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

An abrupt upward shift of the sedimentary facies in the Las Chilcas Formation exists, from limestones and sandstones, in the lower section, to a coarsening upward clastic succession in the upper section culminating with thick (30-50 m) strata of very coarse (up to 50 cm long clasts) sedimentary breccias and conglomerates (deposited coevally with the Caleu pluton emplacement). This feature is indicative of an abrupt change in the sedimentary regime with time. In fact, limestones of the lower section are marine and correspond to a transgressive open carbonate shelf (Martinez-Pardo et al. 1994), whereas the clastic rocks of the upper section could represent a syntectonic molasse deposit that accompanied tectonic basin inversion.

Final considerations

It is worth noting the similarities and differences between the Early Cretaceous rifted continental margin evolution of the West Peruvian Trough (Atherton 1990; Petford & Atherton 1995) and the contemporaneous evolution of this Chilean Coastal Range segment. In both cases, the sequence of events started with extensional basin magmatism and culminated in emplacement of plutons, passing through basin subsidence. However, the Coastal Batholith of Peru is considered to have been derived from partial melting of a 'new crust' (Atherton 1990). In contrast, the Caleutype plutonism cannot be considered as a product of recycling of young lower mafic crust, whose upper-crustal equivalents would be the Veta Negra basaltic rocks. In fact, this assumption is not tenable as the Caleu pluton is isotopically more depleted than the Veta Negra volcanic rocks (Fig. 8).

Finally, it is conceivable that the Caleu plutonism is the turning point between an extensional and a compressional regime. This switch in geodynamic setting would have occurred during the time elapsed between the generation and emplacement of the Caleu magma, as suggested by: (1) the coincidence in the ages of the Caleu pluton emplacement and the climax of the non-deformational very low-grade burial metamorphism, which marks the end of subsidence; (2) the unroofing of the pluton since it was emplaced; (3) the contemporaneity between pluton emplacement and the sedimentary response to the basin inversion, represented by the syntectonic breccias and conglomerates (upper section of the Las Chilcas Formation), containing abundant metavolcanic clasts with very low-grade mineral assemblages. In summary, the Caleu magma would have been generated during the climax of the Early Cretaceous extension and emplaced during the beginning of basin inversion.

This study was mainly financed by FONDECYT grant 1990980. Additional funds were provided by two CNRS-CONICYT grants (1999 and 2000). FONDEF grant 1033 and FONDECYT grant 1031000. We thank J. Richards, R. Tosdal and M. Villeneuve for constructive reviews of the original manuscript. This paper is Contribution Geosciences Azur number 658.

References

ÅBERG, G., AOUIRRE. L., LEVI. B. & NYSTRÖM, J.O. 1984. Spreading-subsidence and generation of ensialic marginal basins: an example from the Early Cretaceous of central Chile. In: KOKELAAR, B.P. & HOWELLS, M.F. (eds) Volcanic and Associated Sedimentary and Tectonic Processes in Modern and Ancient Marginal Basins. Geological Society, London, Special Publications, 16, 185-193.