Silicic volcanism triggered by increased denudation rates in the Quaternary Andean arc of central Chile between 33°50‧-34°30‧S
Résumé
In central Chile (33°50‧-34°30‧S), the Quaternary arc records a distinctive episode of silicic volcanism (SiO2 >70 wt%) that occurred during latest Pleistocene (0.1-1 Ma). This episode is recorded in several eroded and inactive arc centers distributed mostly along the highest summits of the Andes in the same region covered by the modern arc centers. We report new data including field observations, petrography, whole rock chemical and Sr-Nd isotopic analyses, mineral chemistry and geochronology for the Quaternary Andean arc in the region with emphasis in the latest Pleistocene units (0.1-1 Ma). The silicic episode that the latter units represent constitutes a singularity in the common compositions that characterize in the long term this arc segment, as it is preceded and followed by basaltic andesite-andesite-dacite suite compositions. In addition, it occurs coevally with a period of increased denudation resulting from the major uplift event of the Andean orogen in the area. Geologic field markers, supported by thermobarometric estimations, indicate conservative estimates of denudation of ∼1.5 km during this period in the area which in turn accounts for a significant pressure decrease at upper crustal levels that must have affected the shallow reservoirs that fed the arc volcanoes. For evaluating this, thermodynamic numerical simulations have been run at different pressure conditions in order to describe for such magmatic systems the evolution in terms of major element composition, degree of crystallinity and volatile content with the temperature decrease. Overall, results show that the compositional spectra defined by this arc segment can be reproduced by low pressure (<0.75 kbar) crystallization of its most basic members by batch and/or fractional processes, the latter being indispensable to reproduce the most evolved compositions. Results also show that a sudden pressure decrease creates a unique set of conditions that constructively operate for prompting the rapid generation and extraction of silicic melts, among the main factors it is highlighted the sudden devolatilization and enhanced crystal-melt segregation which is expected to lead to the rapid creation of a volatile saturated silicic cap in the magma reservoirs. This in turn can account for a compositional change of the volcanoes that tap them, as is seen in the evolution of the Quaternary Andean arc in the study area. The transitory nature of such silicic volcanic episode, characterized by a return to the basaltic andesite-andesite-dacite suite compositions, follows from the transience of the increased denudation event and marks the reaching of a new steady state for the tectono-magmatic system.