UNIVERSIDADE DE BRASÍLIA - INSTITUTO DE GEOCIÊNCIAS
UNIVERSIDADE DE BRASÍLIA -
INSTITUTO DE GEOCIÊNCIAS
ABSTRACTS
DE CONTRIBUIÇÕES CIENTÍFICAS DO IG/UnB
ANO 2004
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C–O–Sr isotopic stratigraphy of cap carbonates overlying Marinoan-age glacial diamictites in the Paraguay Belt, Brazil
Carlos J. S. de Alvarenga
Roberto V. Santos
Elton L. Dantas
Precambrian Research (2004);
131(1-2): 1-21
Full Text + Links
Keywords: Cap carbonate; Carbon isotope; Glaciation; Neoproterozoic; Sr isotope; Stratigraphy
The Neoproterozoic Paraguay Belt, located along the southeastern border of
the Amazon Craton and eastern border of the Rio Apa Block, comprises a thick
passive margin succession of glaciomarine turbidites, carbonates and
siliciclastic sedimentary rocks that were deformed during the
Brasiliano/Pan-African Orogeny. The carbonate sequence comprises a 1300 m
thick succession of platformal carbonate rocks (Araras Formation), which
directly overlie Marinoan glacial diamictites (Puga Formation). The late
Neoproterozoic age of this carbonate sequence is indicated by the presence of
Cloudina lucianoi and Corumbela werneri found in the upper part
of the Corumbá Group (Southern Paraguay Belt).
Carbon and Sr isotope data were obtained on marine carbonate samples across
three different sections of the Araras Formation.
13CPDB
values of the carbonate from the border of the basin range between −10.5 and
−2.7‰, including a short stratigraphic interval of cap dolomite (22 m)
directly overlying diamictites. The isotopic profile across the basal 200 m of
deep shelf laminated microcrystalline limestone and clay-limestone
(cap-carbonate), that overlie glacial diamictites, also show negative
13CPDB
values (−5.3 and +0.6‰). In contrast with the lower portion of the carbonate
sequence, a 800 m thick unit of shallow-water dolostones forming the upper
part of the carbonate sequence present uniform positive
13CPDB
values ranging from +1.9 to +2.4‰. Dolostone and arenaceous dolostone of the
upper parts of the Araras Formation have high
13CPDB
values (+4.1 to +9.6‰) which are followed by an abrupt decrease in
13C
values, down to −1.0‰.
The carbon isotope profile of the Paraguay Belt is quite different from the
Bambuí Group an other important Neoproterozoic sequences in Central Brazil,
which also overlie glacial sediments. Negative
13CPDB
values are found only within the lowermost meters of these sequence and are
followed by an important and extensive positive carbon isotope excursion
(values ranging from +6.9 to +16.0‰). On the other hand, the
13CPDB
profile observed across the Araras Formation exhibits an approximately 200 m
thick section of carbonates with low
13CPDB
values.
Carbonates of the Araras Formation have 87Sr/86Sr
ratios ranging from 0.70753 to 0.70803, comparable to cap carbonates that
overly Marinoan glacial deposits from other neoproterozoic sequences.
Deep seismic refraction and gravity crustal model and tectonic deformation in Tocantins Province, Central Brazil
Jesus Berrocala,
Yara Marangonia
Nelsí C. de Sáa
Reinhardt Fuckb
José E.P. Soaresb
E. Dantasb
Fábio Perosia
Celia Fernandesa
Keywords: Deep seismic refraction; Crustal structure in Brazil; Crustal deformation; Seismotectonic studies in Brazil
Tectonophysics (2004);
388(1-4): 187-199
Full Text + Links
Interpretation of seismic refraction data in the central sector of
Tocantins Province, Central Brazil, has produced a seismic crustal model with
well-defined upper, intermediate, and lower crust layers having smooth
velocity gradient in each layer. The depths to Moho vary from 32 to 43 km, and
mean crustal P velocity varies from 6.3 km/s, beneath Goiás magmatic arc on
the western side, to 6.4 km/s, below Goiás massif in the central portion and
the foreland fold-and-thrust belt on the eastern side. The behaviour of the
lower crust layer allows an improved understanding of regional gravimetric
features of the central and northern sectors of Tocantins Province and
suggests subduction of the Amazon plate in Central Brazil. In the southeastern
sector, the refraction experiment resulted in the detection of a thinner crust
(38 km) below Brasília fold belt and a thicker crust (41 km) below Paraná
basin and São Francisco craton (42 km). The upper crust beneath Paraná Basin
is around 20 km thick, whereas it is less than 10 km thick below the craton.
These results bring new insights into the geological history of the central
and southeastern sectors of Tocantins Province.
Gravimetric measurements in the central sector of Tocantins Province
delineate a high and a low anomaly separated by a steep gradient with a NE
direction. The axis of the gradient seems to bend still further to NE in the
northern sector of that province, whereas the gravimetric high continues
northwards, defining a separation between them. This suggests that those
features belong to different tectonic processes that occurred during Tocantins
Province orogenesis. The gravimetric model, which incorporates seismically
resolved structure beneath Tocantins Province, better matches the observed
gravimetric data.
Although tectonic movements have only been monitored with high-precision
GPS for short time interval (1999–2001), the results suggest observable
deformations. The main seismicity of Central Brazil, the Goiás–Tocantins
seismic belt, seems to be spatially associated with the large gravimetric high
anomaly and with the observed tectonic deformation.
Stone-line formation processes documented by in situ-produced 10Be distribution, Jardim River basin, DF, Brazil
R. Brauchera
C. V. Limab
D. L. Bourlèsa
J. C. Gasparc
M. L. L. Assadc
a CEREGE UMR 6635, Plateau de l' Arbois 13545, Aix
en Provence, France
b Instituto de Estudos Sócio-Ambientais, IESA Universidade Federal
de Goiás, Goiânia GO, Brazil
c ICC Centro, Campus Univesitário Darcy Ribeiro 70910-900, Brasilia
DF, Brazil
Earth and Planetary Science Letters (2004); 222(2): 645-651
Full Text + Links
Keywords: cosmogenic nuclides; erosion; stone-line
Concentrations of in situ-produced cosmogenic 10Be (T1/2=1.5 Myr) were measured in quartz samples from a quartz vein and its connected stone line from the Jardim River basin, Distrito Federal, Brazil, in order to quantify the processes involved in the landscape evolution of that region. The exponential decrease of the 10Be concentrations along the quartz vein, as well as their systematic increase along the stone-line away from the quartz vein, suggests an autochthonous development. Two models allow to estimate the lateral displacement rate. A plugs model assuming that the sample depth, and hence the production rate, is constant, and a burial model assuming that the sample depth, and hence the production rate, varies through time, yield lateral displacement rate of 37±5 and 68±6 m Myr−1, respectively.
The 3.4–3.5 Ga São José do Campestre massif, NE Brazil: remnants of the oldest crust in South America
E. L. Dantasa
W. R. Van Schmusb
P. C. Hackspacherc
A. H. Fetterc
B. B. de Brito Nevesd
U. Cordanid
A. P. Nutmane
I. S. Williamse
Precambrian Research (2004);
130(1-4): 113-137
Full Text + Links
a Instituto de Geociências,
Universidade de Brasilia, Campus Universitario, Darcy Ribeiro, Brasilia,
70910-900 DF, Brazil
b Department of Geology, University of Kansas, 1475 Jayhawk Blvd.,
Lawrence, KS 66045, USA
c Instituto de Geociências e Ciências Exatas, Universidade Estadual
Paulista (UNESP), Caixa Postal 178, 13506-610, Rio Claro, SP, Brazil
d Instituto de Geociências, Universidade de São Paulo, Caixa Postal
11.348, CEP 05422-970, São Paulo, SP, Brazil
e Research School of Earth Sciences, Australian National
University, Canberra, ACT, Australia
Keywords: Early Archean; Borborema Province in NE Brazil; SHIRIMP U–Pb
The oldest fragment of continental crust recognized in South America occurs as an isolated Archean enclave in northeastern Brazil’s Borborema Province, ca. 600 Ma Brasiliano–Pan African orogenic belt. This Archean fragment, the São José do Campestre massif, is surrounded by large tracts of 2.2–2.0 Ga Paleoproterozoic gneisses and is located more than 600–1500 km from the much larger assemblages of Archean rocks found in the São Fransciso and Amazonian cratons, located to the south and west, respectively. Geochronological studies of the São José do Campestre massif show that its oldest rocks contain zircons with U–Pb ages up to 3.5 Ga and Sm–Nd TDM model ages of more than 3.7 Ga, indicating that they represent "reworked" crust. This older "nucleus" is flanked by both reworked and juvenile 3.25 and 3.18 Ga rocks which are intruded by both 3.00 and 2.69 Ga plutonic bodies. The protracted evolution the São José do Campestre massif is consistent with that of a larger continental mass as opposed to a small crustal fragment that grew in isolation. As such, the São José do Campestre massif is interpreted as representing a detached piece of an evolved craton that became entrained with younger rocks during a subsequent Paleoproterozoic accretionary-orogenic event. This hypothesis is bolstered by the presence of Paleoproterozoic gneisses that envelop the São José do Campestre massif, as well as the existence of ca. 2.0 Ga metamorphic zircon and monazite within its rocks. The occurrence of several different Archean "cratonic basement inliers" within the greater Paleoproterozoic crustal framework of the Borborema Province suggests that cratonic slices spalled off one or more larger Archean masses prior to the ca. 2.2–2.0 Ga Paleoproterozoic orogenic collage. A important challenge is to link these older fragments to their parent cratons. Although results are not unique, the pattern of ages and isotopic signatures observed in the São José do Campestre massif is similar to that seen in parts of the São Francisco Craton, and it is possible that the São José do Campestre massif is a fragment of an Archean continental fragment formed during an episode of continental breakup prior to 2200 Ma.
The Caldas Novas dome, central Brazil: structural evolution and implications for the evolution of the Neoproterozoic Brasília belt
Luiz José Homem D'el-Rey Silva
Percy Boris Wolf Klein
Detlef Hans-Gert Walde
Journal of South American Earth
Sciences(2004); 17(2), 153-169
Full Text + Links
Keywords: Caldas novas dome; Brasília belt; Brasiliano cycle; Neoproterozoic; Structural analysis; Thermal water; Tocantins province
The Caldas Novas dome (Goiás state, central Brazil) lies in the southern
segment of the Neoproterozoic Brasília belt (center of the Tocantins Province)
between the Goiás magmatic arc and the margin of the ancient São Francisco
plate. The core of the dome comprises rocks of the Meso-Neoproterozoic Paranoá
group (passive margin psamitic-pelitic sediments and subgreenschist facies)
covered by a nappe of the Neoproterozoic Araxá group (backarc basin
pelitic-psamitic sediments and volcanics of greenschist facies, bitotite
zone). Hot underground waters that emerge along fractures in the Paranoá
quartzite and wells in the Araxá schist have made the Caldas Novas dome an
international tourist attraction. A recent detailed structural analysis
demonstrates that the dome area was affected by a D1–D3
Brasiliano cycle progressive deformation in the ~750–600 Ma interval
(published U–Pb and Sm–Nd data). During event D1, a pervasive
layer-parallel foliation developed coeval the regional metamorphism. Event D2
(intense F2 isoclinal folding) was responsible for the emplacement
of the nappe. D1 and D2 record a regime of simple shear
(top-to-SE relative regional movement) due to a WNW-ESE subhorizontal
compression (
1).
Event D3 records a WSW-ENE compression, during which the dome rose
as a large-scale F3 fold, possibly associated with a duplex
structure at depth. During the dome's uplift, the layers slid back and down in
all directions, giving way to gravity-slide folds and an extensional
crenulation cleavage. A set of brittle fractures and quartz veins constitutes
the record of a late-stage D4 event important for understanding the
thermal water reservoir.
Kwitko-Ribeiro & Oliveira,2004
Revista Brasileira de Geociências 34(1):117-126, março de 2004
O DEPÓSITO AURÍFERO DE ANTÔNIO PEREIRA, QUADRILÁTERO FERRÍFERO: CONDIÇÕES P-T E NATUREZA DOS FLUIDOS MINERALIZADORES
ROGÉRIO RIBEIRO-KWITKO
CLAUDINEI GOUVEIA DE OLIVEIRA
Palavras-chave: Anticlinal de Mariana, mineralização aurífera, inclusões fluidas, geotermometria
Resumo
O depósito de Antônio Pereira integra o contexto geológico do distrito aurífero de Mariana, o qual inclui ainda outros importantes depósitos auríferos que se distribuem ao longo do traço NW do anticlinal de Mariana, na porção central do Quadrilátero Ferrífero. Os depósitos encontram-se hospedados preferencialmente na interface estrutural (zona de cisalhamento Fundão-Cambotas) que envolvem as unidades Gandarela, Cauê (Supergrupo Minas) e Nova Lima (greenstone belt Rio das Velhas). Os veios quartzo-carbonáticos auríferos ocorrem em fraturas P e T de um sistema deformativo não-coaxial, que se associam a extensões controladas por processos de deslizamento interestratal normal e boudinagem das encaixantes dolomíticas. Estes veios exibem alteração hidrotermal localizada, diagnosticada principalmente por cloritização, silicificação, turmalinização e sulfetação das encaixantes. O ouro ocorre como inclusões e/ou em espaços intragranulares na arsenopirita.Estudos microtermométricos de inclusões fluidas indicam que os fluidos trapeados durante a formação dos veios auríferos são áquo-carbônicos heterogêneos, com médias de XCO2 entre 0,44 e 0,99, tendo contribuições de traços de N2 e H2S na fase gasosa, com valores baixos a moderados de salinidade. Isócoras calculadas para o sistema hidrotermal forneceram valores de pressão compatíveis com profundidades crustais <10 km. A heterogeneidade do grau de preenchimento das inclusões fluidas é indicativa de um processo de precipitação supostamente por mistura de fluidos, envolvendo fluidos metamórficos, com XCO2 elevada e baixa salinidade, e fluidos magmáticos (subordinados), tendo XCO2 mais baixo e salinidade moderada. Geotermômetros de clorita e carbonatos destes mesmos veios auríferos mostram que a temperatura de estabilização da paragênese hidrotermal e, por conseqüência, de precipitação do fluido mineralizador ocorreu por volta de 319±45°C. Semelhanças em termos de mineralogia, composição química do ouro, alteração hidrotermal das encaixantes e geometria de veios auríferos, permitem uma correlação genética entre as depósitos auríferos de Antônio Pereira e Passagem de Mariana.
THE ANTÕNIO PEREIRA GOLD DEPOSIT, IRON QUADRANGLE: P-T CONDITIONS AND THE NATURE OF MINERALIZING FLUIDS
Keywords: Mariana anticline, gold mineralization, fluid inclusion, geothermometry
Abstract
The Antônio Pereira gold deposit occurs along the tectonic contact between the Gandarela and Cauê formations (Paleoproterozoic Minas Supergroup) in the northeastern flank of the Mariana Anticline. The gold-bearing quartz-dolomite-arsenopyrite veins are located in P and T fractures of a non-coaxial deformation system. Interstratal slip and boudinage played an important role in vein emplacement during sin-kinematic to distentional deformation. Gold occurs as veinlets, inclusions and filling interstitial spaces in arsenopyrite. Hydrothermal alteration is restricted to the vein vicinity, and was responsible for the crystallization of chlorite, quartz, sulfides (arsenopyrite) and tourmaline in the host rocks. Ore-vein chlorite and carbonate were used as geothermometers, and indicated an average value of 319±45°C, interpreted as the stabilization temperature of the hydrothermal paragenesis and, consequently, of the fluid precipitation. Fluid inclusion microthermometric studies revealed an inhomogeneous aquo-carbonic fluid with low to moderate salinity, and XCO2 between 0,44 and 0,99. Traces of N2 and H2S have been identified in the gas phase. Isochores calculated for the hydrothermal system indicate pressure values compatible with crustal depth shallower than 10 km. The heterogeneity of fluid inclusion suggests that gold precipitation was induced by fluid mixture, during which high XCO2 and low-salinity metamorphic fluids interacted with subordinate low XCO2 and moderate-salinity magmatic fluids. Similarities in mineralogy, gold / gangue mineral chemistry and ore vein geometry allow a genetic correlation between the Antônio Pereira and Passagem de Mariana gold deposits.
Mafic magmatism associated with the Goiás magmatic arc in the Anicuns region, Goiás, central Brazil: Sm–Nd isotopes and new ID-TIMS and SHIMP U–Pb data
Jorge Henrique Laux a
Márcio Martins Pimentela
Elton Luiz Dantasa
Richard Armstrongb
Alan Armelea
Ariplinio Antônio Nilsonc
Journal of South American Earth Sciences(2004); 16(7): 599-614
Full Text + Links
a Laboratório de
Geocronologia, Instituto de Geociências, Universidade de Brasília, Caixa
Postal 04465, Brasília-DF 70910-000, Brazil
b Research School of Earth Sciences, Australian National
University, Canberra, Australia
c Departamento de Mineralogia e Petrologia, Instituto de
Geociências, Universidade de Brasília, Brasília-DF 70910-000, Brazil
Rocks exposed in the Anicuns region, in the eastern part of the Goiás
magmatic arc, are represented dominantly by amphibolites (metavolcanic and
metaplutonic) and metapelitic rocks. New U–Pb results demonstrate that this
association is Neoproterozoic and that mafic rocks crystallized during two
main periods: ca. 890–815 and 630–600 Ma. Metagabbro and metadiorite samples
JHL-14, JHL-15, JHL-23, AMB-01, and JHL-26B have U–Pb zircon ages of 886±5,
862±5, 815±10, 856±15, and 839±9 Ma, respectively, and compose the older
group. The Late Neoproterozoic intrusive Anicuns-Santa Bárbara gabbro-diorite
and Americano do Brasil suites are coeval. Four samples from the first (SB-01,
JHL-04, JHL-22C, and JHL-19) yield U–Pb ages of 598±8, 612±6, 623±13, and
622±6 Ma, respectively, whereas zircon grains from one norite sample of the
Americano do Brasil complex yield a concordia age of 626±8 Ma. All mafic rocks
investigated present TDM model ages of ca. 1.0 Ga, comparable to
the model ages of metaigneous rocks of the Goiás magmatic arc. The
Nd(T)
values are strongly positive, indicating the depleted nature of the mantle
source (MORB-like), similar to volcanic and plutonic rocks of the arc-type
volcano-sedimentary sequences exposed to the west. The lithological
associations that constitute the supracrustal sequences in the Anicuns area,
however, are more compatible with an origin in an oceanic or forearc setting
than in an intraoceanic arc setting (rocks of andesitic/dacitic/rhyolitic
composition are absent). 147Sm/144Nd ratios for most of
the mafic rocks investigated, however, are smaller than 0.19 and indicate a
relative enrichment in LREE, which is not characteristic of N-MORB mafic
magmas.
Chemostratigraphy of medium-grade marbles of the late Neoproterozoic Serido fold belt, northeastern Brazil
Rielva S. C. Nascimento
Alcides N. Sial
Marcio M. Pimentel
Key words: Vendian, C-isotopes, northeastern Brazil, Seridó Belt, marble
Gondwana Research (July 2004), 7(3):731-744
Abstract
:Contrasting copper and chromium metallogenic evolution of terranes in the Palaeoproterozoic Itabuna–Salvador–Curaçá orogen, São Francisco craton, Brazil: new zircon (SHRIMP) and Sm–Nd (model) ages and their significance for orogen-parallel escape tectonics
Elson P. Oliveiraa
Brian F. Windleyb
Neal J. McNaughtonc
Marcio Pimenteld
Ian R. Fletcherc
Precambrian Research (2004);
128(1-2): 143-165
Full Text + Links
a Departamento de Geologia e Recursos Naturais, Instituto de Geociências, UNICAMP, Campinas 13083-970, Brazil
Keywords: Copper sulphide; Chromite; Geochronology; Palaeoproterozoic orogen; Brazil
The northern segment of the Palaeoproterozoic Itabuna–Salvador–Curaçá collisional orogen in the São Francisco craton (Brazil) comprises two high-grade terranes separated by the 150 km long, N–S-trending Itiúba Syenite. The Curaçá terrane to the west contains copper sulphide deposits hosted by hypersthenite–norite complexes (e.g. Caraíba body), whereas the Jacurici terrane to the east contains chromite deposits hosted by peridotite–norite sills (e.g. Medrado body). SHRIMP U–Pb dating of zircons, combined with a significant major element compositional gap indicates that the Caraíba (2580±10 Ma) and the Medrado (2085±5 Ma) mafic–ultramafic complexes are not related by fractional crystallisation as previously thought. Depleted-mantle Nd model ages for the regional gneisses are also distinct, with the Curaçá terrane having younger ages (2.4–2.8 Ga) than the Jacurici terrane (2.9–3.2 Ga), suggesting that they represent two independent fragments of continental crust accreted during the Palaeoproterozoic orogeny. Emplacements of the Itiúba Syenite (ca. 2084 Ma) and of the Medrado complex were apparently coeval, probably controlled by orogen-parallel block displacement, or escape tectonics, following continent–continent collision.
SHRIMP U–Pb and Sm–Nd ages of the Niquelândia layered complex: Meso- (1.25 Ga) and Neoproterozoic (0.79 Ga) extensional events in central Brazil
Márcio Martins Pimentela
Cesar Fonseca Ferreira Filhoa
Richard A. Armstrongb
Keywords: Niquelândia; Mafic–ultramafic; SHRIMP U–Pb; Neoproterozoic; Rift; Mesoproterozoic
Precambrian Research (2004);
132(1-2):133-153
Full Text + Links
a Instituto de Geociências, Universidade de Brasília, Brasília-DF 70910-900, Brazil
The Niquelândia mafic–ultramafic layered complex, in the central part of the Brasília Belt, is one of three large complexes forming an approximately 350 km long linear NNE belt. The complexes are metamorphosed under granulite to amphibolite facies and have been traditionally interpreted either as part of an important Paleoproterozoic (ca. 2.0–1.8 Ga) rift system or as the exposure of ancient (Archean or Paleoproterozoic) oceanic crust. In both cases, they have been always interpreted as part of the basement of the Neoproterozoic rocks of the Brasília Belt.
New SHRIMP–RG U–Pb data and Sm–Nd mineral isochron age do not support these
models. The results indicate that the eastern part of the complex (Lower
Series) is a Neoproterozoic (797±10 Ma) continental intrusion, heavily
contaminated with older sialic material (Nd(T)
of −5.8, and inherited zircon grains with ages between ca. 1.0 and 1.9 Ga).
The western part (Upper Series) represents a rift-related Mesoproterozoic
layered body (1248±23 Ma). These two different bodies have been tectonically
juxtaposed along a major shear zone. The complex was metamorphosed at 765±8 Ma
as indicated by the age of metamorphic zircon overgrowths in rocks of the
Upper Series. These data confirm previous studies suggesting that the
Niquelândia Complex is made of two distinct intrusions with different ages and
isotopic characteristics. The new data forced the re-interpretation of
previous U–Pb and Re–Os geochronological data for the Niquelândia Complex
which have indicated Paleoproterozoic (ca. 2.0 Ga) crystallisation ages and
suggested that the Mesoproterozoic age of ca. 1.25 Ga represents a metamorphic
event.
Extensional events at approximately 1.25 and 0.8 Ga, although well documented in other continents (e.g. Laurentia, Baltica), are not usual in the Precambrian terrains of South America, suggesting that the continental block including these complexes and associated volcano-sedimentary sequences may be allochthonous, being accreted to the Brasília orogen during the Neoproterozoic.
Schmitt et.al.,2004
Late amalgamation in the central part of West Gondwana: new geochronological
data and the characterization of a Cambrian collisional orogeny in the Ribeira
Belt (SE Brazil) Renata da Silva Schmitta Precambrian Research(2004),
133(1-2): 29-61
Rudolph A. J. Trouwb
William Randall Van Schmusc
Márcio Martins Pimenteld
b Departamento de Geologia, IGEO, Universidade Federal do Rio de
Janeiro—UFRJ, Rio de Janeiro, RJ, Brazil
c Departament of Geology, University of Kansas, 120 Lindley Hall,
Lawrence, KS 66045, USA
d Instituto de Geociências, Universidade de Brasília—UnB, Brasília,
DF, Brazil
Keywords: High-grade metamorphism; U–Pb; Orogeny; Cambrian; Brazil; Gondwana
New U–Pb data reveal that during the mid-Cambrian the central part of West
Gondwana was still undergoing a high-grade tectonometamorphic event
corresponding to collision. The studied area is located in the southeastern
part of the Pan-African–Brasiliano Ribeira Belt, in SE Brazil. The area is
part of the Cabo Frio Tectonic Domain (CFTD) which is limited to the NW by a
major NE–SW striking thrust zone which separates it from the Neoproterozoic
"Oriental terrane", whereas to the SE it is covered by the Atlantic Ocean. The
domain comprises a Paleoproterozoic orthogneissic basement tectonically
interleaved with younger supracrustal rocks, folded and metamorphosed at upper
amphibolite to granulite facies during the mid-Cambrian. The supracrustal
rocks are subdivided in two successions: Búzios (Al-metapelites, calcsilicates
and amphibolites) and Palmital (quartz–feldspathic metasediments with minor
metapelites). These successions were deposited in a deep oceanic environment
between ca. 620 and 525 Ma as indicated by SHRIMP U–Pb data for detrital
zircons and by TDM model ages. The metamorphic peak, defined
by the mineral associations Ky + Kfs in metapelites and Cpx + Grt + Qz in
amphibolites, occurred at minimum pressure of 9 kbar and temperature in excess
of 780 °C. At this stage migmatites were generated by partial melting in all
lithostratigraphic units, including the amphibolites. The metamorphic peak was
also contemporaneous with top to the NW thrusting, testified by mineral and
stretching lineations related to progressive deformation phases D1 and D2. The
metamorphic peak was dated between 525 and 520 Ma, as determined by U–Pb
analyses of zircons of leucosomes. During deformation phase D3, large
recumbent folds developed with NW–SE axes, parallel to the main direction of
movement. The CFTD was juxtaposed at this stage to the "Oriental terrane" by a
major NE–SW striking thrust fault. U–Pb dating of monazites from metapelites
and of sphenes from amphibolites revealed ages of about 510 Ma for the mineral
growth. The sillimanite, aligned as L3, partially replaced kyanite,
indicating a clockwise P–T–t path for the central and
eastern areas of the CFTD.
After docking into the Ribeira belt, during the late Cambrian, the western
limit of CFTD was affected by a transcurrent dextral shear zone that developed
a NE–SW stretching lineation related to D4, under amphibolite facies
conditions. This is recorded in monazites and zircons within this shear zone
with U–Pb ages ranging from 505 to 490 Ma. At this stage, the central and
eastern parts of CFTD were already cooling at a rate of 10 °C/Ma. After
480 Ma, the cooling rate diminished to 5 °C/Ma. A 207Pb/206Pb
age in rutile (480±5 Ma) and a U–Pb zircon age in a post-tectonic pegmatite
(440±11 Ma) mark the stabilization of the area during the Ordovician–Silurian
boundary.
This well-constrained Cambrian collisional orogeny is here denominated the
Búzios Orogeny and is the youngest tectonometamorphic event recorded in the
Brasiliano belts of Brazil. This orogeny is contemporaneous with some other
marginal orogenies (e.g. Pampeana; Ross) that probably accompanied the final
adjustments of the Precambrian continents to complete the formation of
Gondwana in the Ordovician.
U–Pb perovskite ages for brazilian kamafugitic rocks: further support for a temporal link to a mantle plume hotspot track
Patrícia B. A. Sgarbia
Larry M. Heamanb
José Carlos Gaspar c
Keywords: Brazil; Cretaceous; Kamafugite; Minas-Goias alkaline province; U–Pb perovskite ages
Journal of South American Earth
Sciences (2004); 16(8): 715-724
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a Department de
Geologia IGC, Universidade Federal de Minas Gerais, Av. Antonio Carlos 6627-B,
Horizonte 31270-901, Brazil
b Dept. of Earth and Atmospheric Sciences, University of Alberta,
Edmonton, Alberta, Canada, T6G 2E3
c Instituto de Geociências, Universidade de Brasília, 70910-900,
Brasília, DF, Brazil
We present the results of a U–Pb perovskite age study of kamafugites from Mata da Corda (MC) and Santo Antônio da Barra (SAB), Minas-Goiás alkaline province, Brazil. Perovskite crystals were separated from MC mafurites, ugandites, and cognate pyroxenites, as well as from SAB melilite mafurite. The range of ages of Brazilian kamafugitic samples is 15 Ma. The 206Pb/238U perovskite ages generally cluster into three age groupings: 88–90, 80–81, and 75–76 Ma. The two younger periods of kamafugitic magmatism occur in the MC area, whereas the older samples are from the SAB area. These new age results provide the first robust evidence of a progressive eastward younging of mafic alkaline magmatism, most likely related to a mantle plume hotspot track.