Birth, life and death of the Devonian Chaitenia back-arc along the Southwestern Gondwanan margin (southern Chile)

Abstract

The late Paleozoic evolution of SW Gondwana at the current latitudes of 38-40°S involved three different blocks (Chilenia, Chaitenia and North Patagonia), whose origin and tectonic evolution is debated, making paleogeographic reconstructions challenging. Here we address this issue by constraining the tectonic setting and the metamorphic evolution of mafic rocks outcropping in three sectors (Guzmán at 38°33′S, San Carlos at 40°54′S, Los Pabilos at 40°57′S) of the late Paleozoic basal accretionary complex of the Chilean Coastal Cordillera. The geochemical results suggest that their protoliths originated in a back-arc lithosphere correlating with the Chaitenia back-arc while our P–T–t paths reconstructions show that this basin closed diachronically. In the southern part of the basin, the peak metamorphic conditions of ∼ 450 °C and 1.4–1.6 GPa estimated for the San Carlos blueschists are interpreted as representing a cold thermal configuration for the subduction already reached at 335 Ma (in situ ⁴⁰Ar/³⁹Ar on phengite). Comparatively, in its northern part, the Guzmán garnet-bearing amphibolites still followed at the same time a warmer counterclockwise P–T path, interpreted as the initiation of the intra-back-arc subduction, reaching metamorphic peak conditions of ∼ 650 °C and 1–1.2 GPa. The amphibolites further cooled down to 500 °C at 311–299 Ma (in situ ⁴⁰Ar/³⁹Ar on hornblende) and underwent a hydration event under conditions of 480–520 °C and 0.8–1.1 GPa at 277–270 Ma (in situ ⁴⁰Ar/³⁹Ar on actinolite). Our results suggest that the Chaitenia oceanic arc and back-arc system extended about 300 km further north than previously thought, up to the present 38°S latitude. Moreover, we propose that the opening of the Chaitenia back-arc is closely linked to along-strike transition from collision of the Chilenia terrane at the northwest to normal subduction beneath the North Patagonian margin at the southeast, triggering slab curving, tearing and roll-back.