Paleoproterozoic volcanic caldera in the Amazonian craton, northern Brazil: Stratigraphy, lithofacies characterization, and lithogeochemical constraints

Abstract

In the Vila Tancredo Neves locality, Amazonian craton, northern Brazil, occur Paleoproterozoic well-preserved volcano–plutonic centers. The lower 1.88 Ga Sobreiro Formation has overlapped lavas comprising andesite at the base; andesi-basalt and latite at middle; and quartz-latite and rhyolite at the top; besides associated pyroclastic and autoclastic lithofacies. Reveals high-K calc-alkaline and metaluminous signature. This unit shows K, Ba, Sr, and Rb enrichments; and Nb and Ta negative anomalies. Occurs an LREE-enriched pattern relative to HREE. Eu anomaly is absent in the most primitive rocks, whereas the evolved rocks reveal a minor negative Eu anomaly. These chemical characteristics are compatible with arc-related andesites. The upper, fissure-controlled, 1.87 Ga Santa Rosa Formation has peraluminous rhyolites and intrusions of rhyolitic porphyries and granites. These alkali rhyolites erupted in several vents within greater brittle faults, with outflows of ignimbrites and volcaniclastic deposits with variable textures. They reveal HFSE enrichment (Nb and Ta) and strong negative anomalies of LILE (Ba and Sr). Some evolved rhyolitic samples usually attributed to the Santa Rosa Formation are calc-alkaline and have geochemical constraints closer to those of the Sobreiro Formation andesitic rocks. Observations of the volcano–plutonic complex point to a Paleoproterozoic volcanic caldera related to several pulses of felsic and andesitic–rhyodacitic–rhyolitic lavas that comprise a stratovolcano evolution in the pre-caldera stage. Pyroclastic sin-caldera deposits with ignimbrite, ash tuff, crystal-rich tuff, and minor polymictic breccia are less preserved. Later evolved domes, dikes, and volcaniclastic rocks represent the recurrent magmatism in the post-caldera stage. This work supplies new support for more systematic stratigraphic and geochemical studies of these poorly studied, but promising, Proterozoic units.