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 an area between 35.5 and 37.5ºS in the Southern Andean retroarc region, whose age and composition had not been previously analyzed. These bodies are aligned, parallel to the Andean orogenic front some 150 km east of the Present arc front. Even though, these rocks had initially been assigned to the Neogene, new Ar/Ar ages here presented indicate a Late Cretaceous age. Moreover, their chemistry reflects a linkage with the asthenospheric wedge, with poorly evolved arc signature. This , imposes a new interpretation tectonic setting for these latitudes. When compared Late Cretaceous arc rocks with Neogene ones some similarities and differences are found.. Partially superimposed, a Neogene shallow subduction setting had been proposed in the same area,. In this new context we compare these newly identified centers with other nearby Late Cretaceous in 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 south-west and to the younger ones (Late Oligocene-Early Miocene) found in the same location. These volcanic centers may constitute the westernmost Late Cretaceous arc front emplaced over the eastern slope of the Andes and the ones missing in Chilean sector at the same latitudes, during latest Cretaceous/Eocene times. The probable occurrence of two shallow subduction settings, associated with two demonstrable cycles of crustal thickening at the time of foreland arc-expansion is discussed.