UNIVERSIDADE DE BRASÍLIA - INSTITUTO DE GEOCIÊNCIAS
UNIVERSIDADE DE BRASÍLIA - INSTITUTO DE GEOCIÊNCIAS
DISSERTAÇÃO DE MESTRADO No
224
OSMAR SAMIR SERRÃO BAXE
Artigo 1 - Artigo 2GEOCRONOLOGIA DE COMPLEXOS MÁFICO-ULTRAMÁFICOS: EXEMPLO DA SÉRIE SUPERIOR DO COMPLEXO DE NIQUELÂNDIA, BRASIL, E DO COMPLEXO KUNENE, ANGOLA
Data da defesa: 30/03/2007
Área de concentração: Geologia Regional
Orientador: Prof. Márcio Martins Pimentel (UnB)
Banca de examinadores: Prof. Bernhard Manfred Buhn (UnB); Prof. Reinhardt Adolfo
Fuck (UnB)
Palavras Chaves: Complexo Acamadado de Niquelândia; Serie Superior; Zircão U–Pb; Sm–Nd; Suíte Plutônica Kunene; Gabro; Anortosito; Mesoproterozóico
RESUMO
Esta
dissertação reúne um conjunto de dados petrográficos, geoquímicos e
geocronólogicos
ID-TIMS U–Pb em zircão e de Sm-Nd
em rochas da
Série Acamadada Superior
do
complexo de Niquelândia no Brasil,
bem como
da Suíte Plutônica Kunene (SPK), no SW de Angola e NW da Namíbia.
A similaridade de idades de cristalização Série Acamadada Superior do Complexo de
Niquelândia de
1248 ± 23 Ma
para (Pimentel
et al. 2004) e da Suíte Plutônica Kunene com
1371 ± 2.5 Ma para (Mayer et al. 2004), bem como a
similaridade litológica, gabro, troctolito, leucotroctolito e anortosito que
caracterizam estes complexos motivaram esta pesquisa. Contudo, constatou-se que
estes complexos são de natureza distinta e foram formados em ambientes
distintos. A Série Acamadada Superior foi formada em um ambiente de crosta
oceânica, similar ao N e T-MORB, enquanto que a Suite Plutonica Kunene foi
formada a partir da coalêscencia
de diversos pulsos magmáticos em ambiente extensional, anarogênico e
intracontinental.
O Complexo de Niquelândia é constituído por duas intrusões
distintas conhecidas como Série Acamadada Inferior (800 Ma) e Série Acamadada
Superior datada neste estudo por ID-TIMS U–Pb em zircão de gabronorito em 1245 ±
4 Ma e 780 Ma constatado em titanita metamórfica de meta-anortosito. Os valores
positivos de
εNd
(T) indicam que a Série Acamadada Superior
deriva de magma depletado e assinatura geoquímica
similar ao N e T-MORB, sugerindo que a mesma foi formada em crosta oceânica.
Estes novos dados contrastam com os apresentados para a Série Inferior de 800 Ma
que apresenta valores de
εNd
(T) negativos, indicando que a mesma foi fortemente contaminada por material
crustal mais antigo.
Por outro lado, a Suíte Plutônica Kunene (SPK) no SW de
Angola constituída essencialmente por uma suíte plutônica básica, caracterizada
por intrusões gabro - anortosíticas e por uma suíte ácida constituída por
“Granitos Vermelhos”.
Novos
dados de ID-TIMS U–Pb em zircão de 1434 ± 2
Ma para leucogabro da intrusão Uanguembela, e 1403 ± 7 Ma para uma intrusão
mangerítica ao sul de Otchindjau indicam que a suite básica é relativamente mais antiga que a suite
ácida.
Os valores negativos de
εNd
(T) das rochas
básicas (-0.30 to -12.42) e ácidas (-0.67 to -11.02) indicam que a SPK foi
submetida a diferentes graus de contaminação crustal. Contudo, alguns plutons máficos apresentaram
εNd
(T) positivo entre +0.67 to +1.12, sugerindo
que estes magmas derivam de uma depletada.
A suíte
plutônica ácida
é
constituída por granitóides tipo – A, conhecidos genericamente como “Granitos
Vermelhos”, sugerindo que a Suite Kunene foi gerada em um ambiente
intracontinental. A Suíte Plutônica Kunene
é semelhante à
Suíte Plutônica Nain (Labrador, Canadá) e apresenta potencial para hospedar
depósitos magmáticos de Ni-Cu-PGE e Fe-Ti-V.
UNIVERSITY OF BRASILIA- INSTITUTE OF GEOSCIENCES
MSc THESIS No
224
OSMAR SAMIR SERRÃO BAXE
Date of oral presentation: 30/03/2007GEOCHRONOLOGY OF MAFIC-ULTRAMAFIC COMPLEXES: EXAMPLE OF THE NIQUELÂNDIA COMPLES SUPERIOR SERIES, BRAZIL, AND OF THE KUNENE COMPLEX, ANGOLA
Key Words: Niquelândia Layered complex; Upper Series; Zircon U–Pb; Sm–Nd; Kunene; Plutonic Suite; Gabbro; Anorthosite; Mesoproterozoic
ABSTRACT
This work presents petrographic, geochemistry and geochronologic data ID-TIMS U–Pb
in zircon and Sm-Nd isotopic data of the Niquelandia complex Upper Series in
Brazil, and of Kunene Plutonic Suite (KPS) located in SW Angola and NW Namibia.
The similarities of U-Pb ages between the Niquelandia complex Upper Series with
1248 ± 23 Ma (Pimentel et al. 2004) and Kunene Plutonic Suite with 1371 ± 2.5 Ma
(Mayer et al. 2004) and also the litologic similarities, both composed mainly by
gabbro, troctolite, leucotroctolite and anorthosite rocks were the main reasons
for this research investigations to find any relationship between them. However,
this study found out that these Niquelandia complex Upper Series are quite
different from Kunene Plutonic Suite.
The age and tectonic significance of the Barro Alto,
Niquelândia, and Cana Brava layered mafic–ultramafic complexes in Goias, central
Brazil, have been a matter of debate and controversy during the last two
decades. In many models, they have been considered to be representative of
Paleoproterozoic intrusions metamorphosed during the Neoproterozoic, at
approximately 0.79–0.76 Ga. In particular, the Niquelandia Complex has been
described as formed by two different intrusions known as the Lower Series with
800 Ma and the Upper Series, of disputed age. New ID-TIMS U–Pb zircon data and
Sm–Nd data presented in this study suggest that the Upper Series of Niquelandia
Complex crystallized at 1245 ± 4 Ma and was metamorphosed around 780 Ma. The
positive value of εNd (T) indicates that the original magma of the
Upper Series derived from a strongly depleted mantle. Anorthositic rocks have
positive Eu anomaly and gabbroic rocks have REE patterns similar to MORB. These
new data reinforce the idea that the Upper Series of the Niquelandia Complex are
of oceanic nature and, therefore, considerably different from the Lower Series
which present negative εNd (T) values and trace element
characteristics indicating extensive contamination with continental crust.
The Kunene Plutonic Suite (KPS) in SW Angola and NW Namibia
is one of the largest anorthositic complexes in the world, underlying an area of
approximately 18.000 km2. It comprises a basic plutonic suite and an
acid plutonic suite. The basic suite is mainly formed by gabbro-anorthositic
intrusions and layered mafic bodies of different sizes. The acid suite includes
mainly A-type “red granites”. This study presents new ID-TIMS U-Pb and Sm-Nd
data as well as geochemistry and petrologic data for gabbroic, anorthositic and
granitic rocks of the KPS. One sample of a layered gabbroic intrusion (Uanguembela)
of the Otchinjau-Oncócua region was dated at 1434 ± 2 Ma. A mangeritic intrusion
exposed to the east of the Otchindjau layered mafic intrusion yielded the U-Pb
age of 1403 ± 7 Ma. These ages are slightly older than the age of ca. 1.37 Ga
reported previously in the literature, suggesting that the Kunene Complex is
formed by various plutons with different ages, indicating the existence of
distinct magmas which underwent distinct cooling histories and crustal
assimilation in multi-pulse gabbroic and anorthositic intrusions.
The dominantly negative εNd (T) values of the
basic (-0.30 to -12.42) and acid rocks (-0.67 to -11.02) indicate that the KPS
original mafic magmas has been contaminated by crustal material. However, some
mafic plutons yielded positive εNd (T) between +0.67 to +1.12 which
indicates that the magma of some mafic intrusions within the KPS derived from
depleted mantle. The emplacement of the Kunene Complex requires an extensional
setting and a significant thermal anomaly at the margin of the Congo Craton
during the Mesoproterozoic. The Kunene Plutonic Suite resembles the large
Proterozoic anorthosite plutonic suites such as the Nain Plutonic Suite
(Labrador, Canada) and is, therefore, of great potential for Ni-Cu-PGE and
Fe-Ti-V economic magmatic deposits.
Mesoproterozoic Magmatic and Neoproterozoic Metamorphic Ages of the Upper Series
of the Niquelândia Complex, in central Brazil:
further ID-TIMS U–Pb and Sm–Nd isotopic evidence
Osmar S. S. Baxe, Márcio M. Pimentel, César F. Ferreira.
Filho, Elton Dantas
Abstract
The age and tectonic significance of the Barro Alto, Niquelândia, and Cana Brava layered mafic–ultramafic complexes in Goiás have been a matter of debate and controversy during the last two decades. In some models, they have been considered to be representative of Paleoproterozoic intrusions metamorphosed during the Neoproterozoic, at approximately 0.79–0.76 Ga. In particular, the Niquelândia Complex has been described as formed by two different intrusions known as the Lower Series (800 Ma) and the Upper Series, of disputed age. New ID-TIMS U–Pb zircon data and Sm–Nd data presented in this study indicate that the Upper Series of Niquelândia Complex crystallized at 1245 ± 4 Ma and was metamorphosed around 780 Ma. The positive value of εNd (T) indicates that the original magma of the Upper Series derived from a strongly depleted mantle. Anorthositic rocks have positive Eu anomaly and gabbroic rocks have a REE pattern similar to MORB. These new data reinforce the concept that the Upper Series of the Niquelândia Complex is associated to an environment of oceanic crust and, therefore, considerably different from the Lower Series which presents negative εNd (T) values and trace element characteristics indicating extensive contamination with continental crust.
Mesoproterozoic emplacement ages of the Kunene Anorthosite Complex, SW
Angola: ID-TIMS U–Pb and Sm-Nd isotopic evidence
Osmar S. S. Baxe, Márcio M. Pimentel, César F. Ferreira.
Filho, Elton Dantas
Abstract
The Kunene Complex (KC) in southwestern Angola is one of the largest
anorthositic complexes in the world, underlying an area of approximately 18.000
km2. It is part of the so-called Kunene Plutonic Suite (KPS) which
comprises a basic plutonic suite and an acid plutonic suite. The basic suite is
mainly formed of gabbro-anorthositic intrusions and layered mafic bodies of
different sizes. The acid suite includes mainly A-type “red granites”. This
study presents new ID-TIMS U-Pb and Sm-Nd data, as well as geochemistry and
petrologic data for gabbroic, anorthositic and granitic rocks of the KPS. One
sample from a layered gabbroic intrusion (Uanguembela) of the Otchindjau -
Oncócua region was dated at 1434 ± 2 Ma. A mangeritic intrusion to the east of
the Otchindjau layered mafic intrusion yielded the U-Pb age of 1403 ± 7 Ma.
These ages are slightly older than the age of ca. 1.37 Ga reported previously in
the literature, suggesting that the Kunene Plutonic Suite is formed by several
plutons with different ages, indicating the existence of distinct magmas which
underwent distinct cooling histories and crustal assimilation in multi-pulse
gabbroic and anorthositic intrusions.
The dominantly negative εNd (T) values of the
basic (-0.30 to -12.42) and acid rocks (-0.67 to -11.02) indicate that KPS
original mafic magmas were contaminated by crustal material. However, some mafic
plutons yielded positive εNd (T) between + 0.67 to + 1.12 which
indicates that the magma of some mafic intrusions within the KPS derived from
depleted mantle with less important crustal contamination. The emplacement of
the Kunene Complex requires an extensional setting and a significant thermal
anomaly at the margin of the Congo Craton during the Mesoproterozoic. The Kunene
Plutonic Suite resembles the large Proterozoic anorthosite plutonic suites such
as the Nain Plutonic Suite (Labrador, Canada) and is, therefore, of great
potential for Ni-Cu-PGE and Fe-Ti-V deposits.