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

Fission-track dating

Apatite separates from seven samples were mounted in epoxy and polished to expose internal grain surfaces. Spontaneous fission tracks were revealed by etching in 5M HNO^sub 3^ for 20 s at 24 °C. Fission-track ages were determined using the external detector method (Hurford & Green 1982). A thin sheet of low-uranium muscovite was placed in contact with the polished surface of each grain mount to serve as a detector of neutron-induced fission. Samples were irradiated at the Dalhousie University Slowpoke reactor in the presence of a glass neutron dosimeter of known characteristics.

Track densities for both spontaneous and induced fission-track populations were counted at 1000× magnification. Fission-track ages were calculated using a weighted mean zeta calibration factor (Fleisher et al. 1975; Hurford & Green 1982), determined using the Fish Canyon Tuff apatite (obtained from C. W. Naeser) and Durango apatite (obtained commercially) age calibration standards.

Geochronological results of the Caleu pluton

Nine samples of plutonic rocks were selected for dating (Fig. 2). Eight of them were collected to provide cooling ages, and one sample was selected to date the emplacement of the pluton by the U-Pb method. Five samples were dated by the step heating ^sup 40^Ar/^sup 39^Ar method on minerals, and seven by apatite fission-track techniques. Sample locations and descriptions are given in Figure 3 and Table 1, respectively.

U-Pb results

Single-grain zircon analyses were carried out at the MIT laboratory for the Tonalite Zone sample 970121-4, following the procedures given by Schmitz & Bowring (2001). This sample was collected from the southernmost outcrops of this zone and at the lowest elevation (850 m; Table 1 ). Six grains were analysed (Table 2) and the results are shown in the concordia diagram (Fig. 4). Four of the five clustered data (zl, 3, 4, 5, 7) lie on the concordia (z3, 4, 5, 7), but they are not identical in age, spreading over about 3 Ma along the curve. Therefore we are unable to give an anambiguous U-Pb age for this sample. Nevertheless, it is likely that the age is bracketed in the interval 94.2-97.3 Ma.

^sup 40^Ar/^sup 39^Ar results

Amphibole. One amphibole single grain from gabbro (CA99-7) displayed a plateau age of 95.0 ± 2.8 Ma followed by a very high apparent age (last step) that may be caused by excess argon (Fig. 5), possibly trapped in an inclusion. The variable ^sup 37^Ar^sub Ca^/^sup 39^Ar^sub K^ ratios, the ranging from of 4 to 30, demonstrate that the analysed grain is not pure. This ratio, calculated from CaO/K^sub 2^O ratios measured by electron microprobe analysis (EMPA; CaO/K^sup 2^O= 2.179 × ^sup 37^Ar^sub Ca^/^sup 39^Ar^sub K^), varies between 9.1 and 12.6.

Two small amphibole grains from tonalite sample CA99-3 gave plateau ages of 92.1 ± 4.6 and 96.0 ± 3.9 Ma, affected by large error bars. Both age spectra are characterized by a rather constant ^sup 37^Ar^sub Ca^/^sup 39^Ar^sub K^ ratio (c. 13) with a value close to the EPMA ratio (8-10). A single amphibole grain from sample CA99-6, of the same Tonalite Zone, displays a perturbed age spectrum giving a plateau age of 93.2 ±1.1 Ma. The ^sup 37^Ar^sub Ca^/^sup 39^Ar^sub K^ ratio spectrum is variable and contains only two successive steps with values similar to the EPMA ratio. The large error bars are explained by the small size of the amphibole grain. A group of four amphibole crystals from granodiorite sample CA99-4 yields a well-defined plateau age at 93.7 ± 0.6 Ma. The relatively constant ^sup 37^Ar^sub Ca^/^sup 39^Ar^sub K^ ratios with values around five closely approach the EPMA ratios of 6-10.