Ages and cooling history of the Early Cretaceous Caleu pluton: testimony of a switch from a rifted to a compressional continental margin in central Chile

Abstract

The Caleu pluton, in the Coastal Range of central Chile, represents the last magmatic event related to the Early Cretaceous rifting along the western margin of South America. The pluton was emplaced into a c. 10 km thick pile of mainly basalts and basaltic andesites deposited in an Early Cretaceous subsiding basin, and affected by very low-grade metamorphism. The cooling history of the pluton is documented on the basis of U-Pb, Ar-40/Ar-39 step-heating and fission-track dating. The U-Pb date suggests an age of emplacement in the interval 94.2-97.3 Ma. Rapid subsolidus cooling between 550-500 degrees C and 250 degrees C is documented by Ar-40/Ar-39 plateau ages on amphibole, biotite and plagioclase between 94.9 +/- 1.8 and 93.2 +/- 1.1 Ma. Slower subsolidus cooling to c. 100 degrees C is identified at the 94-90 Ma interval by the fission-track thermal model. The geochronological data show that the emplacement of the pluton is coeval with the very low-grade metamorphism of the host rocks. Therefore, this metamorphism is probably not the result simply of burial, but also of a regional thermal gradient related to the plutonism. Exhumation of the pluton started coevally with its emplacement and continued to about 90 Ma, being associated with the closure of the Early Cretaceous rifting. The Caleu plutonism represents an asthenospheric-derived event during maximum extension, and marks a turning point between extensional- and compressional-related magmatism.