Volumetric and compositional estimation of the Choiyoi Magmatic Province and its comparison with other Silicic Large Igneous Provinces

摘要

The Choiyoi Magmatic Province (ChMP) is a major feature of southern South America comprising volcanic and plutonic rocks from the Early Permian to the Early Triassic (286-247 Ma), prior to the Andean subduction. Its outcrops are exposed from northern Chile (similar to 20 degrees S) down to southwestern Argentina in Patagonia (similar to 44 degrees S), and from the Andean region to the foreland in southeastern Argentina. These rocks can be subdivided in three main regions: Andes, La Pampa Permian-Triassic Magmatic Corridor (CMPT-LP) and North Patagonia. Despite the three regions have similar lithological composition and timing, the deformation style and geochemical features of the CMPT-LP rocks suggest that they may represent an event of magmatism distinct from the Andes region. On the other hand, the rocks from North Patagonia and the Andes regions are considered to be part of the same geotectonic setting, the Orogenic Choiyoi, even though North Patagonia exhibits a more complex deformation pattern. A compilation of published ages and lithological data for all the units that compose the ChMP, allowed an estimation of its areal extent (909,250 km(2)), the volume of its volcanic products, and the compositional variation through time. The volcanic products of Choiyoi Magmatic Province (ChMP-V) are composed of 51% rhyolite, 26% dacite, 22% andesite and 1% basalts, but the early stages of the volcanism were dominated by andesitic products. The plutonic rocks make up the 25% of the total area of the ChMP and they are dominated by granites (similar to 65%) and granodiorites (similar to 30%), although the units representing the early stages of the ChMP are composed up to 60% of granodiorites, tonalites and diorites. It is thus evident that the Choiyoi magmatism, in its whole extension, transitioned from dominantly intermediate to more rhyolitic at its final stages. When considering the ChMP-V, their estimated volume (947,553 km(3)) and time span are akin to a Silicic Large Igneous Province (SLIP). However, their lithological composition through time and especially the large amount of intermediate rocks in the first stage of the ChMP-V differ from typical SLIPs, in which rhyolitic and bimodal (rhyolite-basalt) compositions dominate. The largest preserved silicic province identified so far, the Okhotsk-Chukotka Volcanic Belt (OCVB) in Siberia, has nonetheless a compositional pattern very similar to the ChMP-V with an early stage in which the magmatism was dominantly intermediate in composition (similar to 47% of outcrops), followed by a more volumetric and mostly rhyolitic stage. The fact that the OCVB and other silicic provinces related to convergent margins, such as the Sikote-Alin Belt or the Taupo Volcanic Zone have a similar evolution than the ChMP-V, suggests that the Choiyoi Magmatic Province could have been partly generated in a subduction zone setting.