Geochemical and Isotopic Data from Micron to Across-Arc Scales in the Andean Central Volcanic Zone: Applications for Resolving Crustal Magmatic Differentiation and Modification Processes

Plagioclase crystals from silicic (andesitic to dacitic) lavas and domes at Volcán Uturuncu, a potentially active volcano in the back-arc of the Andean CVZ (22.3°S, 67.2°W), exhibit large variations in An contents, textures, and core to rim ⁸⁷Sr/⁸⁶Sr ratios. Many of the isotopic variations can not have existed at magmatic temperatures for more than a few thousand years. The crystals likely derived from different locations in the crustal magmatic system and mixed just prior to eruption. Uturuncu magmas initially assimilated crustal rocks with high ⁸⁷Sr/⁸⁶Sr ratios. The magmas were subsequently modified by frequent recharge of more mafic magmas with lower ⁸⁷Sr/⁸⁶Sr ratios. A typical Uturuncu silicic magma therefore only attains its final composition just prior to or during eruption.

In the Lazufre region of active surface uplift (~25˚14’S; Volcán Lastarria and Cordon del Azufre) closed system differentiation processes are not the only factors influencing silicic magma compositions. ⁸⁷Sr/⁸⁶Sr (0.70651-0.70715) and ²⁰⁶Pb/²⁰⁴Pb ratios (18.83-18.88) are highly elevated and¹⁴³Nd/¹⁴⁴Nd ratios (0.512364 –0.512493) are low relative to similar composition rocks from the “southern Cordillera domain.” These data, along with major and trace element trends, reflect a multitude of differentiation processes and magma sources including crystallization-differentiation of more mafic magmas, melting and assimilation of older crustal rocks, and magma mixing and mingling.

On an arc-wide scale silicic lavas erupted from three well-characterized composite volcanoes between 21^oS and 22^oS (Aucanquilcha, Ollagüe, and Uturuncu) display systematically higher K₂O, LILE, REE and HFSE contents and ⁸⁷Sr/⁸⁶Sr ratios with increasing distance from the arc-front. In contrast, the lavas have systematically lower Na₂O, Sr, and Ba contents; LILE/HFSE ratios; ¹⁴³Nd/¹⁴⁴Nd ratios; and more negative Eu anomalies. Silicic magmas along the arc-front apparently reflect melting of relatively young, mafic composition amphibolitic source rocks with the continental crust becoming increasingly older with a more felsic bulk composition toward the east. We suggest this results from progressively smaller degrees of mantle partial melting, primary melt generation, and crustal hybridization with distance from the arc-front.