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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

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The Caleu pluton consists of three north-south elongated zones (Fig. 3), with contacts that dip subvertcally. The three zones, which define an across-pluton compositional variation characterized by a westward increase in SiO^sup 2^ content, are the Gabbro-Diorite Zone, Tonalite Zone and Granodiorite Zone.

The Gabbro-Diorite Zone has a north-south elongated shape, which occupies 140 km^sup 2^ at the eastern part of the pluton. It includes coarse-grained gabbros and diorites having a colour index between 18 and 30, because of the presence of clinopyroxene, hornblende and biotite.

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Amphibole-biotite-bearing tonalites and quartz diorites are most abundant in the Tonalite Zone, and cover the central part of the pluton as a north-south belt of c. 70 km^sub 2^. The contact between Tonalite Zone and Gabbro-Diorite Zone appears to be gradational, as indicated by a progressive decrease in the colour index (15-20) and in the grain size to medium-grained rocks. However, in some places small batches of quartz diorite intrude rocks of the Gabbro-Diorite Zone. A distinct feature of this zone is the presence of rounded dioritic enclaves of variable size from 5 to 40 cm.

The Granodiorite Zone covers 30 km^sup 2^ at the western part of the body and it extends towards the east as a subhorizontal sill of c. 100m thick, hosted within the Tonalite Zone. Intrusions of small Granodiorite Zone bodies into the Gabbro-Diorite Zone were also observed in the eastern part of the pluton. Unlike the gabbros and diorites. the granodiorites are leucocratic with a colour index between 5 and 10 resulting from the presence of minor amphibole and biotite.

Palaeomagnetic studies in the Caleu pluton (Parada et al. 2002) indicate normal polarity of the remanent magnetization, which is consistent with the long normal polarity Cretaceous superchron. The pluton has a declination of 9.6° and an inclination of -56.6°, which are slightly different from the expected values of 350° and -54.8°, respectively, for Cretaceous rocks. These differences have been explained by a 10-15° tilt of the pluton toward the east or by a 10-20° clockwise rotation. The remagnetized Lo Prado and Veta Negra strata (dipping 30° and c. 80° SW, respectively) at the southwestern margin of the pluton have the same in situ palaeomagnetic direction as those recorded in the adjacent plutonic rocks, suggesting that the pluton intruded the host volcanic formations after their tilting.

Ocoite Group: lithological and metamorphic features

In Bustamante Hill (c. 33°30'S; Fig. 1) the lowest stratigraphie level of the Ocoite Group corresponds to strata of the Lo Prado Formation, c. 2000 m thick, which consist of marine and continental clastic rocks, limestones, and a succession of dacitic ignimbrites and basaltic andesites (Fig. 2; Levi 1969). These rocks have been affected by low-grade non-deformative metamorphism characterized by greenschist-facies mineral assemblages (Levi 1969).

The Veta Negra Formation overlies the Lo Prado Formation between the Bustamanate Hill and Chacana areas (c. 32°40'S; Fig. 1). It is mainly composed of a pile of c. 5-10 km thick basaltic andesites and andesites (Levi 1969; Levi et al. 1982; Vergara et al. 1995), with a distinct porphyritic texture characterized by abundant and large (up to 2 cm) Ca-plagioclase phenocrysts, subordinate augite, titanomagnetite and small amounts of altered olivine crystals.

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

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