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11.
Alcides Nobrega Sial Claudio Gaucher Marinho Alves da Silva Filho Valderez P. Ferreira Marcio M. Pimentel Luiz D. Lacerda Emmanoel V. Silva Filho Wilker Cezario 《Precambrian Research》2010
Two cap carbonates overlying glaciogenic diamictites crop out extensively in the eastern Vaza Barris Domain of the Sergipano Belt, northeastern Brazil. They are represented by carbonates of the Jacoca Formation, resting on top of diamictite of the Ribeiropolis Formation, and by the Olhos D’Agua Formation (carbonates, organic-rich towards the top), which overlies diamictite of the Palestina Formation. These two sequences were deformed and metamorphosed at sub-greenschist facies-conditions during the Brasiliano cycle (650–600 Ma). 相似文献
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Renaud Caby Alcides N. Sial Valderez P. Ferreira 《Journal of South American Earth Sciences》2009,27(2-3):184-196
Unusual high-pressure inner thermal aureoles are described from the Minador and Angico Torto epidote-bearing tonalitic plutons that emplaced into greenschist-facies metasedimentary rocks of the Neoproterozoic Cachoeirinha-Salgueiro belt, northeastern Brazil. The foliated pelitic hornfelses display the mineral assemblage garnet, kyanite, staurolite, muscovite, biotite, plagioclase ± quartz. Rare fibrolite is only found very close to the contacts. Hornfelses display steep mineral lineations and steeply-dipping foliations concordant with magmatic contacts. Leucocratic veinlets containing quartz, oligoclase, garnet, kyanite, staurolite, rutile and ilmenite suggest that limited melting conditions were reached very close to magmatic contacts (T ? 650 °C, P around 8 kbar). These high-pressure hornfelses form a few meters thick, rigid envelopes around the two plutons. Contrary to known examples of kyanite-bearing hornfelses that recorded high-temperature decompression, the nearly isobaric cooling down to ca. 450 °C is constrained by 3.20–3.30 Si contents of retrogressive phengites from both inner hornfelses and ductilely-deformed tonalite at the pluton margins. Isograds and bathograds are, therefore, apparently telescoped due to HP/LT shearing, possibly caused by subsequent differential vertical movements affecting these two solidified plutons. The unusual depth of emplacement of these syn-kinematic calc-alkaline plutons is explained by a tentative geodynamic model involving a pre-620 Ma-subduction setting.ResumenLas aureolas internas que rodean dos plutones tonalíticos emplazados dentro de rocas cajas en facies esquistos verdes del Cinturón-plegado Cachoeirinha-Salgueiro al noreste de Brasil, contienen hornfelses pelíticos foliados con granate, kyanita, estaurolita, muscovita, biotita, plagioclasa ± cuarzo. Fibrolita es rara ó es encontrada solamente cerca de las zonas de contacto. Los hornfelses desarrollaron foliaciones concordantes con buzamiento fuerte cerca de los contactos magmáticos y muestran lineaciones minerales casi verticales. Venillas leucocraticas contienen cuarzo, oligoclasa, granate, kyanita, estaurolita, rutilo e ilmenita sugiriendo que el límite en las condiciones de fusión fueron cercanas a los contactos magmáticos (T ? 650 °C, P entorno 8 kbar). Estos hornfelses de alta presión forman una cobertura rígida de poco metros entorno a los dos plutones. Contrariamente a lo conocido en diferentes ejemplos de hornfelses con kyanita con registro de descompresión, la isóbara de enfriamiento para cerca de 450 °C está controlada por el contenido de Si 3.20–3.30 de fengitas retrógradas, en los hornfelses internos y en las márgenes de los plutones donde las tonalitas están deformadas ductilmente. Isógradas y batógradas son, por lo tanto, aparentemente telescopadas debido a cizalla de alta presión/baja temperatura, posiblemente causado por subsecuentes movimientos verticales diferenciales afectando estos dos plutones solidificados. El emplazamiento a una profundidad inusual de estos plutones calci-alcalinos syn-cinemáticos es explicado por un modelo geodinámico tentativo que involucra un escenario de subducción pre-620 Ma. 相似文献
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Physical controls on sand composition and relative durability of detrital minerals during ultra‐long distance littoral and aeolian transport (Namibia and southern Angola) 
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下载免费PDF全文 Eduardo Garzanti Alberto Resentini Sergio Andò Giovanni Vezzoli Alcides Pereira Pieter Vermeesch 《Sedimentology》2015,62(4):971-996
Sediment in coastal Namibia to southern Angola is supplied dominantly from the Orange River with minor additional fluvial input and negligible modifications by chemical processes, which makes this a great test case for investigating physical controls on sand texture and composition. This study monitored textural, mineralogical and geochemical variability in beach and aeolian‐dune sands along a ca 1750 km stretch of the Atlantic coast of southern Africa by using an integrated set of techniques, including image analysis, laser granulometry, optical microscopy, Raman spectroscopy and bulk‐sediment geochemistry. These results contrast with previous reports that feldspars and volcanic detritus break down during transport, that sand grains are rounded rapidly in shallow‐marine environments, and that quartzose sands may be produced by physical processes. Mechanical wear is unable to modify the relative abundance of detrital components, including pyroxene and mafic volcanic rock fragments traditionally believed to be destroyed rapidly. The sole exceptions are poorly lithified or cleaved sedimentary/metasedimentary rock fragments, readily lost at the transition to the marine environment, and slow‐settling flaky micas, winnowed and deposited offshore. Coastal sediments tend to be depleted in relatively mobile amphibole, preferentially entrained offshore or re‐deposited in sheltered beaches, while less mobile garnet is retained onshore. No detrital mineral displays a significant increase in grain roundness after 300 to 350 km of longshore transport in high‐energy littoral environments from the Orange mouth to south of the Namib Erg, but all minerals get rapidly rounded after passing into the dunefield. Pyroxene and opaques get rounded faster than harder quartz and garnet, but sand mineralogy remains unchanged. Excepting strong transient selective‐entrainment effects, physical processes are unable to modify sand composition significantly. Selective mechanical breakdown can be largely neglected in quantitative provenance analysis of sand and sandstone even in the case of ultra‐long‐distance transport in high‐energy environments dominated by strong persistent winds and waves. 相似文献
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Isotopic and trace-element indications of lithospheric and asthenospheric components in Tertiary alkalic basalts, northeastern Brazil 总被引：1，自引：0，他引：1
Plate reconstructions at 30 Ma place northeastern Brazil over the Fernando de Noronha hotspot, presently 250 km offshore northeastern Brazil. Tertiary basaltic centers in northeastern Brazil may therefore be products of the Fernando de Noronha hotspot. We examined alkalic basalt from seven 30–13 Ma old centers in Rio Grande do Norte and Pernambuco states to assess this possible link. Compositions are primitive, where MgO concentrations range from 12.5 to 17 wt.%, and SiO2 from 41 to 48 wt.%. Trace-element abundances and Sr, Nd, and Pb isotopic compositions compare well with those of ocean island basalt:
=0.7038–0.7051,
=0.51266–0.51280, and
=18.52–19.35. There are correlations among SiO2-undersaturation, incompatible-element abundances, relative percentages of partial melting (based on La/Yb and La/Y ratios), and the degree of isotopic ‘enrichment' inherited from mantle sources. There is also a negative correlation for La/Nb (0.6–0.86) vs. Ba/Nb (6.6–10.6), where lower La/Nb samples represent larger percentages of melting of a source comparatively enriched in radiogenic Sr. We attribute these compositional relationships to the lavas representing mixing of melts mainly from asthenosphere of largely HIMU plus DM characterization, probably the Fernando de Noronha plume, with melts from subcontinental lithosphere that was isotopically closer to EM1. Isotopic and trace-element compositions of the northeastern Brazil basalts are generally similar to those of Fernando de Noronha lavas (12–2 Ma), and some minor trace-element differences observed (e.g., more Zr, Nb, and less Th compared to northeastern Brazil basalts) are probably due to heterogeneity within the asthenospheric plume and to melt contributions from delaminated subcontinental lithosphere that may underlie Fernando de Noronha. 相似文献
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A.N. Sial S. Peralta V.P. Ferreira A.J. Toselli F.G. Aceolaza M.A. Parada C. Gaucher R.N. Alonso M.M. Pimentel 《Gondwana Research》2008,13(4):437-452
Carbon and Sr-isotope profiles in Upper Cambrian platformal carbonate Formations in the Precordillera, western Argentina (Zonda, La Flecha and La Silla Formations), were constructed for three representative sections: (a) Quebrada de la Flecha, Eastern Precordillera, (b) Cerro La Silla, Central Precordillera and (c) Quebrada de La Angostura, northern part of the Central Precordillera.
At Quebrada de La Angostura, upper part of the La Flecha Formation, δ13Ccarb varies continuously up-section from − 2.0 to + 5.6‰ (PDB) and records the SPICE anomaly (+ 5‰) reported for the first time in South America. The peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone that record the onset of a sea-level change.
The Steptoean Zonda Formation dolomites at the Quebrada de la Flecha exhibit a total δ13C range from − 2.7 to + 0.6‰ with discrete positive anomaly about 200 m from the transition to the overlying Sunwaptan La Flecha Formation. Pronounced C-isotope anomaly (− 5.6‰) is observed in the La Flecha Formation at about 300 m below the transition to the La Silla Formation.
At the Cerro La Silla section, the Zonda Formation exhibit δ13C values of − 1‰, increasing slightly at the transition to the La Flecha Formation (− 1 to 0‰). The transition of the La Flecha to the La Silla Formations is characterized by alternation of black shales and dolomitic limestone with a discrete positive C-isotope excursion, probably corresponding to the SPICE.
At the Quebrada de La Flecha, 87Sr/86Sr for the Zonda Formation varies from 0.70924 to 0.70955 and for the La Flecha Formation from 0.70908 to 0.70942. At Cerro La Silla this ratio varies from 0.70914 to 0.70923 for the La Flecha Formation, and from 0.70898 to 0.70980 for the La Silla Formation. At the Quebrada de La Angostura, ratios for the La Flecha carbonates range from 0.70918 to 0.70993. The overall variation of 87Sr/86Sr is consistent with globally reported Upper Cambrian seawater values at ca. 500 Ma.
The unambiguous record of SPICE in the La Flecha Formation at the Quebrada de La Angostura supports a Steptoean age for its deposition and allows precise local, regional, and global stratigraphic correlation. The pronounced negative C-isotope excursion recorded in the La Flecha Formation carbonates at the Quebrada de La Flecha is likely equivalent to that registered in Sunwaptan carbonates of North America and Australia, and might be tied to a global event, as a valuable tool in stratigraphic correlation (SNICE, acronym for Sunwaptan negative isotope carbon excursion). 相似文献
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Global nature of the Paleoproterozoic Lomagundi carbon isotope excursion: A review of occurrences in Brazil,India, and Uruguay 总被引：2，自引：0，他引：2
Anil Maheshwari Alcides N. Sial Claudio Gaucher Jorge Bossi Andrey Bekker Valderez P. Ferreira Antonio W. Romano 《Precambrian Research》2010
Positive carbon isotope excursion is reported from Paleoproterozoic carbonates of the Aravalli Supergroup (northwestern India), the Minas Supergroup (Brazil), and new sections of the Paso Severino Formation (Uruguay). The 2.42 Ga Gandarela Formation, Minas Gerais, Brazil, contains red carbonate-facies BIF grading into dolostones and limestones and yielding δ13C values ranging from −1.6 to +0.4‰ V-PDB. The positive C-isotope excursion (up to + 11‰ V-PDB) in marine shallow-water carbonates in India and Brazil (Jhamarkotra Formation in northwestern India, and Cercadinho and Fecho do Funil formations in Minas Gerais State, Brazil) is comparable to that observed in 2.22–2.1 Ga carbonate successions worldwide that were deposited during the Lomagundi excursion. In Uruguay, δ13C values up to +11.6‰ V-PDB in the deep-water Paso Severino Formation of the Piedra Alta Terrane are compatible with deposition at ca. 2.15 Ga, as indicated by the 2146 ± 7 Ma U–Pb age of dacites occurring at the top of the unit. Negative δ13C values are also present in carbonates of the Paso Severino Formation, but an origin related to organic-matter remineralization cannot be ruled out. Thin carbonate beds in the Rio Itapicuru greenstone belt, Bahia State, Brazil, are associated, as in the Paso Severino Formation, with deep-water black shales and have carbon isotope values up to +9‰ V-PDB. High metamorphic grade carbonates of the Jacurici terrane in the Medrado-Ipueira area, Bahia, Brazil, have carbon isotope values up to +6.9‰ V-PDB, consistent with their minimum age of 2085 ± 5 Ma inferred from the intrusive contact with and the age of the Medrado norite. No evidence was found in India, Brazil, or Uruguay for Paleoproterozoic glacial events recognized in the 2.45–2.22 Ga sedimentary successions worldwide. Unconformities between the Gandarela and Cercadinho formations in Brazil and the banded gneissic Complex and the Lower Aravalli Supergroup in India might explain the absence of glacial record. Compositional and isotopic data presented here for studied Paleoproterozoic carbonate successions allow their integration into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age. The study highlights the global nature of the Lomagundi excursion. Furthermore, it indicates that the Lomagundi excursion is recorded in both shallow-water (Aravalli and Minas supergroups) and deep-water carbonates (Paso Severino Formation and Rio Itapicuru greenstone belt) negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow-water, open-marine and isolated basins. 相似文献
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Olga Solomina Vincent Jomelli Georg Kaser Alcides Ames Bernhard Berger Bernard Pouyaud 《Global and Planetary Change》2007,59(1-4):225
This paper is a comparison and compilation of lichenometric and geomorphic studies performed by two independent teams in the Cordillera Blanca, Peru, in 1996 and 2002 on 66 “Little Ice Age” moraines of 14 glaciers. Using eleven new control points, we recalibrated the initial rapid growth phase of the previously established Rhizocarpon subgenus Rhizocarpon growth curve. This curve was then used to estimate the age of “Little Ice Age” moraines. The time of deposition of the most prominent and numerous terminal and lateral moraines on the Pacific-facing side of the Cordillera Blanca (between AD 1590 and AD 1720) corresponds to the coldest and wettest phase in the tropical Andes as revealed by ice-core data. Less prominent advances occurred between AD 1780 and 1880. 相似文献
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There is a large sociological literature on racial- class-, and economically- based segregation in the United States and there is some evidence that residential segregation by income may have deleterious health consequences for residents of large U.S. cities. The health consequences of segregation in Canadian metropolitan areas, however, remain unknown and the comparison with the U.S. is always compelling. In this paper, we investigate the hypothesis that residential segregation by income may be associated with mortality in Canadian and U.S. metropolitan areas. Given the strong relationship between individual level socioeconomic status and health, it follows that metropolitan areas which isolate individuals economically could produce conditions that severely limit the life chances and therefore the health chances of the most vulnerable. To investigate the association between residential segregation by income and population health, we examined the relationship between working-age mortality and Jargowsky's (1996) neighbourhood sorting index (NSI) for a large group of North American metropolitan areas. We found a relationship between increased segregation and increased mortality for U.S. metropolitan areas but no such relationship for Canadian metropolitan areas. We also determined that income segregation could not be considered in isolation from income inequality – that, in effect, income inequality provides the propensity for meaningful segregation to occur. We further demonstrated the importance of considering both income inequality and income segregation together, especially when the analysis is intended to compare metropolitan areas. We conclude with a discussion of the need for an improved measure of segregation to better reflect the theoretical arguments for the relationship between concentration of poverty and affluence and population health. 相似文献