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, 40 Ar/ 39 Ar 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 8C and 250 8C is documented by 40 Ar/ 39 Ar plateau ages on amphibole, biotite and plagioclase between 94.9 AE 1.8 and 93.2 AE 1.1 Ma. Slower subsolidus cooling to c. 100 8C 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.

Transtension y transpresion del Jurasico Medio-Superior al Cretacico Inferior durante la construccion del arco magmatico en Chile central: evidencia a partir de enjambres de diques maficos.

“…These ages are broadly similar to those obtained along the N-W belt exposed in the Coastal Ranges north of 33ºS (Irwin et al, 1987;Parada et al, 1988). LowerUpper Cretaceous granitoids occupy the eastern slope of the Coastal Ranges and vary in age between 118 and 91 Ma (Corvalán and Munizaga, 1972;Parada et al, 2005). In addition to plutonic rocks, several mafic dike swarms have been recognized along the coastline between 33° and 33°45'S, mainly intruding the Upper Paleozoic basement.…”

El Oroclino del Maipo: Un rasgo estructural de primer orden en la evolución geodinámica Mioceno a Reciente en los Andes de Chile central.

“…Open circles are paleomagnetic sites from this study. Solid circles are previous paleomagnetic studies from Irwin et al (1987), Creixell et al (2006), Parada et al (2005), Beck et al (1986Beck et al ( , 1990, Goguitchaichvili et al (2000), Charrier et al (1996) and Astudillo et al (2009). See tables 1 and 2 for details.…”

“…North of 33ºS, previous studies show no evidence for significant tectonic rotations (Beck et al, 1986(Beck et al, , 1990Parada et al, 2005;Creixell et al, 2006). In contrary, south of 33ºS, both in the Coastal Cordillera and High Andes, clockwise block rotations have been observed and attributed to in situ block rotations in response to oblique convergence (Beck et al, 1986(Beck et al, , 1990 or to deformation of the Andean Cordillera since Early Oligocene-Middle Miocene times (Charrier et al, 1996;Goguitchaichvili et al, 2000).…”

Structural overprint of a late Paleozoic accretionary system in north-central Chile (34°-35°S) during post-accretional deformation

“…On the one hand, considerable acceleration of exhumation was proposed by Willner et al (2005) for the study area for the mid-Cretaceous event suggesting some crustal thickening. This shortening episode seems to have affected the entire forearc at least up to 30°S (Arancibia, 2004;Parada et al, 2005). On the other hand, no marked crustal thickening evidently occurred in the forearc since latest Tertiary times.…”

“…Arancibia (2004) dated ductile reverse shearing along the prominent Silla del Gobernador shear zone further north (32°S) at 109±11 Ma (Ar/Ar; white mica grains). Due to widespread apatite fi ssion track ages in north-central Chile around ~100 Ma a mid-Cretaceous shortening event, responsible for the closure of Mesozoic basins and associated with an increased erosion rate, is now widely accepted (e.g., Parada et al, 2005). Thus, the mid-Cretaceous shortening episode affected a wider part of the forearc causing closure of the Mesozoic basins and acceleration of exhumation Belmar et al, 2004).…”