Abstract Establishing the frontal extent of ancient flat subduction events from the geologic record can be challenging. This difficulty arises because magmatic activity in the arc typically ceases during complete slab flattening, and other meaningful proxies are usually absent. To address this issue, we examine early Paleocene intraplate magmatic units in central Patagonia, specifically, the La Angostura and Pagasartundua basalts. These basalts erupted and were emplaced during the final phase of a significant flat subduction event, referred to as the Nalé flat slab. The small outcrops of these units are composed of metaluminous alkaline basalts, whose origin would be related to the decompression melting of the sub‐slab asthenosphere. This melting likely occurred due to local slab gaps in the frontal section of a flattened slab, where the oceanic lithosphere resumed a steep angle. The resulting mantle primitive melts would have caused partial melting of minor portions of the upper slab (including eclogitized components). The interaction of these two end‐member melts results in slight hybridization, as evidenced by positive anomalies in Cs, Pb, and Li, subtle Ta depletion, and local Th enrichment in the basalts. By recognizing these basalts and their geochemical characteristics in relation to the geodynamic context, we can establish the maximum frontal extent of a large‐scale Late Cretaceous‐Paleocene flat subduction event. Thus, this case represents the first ancient flat slab that is frontally constrained by slab gap‐related intraplate magmatism, a pattern that could be replicated in other similar settings worldwide.
Navarrete et al. (Sun,) studied this question.