Magmatism and tectonics in continental Chiloé, Chile (42°-42°30′S)

The Chiloé-Chonos region seems to preserve the oldest depositional events in the fore-arc accretionary complex of the Southeast Pacific margin. There are isolated occurrences of low-grade metamorphic rocks, including slates with a Devonian trilobite fauna and schists that give Rb-Sr evidence of a ca. 290 Ma metamorphism. Pillow basalts and ultramafic rocks may represent parts of the Pan-Thalassic ocean floor on which the Palaeozoic sediments were laid down. Emergence of a magmatic arc is indicated by Jurassic to Early Cretaceous volcanogenic and marine deposits. During the mid-Cretaceous climax of plutonic activity, these were intruded by monzogranites, which here constitute the eastern portion of the North Patagonian batholith. They give Rb-Sr isochron ages of 120-100 Ma (Barremian-Albian). Initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7040-0.7045, and ε{lunate}_Ndt values of +0.5 to +1.5, indicate a simple petrogenesis with a mantle source. The western part of the batholith is petrologically more primitive, being composed predominantly of tonalite, diorite and gabbro, and initial ⁸⁷Sr/⁸⁶Sr ratios are more variable. Late Cenozoic movement of the Liquiñe-Ofqui fault zone (LOFZ) generated deep pull-apart basins to the west of the uplifted batholith/basement complex. These were filled by thick marine sequences of volcanogenic debris, indicating the wide extent of a mainly rhyolitic volcanic field during Miocene times. Pliocene tonalite and granodiorite plutons (dated by a Rb-Sr whole-rock isochron at 4.7 ± 0.5 Ma) and Holocene andesite-basalt stratovolcanoes are located along the LOFZ. The latter feature has thus been a major influence on the tectonic evolution of the area. There is no evidence for major post-Palaeozoic compression or crustal shortening.