Sr and Nd isotope variations along the Pleistocene San Pedro - linzor volcanic chain, N. Chile: tracking the influence of the upper crustal altiplano puna magma body

Abstract

Subduction-related magmas that erupted in the Central Andes during the past 10 Ma are strongly affected by crustal assimilation as revealed by an increase in Sr-87/Sr-86 isotope ratios with time that in turn are correlated with increased crustal thickening during the Andean orogeny. However, contamination is not uniform and can be strongly influenced locally by crustal composition, structure and thermal condition. This appears to be the case along the NW-SE San Pedro - Linzor volcanic chain (SPLVC) in northern Chile, which straddles the boundary of a major zone of partial melt, the Altiplano_Puna Magma Body (APMB). Herein we report Ar-40/Ar-38 ages, compositional and isotope data on lavas from the SPLVC that track the influence of this zone of partial melting on erupted lavas with geochronological and geochemical data. Ages reported here indicate that SPLVC has evolved in the last 2 M.y., similar to other volcanoes of the Western Cordillera (e.g. Lascar, Uturuncu, Putana). Sr-87/Sr-86 ratios increase systematically along the chain from a minimum value of 0.7057 in San Pedro dacites to a maximum of 0.7093-0.7095 for the Toconce and Cerro de Leon dacites in the SE. These changes are interpreted to reflect the increasing interaction of SPLVC parental magmas with partial melt within the APMB eastwards across the chain. The Sr-87/Sr-86 ratio and an antithetic trend in Nd-143/Nd-144 is therefore a proxy for the contribution of melt from the APMB beneath this volcanic chain.

Similar Sr-87/Sr-86 increases and Nd-143/Nd-144 decreases are observed in other transects crossing the boundary of the APMB. Such trends can be recognized from NW to SE between Aucanquilcha, Ollague, and Uturuncu volcanoes, and from Lascar volcano to the N-S-trending Putana-Sairecabur-Licancabur volcanic chain to the north. We interpret these isotopic trends as reflecting different degrees of interaction of mafic parental melts with the APMB. High Sr-87/Sr-86, and low Nd-143/Nd-144 reveal zones where the APMB is thicker (similar to 20 km) and more melt-dominated (similar to 25% vol. partial melt) while lower 87Sr/86Sr, and higher Nd-143/Nd-144 reveal thinner marginal zones of the APMB where lower contents of partial melt (< 10% vol) involves reduced interactions. The lowest Sr-isotope ratios, and higher Nd-isotope ratios (where available) occur in magmas erupted outside the APMB (e.g. San Pedro, Lascar and Aucanquilcha volcanoes), indicating a diminished influence of crustal partial melts on parental mafic magmas. These geochemical parameters provide a useful tracer for the extent and significance of crustal partial melt bodies in magma genesis in the Central Andes.