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Abstract: The Planalto da Serra ultramafic-alkaline rocks are represented by several plugs, lava-flows and N60–80E trending dykes which are mainly represented by glimmerites, harzburgites and carbonatites. These rocks intrude the Cryogenian basal unit of the Northern Paraguay Belt in the Rio dos Cavalos rift (southeastern part of the Amazonian Craton). 40Ar/39Ar dating on the phlogopite from glimmeritic dykes and plugs yielded an age of ca. 600 Ma, which is consistent with Rb/Sr and Sm/Nd determinations on the other rock-types. These age determinations make it difficult to establish a geochronological and genetic relationship between the Planalto da Serra rocks and the Cretaceous bodies of the “Azimuth 125° Lineament”, and it suggests there is a relationship between its emplacement and the beginning of the extensional tectonics responsible for the separation of the Amazonian and Laurentia Cratons. This age also indicates that the onset of the deformation of the Cuiaba Group may be older than 600 Ma. [Copyright &y& Elsevier]

Abstract: The Jaguarão stratoid dacites (Rio Grande do Sul, Brazil) are limited in areal extent, are comprised of about 3.2km3 of preserved erupted material, and outcrop only in areas of the region underlain by mylonitic and ultramylonitic rocks. They are S-type volcanic rocks containing cordierite, orthopyroxene, plagioclase, and ilmenite as liquidus phases, and partially melted granite, gneiss, and migmatite enclaves that are very similar to the Precambrian basement rocks. The Jaguarão lavas have distinct geochemical signatures and Sr–Nd isotopes with respect to other volcanic rocks of the region. Available geochronological data for Jaguarão dacites range between 157±5Ma and 139.6±7.4Ma. Considering the errors, the younger ages obtained for Jaguarão lavas overlap the 138–128Ma age of rocks of the Serra Geral Group, and thus indicate that the dacites were erupted prior to the break-up of Gondwana in this region. Petrographic, mineralogical, and petrochemical data, as well as the tectonic context of the Jaguarão lavas, suggest that magma genesis was linked, at least in part, to friction melts. The dacitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartz-feldspathic source terrane, leaving a granulite facies residue in subsurface. These melts were probably generated as a consequence of crustal thinning linked to simple shear extension just prior to Gondwana break-up and rifting of the southern Atlantic Ocean. [Copyright &y& Elsevier]

The Penatecaua magmatism (∼201 Ma) is part of the Central Atlantic Magmatic Province (CAMP) and is represented by voluminous sills in the Amazonas Basin, north Brazil. The sills appear south of the Amazonas River, particularly in the Medicilândia, Placas, and Rurópolis cities. To the north of the river, near Monte Alegre and Alenquer, smaller sills and NNE-SSW dikes prevail. Paleomagnetic data from 28 sampling sites of sills and dikes from all areas gave consistent results of normal polarity. Despite the large area of occurrence, the VGPs show small dispersion, consistent with a very brief emplacement time, as indicated by the radiometric ages. However, some sites, mainly from Alenquer and the southern sills, gave anomalous directions that may represent the record of a transitional geomagnetic field. The calculated paleomagnetic pole includes former data from Guerreiro and Schult (1986) plotting at 260.1°E 77.5°S (N = 30; A 95 = 3.8°; k = 48) and agrees with other high-quality CAMP poles for South America. [Display omitted] • Paleomagnetic investigation of 28 sites from the Penatecaua magmatism, the CAMP in the Amazonas Basin • Paleomagnetic data agree with the radiometric indication of a very brief emplacement of the Penatecaua magmatism. • Evidence of an excursion or even a field reversal between 201.5 and 201.3 Ma. • High-quality Early Jurassic paleomagnetic pole from South America well constrained by high precision radiometric data. [ABSTRACT FROM AUTHOR]

The magmatic rocks from Alto Paraguay (High Paraguay River extensional lineament), western Apa craton, mainly consist of several major circular alkaline complexes and some rhyolitic domes and ignimbrites. The former are characterized by intrusive Na–alkaline rock-types (nepheline syenites and syenites and effusive equivalents) topped by lava flows and ignimbrites. Two main evolved suites were defined using petrochemical and Sr– isotope data: an agpaitic suite in the north and a miaskitic suite in the south. The domes of subalkaline rhyolitic lavas and ignimbrites occur to the north of the alkaline complexes, along the Paraguay River, near the town of Fuerte Olimpo. The emplacement ages of the alkaline complexes were constrained using the K–Ar, Ar–Ar, Rb–Sr and Sm–Nd dating methods on whole rocks and/or mineral separates (amphibole, alkali feldspar and biotite). Ages are quite variable (Upper Permian to Middle Triassic), with average K–Ar and Ar–Ar ages of 248.8 ± 4.8 and 241.8 ± 1.1 Ma, respectively, and Rb–Sr and Sm–Nd age data giving best values from 248 ± 4 to 244 ± 27 Ma and from 256 ± 3 to 257 ± 3 Ma, respectively. In contrast, the Fuerte Olimpo volcanics show a Mesoproterozoic age (1.3 Ga, K–Ar and Ar–Ar radiometric methods; and 1.42 ± 0.24 to 1.30 ± 0.03 Ga, Rb–Sr and Sm–Nd methods, respectively). Rb–Sr systematics ( 87 Sr/ 86 Sr initial ratios ≤ 0.7038) highlight a relatively “primitive” character of the Na–alkaline magmatic source(s), in contrast with the “crustal” values ( 87 Sr/ 86 Sr initial ratio ∼ 0.7105) of the Fuerte Olimpo rhyolites. Thus, magmatism in the Alto Paraguay area is related to two extensional events: a younger event corresponding to the Permian–Triassic alkaline rocks, and an older event connected to the Precambrian volcanic acidic rocks. [ABSTRACT FROM AUTHOR]