El Batolito de Colangüil (29-31°S) cordillera frontal de Argentina: estructura y marco tectonico

Abstract

ABSTRACT. The Colangüil Batholith (CB) is located on the eastern border of the Cordillera Frontal (29-31°S), northern San Juan Province. Intrusion took place between 264 and 247 Ma, after the San Rafael orogenic phase (SROP). It consists of 61 % granodiorites and 39% granites and silicic longitudinal dike swarms with minor mafic dikes. Global composition is different from circumpacific batholiths which are more mafic. Most of the CS was intruded into the Rio Blanco-Agua Negra sedimentary basin of Upper Carbonilerous-Asselian age. The SROP took place between the end of the sedimentation in this basin (Asselian) and the beginning of the intrusion of the CB, being 280-270 Ma its most probable age. The first batholith intrusions are granodiorites, being roughly coeval with the first eruptions of the Choiyoi volcanics, which consist of andesitic to dacitic ignimbrites. Both sequences evolved from intermediate to silicic and high silicic composition. The compressive stresses of the SROP folded and thrusted a wide belt along the Cordillera Frontal. Degree of deformation decreased gradually to the east, and thus the eastern Precordillera and Sierras Pampeanas were not deformed. The selective control of deformation was favored by thermal weakening. As a consequence of the SROP, the crust was thickened giving rise to an extensional regime which was enhanced by the high thermal gradient. CB was intruded following the tensional fractures. Thus, the relationship between the SROP and the subsequent extensional regime explains the selective localization of the Permian magmatism in Cordillera Frontal, along the most deformed belts. Previous to the emplacement of the CB, a subducting regime was active and probably ended at the same time the SROP was no longer active. CB is not related to the subductive regime because when the motion of the slab being subducted stopped, it became thermally equilibrated, the dehydration processes ceased, and the convective flow in the astenospheric wedge stopped. However, the thermal anomaly generated during this process was preserved even though the motion of the subducing slab was unimportant. Heating of the thickened crust was favored on account of the motionless state of the lithosphere giving rise to a basic underplate which melted the base of the thickened crust.