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

From geochronological data (c. 94 Ma) obtained on minerals in equilibrium with peak burial conditions of metamorphism, Aguirre el al. (1999) proposed that the very low-grade metamorphism of the Veta Negra flows was the result of burial conditions only, and was not related to the numerous and large igneous bodies (including the Caleu pluton) intruding the Veta Negra Formation. This hypothesis was supported by the fact that the metamorphic mineral assemblages are a direct function of the stratigraphie depth (Levi 1969; Aguirre et al. 1989; Levi et al. 1989), and are not conformable with exposed boundaries of the pluton. However, without any precise knowledge of the emplacement age of the pluton and the age of the metamorphism of the host rocks, it was hard to establish any reliable time relationship between them. The geochronological data obtained for the Caleu pluton clearly show that its emplacement and cooling (dated between 93.2 ± 1.1 and 94.9 ± 1.8 Ma) is coeval with the low-grade metamorphism (Fig. 2) dated between 93.5 ± 0.6 and 94.2 ± 1.2 Ma by ^sup 40^Ar/^sup 39^Ar on aduiaria in the Bustamante Hill section of the Veta Negra Formation (Aguirre et nl. 1999). Therefore, the low-grade metamorphism dated may not be the result of burial alone, but may also involve a rise in the regional thermal gradient related to at least one magmatic event. The Caleu pluton was thus emplaced at the end of the Veta Negra basin subsidence, which coincided with the peak of very low-grade burial metamorphism. It is interesting to note that the temporal and spatial relationships between plutonism and low-pressure-high-temperature metamorphism has been described elsewhere as a consequence of an underlying thermal event or as a direct product of the advection of heat caused by plutonism (see Lux et al. 1986; Barton & Hanson 1989). The results of our study indicate that these relationships and their thermal implications can be validly applied to the development of low- to very low-grade metamorphism.

Subsolidus cooling of the plu ton

Good agreement is observed for most of the ^sup 40^Ar/^sup 39^Ar plateau ages obtained on biotite, amphibole and plagioclase from samples CA99-1, -3, -4, -6 (biotites CA99-7 excepted) mainly ranging from 93.2 ± 1.1 to 94.9 ± 1.8 Ma, if we exclude plateau ages affected by large error bars (concordant, nevertheless, with this age range). These ages are also coincident with the U-Pb zircon age estimated in the interval 94.2-97.3 Ma.

The significantly different plateau ages of 96.0 ± 0.6 and 96.0 ± 0.8 Ma obtained on biotites from sample CA99-7, the only sample located near the pluton margin, can be explained by an earlier cooling of this marginal zone of the pluton, too old ages owing to undetectable excess argon (the ^sup 36^Ar/^sup 40^Ar v. ^sup 40^Ar/ ^sup 39^Ar inverse isochron plots display initial atmospheric ratios), or chloritization of the biotite. Because of the concordance of precise ages obtained on amphibole (93.7 ± 0.6 Ma) and plagioclase (94.0 ± 0.3 Ma) from samples CA99-4 and CA99-3 located in the core region of the pluton, which have a estimated closure temperature off. 500-550 °C (McDougall & Harrison 1999) and 250°C (Berger & York 1981), respectively, it is unlikely that the age difference obtained on biotite of about 2 Ma, measured on core and margin of the pluton, is real. Chloritization of the CA99-7 biotite is observed in thin section, but its effect seems low, as shown by the age spectra. Nevertheless, one of the biotites gives a weighted mean age of 94.9 ± 1.1 Ma on the five high-temperature steps (corresponding to 41% of the total ^sup 39^Ar released), which may give a more valid age for chloritization (Ruffet et al. 1991).