Evolution of the volcanism in the northwestern part of meseta de Somuncura, Patagonia, Argentina

摘要

The Meseta de Somuncur acute accent a is the largest intraplate mafic volcanic field of north Patagonia, Argentina, whose origin is the subject of an ongoing debate and remains uncertain. This contribution presents petrographic, geochemical, and isotopic characteristics of trachybasalts and subordinated basaltic trachyandesites lava flows and basanite volcanic rocks located in the northwestern part of the Meseta de Somuncur acute accent a. The trachybasalts and subordinated basaltic trachyandesites lava flows have OIB signature and based on their isotopic composition (0.70406-0.70422 of 87Sr/86Sr and 0.512787-0.512817 of 143Nd/144Nd) and the petrological modelling, they derived from 1 to 5 of partial melting of a spinel-bearing pyroxenitic source, with an isotopic composition equivalent to 97 of DM and 3 of EM1. The basanites comprise a volcanic cone and lava flows of short extension located at the top of the volcanic stratigraphy of the meseta. They carry peridotite xenoliths and are slightly enriched compared with OIB standards. According to their isotopic composition (0.70407 of 87Sr/86Sr and 0.512772 of 143Nd/144Nd) and the petrological modelling, they derived from 0.1 to 1 of partial melting of a garnet-bearing peridotite, with an isotopic composition equivalent to 97 of DM and 3 of EM1, which was affected by metasomatism prior to its melting. Although both studied lithologies show mobile element ratios that suggest slab influence, that is more evident in the trachybasalts than in the basanites. These two groups of rocks were attributed to the early Miocene Vulcanitas Corona Chico unit. However, the new information presented in this contribution shows that the trachybasalts and subordinated basaltic trachyandesite lavas have geological characteristics equivalent to the Vulcanitas Corona Chico unit, while the basanites show differences with this unit. Therefore, we recommend to exclude the basanites from this group and assign them to a different volcanic unit, probably younger than the Vulcanitas Corona Chico unit. This new information and the geochemical data previously published for the basic volcanic units cropping out in the northwestern area of the meseta show an increase in the alkalinity with time, resulting in lower degrees of partial melting in the youngest lavas. In addition, geochemical and isotopic data suggest that the lavas from different volcanic units have distinct mantle sources. Although the volcanic rocks from Meseta de Somuncur acute accent a have some geochemical characteristics that could be explained by the presence of rising mantle plume(s), the model that so far best explains the geochemistry and isotopic composition of the volcanic rocks in this region is one that invoke the presence of a stagnant slab below Patagonia.