Sr–Pb isotopes signature of Lascar volcano (Chile): Insight into contamination of arc magmas ascending through a thick continental crust

Abstract

In the Central Volcanic Zone of the Andean cordillera (CVZ), thickening of the continental crust during the last 30 Ma led to the production of increasingly differentiated magmas. Better constrain on the interactions between ascending magmas and the thick continental crust are needed to understand the contamination processes and their implications on the geochemical diversity of arc magmas.Lascar volcano figures among the most active Quaternary volcano of the CVZ and yet geochemical studies on its volcanic products remain scarce. We lead major-, trace elements and Pb–Sr isotopes measurements on 10 new Lascar samples representative of the different petrologic types (andesitic to dacitic).206Pb/204Pb (18.770–18.823), 207Pb/204Pb (15.636–15.655), 208Pb/204Pb (38.711–38.809) and 87Sr/86Sr isotope ratios (0.705731–0.706564) show small variations within the previously defined isotopic field of Lascar. In Pb–Sr or Pb–Pb isotope space, the data points show a triangular distribution that requires the contribution of at least three end-member components at the source of Lascar magmas. We built up a two-stages three-components assimilation fractional crystallization model (AFC) that reproduces the compositional range of Lascar magmas. It allows us to constrain the composition and nature of the three end-member components: (BA) a common mantellic source with low 87Sr/86Sr (<0.70575), low 206Pb/204Pb (<18.768), 207Pb/204Pb (<15.637) and 208Pb/204Pb (<38.711), (LC) a lower crust and/or slab component radiogenic in Sr (87Sr/86Sr > 0.70656) and in Pb (206Pb/204Pb > 18.82; 207Pb/204Pb > 15.647; 208Pb/204Pb > 38.79), (UC) a felsic upper continental crust (>65.1 wt% of SiO2) with similar Sr–Pb isotope composition as LC (87Sr/86Sr > 0.70643; 206Pb/204Pb > 18.823; 207Pb/204Pb > 15.655; 208Pb/204Pb > 38.809). Both BA and LC components occur in lavas with <57 wt% SiO2.This model predicts that Lascar magmas assimilated ≤30% of lower crust and ≤29% of upper crust. Our conclusions are in good agreement with the CVZ literature that reports the contribution of three ubiquitous end-member components at the source of CVZ magmas, with similar natures and origins.