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Almeida V.V.,Geological Survey of Brazil CPRM | Almeida V.V.,University of Sao Paulo | Janasi V.A.,University of Sao Paulo | Svisero D.P.,University of Sao Paulo | Nannini F.,University of Sao Paulo
Central European Journal of Geosciences | Year: 2014

Alkali-bearing Ti oxides were identified in mantle xenoliths enclosed in kimberlite-like rocks from Limeira 1 alkaline intrusion from the Alto Paranaíba Igneous Province, southeastern Brazil. The metasomatic mineral assemblages include mathiasite-loveringite and priderite associated with clinopyroxene, phlogopite, ilmenite and rutile. Mathiasite-loveringite (55–60 wt.% TiO2; 5.2–6.7 wt.% ZrO2) occurs in peridotite xenoliths rimming chromite (∼50 wt.% Cr2O3) and subordinate ilmenite (12–13.4 wt.% MgO) in double reaction rim coronas. Priderite (Ba/(K+Ba)< 0.05) occurs in phlogopite-rich xenoliths as lamellae within Mg-ilmenite (8.4–9.8 wt.% MgO) or as intergrowths in rutile crystals that may be included in sagenitic phlogopite. Mathiasite-loveringite was formed by reaction of peridotite primary minerals with alkaline melts. The priderite was formed by reaction of peridotite minerals with ultrapotassic melts. Disequilibrium textures and chemical zoning of associated minerals suggest that the metasomatic reactions responsible for the formation of the alkali-bearing Ti oxides took place shortly prior the entrainment of the xenoliths in the host magma, and is not connected to old (Proterozoic) mantle enrichment events. © 2014, Versita Warsaw and Springer-Verlag Wien.

Japsen P.,Geological Survey of Denmark | Bonow J.M.,Geological Survey of Denmark | Green P.F.,Geotrack International | Cobbold P.R.,French National Center for Scientific Research | And 4 more authors.
Bulletin of the Geological Society of America | Year: 2012

It is a common assumption that elevated passive continental margins have remained high since rifting and breakup. Here, we show that the Atlantic margin of NE Brazil has under gone a more complex history. Our synthesis of geological data, landscape analysis, and paleothermal and paleoburial data reveals a four-stage history: (1) After Early Cretaceous breakup, the margin under went burial beneath a thick sedimentary cover; (2) uplift episodes in the Campanian and Eocene led to almost complete removal of these deposits; (3) the resulting large-scale, low-relief erosion surface (peneplain) was deeply weathered and finally reburied at the Oligocene-Miocene transition; and (4) Miocene uplift and erosion produced a new, lowerlevel peneplain by incision of the uplifted and re-exposed Paleogene peneplain. Previous studies have identified aspects of this interpretation, but we have defined the absolute timing and magnitude of discrete events of burial and exhumation that followed Early Cretaceous rifting and Eocene-Oligocene peneplanation. We suggest that a late sedimentary cover protected Paleogene weathering profiles until the present day. The uplift phases in Brazil are synchronous with uplift phases in Africa and the Andes. The Andean phases coincided with rapid convergence on the western margin of South America, and the Campanian uplift coincided with a decline in spreading rate at the Mid-Atlantic Ridge. Consequently, we suggest that both vertical movements and lateral changes in the motion of the plates have a common cause, which is lateral resistance to plate motion. © 2012 Geological Society of America.

Baggio S.B.,Federal University of Rio Grande do Sul | Hartmann L.A.,Federal University of Rio Grande do Sul | Andrade R.H.P.,Superintendencia do Mato Grosso do Sul | Rizzotto G.J.,Geological Survey of Brazil CPRM | And 3 more authors.
Ore Geology Reviews | Year: 2015

Thousands of silica gossans are exposed at the top of the Lower Cretaceous basalt hills in the Paraná volcanic province, strongly indicating the presence of calcite, amethyst and agate geode deposits along with native copper mineralization. The Embrapa silica gossan in the northwestern portion of the province (Campo Grande region) is an excellent example of such novel geological structure in the continental flood basalts. This silica gossan has a size of 450 × 350 m standing out as a treeless area in the densely wooded savanna and makes part of the stratigraphy of six basalt flows of Paranapanema intermediate-Ti chemical type. The base of the volcanic column is constituted by two Pitanga types and the overlying column is Paranapanema type. Every basalt flow has a silicified sand layer or breccia at the top and these are fed by abundant sand dikes. The Anel Viário Norte (AVN) flow is the most intensely altered by hydrothermal fluids producing voluminous secondary calcite infillings in the amygdales and fractures. In this region the basalts contain higher copper content than the average of the volcanic province. The studied silica gossans display negative anomalies in gamma spectrometry as a response to K, U and Th depletion during alteration. We propose a new exploration methodology by observing GoogleEarth images complemented with field studies and geochemistry to readily locate favorable areas for amethyst and agate geode deposits and native copper mineralization. © 2015 Elsevier B.V.

Rizzotto G.J.,Geological Survey of Brazil CPRM | Hartmann L.A.,Federal University of Rio Grande do Sul
Lithos | Year: 2012

We document the first-known Mesoproterozoic ophiolite from the southwestern part of the Amazon craton, corresponding to the Trincheira Complex of Calymmian age, and propose a tectonic model that explains many previously enigmatic features of the Precambrian history of this key craton, and discuss its role in the reconstruction of the Columbia supercontinent. The complex comprises extrusive rocks (fine-grained amphibolites derived from massive and pillowed basalts), mafic-ultramafic intrusive rocks, chert, banded iron formation (BIFs), pelites, psammitic and a smaller proportion of calc-silicate rocks. This sequence was deformed, metasomatized and metamorphosed during the development of the Alto Guaporé Belt, a Mesoproterozoic accretionary orogen. The rocks were deformed by a single tectonic event, which included isoclinal folding and metamorphism of the granulite-amphibolite facies. Layered magmatic structures were preserved in areas of low strain, including amygdaloidal and cumulate structures. Metamorphism was pervasive and reached temperatures of 780-853 °C in mafic granulites and 680-720 °C in amphibolites under an overall pressure of 6.8. kbar. The geochemical composition of the extrusive and intrusive rocks indicates that all noncumulus mafic-ultramafic rocks are tholeiitic basalts. The mafic-ultramafic rocks display moderate to strong fractionation of light rare earth elements (LREE), near-flat heavy rare earth element (HREE) patterns and moderate to strong negative high field strength element (HFSE) anomalies (especially Nb), a geochemical signature typical of subduction zones. The lowest units of mafic granulites and porphyroblastic amphibolites in the Trincheira ophiolite are similar to the modern mid-ocean ridge basalt (MORB), although they locally display small Ta, Ti and Nb negative anomalies, indicating a small subduction influence. This behavior changes to an island arc tholeiite (IAT) signature in the upper units of fine-grained amphibolites and amphibole rich-amphibolites, characterized by progressive depletion in the incompatible elements and more pronounced negative Ta and Nb anomalies, as well as common Ti and Zr negative anomalies. Tectono-magmatic variation diagrams and chondrite-normalized REE and primitive mantle normalized patterns suggest a back-arc to intra-oceanic island arc tectonic regime for the eruption of these rocks. Therefore, the Trincheira ophiolite appears to have originated in an intraoceanic supra-subduction setting composed of an arc-back-arc system. Accordingly, the Trincheira Complex is a record of oceanic crust relics obducted during the collision of the Amazon craton and the Paraguá block during the Middle Mesoproterozoic. Thus, the recognition of the Trincheira ophiolite and suture significantly changes views on the evolution of the southern margin of the Amazon craton, and how it can influence the global tectonics and the reconstruction of the continents. © 2012 Elsevier B.V.

Rizzotto G.J.,Geological Survey of Brazil CPRM | Santos J.O.S.,Redstone Resources | Hartmann L.A.,Federal University of Rio Grande do Sul | Tohver E.,University of Western Australia | And 2 more authors.
Journal of South American Earth Sciences | Year: 2013

A major Mesoproterozoic paleo-plate boundary in the southwestern Amazonian Craton, the Guaporé Suture Zone, is investigated by U-Pb zircon geochronology, Sr-Nd isotope geochemistry and aeromagnetic data. This suture zone is constituted dominantly by ophiolitic mafic-ultramafic rocks of the Trincheira Complex, and minor proportion of tonalites of the Rio Galera and São Felipe complexes, Colorado Complex, amphibolites of the Rio Alegre Terrane and syn- to late-kinematic mafic to felsic plutonic rocks. The ophiolitic Trincheira Complex formed during an accretionary phase from 1470 to 1430Ma and was overprinted by upper amphibolite-granulite facies metamorphism during the collisional phase of the Ectasian followed by syntectonic emplacement of gabbro and granite plutons (1350-1340Ma). The ophiolites were intruded by syntectonic tonalitic-plagiogranitic plutons ca. 1435Ma. Mafic-ultramafic rocks of the Trincheira ophiolites show moderate to highly positive initial epsilon Nd (t=1.46Ga) values (+2.6 to+8.8) and very low initial 87Sr/86Sr ratio (0.7013-0.7033). It is suggested that these magmas originated from a depleted mantle source in an island-arc-back-arc setting. The identification of a fossil ophiolite in the Guaporé Suture Zone early as 1470-1435Ma and later collisional phase, as late as 1350Ma, marks the impingement of the proto-Amazonian Craton against the Paragua Block, before the formation of the Rodinia supercontinent. The results provide important insights into the geodynamic history of the SW Amazonian Craton, with evidence for both accretionary orogen and subduction of oceanic lithosphere in the Mesoproterozoic, and provide information that allows other workers to evaluate the configuration of supercontinents. © 2013 Elsevier Ltd.

Hartmann L.A.,Federal University of Rio Grande do Sul | Wildner W.,Geological Survey of Brazil CPRM | Duarte L.C.,Federal University of Rio Grande do Sul | Duarte S.K.,Federal University of Rio Grande do Sul | And 4 more authors.
Geological Magazine | Year: 2010

Geochemical studies of the six lowermost lava flows of the Cretaceous Serra Geral Formation (Parań volcanic province) in Quaraí (Brazil) and Artigas (Uruguay) were combined with flow-by-flow field studies of structures and scintillometric profiles to establish a consistent regional stratigraphic framework over at least 100 km. This greatly improves exploration capability for amethyst and agate geodes. A basalt, colada Mata Olho (Alegrete facies, Serra Geral Formation), was the first lava to flow over the ancient Botucatu desert in the region, but an andesite, colada Cataln, overstepped this basalt in many places, perhaps palaeohighs. Four basaltic andesites complete the lava stratigraphy in this formation, adding up to 300 m of lavas. The stratigraphic sequence of contrasting lava compositions is 51.0 wt% SiO2 in the first lava, followed by 57.5, 52.5, 56.0, 53.0 and finally 54.5 wt% SiO 2. Overall MgO variation is between 2 and 7 wt%. All lavas in the two districts are low-Ti (<2.0 wt% TiO2) of the Gramado type. The characteristic contents of most major and trace elements (124 rock samples analysed) allow the ready identification of each lava. Contrasting rock chemistry also results in strong variation in scintillometric values (270 points measured in the field and nineteen continuous borehole profiles); from bottom to top of the stratigraphy, the cps values are 49 ± 3.2, 123 ± 10.3, 62 ± 4.7, 94 ± 4.6, ∼45 and ∼85. Colada Cataln has the structure of aa lava, particularly the contorted igneous banding and autobreccias in the upper and lower crusts. In some places, a 2 m thick, silicified sandstone layer lies on top of some coladas, and silicified sandstone forms breccias with volcanic rocks. Geochemistry of the six lavas indicates complex evolution, involving melting of lithospheric mantle, injection into the crust and assimilation of crust followed by fractional crystallization. This study indicates the possibility of world-class deposits of amethyst geodes on the Brazilian side of the border with Uruguay. Copyright © Cambridge University Press 2010.

Hartmann L.A.,Federal University of Rio Grande do Sul | Da Cunha Duarte L.,Federal University of Rio Grande do Sul | Massonne H.-J.,University of Stuttgart | Michelin C.,Federal University of Rio Grande do Sul | And 11 more authors.
International Geology Review | Year: 2012

The opening and filling of cavities in rocks are the major processes related to the generation and sealing of porosity in ore deposits. This study documents three stages of opening and filling of vesicles and geodes in the basalts and rhyodacites of the southern Paraná volcanic province. Each step detailed here is actually part of a sequence of minor hydrothermal events. First, lava degassing at high temperature (1150°C) formed small (<4 cm) vesicles in the crusts of flow units. In sequence, these vesicles were partly to fully filled at low temperature (30-150°C) by hydrothermal minerals, particularly clays and zeolites; this process also sealed the porosity of the lava. Second, the injection of fluidized sand generated new cavities, which were partly filled with sand; the newly formed porosity was sealed by the low-temperature fluid. Third, intense alteration of the basalt or rhyodacite core into a claystone favoured the opening of small to giant protogeodes (0.1 mm to 4 m) by dissolution; cooling of the fluid led to the precipitation of hydrothermal minerals, particularly the spectacular amethyst, calcite, and gypsum-bearing geodes. © 2012 Copyright Taylor and Francis Group, LLC.

Souza V.D.S.,University of Brasilia | De Souza A.G.H.,Geological Survey of Brazil CPRM | Dantas E.L.,University of Brasilia | Valerio C.D.S.,Federal University of Roraima
Brazilian Journal of Geology | Year: 2015

The Mesoproterozoic K'Mudku event (1490 - 1147 Ma) is represented by a britlle-ductile shear belt that cuts across the Paleoproterozoic units in the southernmost Guyana shield, central-north Amazon craton. This event produced mylonitization and cataclasites at low/medium-to high-grade metamorphic, and local within-plate magmatism. In the Amazonas State, Brazil, A-type magmatism chronologically associated to K'Mudku has been reported for the Pedra do Gavião and Samaúma syenogranites. However, the spatial relationship between K'Mudku event and A-type magma generation are not yet adequately clarified in the region. The Pedra do Gavião syenogranite is a high-K alkaline, metaluminous, reduced A-type granite with a post-collisional to within-plate geochemical signature. It has U-Pb zircons crystallization age of 1218 Ma and inherited zircons with ages between 1820 and 1720 Ma, which, together with the Sm-Nd data, suggest melting of Paleoproterozoic basement rocks of the Cauaburi Complex (1810 - 1780 Ma) regional unit. These data demonstrate that the effects of the A-type magmatism associated to the end of the Grenvillian-Sunsas orogeny, reported primarily in the southwestern margin of the Amazon craton, may also be extended for the central-northern part of the Amazon craton. Probably the generation or emplacement mechanisms of A-type magma occurred with some degree of involvement in the final stages of the K'Mudku event. However, this tectonic framework conception still needs more geological and geophysical investigations. Therefore, these news data should instigate to the return of geological research in the region, as well as to debate on the tectonic evolution and A-type granites production during the Ectasian-Stenian period in the central-north Amazon craton. © 2015 Brazilian Journal of Geology.

Geostatistical modeling, using airborne and borehole electromagnetic data, was used to estimate electrical conductivity in groundwater within fractured paragneisses and migmatites in a semi-arid climate in northeastern Brazil. Despite the geologic heterogeneity of crystalline aquifers, the use of high resolution helicopter electromagnetic (HEM) data enabled the characterization of groundwater electrical conductivity where data from drilled wells were insufficient. The tacit assumption is that HEM measurements can be used to relate rock electrical conductivity to groundwater electrical conductivity. In this study, the HEM data were used as an external drift variable in non-stationary estimation and stochastic simulation to identify the variability of groundwater electrical conductivity. Validation tests, comparing predicted values for groundwater conductivity with measurements in new wells, confirmed the success of these models in locating fresh groundwater sources in crystalline bedrock. © 2010 Springer-Verlag.

Scandolara J.E.,Geological Survey of Brazil CPRM | Ribeiro P.S.E.,Geological Survey of Brazil CPRM | Frasca A.A.S.,Geological Survey of Brazil CPRM | Fuck R.A.,University of Brasilia | Rodrigues J.B.,Geological Survey of Brazil CPRM
Journal of South American Earth Sciences | Year: 2014

This study presents new geochemical data on rocks from the Vespor suite, an important mafic unit from the Juruena arc, Roosevelt-Juruena terrain, SW Amazonian craton, northwest Mato Grosso, Brazil, attempting to define their tectonic setting and type of mantle source. The Juruena arc may be part of a magmatic belt (Jamari and Juruena arcs) at the southwestern Amazonian craton during assembly of the Columbia supercontinent. The investigated rocks represent a Paleoproterozoic subduction-related mafic suite of sigmoidal bodies, composed mainly of gabbro, norite, gabbronorite and diorite, that underwent amphibolite facies metamorphism. Here we present also preliminary petrology aspects and zircon U-Pb geochronology. Geochemical character and variation trends of major and trace elements as well as selected trace element ratios suggest that Vespor suite rocks have a tholeiitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Gabbros, norites and gabbronorites are characterized by enrichment of LILE and weakly to moderately differentiated HFSE patterns, suggesting their deviation from an enriched heterogeneous lithospheric mantle source. Vespor suite rocks are characterized by depletion of Nb-Ta, P and Ti, with flat distribution of HFSE, markedly large variations in most of the LILE, positive anomalies displayed by Ba, K, Th, Sr, Pb and weak negative anomalies of Hf-Zr. These features reflect limited degrees of crustal contamination associated with a subduction-related magma process where the mantle wedge was chemically modified. In addition, the enrichment in LILE and Pb, low values of the ratios (Lan/Smn - 0.83 to 4.58) and (Nbn/Lan - 0.04 to 0.45), but high Th/Yb ratios, gently to moderately sloping REE profiles (La/Ybn=2.53-7.37), negative anomalies in HFSE (Ta, Zr, Hf, and Ti), and positive anomalies in LILE (Th, Ba, Sr), suggest derivation from a metasomatized lithospheric mantle source above a subduction zone with weak crustal contamination. Both the composition of the mantle source and the degree of partial melting that produced the parental magmas of these rocks, determined by using REE abundance and ratios, indicate that gabbroic/dioritic melts were generated at different degrees of melting of the source: about 5-20% partial melting of a garnet-spinel lherzolite, around 1-10% partial melting of spinel lherzolite source, and approximately 1-5% partial melting of intermediate source composition, and crystallizing between 1.773 and 1.764Ma. © 2014 Elsevier Ltd.

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