Orosirian magmatism in the Tapajós Mineral Province (Amazonian Craton): The missing link to understand the onset of Paleoproterozoic tectonics
Résumé
The Tapajós Mineral Province (TMP) located at south-central Amazonian Craton is dominated by igneous suites that show geochemical and petrologic evidence of a long-lived subduction process accounting for continental crust generation. The process lasted for approximately 100 Ma and yielded in different intrusive and extrusive manifestations within the 1.99-1.89 Ga interval. At its initial stage, arc magmatism is characterized by a strong crustal component on the magmas possibly due to subcrustal-erosion upon the onset of subduction. After this phase, metaluminous primary melts provided by the subduction zone gradually differentiate to peraluminous compositions, the case for the ca. 1.98-1.95 Ga Creporizão Intrusive Suite (CRP) and its extrusive correspondences. Geochemical modelling reveals that the first bursts of magmatism at the TMP (at ca. 1.99 Ga) evolved through pyroxene fractionation, whereas the following pulses are amphibole-bearing. As the arc evolves, the mantle wedge gets progressively metasomatized and hydrated, eventually melting to generate the high-K metaluminous to mildly peraluminous granitoids of the ca. 1.89 Ga Parauari Intrusive Suite (PAR), accompanied by more evolved ɛNd(1.88). The first pulses of the Iriri Group volcanism are synchronous with PAR, nevertheless, Nd isotopes point to different sources. Around 1.87 Ga, slab break off provokes astenospheric upwelling, which is responsible for destabilizing and melting of the subcontinental-lithospheric-mantle severely metasomatized by the previous magmatic events, generating shoshonitic and metaluminous melts with variable degrees of crust assimilation, translated into mildly negative ɛNd(1.86) of the Maloquinha Intrusive Suite (MLQ) and Iriri volcanic rocks at an early-anorogenic tectonic setting. Geochemical modelling and the depletion in Ba, Sr and Eu of MLQ and part of Iriri are compatible with early extraction of plagioclase and support the relatively dry evolution of these units. In addition, fractional crystallization alone cannot account for the geochemistry variability of MLQ and Iriri, reinforcing crust assimilation on their petrogenesis. In terms of metallogenetic potential, CRP and PAR have petrologic evolutions consistent with amphibole and magnetite crystallization, pointing to hydrated and oxidized magmas that might be considered as favorable for the formation of magmatic-hydrothermal mineral deposits (porphyry Ausbnd Cu (Mo) and epithermal systems). Lastly, the magmatic events younger than 1.89 Ga might act incorporating residual Au-rich sulfide from the mantle, explaining the Au-only characteristic of many mineral deposits described at the TMP.