The role of crustal thickness on magma composition in arcs: An example from the pre-Andean, South American Cordillera

We explore the temporal evolution of pre-Andean Cordilleran arcs in central and northwestern Argentina and northern Chile (27°-34°S) with a focus on the geochemical characteristics of the episodic magmatism and the relationship with crustal thickness. A compilation of ca. 5000 U-Pb bedrock and detrital zircon ages, with ages from 600 to 130 Ma, reveals seven temporal magmatic accretions with main peaks at ~526 Ma (Pampean); ~471 Ma (Famatinian); ~340 and ~305 Ma (Early Gondwanan); ~253 and ~221 Ma (Gondwanan); and ~ 160 Ma (Early Andean). We show that most of the magmatic episodes were developed in 30-50 km thick crust. Magmatism experienced fractional crystallization (r = 0) combined with variable degrees of assimilation (r > 0) that mostly did not exceed 30% crust. We conclude that increasing crustal thickness not only promotes more extensive differentiation by fractionation but also establishes conditions near host rock solidi at shallow crustal levels promoting further contamination in magmas.