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

Biotite. Laser-probe analyses were carried out on five single grains of biotite from three samples. The ^sup 40^Ar/^sup 39^Ar age spectra displayed by the biotites from a sample of gabbro CA99-1 give two well-defined plateau ages of 94.3 ± 1.4 and 93.4 ± 0.8 Ma (Fig. 5). Two biotites from gabbro located near the pluton margin (sample CA99-7) gave more disturbed age spectra, which, nevertheless, define plateau ages of 96.0 ± 0.6 and 96.0 ± 0.8 Ma. These slight disturbances may be the result of a weak chloritization of the mineral that is observed in thin sections. A small grain of biotite from tonalite (sample CA99-3) also yielded a disturbed age spectrum, but still defined a plateau age of 94.9 ± 1.8 Ma.

Plagioclase. A plagioclase bulk sample from tonalite CA99-3 displays a plateau age of 94.0 ± 0.3 Ma containing over 80% of the ^sup 39^Ar released (Fig. 5). This age is concordant with the ^sup 36^Ar/ ^sup 40^Ar v. ^sup 40^Ar/^sup 39^Ar correlation diagram age of 93.7 ± 0.6 Ma (not shown; initial ^sup 40^Ar/^sup 36^Ar ratio of 296.8 of atmospheric composition, MSWD = 2.5) calculated on the plateau fraction. The ^sup 37^Ar^sub Ca^/^sup 39^Ar^sub K^ ratio ranges from 0.2 to 2.0.

Detailed ^sup 40^Ar/^sup 39^Ar analytical results for biotites, amphiboles and plagioclases are given in Table 3.

Fission-track results

Apatite mineral separates from two gabbros (970118-3, CA99-1), three tonalites (970121-4, 970524-3, CA99-7) and two granodiorites (970616-1, CA99-4) from the samples dated by ^sup 40^Ar/^sup 39^Ar were dated by the fission-track method. Apatite in three samples contained substantial dislocations and pitting, and was variously zoned. The apatite fission-track age data given in Table 4 are therefore considered to be highly reproducible. The pooled fission-track ages (error at the 2σ level) of the samples are: 92.7 ± 6.2 Ma and 93.1 ± 2.6 Ma for the Gabbro-Diorite Zone samples; 90.4 ± 7.6 Ma, 95.7 ± 6.2 Ma and 96.5 ± 5.4 Ma for the Tonalite Zone samples; and 82.0 ± 5.6 Ma and 91.9 ±5.1 Ma for the Granodiorite Zone samples. Six of the analysed samples contain very long (unannealed) confined tracks, and a long mean confined track length, indicating that these samples cooled relatively rapidly. The oldest age (CA99-7) is based on only seven relatively dislocation-free grains. Most of the ages reported here are very similar to the previous apatite fission-track age of 91 ± 3.2 Ma reported by Gana & Zentilli (2000) for the same pluton. In addition, the ages of the Caleu pluton are close to apatite fission-track ages obtained on Jurassic and Palaeozoic granitoids (98 ± 10 to 106 ± 8 Ma; Gana & Zentilli 2000) that crop out c. 70 km west of the Caleu pluton, along the coastline south of Valparaiso.

Interpretation and tectonic significance of the geochronological data

Magmatism and regional thermal history

A magmatic quiescence is recognized between the Veta Negra volcanism and the Caleu plutonism. In fact, plagioclases from basaltic flows of the Veta Negra Formation at the Bustamante Hill and Chacana sections have been dated around 119 ± 1.2 Ma by the ^sup 40^Ar/^sup 39^Ar method (Aguirre et al. 1999; Fuentes et al. 2001), which are about 24 Ma older than the Caleu pluton (Fig. 2). The only magmatic activity occurring between these two events is the small volume of andesites, rhyolite and volcaniclastic intercalations in the lower and upper sedimentary section (c. 110-106 and 101-95 Ma, respectively) of the Las Chilcas Formation.