Sills and mafic dyke swarms are an important tool for understanding geodynamic processes once they mark the beginning of large extensional tectonic events, but also they are fundamental indicators of nature and evolution of mantle sources through geological time. In the S-SW Amazon Craton, Proterozoic sills and dyke swarms are reported in Eastern Bolivia, and in the Brazilian states of Mato Grosso and Mato Grosso do Sul. There are examples, such as the dyke swarms of the Huanchaca, Rancho de Prata, and Rio Perdido intrusive suites as well as mafic sills of the Huanchaca, and Salto do Céu suites, and Rincón del Tigre Complex. This work aims to characterize the nature, petrological evolution and tectonics of the mafic magmatic event related to tafrogenetic events that are responsible for the break-up or attempted break-up of continental crust. Several tools were used in order to clarify this issue, such as geological mapping, petrographic, lithogeochemical and geochronological (U-Pb IDTIMS and Ar-Ar) analysis. The studied units are sited in the municipalities of Vila Bela da Santíssima Trindade, Nova Lacerda, Conquista D‟Oeste, and Salto do Céu in Mato Grosso, and in Porto Murtinho and Caracol in Mato Grosso do Sul. Rocks of the Salto do Céu suite occur in the municipalities of Salto do Céu and Rio Branco (MT), and outcrop as sills and lava flows. Sills are emplaced into pelitic rocks of the Aguapeí Group usually with shallow dips towards WSW. Lava flows overly the same sedimentary unit and show internal vertical structures and flow-top structures that are typical of thin basaltic flows. Vesicles and amygdales are commonly observed along with flow-folds and breccias. Petrographically, these rocks are mesocratic to melanocratic, greenish-gray to black, and equigranular varying from very fine- to medium-grained. Sills consist of diabases and massif gabbros that under the microscope show ophitic, sub-ophitic, intergranular, and coronitic textures. They are essentially composed of plagioclase and pyroxene having its accessory assemblage represented by opaques, acicular apatite and subhedral sphene. Lava flows, in turn, consist of basalts and diabases that commonly displays ophitic, sub-ophitic, hyalophitic, porphyritic or amygdaloidal textures in a pseudo-trachytic groundmass; some samples exhibit vitrophyric texture. The main components are plagioclase, pyroxene, and relict glass. Amygdales are rounded to ellipsoidal filled with fibrous to fibro-radiated material which is composed of zeolites, chlorite, fluorite, and opaques. Sills and lava flows have tholeiitic affinity, and are classified as intraplate basalts. This suite shows a U-Pb (ID-TIMS) baddeleyite age of 1439 ± 4 Ma. 40Ar-39Ar analysis of plagioclase and amphibole provided a plateau age of 1021 ± 5 Ma, and an integrated age of 1385 ± 9 Ma, respectively. Numerous mafic dykes of the Rancho de Prata Intrusive Suite occur in the surroundings of Nova Lacerda and Conquista D‟Oeste (MT) along an array about 30 km-wide and 150 km-long trending NNW. They occurs as parallel dyke swarms striking N30°–40°W with steep dips. There are no records of deformation or metamorphism on these rocks which occur in intrusive contact with gneissic, granitic and metavulcanossedimentary rocks of the basement. These mafic dykes consist of gabbros, diabases, and basalts, very fine to medium-grained, which exhibits phaneritic, aphanitic to porphyritic textures. They are melanocratic dark-gray to black, with massif structure, in places with discrete foliation parallel to the dyke walls. Microscopically, these rocks are holo- to hypocrystalline, and show porphyritic, intergranular, and subophitic to ophitic textures, and are essentially composed of plagioclase, clinopyroxene and orthopyroxene, olivine and amphibole. Dark-brown intergranular glass is seldom observed in basalts. Lithogeochemical studies allow us to classify these rocks as basalts and andesiticbasalts. The magmatism is sub-alkaline to tholeiitic whose chemical affinity is compatible with continental basalts. Two groups are observed in rare earth elements distribution patterns: one strongly fractionated and enriched in light ETR, and another one weakly fractionated with medium La/Yb ratios, respectively, 3.22 and 1.26. A U-Pb (ID-TIMS) baddeleyite age of 1387 ± 17 Ma was obtained for the dyke swarms. 40Ar-39Ar analysis of plagioclase provided plateau ages of 967 ± 5 Ma and 980 ± 7 Ma. However, 40Ar-39Ar age-spectrum data for amphibole is heterogeneous, therefore provide integrated ages of 1495 ± 8 Ma and 1509 ± 7 Ma. Sills and mafic dykes of the Huanchaca Intrusive Suite are sited in the portion of the Paraguá Terrane which is not affected by the Sunsás Orogeny (1.1 to 0.9). Dykes occur emplaced into the basement rocks underlying the Aguapeí Group that are represented by the Mesoproterozoic granites Guaporeí and Passagem that form part of the Pensamiento Granitoid Complex, as well as by the Paleoproterozoic orthogneisses Shangri-lá and Turvo that occur within the Chiquitania Metamorphic Complex; sills, in turn, are emplaced into the pelites and sandstones of the Vale da Promissão Formation (Aguapeí Group). Sills outcrop as blocks and low-lying outcrops in abrupt and parallel contacts to the layering of sedimentary rocks. On the other hand, dykes outcrop as small and discontinuous trending-ENE crests, or as single, rounded and angular blocks in the granitic-gnaissic terrane whose main orientation varies between N70°-90°E. Sills consist of gabbros and diabases, are greenish-gray to black in colour, and fine- to medium-grained. Optically, these are holocrystalline with sub-ophitic to ophitic texture, and rare intergranular texture. Cumulate rocks of restricted occurrence were identified with paragenesis and textures similar to each other whose difference is the presence of olivine and high content of mafic minerals. These rocks are essentially composed of plagioclase, pyroxene, amphibole, opaques, and in a few of them, alkali-feldspar and quartz displaying graphic intergrowth are also observed. Dykes are dark-gray to greenish-gray with grain size decreasing from the rock wall towards the center of the body from very fine-grained or glassy to medium-grained, respectively. They are classified as diabases and basalts, respectively, holo to hypocrystalline, and have an essential composition of plagioclase, pyroxene and olivine. Under the microscope, diabases show inequigranular, sub-ophitic, and subordinate ophitic textures, and are fine- to medium-grained, while basalts display porphyritic, glomeroporphyritic, and textures vitrophyric, and rarely intersertal to hyalophitic textures. Chemically, dykes and sills are classified into sub-alkaline andesitic basalts (tholeiitic) formed in intraplate settings. REE patterns show that sills are richer in total REE relative to the dykes, as well as show significant vertical variation with respect to the REE pattern envelope, yet parallel to it. Ar-Ar plateau ages were obtained for the sills both from plagioclase (948 ± 5 Ma), and amphibole (1113 ± 11 Ma). A U-Pb (ID-TIMS) baddeleyite age of 1111.5 ± 1.9 Ma was also obtained for sills. The dyke swarms that form part of the Rio Perdido Intrusive Suite occur emplaced into Paleoproterozoic rocks sited in the Rio Apa Terrane (SW of Mato Grosso do Sul), and Paraguay. Dykes are tabular to lenticular, 1 to 30 m thick, generally striking N70°-90°E and N70º-90ºW. They exhibit abrupt and discordant contact with respect to the general NS trend. Dykes consist of very fine- to fine-grained diabases, and fine- to medium-grained microgabbros, both with no evidence of ductile deformation and metamorphism. Under the microscope, they are holocrystalline with ophitic to sub-ophitic, intergranular, and, in places, porphyritic textures, as well as quench textures in which they display swallow-tail shape. They contain essential plagioclase, pyroxenes and olivine, and show a tholeiitic trend with FeOt enrichment relative to MgO for relatively constant alkali contents. They are classified as basalts and andesitic basalts that are similar to Phanerozoic intraplate basalts. REE patterns show strong fractionation of light REE relative to the heavy, with La/Yb ratios varying between 2.8 and 6.2 and Eu anomalies subtly negative or absent. Recent U-Pb (ID-TIMS) data on baddeleyite provided an age of 1110 Ma. The Rincón del Tigre Igneous Complex is a thick layered intrusion that intrudes into the Sunsás Group (below), and into the Vibosi Group (above). Its name is due to the region of Rincón del Tigre in Bolivia, and is characterized as an igneous event related to the Sunsás Orogeny. It is divided into three units: Ultramafic (basal), Mafic (intermediate), and Felsic (superior). The Ultramafic Unit is composed of serpentinized dunite, harzburgite, olivine bronzite, bronzite picrite, and melanorite, while the Mafic Unit is composed of norite and gabbro. The Felsic Unit is represented by granophyres. A U-Pb (ID-TIMS) baddeleyite age of 1110.4 ± 1.8 Ma was obtained from the Felsic Unit, and show chronological similarity to the syn- and postorogenic granitic suites that occur in the Sunsás-Aguapeí province sited in Bolivia, and Brazil. Based on K-Ar ages varying between 1006 and 875 Ma, the units above were attributed to a single magmatic event and interpreted as a LIP that formed during an attempted breakup of Rodinia. Now, based on new precise geochronologic data (U-Pb TIMS on baddeleyite, and Ar-Ar on amphibole and plagioclase), and field and petrological data, this hypothesis is not supported anymore. There were two fissural magmatic events prior to the agglutination of this supercontinent: the older one with ages of 1439 and 1387 Ma, and the younger one around 1110 Ma old. By taking into account the evolution of the Amazon Craton, the older episode is marked by dyke swarms of the Rancho de Prata suite as well as lava flows and sills of the Salto do Céu suite, likely associated with post-orogenic stages of the Santa Helena Magmatic Arc in the Jauru Terrane; the younger event, which have occurrence restricted to the Paraguá and Rio Apa Terranes, is represented by the Huanchaca, and Rio Perdido suites and Rincón del Tigre Complex, and form part of a Stenian LIP sited in the south-southwestern Amazon Craton. This LIP evolved from an attempted break-up of continental crust that resulted in the formation of the Aguapeí Aulacogen. The Sunsás and Aguapeí Belts mark the period of agglutination of Rodinia, and are responsible for the metamorphism and deformation observed in part of this Stenian LIP.
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