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The region of the São Francisco river valley in eastern Brazil encompasses two main components of the geologic framework of the South American continent: the São Francisco craton and its marginal orogenic belts. Cratons, as the oldest, differentiated and relatively stable pieces of the continental lithosphere, preserve a substantial part of the Earth's memory. Orogenic belts, on the other hand, record collisional processes that occurred during a limited time span. Because of their topographic relief, mountain belts developed along craton margins provide however access to rock successions not exposed in the low lands of the adjacent cratons. The combination of geologic information obtained in cratonic domains and their marginal orogenic belts thus form the basis for deciphering substantial periods of Earth’s history. Corresponding to the most intensively studied portion of the Precambrian nucleus of the South American plate, the São Francisco craton and its margins host a rock record that spans from the Paleoarchean to the Cenozoic. Precambrian sedimentary successions that witness ancient Earth processes - many of them of global significance - are especially well preserved and exposed in this region. With all these attributes the São Francisco craton together with its fringing orogenic belts can be viewed as a ‘continent within a continent’ or a ‘continent in miniature’.

The Las Lozas volcanic sequence, which crops out at northwestern border of the Famatina belt-southeastern Puna, NW Argentina, is constituted mainly by rhyolites and a lesser volume of basalts and trachytes, and volcanoclastic deposits. These rocks, previously considered of Early Paleozoic age, are now assigned to the Lower Pennsylvanian (320 Ma U-Pb age). They represent a bimodal volcanic succession that plot in the subalkaline/tholeiitic (rhyolites), alkaline basalts (basalts) and alkaline (trachytes) fields on the total alkali-silica diagram. The basalts display features comparable to transitional MORB and within-plate tholeiites, with contributions from a mantle source affected by crustal contamination. The acid members also show geochemical affinities to within-plate magmas, and their composition suggest a derivation from continental crustal material with mantle source interaction or a juvenile essentially mantle derived crust. The 320 Ma age from the Las Lozas volcanic succession as well as the 342 and 348 Ma U-Pb ages, from rocks in the nearby Cazadero Grande section, to the south, and the U-Pb ages from Sierra Pampeanas granites (332-357 Ma) highlight the importance achieved by Carboniferous magmatic activity in that region, framed between 320 and 350 Ma. Low strontium initial ratios from the Las Lozas (0.70479-0.70164) indicate a predominant contribution by a juvenile component, while the ratios in the nearby Cazadero Grande (0.71433-0.71233) and Sierras Pampeanas granites (0.717 to 0.7124) point to an input by enriched sources with restricted contribution of a mantle component. Nd isotopes from a basalt from the Las Lozas section (εNd(320) with +3.11, T DM with 774.6 Ma) indicates a possible asthenospheric source, with evidence for some mixture between juvenile and reworked crustal material. In contrast Sr-Nd isotopes in a rhyolite from Cazadero Grande (εNd(t) of -2.91 and -0.3, T DM of 1.09 and 1.1 Ga) and of Sierra Pampeanas granites (εNd(t) of -0.6, T DM of 1.19 and 1.1 Ga) suggest a crustal source with minor juvenile input. Taking in account the age difference in the region between Mississippian and the Lower Pennsylvanian magmatism, this would indicate a change in the magma source consistent with a more pronounced extensional tectonic regime for the Lower Pennsylvanian. This assumption need to be considered together with observations along a transect at 27°30'S, where the Carboniferous Eastern Sierras Pampeanas granitic rocks show, regardless of age, a greater contribution ofjuvenile material of mantle character to the west. Among these rocks, with a major juvenile component, are those of the Las Lozas succession as well the Cerro Gloria Granite, the eastern manifestations of the Carboniferous Eastern Sierras Pampeanas magmatism. From the foregoing emerges that the contribution of juvenile material could be continuously varied in the region through the Carboniferous, due to varying lithospheric extension. An example of this arise from the presented data of the Lozas succession, which indicate that this rocks resulted from reworking of supra-crustal material with input of juvenile magmas, linked to the change in the lithospheric extension. La sucesión volcánica que aflora en Las Lozas, noroeste del Sistema de Famatinasureste de la Puna (noroeste Argentina), está constituida principalmente por riolitas y en menor volumen por basaltos, traquitas y depósitos volcanoclásticos. Estas rocas, previamente consideradas de edad paleozoica inferior, son asignadas actualmente al Pensilvaniano Inferior (edad U-Pb 320 Ma). Representan una sucesión volcánica bimodal encuadrada en los campos subalcalino/toleítico (riolitas) y alcalino (basaltos y traquitas) sobre la base álcalis-sílice. Los términos básicos muestran características comparables y transicionales a MORB-toleitas de intraplaca, con una fuerte contribución mantélica y contaminación cortical. Los miembros ácidos también muestran afinidades con magmas de intraplaca, sugiriendo su derivación de la interacción de material cortical con una fuente mantélica o de una corteza juvenil esencialmente derivada del manto. La edad U-Pb de 320 Ma obtenida para la sección de Las Lozas, al igual que las edades de U-Pb 342 y 348 Ma reportadas para rocas aflorantes en la zona de Cazadero Grande, hacia el sur; junto con las edades U-Pb reportadas para granitos de las Sierra Pampeanas (332-357 Ma) confirman la importancia alcanzada por la actividad magmática carbonífera, encuadrada entre 320 y 350 Ma, en dicha región. Las bajas razones iniciales de estroncio en las rocas volcánicas de Las Lozas (0,70479-0,70164) indican contribución de un componente juvenil, mientras que las mismas relaciones observadas en las riolitas de Cazadero Grande (0,71433-0,71233) y en los intrusivos graníticos de Sierras Pampeanas (0,717-0,7124), apuntan a magmas con participación de componentes corticales. Los valores isotópicos de Nd sobre un basalto en Las Lozas (εNd(320) con 3,11, T DM con 774,6 Ma) permiten argumentar una fuente astenosférica, con evidencia de mezcla entre elementos juveniles y corticales. Los isótopos Sr-Nd determinados, en cambio, para una riolita que aflora en Cazadero Grande (εNd(t) 2,91 y -0,3, T DM: 1,09 y 1,1 Ga) y en granitos de las Sierras Pampeanas (εNd(t): -0,6, T DM: 1,19 y 1,1 Ga) sugieren participación cortical. Dada la diferencia de edades en la región entre el magmatismo misisipiano y pensilvaniano inferior, dichas variaciones indicarían un cambio en la fuente magmática, consistente de un régimen tectónico extensional más pronunciado durante el Pensilvaniano Inferior. Esta presunción debe ser considerada teniendo en cuenta las observaciones a lo largo de una transecta a los 27°30'S, donde las rocas graníticas carboníferas de las Sierras Pampeanas Orientales muestran, sin importar la edad, un mayor aporte hacia el oeste de material juvenil de carácter mantélico. Entre estas rocas, con una mayor componente juvenil, se encuentran: la sucesión de Las Lozas y el Granito Cerro Gloria, manifestaciones occidentales de dicho magmatismo. De lo anterior se desprende que la contribución de material juvenil podría variar continuamente durante ese tiempo como respuesta a una extensión litosférica variable. Un ejemplo de esto surge de los datos de la sucesión de Las Lozas presentados, los que indican a la misma como resultado del retrabajo de material supracortical, con el aporte de magmas juveniles asociados a cambios extensionales de la litósfera.

The Las Lozas volcanic sequence, which crops out at northwestern border of the Famatina belt-southeastern Puna, NW Argentina, is constituted mainly by rhyolites and a lesser volume of basalts and trachytes, and volcanoclastic deposits. These rocks, previously considered of Early Paleozoic age, are now assigned to the Lower Pennsylvanian (320 Ma U-Pb age). They represent a bimodal volcanic succession that plot in the subalkaline/tholeiitic (rhyolites), alkaline basalts (basalts) and alkaline (trachytes) fields on the total alkali-silica diagram. The basalts display features comparable to transitional MORB and within-plate tholeiites, with contributions from a mantle source affected by crustal contamination. The acid members also show geochemical affinities to within-plate magmas, and their composition suggest a derivation from continental crustal material with mantle source interaction or a juvenile essentially mantle derived crust. The 320 Ma age from the Las Lozas volcanic succession as well as the 342 and 348 Ma U-Pb ages, from rocks in the nearby Cazadero Grande section, to the south, and the U-Pb ages from Sierra Pampeanas granites (332-357 Ma) highlight the importance achieved by Carboniferous magmatic activity in that region, framed between 320 and 350 Ma. Low strontium initial ratios from the Las Lozas (0.70479-0.70164) indicate a predominant contribution by a juvenile component, while the ratios in the nearby Cazadero Grande (0.71433-0.71233) and Sierras Pampeanas granites (0.717 to 0.7124) point to an input by enriched sources with restricted contribution of a mantle component. Nd isotopes from a basalt from the Las Lozas section (εNd(320) with +3.11, T DM with 774.6 Ma) indicates a possible asthenospheric source, with evidence for some mixture between juvenile and reworked crustal material. In contrast Sr-Nd isotopes in a rhyolite from Cazadero Grande (εNd(t) of -2.91 and -0.3, T DM of 1.09 and 1.1 Ga) and of Sierra Pampeanas granites (εNd(t) of -0.6, T DM of 1.19 and 1.1 Ga) suggest a crustal source with minor juvenile input. Taking in account the age difference in the region between Mississippian and the Lower Pennsylvanian magmatism, this would indicate a change in the magma source consistent with a more pronounced extensional tectonic regime for the Lower Pennsylvanian. This assumption need to be considered together with observations along a transect at 27°30'S, where the Carboniferous Eastern Sierras Pampeanas granitic rocks show, regardless of age, a greater contribution ofjuvenile material of mantle character to the west. Among these rocks, with a major juvenile component, are those of the Las Lozas succession as well the Cerro Gloria Granite, the eastern manifestations of the Carboniferous Eastern Sierras Pampeanas magmatism. From the foregoing emerges that the contribution of juvenile material could be continuously varied in the region through the Carboniferous, due to varying lithospheric extension. An example of this arise from the presented data of the Lozas succession, which indicate that this rocks resulted from reworking of supra-crustal material with input of juvenile magmas, linked to the change in the lithospheric extension.

Texto completo: acesso restrito. p. 775-785 Submitted by Suelen Reis (suelen_suzane@hotmail.com) on 2012-12-10T16:17:02Z No. of bitstreams: 1 1-s2.0-S0895981102001293-main.pdf: 470146 bytes, checksum: 27f6bc0665ccec0fdb17aca9157ff51f (MD5) Made available in DSpace on 2012-12-10T16:17:02Z (GMT). No. of bitstreams: 1 1-s2.0-S0895981102001293-main.pdf: 470146 bytes, checksum: 27f6bc0665ccec0fdb17aca9157ff51f (MD5) Previous issue date: 2003 The Gavião block in the northern portion of the São Francisco craton, northeast Brazil, is one of the oldest Archean fragments of the South American platform. It underwent polycyclic evolution from very old juvenile components dated between 3400 and 3000 Ma. The studied parts of the Umburanas greenstone belt (UGB) formed over ‘stable’ continental crust. SHRIMP U/Pb isotopic analyses of detrital zircons from conglomeratic quartzites of the UGB's lower unit yield dates between 3335 and 3040 Ma. These dates indicate provenance from crustal materials of different ages and a maximum age of deposition as young as 3040±24 Ma (single zircon analysis) but certainly after 3147±16 Ma (three analyses). 207Pb/206Pb zircon evaporation analyses from a meta-andesite of the intermediate unit of the UGB yield 2744±15 Ma, which is interpreted as the time of magmatic crystallization. The Sm/Nd whole-rock systematics of three metakomatiites from the base of the lower unit indicate that they are isotopically disturbed. Additional Sm/Nd analyses of one metabasalt (lower unit) and two meta-andesites (intermediate unit) were compared with those of the country rocks. Their similarity suggests that crustal contamination processes play an important role in the formation of volcanic rocks. As a whole, the isotopic picture is in agreement with an ensialic tectonic setting of the studied parts of the UGB.