Tectonic setting, age and emplacement mechanisms of the end-Cretaceous to Palaeocene magmatic arc system, Precordillera of Vallenar, Northern Chile
The emplacement of the end-Cretaceous to Palaeocene magmatic arc in the Precordilleria of Vallenar occurred during a fundamental switch in deformation style of the upper plate of the Andean margin from extension/transtension to contractional deformation. This switch in regional tectonics is shown by combining precise [sup]40 Ar/[sup]39 Ar geochronology with microstructural and structural field observations that demonstrate a direct link between the ages for emplacement, cooling and subsequent deformation of end-Cretaceous to Palaeocene magmatic arc. The emplacement of the Las Campanas pluton during upper plate extension, 70.6 ± 1.3 Ma, was accommodated by a floor-subsidence mechanism that normally reactivated the east-dipping Agua de los Burros extensional growth fault. This generated a steep upper amphibolites-facies, syn-plutonic, brittle-ductile, dip-slip shear zone that is located on the pluton's western side and exhibits a consistent east-down sense of shear. [sup]40 Ar/[sup]39 Ar hornblende and biotite ages from the pluton interior and shear zone are the same, within error, which demonstrates rapid cooling of the pluton. The emplacement of the Chehueque pluton, 69.1 ± 0.62 Ma to 66.75 ± 0.62 Ma was also accommodated by floor-subsidence mechanisms during upper plate extension, but a switch from regional extension/transtension to contraction can be occurred prior to cooling. The Chehueque pluton has an upper-amphibolite facies brittle-ductile shear zone along its western margin, similar to that of Las Campanas. However, this shear-zone is much wider and shows both east-down and east-up sense of shear indicators. This duality in shear sense is attributed to two phases of deformation during the emplacement and subsequent cooling in this part of the Palaeocene magmatic arc. The initial extensional phase is directly linked to floor-subsidence emplacement mechanisms that generated passive down-bending in the host rocks, towards the pluton. This was closely followed by deformation linked to the initial stages of regional contraction that superimposed an east-up sense of shear; these kinematic indicators can be matched between the host rocks and pluton. This difference in syn-plutonic deformation style and the ages of the Las Campanas and Chehueque plutons therefore constrains the time of the switch from regional extension/transtensional deformation to contractional deformation to a period of c. 4 Myr in latest Cretaceous to early Palaeocene time in the Precordillera of Vallenar.