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

The high concordance of U-Pb zircon age and plateau ages obtained from amphibole CA99-4 and plagioclase CA99-3 strongly argues for fast cooling of the pluton immediately after its crystallization, to host rock temperature at the site of emplacement (Fig. 6). This last temperature may be deduced from the thermal gradient obtained from the c. 93 Ma metamorphism of the Veta Negra Formation at Bustamante Hill, located about 35 km further south (Fig. 1; Aguirre et al. 1999). The use of this information seems justified by the homogeneity of the mineral parageneses (primary and secondary) of the Veta Negra Formation over the whole region, from Bustamante Hill to the Chacana area, 25 km north of the Caleu pluton (Fig. 1; Fuentes et al. 2001). Based on (1) the assumed depth of emplacement of the pluton at about 7 km (Parada et al. 2002), and (2) a thermal gradient of c. 40-45 °C km^sup -1^, a temperature of the host rocks of c. 280-315 °C at the depth of the pluton emplacement is deduced. The rapid cooling of the pluton at c. 94 Ma ended only when pluton and host rocks cooled together below 250 °C (blocking temperature of Ar retention in plagioclase; see Fig. 6). Whereas U-Pb and ^sup 40^Ar/^sup 39^Ar data apparently indicate a rapid cooling rate of the Caleu pluton, the fission-track ages seem to correspond to a slower cooling event(s) as shown by the plots of the fission-track age data in the cooling diagram (Fig. 6).

Exhumation evidence for the Caleu pluton

An exhumation event, defined as the vertical displacement of rocks with respect to the surface (England & Molnar 1990; Stüwe & Barr 1998), may have started coevally with the emplacement of the pluton. Although the individual errors on apatite fission-track ages are too large to constrain the cooling path, a thermal model applied to three apatite sample datasets (samples CA99-1, CA99-4 and CA99-7) using a FORTRAN algorithm developed by Willett (1992, 1997) and modified by Issler (1996) allows more useful information to be obtained. Distribution of fission-track single-grain ages in the samples pass the χ^sup 2^ test. It is assumed therefore that they are composed of only one population of fission-track ages and that the age and length data represent a single modelable history. Figure 7 shows the preferred calculated thermal history (cooling-only model) for the three samples since 100Ma. The results indicate that the samples appear to have cooled at c. 90 Ma from temperatures high enough to cause substantial apatite fission-track annealing, and have not subsequently experienced any significantly elevated temperatures. A cooling rate of 5°CMa^sup -1^ is obtained from Figure 6 for the 100-80 Ma interval. No significant variation in the very slow cooling rate is apparent after 80 Ma, as shown in Figure 7.

In conclusion, if the fission-track model is correct, it suggests exhumation of the pluton and host rocks between about 94 and 90 Ma. Nevertheless, our data do not allow us to determine whether the exhumation of the pluton occurred continuously along cooling path A (Fig. 6), as it cooled from its level of emplacement, or episodically following cooling path B as a result of intermediate temperature stabilization with a significant break in the cooling curve. The coincidence in time between the exhumation of the pluton and the deposition of the thick strata of conglomerates and breccias (molasse-type deposits) of the upper section of the Las Chilcas Formation (Fig. 2) suggests a cause-and-effect relationship.