Magmatic Sources, Setting and Causes of Eocene to Recent Patagonian Plateau Magmatism (36°S to 52°S Latitude)

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Continental back-arc and within-plate basaltic volcanic provinces provide a view into the evolution of the mantle beneath continents and the mass flux of materials that are subducted, but not extracted by arc volcanism. One of the largest continental mafic provinces is the Patagonian province where large mafic plateau volcanic events ranging in age from late Paleocene to Recent have occurred in the absence of major extension. The largest events produced the Eocene Posadas Formation (∼46°S to 50°S), the late Oligocene to early Miocene Somuncura magmatic province (41°S to 43°S plus similar age magmas up to 46°S), the late Miocene to Pliocene lavas east of the Chile Triple Junction (∼46°S to 49°S), and the Payunia magmatic province (36°S to 38.5°S). Tectonically, the Posadas and Triple Junction provinces have been respectively associated with Eocene and Miocene-Recent collisions of the Farallón-Aluk and the Nazea-Antarctic spreading ridges with the Chile trench, the Somuncura province with a mantle thermal anomaly linked to late Oligocene/early Miocene changes in plate convergence vectors, and the Payunia lavas with steepening of a formally shallow subduction zone. Between 45°S-50°S where Tertiary arc volcanism was largely absent, the plateau magmas show almost no trace element affinity with arc rocks as indicated by Ba/La ratios < 20 (arcs > 20) and La/Ta ratios <18 (arcs > 25). The same is true for pre-29 Ma magmas in the Somuncura and pre-20 Ma magmas in the Payunia regions. Associated magmas erupted near the arc can have higher ratios. This situation contrasts with post-29 Ma Somuncura and post-20 Ma Payunia magmas north of 45°S in which Ba/La ratios over 30 and La/Ta ratios < 20 extend far to the east, and lavas with La/Ta ratios > 20 occur to the west. These higher ratios fit with the introduction of arc components into the mantle magma sources as a result of interaction with a Paleogene subducting slab in the Somuncura region and dehydration of a shallowly subducting slab in the Payunia region. At the same time, Nd and Sr isotopic ratios of most of the plateau lavas plot in the OIB field (most ∑ND = +5 to -1; 87Sr/86Sr = 0.7035 to 0.7048) with Posadas magmas being the most depleted, and Somuncura and some Triple Junction magmas being the most enriched. Across the plateau, these tectonic, chemical, and isotopic data are consistent with the large volume plateau magmas being due to tectonic perturbations that provoke melting in a hot mantle that has been on the verge of melting since the Mesozoic break-up of Gondwana.

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