Mechanism and depth of emplacement of the Cerro Cristales Pluton, Cordillera de la Costa de Antofagasta, Chile.

Abstract

The Cerro Cristales Pluton, of late Jurassic-early Cretaceous age, is exposed in the Coastal Cordillera south of Antofagasta (24-25ºS). The pluton shows a petrographical and structural zonation, defined by a strongly foliated mainly tonalitic margin and an isotropic to nalitic core.The foliation of the margin has a magmatic character and it is defined by a subparallel orientation of plagioclase crystals. The geometry of the foliation is characterized by a planar arrangement of minerals, stretching lineation was not developed. Deformed elliptic amphibolitic enclaves are parallel with the magmatic foliation and plot in the field of oblated ellipsoids on a Flinn-diagram. This oblated geometry and the absence of mineral lineation can be interpretad as the result of pure shear deformation induced by a lateral expansion of the pluton during its emplacement. The isotropic core is interpretad as the magma conduit that allowed the volumetric expansion of the pluton. Geobarometric determinations, show that crystallization pressure was near 3.6 kbar, suggesting that the pluton was emplaced at a depth of nearly 13 km. Field relations show that the pluton was emplaced along a major north-south sinistral shear zone developed in Jurassic gabbros and volcanic rocks of the La Negra Formation. The activity of the shear zone is older than 153 Ma, but was still active during the intrusion of the pluton near 143-145 Ma.