Crustal and Subcrustal Magmatic Components of Cenozoic Andean Arc Magmas in Central Chile (32°45’-34°30’S)

Abstract

In Central Chile (32°45’-34°30’S) there is a protracted Cenozoic arc magmatic activity that developed almost uninterruptedly between,at least, the middle Eocene and up to the present. This is recorded in extensive continental and mainly volcanic deposits, intrusiveunits, and the current volcanics which are widely distributed from the axis of the main Andes Cordillera to the west up to the easternportion of the Central Depression. The evolution of the margin throughout this period was initially framed by an extensional event,between (middle Eocene?) Oligocene - early Miocene, characterized by an intra-arc rift system developed over a thin crust (~35 km;Charrier et al., 2002; Nyström et al., 2003; Kay et al., 2005). The system was then inverted in the early Miocene under a compressionalregime that was active until today and responsible for the constructional processes of the modern Andes in the region (e.g. Giambiagi& Ramos, 2002; Farías et al., 2010). Overall, arc igneous rocks associated with this evolution show compositional variations that aremarked by an increasing crustal signal, with a progressive migration of the magmatic loci to the east, from the westernmost Oligocenefrontal arc deposits (current Central Depression) up to the Holocene active arc axis (Andean drainage divide, ~80 km in currentdistance; Charrier et al., 2002; Nyström et al., 2003; Kay et al., 2005).Along with new determinations presented herein, the numerous studies developed in Central Chile in the last decades provide anextensive compositional characterization of Cenozoic arc magmas in terms of whole rock geochemistry and Sr, Nd, Hf and O isotopiccompositions. This allows for a comprehensive interpretation of the major magmatic components, the crustal vs subcrustal sources,and the dynamic variations recorded throughout the evolution of the Andean margin. Four major components involved in the magmagenesis of this arc segment have been identified: (i) the subcrustal Subduction Modified Depleted Mantle, which sources the juvenilearc magmatic additions prompting extensive MASH-type magmatic processes in the deep continental crust, (ii) a crustal Deep EvolvedComponent, here ascribed to the Cuyania terrane Grenvillian basement, (iii) a Juvenile Low-δ18O Component, which likely correspondsto an oceanic crust type domain located deep within the continental crust, and (iv) a Late Paleozoic – Early Triassic Component, likethe Frontal Cordillera crystalline basement, located at medium to upper crustal levels underneath the Mesozoic cover of the mainAndes Cordillera in this region. The influence of these components is variable depending on the time and location of the arc magmaticunits. In order to evaluate their relative contributions, geochemical models are being tested based on thermodynamic simulations ofmineral phase equilibrium, melt compositions and isotope partitioning models. Overall, the composition of Cenozoic arc magmas inCentral Chile is strongly controlled by the continental crust, both after (i) the constitution and architecture of this domain and itsreshaping throughout the evolution of the Andean margin and by (ii) the governing tectonic regime.