Observations on the Origin of Across-Strike Geochemical Variations in Quaternary Silicic Lava Flows from the Andean Central Volcanic Zone: Comparision of Data from Individual Eruptive Centers

In an effort to better understand the origin of across-strike K₂O enrichments in silicic volcanic rocks (60-68 wt.% SiO₂) from the Andean Central Volcanic Zone, we compare geochemical and isotopic compositions of Quaternary (<1.0 Ma) lava flows erupted at three well-characterized composite volcanoes situated along a narrow southeast striking transect between 21^oS and 22^oS. From northwest to southeast these are volcáns Aucanqilcha, Ollagüe, and Uturuncu. Aucanqilcha is located on the arc front entirely within Chile; Ollagüe is ~25 km east of the arc front in the transition zone between the arc front and Bolivian Altiplano; Uturuncu is ~75 km east of the arc front on the Altiplano. Trends observed include the following. At a given SiO₂ content lavas erupted with increasing distance from the arc front display systematically higher K₂O, P₂O₅, TiO₂, Rb, Th, Y, REE and HFSE contents; Rb/Sr elemental ratios; and Sr isotopic ratios (0.7055 - 0.7165). In contrast, the lavas display systematically lower Al₂O₃, Na₂O, Sr, and Ba contents; Ba/Nb, Ba/Zr, K/Rb, and Sr/Y elemental ratios; and Nd isotopic ratios (0.51239 - 0.51214). In addition, Eu anomalies become progressively more negative toward the SE. It is unlikely that these trends solely reflect regular across-strike variations in mantle source compositions or degrees of melting for parental magmas given the highly modified isotopic ratios relative to primitive intra-oceanic arc magmas, which requires extensive contamination by or derivation within the continental crust. Instead, these data favor a model in which lower crustal source rocks for the lavas become progressively older and more feldspar-rich with increasing distance from the arc front. In this regard, our preliminary interpretation is that silicic magmas erupted along the arc front reflect melting of relatively young, mafic composition amphibolitic source rocks with a garnet- (but not feldspar-) rich residual mineralogy and that the lower crust becomes increasingly older with a more felsic bulk composition in which residual mineralogies are progressively more feldspar-rich, but garnet-poor. One implication of this interpretation is that large-scale regional trends in magma compositions at continental volcanic arcs may reflect a process wherein the continental crust becomes strongly hybridized beneath frontal arc localities due to protracted intrusion of subduction-derived basaltic magmas, with a diminishing effect behind the arc front because of smaller degrees of mantle partial melting and primary melt generation.