Late Cretaceous arc rocks in the Andean retroarc region at 36.5°S: Evidence supporting a Late Cretaceous slab shallowing

Abstract

This work is focused in a group of subvolcanic bodies and lava domes located in the retroarc region of the southern Central Andes in an area between 35.5 and 37.5°S, whose age and composition were poorly constrained. These bodies are aligned, parallel to the Andean orogenic front some 150 km east of the Present arc front. These rocks had initially been assigned to the Neogene, but new Ar/Ar ages here presented indicate a Late Cretaceous age. Moreover, their chemistry reflects a linkage with the asthenospheric wedge, with weakly evolved arc signature. This imposes a new interpretation of the tectonic setting for these latitudes. When compared Late Cretaceous arc rocks with Neogene ones some similarities and differences are found. Partially superimposed along this segment of the Andes, a Neogene shallow subduction setting had been proposed in the area. In this context we compare these new identified centers with other nearby of Late Cretaceous age, as well as with arc-related Neogene volcanic rocks. Our analysis showed that these centers may have evolved under different conditions in relation to the slightly older ones located immediately to the southwest and to the younger ones (Late Oligocene–Early Miocene) found in this segment These volcanic centers may constitute the westernmost Late Cretaceous arc front emplaced over the eastern slope of the Andes in coincidence with a volcanic gap during the latest Cretaceous/Eocene times time in the adjacent Chilean sector at these latitudes. The occurrence of two shallow subduction episodes along the same segment, associated with two definite cycles of crustal thickening at the time of foreland arc expansion is discussed.