Fissural mafic magmatism on southwestern Amazonian Craton: Petrogenesis and 40Ar-39Ar geochronology

Expressive occurrences of sills and mafic dyke swarms as a result of tafrogenetic processes that led to break-up or attempted break-up of continental crust are found on the south and southwestern Amazonian Craton. This work focus on the magmatic-tectonic processes that formed the Salto do Ceu, Huanchaca, and Rancho de Prata suites. The Salto do Ceu Suite, with a U-Pb (TIMS) baddeleyite age of 1439 +/- 4 Ma, occurs as sills and lava flows intruding and, in places, overlaying the sedimentary rocks of the region of Rio Branco (state of Mato Grosso, Brazil). Plagioclase and amphibole Ar-Ar geochronological data provided a plateau age of 1021 +/- 5 Ma, and an integrated age of 1385 +/- 9 Ma, respectively. The Rancho de Prata Suite consists of mafic dyke swarms intruded into the basement rocks of the Jauru Terrane, with a U-Pb (TIMS) baddeleyite age of 1387 +/- 17 Ma. Plagioclase Ar Ar data yielded a plateau age of 967 +/- 5 Ma, while amphibole Ar-Ar data provided an integrated age of 1495 +/- 8 Ma. Sills and mafic dykes of the Huanchaca Intrusive Suite are situated in the eastern portion of the Paragua Terrane which is not affected by the Sumas Orogeny (1.1-0.9 Ga). Dykes occur emplaced into the basement rocks underlying the Aguapef Group, whereas sills are intruded into pelites and sandstones of the Vale da Promissao Formation (Aguapei Group). Ar Ar ages obtained for the sills provided both a plateau age of 1041 +/- 6 Ma (plagioclase) and an integrated age of 1113 +/- 11 Ma (amphibole), A U-Pb (TIMS) baddeleyite age of 1111.5 +/- 1.9 Ma was obtained for the sills as well. The units studied here are composed of gabbros, diabases, and basalts. Regardless their distinct crystallization ages, all of them are tholeiitic with high iron enrichment, which indicates accentuated mantle melting in distensive tectonic regimes. Their chemical characteristics mainly suggest affinity to intraplate basalts. Significant variations in the patterns of rare earth elements (REE) are observed among these suites, yet all of them show enrichment of light REE relative to the heavy REE. The main difference among ratios (Zr versus incompatible element) obtained for each unit combined with interpretations from variation and classification diagrams, as well as available geochronological data, allow us to suggest that these rocks derived from a heterogeneous mantle sources. The integration of results from this research allowed us to recognize two distinct magmatic events in the south-southwestern Amazonian Craton: an older one with age between 1387 and 1439 Ma, and a younger one around 1110 Ma old. The former event, which gave rise to the Salto do Ceti and Rancho de Prata suites, would be associated with post-orogenic stages of the Santa Helena Magmatic Arc in the Jauru Terrane, while the younger event, which is represented by the Huanchaca and Rio Perdido suites, and Rincdn del Tigre Complex, forms a Stenian Large Igenous Province (LIP) evolved during an attempted break-up of continental crust that resulted in the set-up of the Aguapei Aulacogen.