Geochemistry and origin of volcanic rocks of the Andes (26°–28°S)

Mesozoic to Recent volcanic rocks from a transect of the Central Andes between latitudes 26 ° and 28 ° South in northern Chile and Argentina show chemical and temporal zonation with respect to the Peru-Chile trench. Jurassic to Eocene lavas occur closer to the trench and are comparable to calc-alkaline rocks of island arcs. Eastwards they are followed by Miocene to Quaternary sequences of typical continental margin calc-alkaline rocks which have higher contents of K, Rb, Sr, Ba, Zr, and REE and also higher K/Na and La/Yb ratios. The rocks occurring farthest from the trench have shoshonitic affinities. The distribution of major and trace elements is consistent with a model in which magmas were derived by anatexis of an upper mantle source already enriched in LILE and located above the descending oceanic slab. It is suggested that the chemical variations across the volcanic belt reflect systematic changes in the composition of the magmas due to a decreasing degree of partial melting with increasing depth, and probably also due to the heterogeneity of the source materials.