Long-term compositional and eruptive transitions in an early Miocene volcanic system, Central Chile (33°S)

The reconstruction of long-term magmatic and eruptive evolution in arc settings is often limited by the incomplete preservation of volcanic stratigraphy, as sequences in active tectonic regions are commonly disrupted and eroded, making it difficult to link these variations to underlying geodynamic processes. This contribution presents a detailed reconstruction of the prolonged (~6 Myr) volcanic evolution recorded in a dissected Miocene arc system, based on detailed stratigraphic analysis, whole-rock geochemistry, and U-Pb zircon geochronology. The >2500 m-thick volcanic succession documents diverse eruptive and compositional phases in three stratigraphic units, revealing remarkable changes in magma composition and eruptive style over time. The lower unit is composed of andesitic lavas and pyroclastic breccias, indicating explosive-effusive volcanism prior to 22.6 Ma. Zircon ages from the base of the middle unit confirms the onset of an episode dominated by rhyolitic domes between 22.6 and 18.6 Ma. This is followed by the construction of a composite andesite-basaltic edifice (upper unit), characterized by lava flows and volcanoclastic sequences, with the youngest zircon crystallization ages at ca.15 Ma. The enrichment in incompatible elements in rhyolitic and late andesitic rocks, combined with isotopic evidence for increased crustal assimilation and subducted sediments input within the source, points to a progressive modification of the magmatic system between 22 and 18 Ma. These geochemical and isotopic signatures coincide with the onset of a well-established crustal thickening event in central Chile during this period. This case illustrates how major magmatic changes and eruptive transitions can align with the transition from extensional to contractional tectonics within a single arc segment.