Few localities in the Patagonian Andes expose remnants of the Mesozoic Chilean paleo-accretionary complex. We focus on the Diego de Almagro Island high-pressure/low-temperature (HP/LT) Complex, a pluri-kilometer thick sequence comprising metavolcanic rocks with oceanic affinities and metasedimentary rocks. In this study, the deepest segments of the Chilean subduction interface in Patagonia are characterized for the first time. Despite its apparent homogeneity, the complex is actually composed of two tectonic units with distinct ages of metamorphism and thermal evolution: the garnet amphibolite (GA) and the underlying blueschist (BS) units. The GA unit mafic rocks exhibit epidote, phengite, titanite, rutile, chloritoid and paragonite inclusions in prograde garnet I, diopside + albite intergrows replacing omphacite inclusions in garnet II, and relict omphacite (XJd45) included in edenitic-pargasitic amphiboles. Thermobarometric results show that these rocks were buried along a relatively cold prograde path (c. 11 °C/km) and reached eclogite-facies near peak pressure conditions (c. 550–600 °C, 1.6 GPa). The GA unit underwent a pervasive stage of amphibolitization during decompression at c. 1.3 GPa. Field and petrological observations, together with multi-mineral Rb-Sr dating, indicate that amphibolitization of the GA unit took place along the subduction interface at c. 120 Ma in a slightly warmer subduction regime (c. 13–14 °C/km), in agreeement with the formation of coetanoeus amphibolites at c. 35 km. The underlying BS unit (i) yields four consistent Rb-Sr deformation ages of c. 80 Ma, i.e. 40 Ma younger than the overlying rocks from the GA unit; (ii) exhibits slightly cooler peak metamorphic conditions (c. 520–550 °C, 1.6 GPa) indicating burial along a prograde path of c. 10 °C/km (iii) does not show amphibolite-facies overprint as seen in the GA unit. After a long residence time under amphibolite-facies conditions, the amphibolitized rocks of the GA unit partly recrystallized during blueschist-facies deformation at around 80 Ma, as shown by silica-richer rims around phengite and glaucophane overgrowths around hornblende. These new results: (i) shed light on the dynamics of the deep Chilean paleo-accretionary wedge, (ii) reveal that the GA unit underwent two stages of exhumation, likely juxtaposed with the BS unit near 40 km depth, and (iii) suggest that the subduction thermal gradient on Diego de Almagro Island fluctuated between c. 10–11 °C/km and 13–14 °C/km during Cretaceous times.
Áreas temáticas de ASJC Scopus
- Geoquímica y petrología