by source region contamination of subarc asthenosphere, with contaminants derived from subducted pelagic sediments and seawater-altered basalts by dehydration of subducted oceanic lithosphere. In the northern extremity of the SVZ between 33° and 34°S, basaltic andesites and andesites have higher ⁸⁷Sr/⁸⁶Sr,Rb/Cs, and Hf/Lu, and lower ¹⁴³Nd/¹⁴⁴Nd than basalts and basaltic andesites erupted farther south in the SVZ, which suggests involvement of components derived from the continental crust.

In the AVZ, the most primitive sample, high-Mg andesite from the southernmost volcanic center in the Andes (54°S) has Sr and Nd isotopic compositions and KR/b and Ba/La similar to MORB. The high La/Yb of this sample suggests formation by small degrees of partial melting of subducted MORB with garnet as a residue. Samples from centers farther north in the AVZ show a regionally regular northward increase in SiO₂, K₂O, Rb, Ba,Ba/La, and ⁸⁷Sr/⁸⁶Sr/ and decrease in MgO, Sr,KR/b,Rb/Cs, and ¹⁴³Nd/¹⁴⁴Nd, suggesting increasingly greater degrees of fractional crystallization and associated intra-crustal contamination.