Geochemical Variations of Ancient Magmatic Arcs in the Andean Forearc of Northern Chile

Continuous subduction, subduction angle changes, convergence in different directions, and variations in the degree of coupling between oceanic lithosphere beneath the South American continental margin are first-order factors in the Andean orogen setting, and commonly linked to the development of magmatic arcs through time. The Andean forearc of northern Chile includes Late Paleozoic to Paleogene subduction-related volcanic sequences and intrusive complexes. Notably, a remarkable compositional variability has been recognized for the Triassic to Paleocene magmatism. The petrogenetic processes that led to the formation of variable lithologies for this period, however, remain under-explored. Well-preserved remnants of the Triassic to Paleocene magmatism can be found between the Coastal Cordillera and the Domeyko Cordillera in the Antofagasta region (22°30' - 23°40'S). Here we provide a timely update for this magmatism. The studied volcanic and intrusive samples (n = 25) show subalkaline to alkaline affinities, varying from tephrite to rhyolitic compositions. New whole-rock element data and Sr and Nd isotope ratios have wide ranges of e.g., 45.279.5 wt% SiO2, 0.114.2 wt% MgO, Sr = 351370 ppm, Cr = 3.8794 ppm, 87Sr/86Sri = 0.6950.712, and Ndi = -3.45.1. It is also noted that these compositional variations are unrelated to the spatial distribution of the samples, but in part related with their ages. Crustal contamination is not a dominant process in the petrogenesis of the studied samples. The pronounced negative Nb-Ta anomaly, high incompatible element concentrations, and relatively high Sr isotope ratios are thus all consistent with an enriched mantle lithosphere source. However, their high Nd isotope compositions agree with depleted mantle sources. This apparent compositional contradiction probably represents a heterogeneous sub-continental lithospheric mantle (SCLM) source, which has been modified by both depleted and enriched slab-derived melts/fluids. The compositional variability revealed by the data, on the other hand, reflects different degrees of partial melting and metasomatism of the SCLM, but also appears to retain a record of subduction margin changes in time.