Plio-Holocene magmatic evolution during a steepening subduction regime

Abstract

From the late Pliocene to Holocene times, magmatism in the Southern Central Andes (34–38.5 ◦S) displayedsignificant geochemical changes that show a systematic variation with both time and location. These changeshave primarily been linked to the steepening of the Nazca plate after its shallow subduction configuration in thelate Miocene. As a result, Plio-Holocene magmatism developed in three N-S magmatic belts, located within thepresent-day arc front, the rear-arc, and the back-arc areas. This work presents the compilation of geochemicaldata (major and trace elements, and Sr-Nd-Pb isotopes) and geochronological data (Ar-Ar, K-Ar, U-Pb) from thePlio-Holocene rear-arc and back-arc magmatic sequences to evaluate the magmatic evolution since the beginningof the Nazca plate steepening in the latest Pliocene.The back-arc volcanic sequences show a decrease in arc-like geochemical features and an increase in theconcentration of incompatible elements from the northern to the southern back-arc units. This change has primarilybeen attributed to variations in the mantle source, with southern magmatism associated with an EM-1 OIBtype source and northern volcanism related to an N-MORB mantle source. However, other processes, such aschanges in the thermal structure of the mantle wedge, along-strike variations in crustal thickness, and forearcsubduction erosion, are essential to explain other geochemical features. On the other hand, the geochemistry ofthe Plio-Holocene rear-arc magmatism slightly reflects the geochemical patterns of the back-arc magmatism, asthe northern rear-arc units present arc-like features, while the southern rear-arc shows an incipient intraplatelikesignature. Thus, regarding the varying distances to the trench between back-arc and rear-arc units, firstordercontrols such as heterogeneities in ambient mantle composition and the thermal structure of the mantlewedge have influenced the magmatism of the Southern Central Andes.