Abstract

The Devonian magmatism in the north Patagonian segment of SW Gondwana (40°-43°S) was generated in two partly coeval belts (1); an older one located inboard of the continental landmass (the San Martín Arc), and another emplaced in a peri-Gondwanan position (the Chaitenia Island Arc). This tectono-magmatic configuration resulted after the opening of a marginal back-arc basin between both arcs in late Lower Devonian, whereas its subsequent closure probably occurred before Upper Mississippian (2). The Zarao metarhyolite is a subvolcanic body located in the eastern slope of the Coast Range (c. 41°18’S), has a U-Pb SHRIMP age of c. 393 Ma and is one of the older igneous units of the Chaitenia Island Arc (3). The metarhyolite body is tectonically interleaved with metabasaltic rocks with similar metamorphism and structure, and thus grouped as the Zarao Metavolcanic Rocks (ZMR). Although the crystallization age of metabasalts is unknown, the Devonian detrital zircon component is dominant in metasedimentary rocks in the area (3). Whole-rock major and trace element analysis together with Sr-Nd-Pb isotope determinations were performed in one metarhyolite and four metabasalts of the ZMR. The metabasalts show sub-alkaline affinities, resembling enriched and depleted MORB-like types, and display Nb negative anomalies, these are more subtle in the depleted MORB-like metabasalts. The metarhyolite is alkaline, has a flat chondrite-normalized HREE pattern together with a marked Eu negative anomaly, and NbTi negative anomalies. The Nd-Pb isotope composition (t=393 Ma) show an Enriched Mantle 1 (EM1) source-type for the enriched metabasalts and the metarhyolite, wheras a mantle source change towards Depleted MORB Mantle (DMM) composition, signaled by depleted metabasalts. We interpret that the ZMR was formed during different stages of lithospheric thinning over a suprasubduction zone. During the onset of extension, the stretched continental lithosphere located in the forearc of the San Martín Arc melted under low pressure conditions and generated enriched basaltic magmas that reached the surface (enriched MORB-like metabasalts); stagnant magma batches within the attenuated crust underwent fractionational crystallization processes causing the formation of rhyolitic magmas (metarhyolites) at c. 393 Ma. The mature stages of the back-arc marginal basin are represented by the depleted MORB-type metabasalts, which formed due the partial melting of a DMM source in the expansion center located at the rear of the Chaitenia arc. Geophysical surveys have identified a dense block below the Cenozoic cover located to the east (4), and Devonian U-Pb detrital ages have been reported in metasedimentary rocks cores recovered from boreholes ((1), (3)). Thus, the tectonic slice comprising the ZMR might represent scraped fragments of this buried block. 1) Hervé et al. (2016). Journal of the Geological Society 173(4): 587–602. (2) Rapela et al. (2021). Gondwana Research 96: 1-21. (3) Hervé et al. (2018). Andean Geology 45(3): 301–317. (4) Maksymowicz et al. (2022). Solid Earth 14: 117-136.