Emplacement mechanism of the Sierra de Varas Norte Pluton, Antofagasta Region, Chile.

Abstract

The Sierra de Varas Norte pluton is a tonalitic intrusive body with an elliptical shape on the horizontal, it has a 12 km long axis by a recostructed 8 km width. It has a funnel shape in vertical section and is calcalkaline in composition. It was emplaced during the Late Carboniferous into a stratigraphic succession of Pennsylvanian to Permian? age which belongs to the Sierra de Varas Formation. The pluton contains a concentric, steeply inclined magmatic foliation enhanced by concordant 'schlieren', strangled synplutonic microdioritic dykes and microdioritic enclaves. The enclaves are more abundant towards the pluton margin and they are probably related to the dykes. Ellipticity measurements on the enclaves yield an oblated strain ellipsoid with Flinn parameter k=0,5, characterized by an X axis longer than the Y axes, with the XY plane contained in the magmatic foliation. In the deformed country rock an oblate strain ellipsoid also with Flinn parameter k=0,5 was obtained from deformed oncolites. The folding, axial plane cleavage and contact metamorphism in the country rock are more intense towards the contact with the pluton. This strongly suggests a mostly lateral flattening. The pluton and the country rock fabrics are interpreted to be concurrent and derived from a ballooning forceful emplacement of the pluton in shallow levels of the continental crust. The stretched oblate form of the strain ellipsoid (X longer than Y), and also the plunge towards the centre of the pluton of the stretching lineation in the country rock , suggest that the pluton-country rock system retained the memory of a diapiric ascent.