Late-Triassic fore-arc subduction complexes: Chonos and Viedma metamorphics, Patagonia, Chile

New detrital zircon U-Pb chronologic data (two detrital zircon U-Pb laser ablation data; UNAM laboratory) from the Viedma Nunatak (VNU), Campo de Hielo Sur, combined with existing U-Pb zircon data for this nunatak (Suárez et al., 2019) and the Chonos Metamorphic Complex (CMC; Hervé and Fanning, 2001), allow us to suggest a different tectonic interpretation for the former. The coeval Upper Triassic CMC and VNU,  has been interpreted as a subduction complex with an associated magmatic arc towards the east represented by the Central Patagonian Batholith and Subcordilleran  Plutonic Belt (see Suárez and Márquez, 2007; Navarrete et al., 2019) and as a back arc association (Suárez et al., 2019) respectively. The latter authors, argue that “Trench or forearc basins such as the Chonos basin present a large population of detrital (zircon) ages close to the sedimentation age without a large proportion of older zircons, while basins in a backarc position present an input of older detrital sources from the adjacent basement outcrops. Given the broad range of detrital ages found in the Nunatak Viedma Unit it seems that the Viedma basin was probably in a backarc position”. However, the U-Pb zircon detrital analyses of metasedimentary samples from the Chonos Metamorphic Complex (CMC) (Hervé and Fanning, 2001) and from the “Viedma Nunatak Unit” (our new ages and from Suárez et al., 2019), do not show major differences in the age distribution of the detrital zircon grains as pointed out by the latter authors, instead, exhibit a comparable age distribution, both in the Triassic and in older age ranges. This suggests a comparable source and that the CMC and the VNU would have been deposited in a comparable basin, contrary to what Suárez et al. (2019) stipulates. Considering that the currently accepted setting for the CMC is a subduction complex, then we extrapolate a similar subduction tectonic setting for the VNU instead of that of a back-arc basin. Furthermore, the associated Triassic magmatic arc of the CMC subduction complex has been interpreted to be located far to the east of the Viedma Nunatak (Central Patagonian Batholith and continuing into de Early Jurassic, the Subcordilleran  Plutonic Belt;  Navarrete et al., 2019), in which case, the associated arc could not have been a topographic barrier between the Chonos trench and the Viedma backarc basin as it can be inferred from the current hypothesis (unless, it was a double-arc system for which there is no evidence).

References

Hervé, F., Fanning, C.M., 2001. Late Triassic detrital zircons in meta-turbidites of the Chonos Metamorphic Complex, southern Chile. Revista Geológica de Chile, 28(1), 91-104. http://dx.doi.org/10.4067/S0716-02082001000100005 

Navarrete C.A, Gianni G.B. C., Encinas, A., Márquez M., Kamerbeek,Y., Valle M., Folguera, A. 2019. Triassic to Middle Jurassic geodynamic evolution of southwestern Gondwana: From a large flat-slab to mantle plume suction in a rollback subduction setting. Earth-Science Reviews, 194, 125-159, https://doi.org/10.1016/j.earscirev.2019.05.002

Suárez, R.J., Ghiglione, Matí.C., Calderón, M., Sue, C., Martinod, J., Guillaume, B., Rojo, D., The metamorphic rocks of the Nunatak Viedma in the Southern Patagonian Andes: Provenance sources and implications for the early Mesozoic Patagonia-Antarctic Peninsula connection, Journal of South American Earth Sciences (2019), doi: https://doi.org/10.1016/j.jsames.2018.12.015.