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

Journal of the Geological Society, Mar 2005 by Parada, Miguel A, Féraud, Gilbert, Fuentes, Francisco, Aguirre, Luis, Et al

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

Keywords: Chile, Caleu pluton, absolute age, exhumation, extension.

Widespread rifting and generation of ensialic basins from Colombia in the north to southernmost Chile are the most outstanding features characterizing the evolution of the active western margin of South America during the Cretaceous. These events have been interpreted as the outcome of trench retreat that led to partial or total crustal attenuation at the continental margin (Pankhurst et al. 1988), asthenospheric upwelling to fill the gap (Aguirre et al. 1989), bimodal volcanism, subsidence, burial metamorphism and plutonic activity (Åberg et al. 1984; Vergara et al. 1995; Aguirre et al. 1999). The imprints of such events are at present recorded as volcano-plutonic lineaments extending for thousands of kilometres along the Andean Range (Dalziel 1981; Åberg et al. 1984; Aguirre et al. 1989).

In Peru, this type of lineament is represented by the West Peruvian Trough, which evolved by 'encratonic' spreading involving marginal basin development (Atherton et al. 1983, 1985). A composite marginal basin (Huarmey-Cañete) was the site of intense volcanism, which deposited upwards of 9000 m of volcanic and volcanogenic material from the Tithonian to the Albian, with maximum subsidence in the Albian (Atherton et al. 1985). These rocks were later affected by low-grade metamorphism under high geothermal gradients. The lineament-constrained multiple Peruvian Coastal Batholith forms the core of the Western Cordillera, occupying, for the most part, a c. 50 km wide band lying within the exposed Albian rocks of the marginal basin (see Pitcher & Cobbing 1985; Atherton 1990).

In north and north-central Chile, between latitudes 25°S and 36°S, bimodal volcanic activity took place along the rifted continental margin during the Early Cretaceous (c. 120 Ma), generating a c. 1200 km long uninterrupted belt with an average width of 30 km and thickness of 3-13 km. These volcanic rocks were later affected by a very low-grade metamorphic event dated in adularia at 93.1 ± 0.6 and 94.2 ± 1.2 Ma (Aguirre et al. 1999). Granitoid complexes with ages around 90-96 Ma, bearing a chemical mantle signature akin to the volcanic material, are spatially related to this volcanic belt.

In Chile, the temporal relationships between the granitoids, the basinal volcanic rocks, the metamorphic phenomena, and the exhumation of these volcano-plutonic lineaments have been poorly explored. In north-central and central Chile, the inception of extensional basins during the Cretaceous and their subsequent evolution is the most outstanding orogenic event of the Mesozoic, which led to the rise of the Andean mountain chain along this margin of South America.

The aim of this study is to provide evidence from zircon U-Pb, hornblende, biotite and plagioclase ^sup 40^Ar/^sup 39^Ar, and apatite fission-track dating about the timing of magmatism and subsolidus cooling of the Caleu pluton, a c. 340 km^sup 2^ composite intrusion emplaced in a Lower Cretaceous succession affected by very low-grade metamorphism. The geological setting of the pluton is favourable for establishing temporal relationships between the plutonism, the very low-grade metamorphism of the volcanic host rocks and the tectonic processes that took place during the Early Cretaceous, a key period of the Andean geodynamic evolution in central Chile.

Geological setting

The Early Cretaceous magmatism of the Coastal Range segment (32°30'-33°30'S) of central Chile is represented by the Caleu pluton and its host volcanic-sedimentary succession (Fig. 1). The Caieu pluton is a good example of the numerous Early Cretaceous plutons of the Coastal Batholith of central Chile. These Early Cretaceous plutons form part of one of the largest Andean magmatic provinces, which extends as a continuous belt for over c. 750 km, from 27° to 34°S. In the studied segment of the Coastal Range, Early Cretaceous granitoids and volcanic rocks crop out over an area of more than 6000 km^sup 2^. The Early Cretaceous plutonism corresponds to the eastern belt of the Coastal Batholith segment, which is made up of three north-south belts with age decreasing eastward from late Palaeozoic to Jurassic to Early Cretaceous.