U–Pb geochronology and geochemistry of grenville-age plutons in the Sunsas Belt - Bolivia, SW Amazonian Craton: Tectonic and magmatic implications

Abstract

The Sunsas syn-to post-orogenic high-K calc-alkaline magmatism is crucial to understand the tectonic and thermodynamic history of the Sunsas Orogeny in the southwestern part of the Amazonian Craton. This orogeny is correlated with the Grenville orogenic cycle (c. 1.3 to 0.95 Ga) of Laurentia. Three granitic intrusions from Sunsas belt were classified as fractionated I-type and hybrid A-type granites. The first, the Nomoca granodiorite presents calc-alkaline affinity with peraluminous-magnesian composition (Mg0 = 1.61 wt%), and typical characteristics of intermediate magmas (SiO2 = 67.3 w%). The La Asunta and Nocemano syenogranites have calc-alkaline affinity with peraluminous-ferrous composition (FeO = 2.26–3.31 wt%) and characteristics of silica-rich magmas (SiO2 = 70.4–74.9 w%). Geochemical fractionation patterns suggest that the granites were derived from the reworking of two crustal protolith sources in the same tectonic environment. The Nomoca, La Asunta and Nocemano intrusions yield crystallization ages between 1.17 and 1.08 Ga. Inherited zircon cores with 3.63 and 2.91 Ga Archean ages from Nomoca and Nocemano intrusions may be correlated to the Superior Province evolution, which is located in the south-central part of the Canadian Shield. These U–Pb ages strengthen the geotectonic model for the Rodinia supercontinent with Laurentia-Baltica-Amazonia configuration. Paleoproterozoic (2.12–1.69 Ga) and Mesoproterozoic (1.4–1.3 Ga) U–Pb inherited zircon ages from these intrusions support intensive reworking of the Paraguá basement. A concordant age of 555 Ma for zircon rims suggests reworking of the Sunsas Province during the Neoproterozoic Brasiliano Orogeny.