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 rocks of the Veta Negra Formation have been affected by very low-grade, non-deformative metamorphism. The proportion of secondary minerals (in veinlets, amygdules, and primary mineral and glass replacement) increases downward in the pile, and the mineral assemblages characterize a gradual change of metamorphic facies from a pumpellyite-bearing zeolite facies at the top to a prehnite-pumpellyite facies in the middle and bottom (Levi et al. 1989). Regional metamorphic facies boundaries are parallel or subparallel to bedding and not to the exposed contacts with contemporaneous or younger granitoids in the area. Based on the physical conditions of the metamorphism, Aguirre et al. (1989) calculated a thermal gradient of around 20-30 °C km^sup -1^. However, a gradient of about 40-45 °C km^sup -1^ is deduced from the P- T conditions of the very low-grade metamorphism obtained from low-variance assemblages contained in amygdules of Veta Negra lavas (Aguirre et al. 1999).

Analytical procedures for ^sup 40^Ar/^sup 39^Ar and fission-track dating

^sup 40^Ar/^sup 39^Ar dating

Samples from the Caleu pluton were analysed by the ^sup 40^Ar/^sup 39^Ar step heating procedure on single grains of amphibole and biotite, a cluster of four grains of amphibole, and bulk samples of plagioclase. The samples were altered to different degrees, and we chose to analyse single grains despite the resulting increased errors. Grain sizes for single-grain and multi-grain analyses are of the order of 180-500 µm long. The grain sizes for the plagioclase bulk samples are 250-500 µm long and the analysed weight was 35-42 mg. Single grains of amphibole were separated using a magnetic separator and heavy liquids; biotite crystals were hand picked from crushed whole rocks; and plagioclases were separated using a magnetic separator, then carefully selected under a binocular microscope, so as to select only transparent grains. The samples were irradiated in the nuclear reactor at McMaster University (Hamilton, Canada), in position 5c. The total neutron flux density during irradiation was 8.8 × 10^sup 18^n cm^sup 2^, with a maximum flux gradient estimated at ±0.2%. We used the Fish Canyon sanidine (FCs) as a flux monitor with an age of 28.02 Ma (Renne et al. 19981. The plagioclase bulk sample analyses were performed with a mass spectrometer composed of a 120° M.A.S.S.E. tube, a Baür-Signer GS 98 source and a Balzers electron multiplier. The gas extraction of single grains and small clusters of grains was carried out by heating with a Synrad 48-5 CO2 laser. The mass spectrometer used was a VG 3600 working with a Daly detector system. Typical blank values of the extraction and purification laser system are in the range (30-90) × 10^sup -14^, (0.3-8) × 10^sup -14^, (10-20) × 10^sup -14^, and (0.3-7) × 10^sup -14^ cm^sup 3^ STP (standard temperature and pressure conditions) for the masses ^sup 40^Ar, ^sup 39^Ar, ^sup 37^Ar and ^sup 36^Ar, respectively, measured every third step. The criteria for defining plateau ages were the following: (1) it should contain at least 70% of released ^sup 39^Ar; (2) there should be at least three successive steps in the plateau; (3) the integrated age of the plateau should agree with each apparent age step of the plateau within a 2σ error confidence interval. Uncertainties on the apparent ages of each step (see Table 3 and age spectra) are quoted at the 1σ level and do not include the errors of the age of the monitor. All other quoted uncertainties are given at the 2σ level. The error of the ^sup 40^Ar*/ ^sup 39^Ar^sub K^ ratio of the monitor is included in the plateau age error calculation. We present here only data giving plateau ages.