The metamorphic rocks of the Nunatak Viedma in the Southern Patagonian Andes: Provenance sources and implications for the early Mesozoic Patagonia-Antarctic Peninsula connection

摘要

The Nunatak Viedma within the Southern Patagonian Icefield has been considered as a volcanic center based on its geomorphologic features, despite the fact that field explorations by Eric Shipton determined its metamorphic nature 70 years ago. We carried out fieldwork to characterize this isolated outcrop and performed the first U-Pb dating in detrital zircons from the basement rocks located inside the Southern Patagonian Icefield. We recognized very-low grade metamorphic rocks, corresponding principally to metapelites and metapsammites, and scarce metabasites. Detrital zircons in three metapsammitic samples (composite group of 240 grains) yielded prominent age population peaks at similar to 1090, similar to 960, similar to 630, similar to 520, similar to 480-460, similar to 380, similar to 290-260, similar to 235-225 Ma that are typical of Gondwanide affinity, and youngest grains at 208 Ma. Maximum depositional ages of 225, 223 and 212 Ma were calculated for each sample from the youngest cluster of ages. This distinctive and novelty Late Triassic age justifies differentiate the Nunatak Viedma Unit from the Devonian-early Carboniferous and Permian-Early Triassic (?) belts of the Eastern Andean Metamorphic Complex. Possible primary source areas for the detrital zircons are outcropping in southern Patagonia, the Antarctic Peninsula, and the Malvinas Islands. Additionally, secondary sources could be part of the erosion and recycling of metasediments from the Eastern Andean Metamorphic Complex. We propose that the cluster of Triassic ages is related to the volcanic arc emplaced along the Antarctic Peninsula and active at that time when was still attached to southern Patagonia during the Triassic. The dynamics of the early Mesozoic orogen is also discussed.