Ar-40/Ar-39 dating of volcanism and subsequent very low-grade metamorphism in a subsiding basin: example of the Cretaceous lava series from central Chile

Abstract

Ar-40/Ar-39 geochronological method is applied to date both emplacement and subsequent very low-grade metamorphism of a thick lava series, the Veta Negra Formation from the Cordon de Chacana area, in the Coastal Range of central Chile. On the same lava series, and even on the same rock sample, it was possible to measure apparently valid ages of both the emplacement of lava flows (by dating transparent plagioclase) and the very low-grade metamorphic event (by dating sericite and/or adularia). Sericite was dated on step heated single grains of strongly altered plagioclases, the radiogenic argon from remaining fresh plagioclase appearing as negligible on most part of the age spectrum. The concordance between one plateau age of 118.7 +/- 0.6 Ma displayed by fresh plagioclase and a previously measured plateau age on plagioclase from one lava flow from a region due west from Santiago [Bustamante Hill, Aguirre, L., Feraud, G., Morata, D., Vergara, M., Robinson, D., 1999. Time interval between volcanism and burial metamorphism and rate of basin subsidence in a Cretaceous Andean extensional setting. Tectonophysics 313, 433-447] may indicate that the volcanic activity of the Veta Negra Formation was synchronous along the 80-km long latitudinal segment studied. Ages of 96.8 +/- 0.2 and 97.0 +/- 1.6 Ma displayed by adularia and sericite (respectively) are slightly older than ages of 93-94 Ma given by adularia from the Bustamante Hill. These data allow to define a period of about 22 Ma between volcanism and metamorphism. Because of the contemporaneity of some metamorphic minerals and Cretaceous plutons of the region, the very low-grade metamorphic event may be the result of the additional effect of burial and a regional thermal event. Nevertheless, a significant effect of burial (relatively to the contribution of high thermal gradient event(s)) seems revealed by an apparent age gradient on very low-grade minerals with the stratigraphic depth.