Journal of South American Earth Sciences 27 (2009) 235–246
Contents lists available at ScienceDirect
Journal of South American Earth Sciences
journal homepage: www.elsevier.com/locate/jsames
Crustal growth of the central-eastern Paleoproterozoic domain, SW Amazonian
craton: Juvenile accretion vs. reworking
Moacir José Buenano Macambira a,*, Marcelo Lacerda Vasquez b, Daniela Cristina Costa da Silva c,
Marco Antonio Galarza a, Carlos Eduardo de Mesquita Barros d, Julielson de Freitas Camelo e
a
Laboratório de Geologia Isotópica – Para-Iso, Instituto de Geociências, Universidade Federal do Pará, Caixa Postal 8608, 66075-110 Belem, PA, Brazil
Companhia de Pesquisa de Recursos Minerais, Av. Dr. Freitas, 3645, 66095-110 Belém, PA, Brazil
c
Programa de Pós-graduação em Geologia e Geoquímica, Universidade Federal do Pará, Caixa Postal 8608, 66075-110 Belém, PA, Brazil
d
Universidade Federal do Paraná, Departamento de Geologia, Centro Politécnico, Caixa Postal 19001, 81531-990 Curitiba, PR, Brazil
e
Mineração Rio do Norte S.A., Porto Trombetas, PA, Brazil
b
a r t i c l e
i n f o
Article history:
Received 2 August 2007
Accepted 6 February 2009
Keywords:
Trans-Amazonian cycle
Zircon
Nd isotopes
Amazonian craton
Paleoproterozoic
a b s t r a c t
The Trans-Amazonian cycle was an important rock-forming event in South America, generating voluminous juvenile and reworked fractions of continental crust. The Bacajá domain, in the southern sector of
the Maroni-Itacaiúnas Province in the Amazonian craton, is an example of the Trans-Amazonian terranes
adjacent to the Archean Carajás block. Zircon Pb-evaporation and whole-rock Sm–Nd analyses were carried out on representative samples of six lithological units, and allowed the proposal of a comprehensive
tectonic-magmatic evolutionary sequence for the central and eastern parts of this domain, from the Neoarchean to the Rhyacian. Gneisses with ages of ca. 2.67 and 2.44 Ga are the oldest rocks recorded in the
region, and probably represent remnants of island and continental arcs. The Três Palmeiras succession,
emplaced between 2.36 and 2.34 Ga, hosts gold deposits and represents the first record of Siderian supracrustal rocks in the Amazonian craton. It was probably part of an island arc/ocean floor accreted to a craton margin. Rhyacian granitogenesis lasted for ca. 140 My (2.22–2.08 Ga), marking different stages of the
Trans-Amazonian cycle. The first stage is represented by continental arc granitoids formed by melting of
Archean crust at 2.22–2.18 Ga. The second is characterized by the production of juvenile material
between 2.16 and 2.13 Ga. The third and final stage at ca. 2.08 Ga is represented by a large volume of
granitoids originated from either juvenile material or reworked crust during compressive stresses. Nd
isotopes reveal that juvenile rocks dominated in the northern part of the domain, whereas those formed
from reworked crust predominate in the south. The present-day configuration of the Bacajá domain
results from collision against the Archean Carajás block at the end of the Trans-Amazonian cycle.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
The geotectonic model of evolution suggesting that the Amazonian craton (Guiana and Central Brazil shields) represents a collage
of Proterozoic belts or geochronological provinces surrounding Archean nuclei was first presented in the seventies (e.g. Amaral,
1974; Cordani et al., 1979). Presently, this model is considered to
be the most appropriate to explain the main general features of
the craton, and has been updated by several authors (Lima, 1984;
Teixeira et al., 1989; Tassinari and Macambira, 1999, 2004; Tassinari et al., 2000; Dall’Agnol et al., 2000; Santos et al., 2000,
2006). The division of the craton into provinces (Fig. 1) mainly
takes into account the geochronology of the regional basement,
as well as general geological and geophysical features (e.g. Tassi* Corresponding author. Tel.: +55 91 3201 7483; fax: +55 91 3246 2323.
E-mail address: [email protected] (M.J.B. Macambira).
0895-9811/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsames.2009.02.001
nari and Macambira, 1999). The boundaries between these
geochronological provinces are key areas for understanding the
growth of the craton and of the provinces themselves, which
have their own geochronological, tectonic and lithological
characteristics.
The Trans-Amazonian cycle was an important rock-forming
event in the South American Platform (e.g. Cordani and Sato,
1999). The southern part of Maroni-Itacaiúnas Province, which is
the Bacajá domain, is a special example of the Trans-Amazonian
terranes since it makes contact with the Archean Carajás block
(Fig. 1), included in the Central Amazonian Province (e.g. Tassinari
and Macambira, 2004). Mapping projects carried out by CPRM resulted in conflicting proposals for the location of the boundary between the Archean and Paleoproterozoic domains (e.g. Santos,
2003; Faraco et al., 2005; Santos et al., 2006). Apart from this question, it is also important to take into account the internal structure,
composition and evolution of the provinces themselves.
236
M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
60º W
50º W
60º W
Amazonian craton
Guyana
Suriname
French Guiana
Venezuela
Colombia
Guia
na sh
i
ield
Atlantic
Ocean
Guiana Shield
Ecuador
0º
Amazon
Basin
ield
il sh
Braz
Central
tralBrazil Shiel
Cen
Peru
Brazil
g
Pacífic
Ocean
b
(Iricoumé)
Solimões
Basin
500 km
Bolivia
Atlantic
Ocean
Amazon
Basin
0º
c
b
(Xingu)
a
10º S
a - Carajás block
b - Xingu-Iricoumé block
c - Bacajá domain
500 km
Geochronological Provinces
Neoproterozoic belt
Sunsás (1.25-1.0 Ga)
Ventuari-Tapajós (1.95-1.8 Ga)
Rondoniano-San Ignacio (1.5-1.3 Ga)
Maroni-Itacaiúnas (2.2-1.95 Ga)
Rio Negro-Juruena (1.8-1.55 Ga)
Central Amazonian (> 2.5 Ga)
Figure 2
Fig. 1. Sketch map showing the geochronological provinces of the Amazonian craton (based on Tassinari and Macambira (2004)) and the location of the study area.
This work presents new isotope data (Pb-evaporation on zircon
and whole-rock Sm–Nd) for rocks cropping out in the central and
eastern parts of the Bacajá domain, Pará state, in order to better
characterize the age and origin of these rocks. Additionally, we
hope to clarify the formation and evolution of the southernmost
part of the Maroni-Itacaiúnas Province and its nature, whether
by juvenile accretion, or by reworking of the rocks involved in
the Trans-Amazonian cycle.
2. Regional geological setting
According to recent studies (e.g. Tassinari et al., 2000; Tassinari
and Macambira, 2004; Santos et al., 2000, 2006), Archean terranes
constitute the southeasternmost part of the Amazonian craton
(Central Amazonian Province), and are surrounded by Proterozoic
provinces, which become progressively younger southwestwards
(Fig. 1). Tassinari and Macambira (2004) defined the Central Amazonian Province as the oldest continental crust of the craton, which
was not affected by the Trans-Amazonian cycle. Following Tassinari and Macambira (1999), Dall’Agnol et al. (1999a), and Tassinari
et al. (2000), Tassinari and Macambira (2004) divided the province
into two segments: the Carajás and the Xingu-Iricoumé blocks
(Fig. 1). The first comprises a 3.00–2.85 Ga granite-greenstone
basement covered, in its northern part, by a ca. 2.76 Ga volcanosedimentary sequence hosting the most important mineral deposits (Cu, Fe, Au, Mn etc.) of the craton. All the Archean rocks of the
Carajás block have TDM(Nd) ages between 3.2 and 2.86 Ga. The Xingu-Iricoumé block is a NW–SE segment located in the central part
of the craton, and is partially covered by Phanerozoic sedimentary
rocks of the Amazon basin. It represents the least studied part of
the Amazonian craton. Paleoproterozoic granitoids and volcanic
rocks, which dominate in this block, are largely covered by sedimentary sequences. Geochronological data for the regional basement are not available, but it has been considered to be of pre‘‘Trans-Amazonian” age (>2.5 Ga) (Tassinari and Macambira,
2004). The Archaean age for the rarely exposed metamorphic basement is based on a few TDM(Nd) ages of the Paleoproterozoic granitoids and volcanic rocks, which were probably formed by melting
of the basement.
The Maroni-Itacaiúnas Province (2.2–1.95 Ga) borders the
northeastern and northern parts of the Central Amazonian
M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
Province. It was formed during the Trans-Amazonian orogenic cycle, but several Archean inliers are recognized within the Paleoproterozoic rocks. The province is characterized by widespread
exposures of greenschist to amphibolite facies metavolcanic and
metasedimentary units, as well as by granulitic and gneissic-migmatitic terranes.
Apart from the two provinces described above, Tassinari and
Macambira (2004) revised some province boundaries, while maintaining others proposed in previous works (Fig. 1), which are: Ventuari-Tapajós (1.95–1.8 Ga), Rio Negro-Juruena (1.8–1.55 Ga),
Rondonian-San Ignacio (1.55–1.3 Ga) and Sunsas (1.3–1.0 Ga). Santos et al. (2000) suggested other names and limits for the geochronological provinces of the Amazonian craton, which were also
updated in recent publications (e.g. Santos et al., 2006).
The Maroni-Itacaiúnas Province can be divided into several domains according to their geological features and geographical distribution. The Bacajá domain (Fig. 2) borders the northern part of
the Carajás block (Central Amazonian Province). Its northern part
is covered by rocks of the Amazon basin, and its eastern part by
the Grajaú basin and the Neoproterozoic Araguaia belt. The domain
extends westwards parallel to the southern margin of the Amazon
basin, and is covered here by the Paleoproterozoic volcanic rocks of
the Central Amazonian Province. The Bacajá domain is comparatively less well studied than the Carajás block. Its central-eastern
part, the object of this work, is composed of deformed granitoids,
granulites, gneisses and the Três Palmeiras and São Manoel greenstone belts which are discussed below.
237
as in the Itatá Amphibolite, and in the Bacajá Micaschist. According
to Faraco et al. (2005), the Três Palmeiras greenstone belt encompasses the last two units. Siderian granitoids are grouped into the
Jacaré Complex, whereas Rhyacian granitoids are represented by
the Valentin Complex, as well as the Felício Turvo and Bacajá granites. The map presented by Faraco et al. (2005) will be used as the
geological background of this study. In spite of some divergences in
relation to our data regarding rock classification, and the locations
of the contacts between the lithological units, we maintain this
map because it is the most recent and the most complete available.
For this reason, in this work geographical references to sample
locations are preferred, rather than the geological units and contacts proposed by Faraco et al. (2005).
Barros et al. (2007) studied a NW–SE oriented area, parallel to
the BR230 road in the northeastern part of the Bacajá domain,
and described monzogranites and granodiorites, with subordinate
tonalites, syenogranites and scarce quartz diorites. These rocks are
rather homogeneously deformed at the regional scale, with foliations striking N60 W and WNW–ESE. Primary subvertical and
flat-lying igneous layering are transposed to high-temperature secondary foliations and mylonite zones. According to these authors
the development of these structures was controlled by progressive
deformation under decreasing temperatures, characterizing the
syntectonic emplacement of these granitoids during regional
shortening. Taking into consideration the age of the granitoids
(2076 ± 6 Ma, Pb-evaporation zircon age), they proposed an evolution related to a continental arc environment developed during soft
amalgamation of continental plates at the end of the Trans-Amazonian cycle.
3. Geology of the central-eastern part of Bacajá domain
Few studies have been carried out in the eastern Bacajá domain.
The RADAM project (Silva et al., 1974; Issler et al., 1974) produced
the first geological map of the region when, based only on K–Ar
and Rb–Sr data, it was speculated that the Trans-Amazonian cycle
had affected older rock units. Later on, Jorge João et al. (1987) and
Santos et al. (1988) studied the northwestern part of this region.
Their investigation recognized several lithostratigraphic units such
as: the Bacajaí Granulite, the Três Palmeiras Metamorphic Suite
(greenstone belt), the Anapu Granodiorite, the Oca Granodiorite,
and the João Jorge Granite. The second study cited presented Rb–
Sr data, and suggested that the domain was formed by Paleoproterozoic reworking of gneisses, as well as juvenile additions
represented by the mafic metavolcanic rocks of the Iriri-Xingu
region.
Some local studies were carried out on the central and the western parts of the Bacajá domain (Fig. 2). In the central part in the
Manelão gold mine, Souza et al. (2003) and Souza and Kotschoubey
(2005) described the poly-metamorphic regional basement of the
Xingu Complex (Silva et al., 1974), and the São Manoel volcanosedimentary sequence, both of them intruded by the Felício Turvo
Granite. For the northwestern part of the domain, in the Iriri-Xingu
area, Vasquez et al. (2008) and Santos (2003) presented new Pbevaporation and U–Pb SHRIMP zircon data for granitoids and
gneisses which indicated ages between 2.50 and 2.07 Ga.
Faraco et al. (2004, 2005) reviewed the geology of the eastern
Bacajá domain (Fig. 2) and proposed new lithostratigraphic units
which are usually elongated along NW–SE and WNW–ESE trends.
These are: the Direita Granulitic Suite composed by foliated
quartz-feldspar granulites; the calc-alkaline Bacajaí Charnockitic
Complex; the Ipiaçava Kinzigitic Complex including rocks with garnet, biotite and sillimanite; the Rio Preto Piriclasite represented by
tholeiitic to calc-alkaline mafic granulites formed at high temperature and pressure; and the Cajazeiras Enderbitic Complex comprising calc-alkaline granulites. Metavolcano-sedimentary rocks
were included in the Misteriosa and São Manoel groups, as well
4. Analytical methods
Zircon from seven samples, and 15 whole-rock samples from
the central-eastern part of the Bacajá domain were analyzed by
Pb-evaporation and by Sm–Nd methods, respectively, at the Isotope Geology Laboratory of the Federal University of Pará (ParáIso), Brazil, using a Finnigan MAT 262 mass spectrometer.
For the Pb-evaporation technique (Kober, 1986, 1987), zircon
crystals were concentrated by conventional methods of heavy mineral separation, and then were hand-picked. In this technique, the
individual zircon grain is encapsulated in the Re-filament used for
evaporation, which was placed directly in front of the ionization
filament. Both filaments are introduced into the mass spectrometer. The evaporation filament is heated to evaporate the Pb from
the zircon, and the Pb liberated is condensed on the cold ‘‘ionization” filament. Three evaporation steps, each of a maximum of
5 min, are performed at 1450, 1500 and 1550 °C. After each evaporation step, the temperature of the ionization filament is raised to
the point of Pb emission, and the isotopic measurements are
dynamically made with the ion counter of the instrument. The
intensities of the emission of each Pb isotope were measured in
one cycle by peak stepping through the 206–207–208–206–207–
204 mass sequence for five mass scans, defining one data block
with eight 207Pb/206Pb ratios. Five blocks are usually recorded for
each evaporation step. The weighted 207Pb/206Pb mean for each
block is corrected for common Pb using appropriate age values derived from the two-stage model of Stacey and Kramers (1975), and
the corrected block is used for sample age calculation. Blocks yielding a 204Pb/206Pb mean above 0.0004, and those that scatter more
than two standard deviations (2r) from the mean age value are
discarded. The calculated age for a single zircon grain and its error,
according to Gaudette et al. (1998), is the weighted mean and standard error of the accepted blocks of data. The same procedure is
adopted to calculate the age for a rock sample from a set of cogenetic grains. The ages are presented with 2r error.
238
M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
Fig. 2. Geological map of the central-eastern Bacajá domain (based on Faraco et al. (2005)) with location of dated samples.
239
M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
For the Sm–Nd analysis, a mixed 150Nd–149Sm spike is added to
ca. 100 mg of rock powder and attacked with HF + HNO3 in Teflon
vials inside PARR containers at 150 °C for one week. After evaporation, new additions of HF + HNO3 are made, the solutions are dried,
followed by dissolution with HCl (6 N), drying, and finally dissolution with HCl (2 N). After the last evaporation, the REE are separated from other elements by cation exchange chromatography
(Dowex 50WX-8 resin) using HCl (2 N) and HNO3 (3 N). After that,
Sm and Nd were separated from the other REE by anion exchange
chromatography (Dowex AG1-X4 resin) using a mixture of HNO3
(7 N) and methanol. The isotopic measurements are statically acquired using the Faraday cups of the mass spectrometer, and Nd
data are normalized to a 146Nd/144Nd ratio of 0.7219. Procedure
blanks were <100 pg for Sm and <400 pg for Nd. The La Jolla Nd
standard yielded a 143Nd/144Nd ratio of 0.511844 ± 22 (2r) based
on four analyses. The crustal residence ages were calculated using
the model of De Paolo (1988) for the depleted mantle (TDM).
Age calculation was done using the software Isoplot (v.2.49) of
Ludwig (2001) and others developed in the Pará-Iso.
5. Zircon ages
Seven rock samples from different igneous and metaigneous
lithological units of the central-eastern Bacajá domain were dated
by the zircon Pb-evaporation technique. The results revealed ages
within a ca. 0.6 billion years interval. Combined with the field
and petrographic data, as well as with previous geochronological
results, the rock units can be separated into six groups representing different stages of the magmatic and tectonic evolution of the
Bacajá domain. Although the rocks show different degrees of recrystallization and deformation, the general features of the zircon
grains, the similarity with some previous results, and the accuracy
of the new ages allow them to be interpreted as the crystallization
ages of the grains and, consequently, the emplacement ages of the
bodies. These results will be discussed below, from the oldest to
the youngest rock units.
5.1. Tonalitic gneiss
Sample MDM03A was collected at the Manelão gold mine located on the WNW–ESE Bacajá transcurrent shear zone (Fig. 2). It
is an orthogneiss included by Souza et al. (2003) and Souza and
Kotschoubey (2005) in the Xingu Complex (Silva et al., 1974). It
is a light gray, fine to medium-grained, banded hornblende-biotite
tonalitic gneiss. Under the microscope, this gneiss shows polygonal
granoblastic quartz-feldspar arrays and lepido-nematoblastic
aggregates of mafic minerals. Hornblende and epidote are subordinate, and apatite, titanite, zircon and opaque are accessory
minerals.
Selected zircon crystals are prismatic, bipyramidal, light brown
to colorless, semitransparent, and show few inclusions, fractures
and metamictization features. Some crystals have rounded edges
and are sometimes drop-shaped. Five crystals were analyzed yielding individual ages varying from 2674 to 2664 Ma, and a mean age
of 2671 ± 3 Ma (Table 1, Fig. 3). The 40 blocks and 270 isotopic
Table 1
Zircon Pb-evaporation isotopic data from rocks of the central-eastern part of the Bacajá domain. Only results included in the age calculation are presented. In the Ratios column,
xxx/yyy is the total isotopic ratios measured (xxx) and used (yyy) in age calculation.
Sample/grain
Ratios
204
MDM03/3
MDM03/5
MDM03/6
MDM03/9
MDM03/10
24/62
34/62
40/114
106/114
66/88
270/440
16/54
34/44
40/100
32/70
122/268
30/38
36/72
32/66
34/34
62/86
8/8
202/304
32/64
38/62
34/70
104/196
40/54
36/48
72/72
20/20
62/62
34/42
264/298
16/24
32/62
16/46
8/12
72/144
40/78
38/54
20/50
98/182
MDM01/1
MDM01/2
MDM01/12
MDM01/13
MDM07C/1
MDM07C/2
MDM07C/4
MDM07C/7
MDM07C/8
MDM07C/10
MJ36/1
MJ36/2
MJ36/5
MCM18/4
MCM18/8
MCM18/9
MCM18/10
MCM18/11
MCM18/12
MDM02/2
MDM02/3
MDM02/4
MDM02/6
MCM58/1
MCM58/2
MCM58/4
207
(
206
Pb/
Pb)c and (
208
Pb/
206
Pb/206Pb
±2r
(208Pb/206Pb)c
±2r
(207Pb/206Pb)c
±2r
Age (Ma)
±2r
0.000009
0.00002
0.000005
0.000013
0.000071
4
4
4
2
47
0.13606
0.12147
0.15041
0.13313
0.15042
35
31
57
81
134
55
23
37
44
17
0.000248
0.000019
0.000027
0.000024
26
4
4
3
0.18409
0.16612
0.16481
0.14657
117
69
6
34
0.000099
0.000019
0.000019
0.000052
0.000017
0.000021
11
7
3
7
13
2
0.08815
0.15248
0.13579
0.10401
0.12469
0.11289
63
38
43
95
37
66
0.000034
0.000027
0.000011
4
2
4
0.20205
0.21637
0.24534
69
58
64
0.000062
0.000118
0.000039
0.000076
0.000039
0.000014
8
22
15
76
3
5
0.11353
0.11915
0.10852
0.09662
0.07653
0.10268
71
103
103
181
58
13
0.000349
0.000154
0.00004
0.000133
37
2
18
2
0.10198
0.13494
0.09334
0.12484
73
57
38
174
0.000110
0.000041
0.000053
12
13
6
0.35182
0.17094
0.07279
39
45
67
0.18177
0.18158
0.18122
0.18208
0.18225
207
Pb/206Pb
0.15922
0.15899
0.15842
0.15809
207
Pb/206Pb
0.15055
0.15114
0.15136
0.15123
0.15118
0.15159
207
Pb/206Pb
0.13681
0.13714
0.13712
207
Pb/206Pb
0.13395
0.13362
0.13447
0.13311
0.13381
0.13411
207
Pb/206Pb
0.12924
0.12902
0.12945
0.12832
207
Pb/206Pb
0.12492
0.12822
0.12856
207
Pb/206Pb
2669.5
2667.5
2664.3
2672.3
2673.8
2671.2
2447.6
2445.2
2439.1
2435.6
2438.5
2352.4
2359.1
2361.7
2360.2
2359.6
2364.2
2359.0
2187.5
2191.7
2191.4
2190.8
2150.6
2146.3
2157.4
2139.6
2148.8
2152.7
2153.9
2088.0
2084.9
2090.8
2075.5
2084.7
2079.2
2073.4
2078.8
2076.9
5.0
2.1
3.4
3.9
1.6
2.6
7.4
5.8
2.4
2.5
3.9
3.7
3.0
4.6
6.3
1.8
6.0
2.3
4.9
4.5
2.5
2.1
4.0
12.7
5.2
8.2
3.1
8.4
3.8
9.3
2.8
8.9
7.9
4.0
4.4
4.9
7.8
3.0
Pb)c = ratio corrected for common Pb.
mean age=
69
54
23
24
mean age=
32
26
41
56
16
53
mean age=
39
35
2
mean age=
31
97
4
63
24
65
mean age=
68
21
66
58
mean age=
18
36
57
mean age=
240
M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
2678
box heights are 2 σ
Tonalitic gneiss
2458
MDM-03
MDM3/3
box heights are 2 σ
Quartz-monzodioritic gneiss
MDM-01
2454
2674
MDM01/2
2450
2670
MDM03/6
Age [Ma]
Age [Ma]
MDM03/10
MDM03/9
2666
2446
MDM01/12
2442
MDM01/1
MDM3/5
2438
2662
2434
2658
Mean = 2671 ± 3 Ma
MSWD = 5.6
2430
Zircon
Zircon
box heights are 2 σ
2374
MDM-07C
Metandesite
2197
MDM01/13
Age = 2439 ± 4 Ma
MSWD = 5.7
box heights are 2 σ
Monzogranite
MJ-36
2370
2195
MDM07C/4
2366
2193
MDM07C/8
2362
2358
MDM07C/10
Age [Ma]
Age [Ma]
MJ36/5
MDM07C/7
MJ36/1
2191
2189
2187
MDM07C/2
MJ36/2
2354
2185
2350
2183
Age = 2359 ± 2 Ma
MSWD = 3.5
MDM07C/1
2346
Age = 2191 ± 2 Ma
MSWD = 1.1
2181
Zircon
Zircon
box heights are 2 σ
Granodiorite
2165
box heights are 2 σ
MCM-18
Granodiorite
MDM02/4
2100
MCM18/9
MDM-02
MCM18/8
2090
2145
MCM18/11
MCM18/12
2135
2125
MDM02/3
2080
MDM02/2
2070
Mean = 2154 ± 4 Ma
MSWD = 3.2
MDM02/6
MCM18/10
Zircon
2090
Age [Ma]
Age [Ma]
2155
MCM18/4
2060
box heights are 2 σ
Granodiorite
Age = 2085 ± 4 Ma
MSWD = 2.6
Zircon
MCM-58
2086
Age [Ma]
2082
MCM58/2
2078
2074
MCM58/1
MCM58/4
2070
2066
Age = 2077 ± 3 Ma
MSWD = 1.7
Zircon
Fig. 3. Zircon Pb-evaporation age diagrams for rocks from the central-eastern part of Bacajá domain.
M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
ratios, mostly obtained at the highest evaporation temperature
(1550 °C), show a very homogeneous pattern, yielding a well-defined mean age.
5.2. Quartz-monzodioritic gneiss
Sample MDM01 is a gneissic quartz-monzodiorite collected at
Belmonte (Fig. 2), where the outcrop is intruded by a pink medium
to coarse-grained leucogranite (possibly a leucossome vein?). The
gneiss is fine to medium-grained, light gray and banded, with granoblastic and lepidoblastic textures. The fabric is marked by elongated crystals of quartz and feldspar. Hornblende and biotite are
subordinate, and the accessory minerals are apatite, zircon and
allanite.
Zircon crystals are prismatic, semitransparent, light brown to
colorless, and show fractures and metamictization features. Different types of inclusions are present: long and colorless (fluid?),
round, dark brown, irregular etc. The analytical results for four
grains yielded a mean age of 2439 ± 4 Ma Table 1, Fig. 3) from 17
blocks and 122 isotopic ratios. Crystal # 6 was rejected since it
has a slightly older age (2457 ± 5 Ma) and most likely represents
an inherited grain.
A similar early Siderian age (2440 ± 7 Ma obtained by Pb-evaporation on zircon) was reported by Vasquez et al. (2005) for an enclave of quartz dioritic gneiss hosted by a porphyroclastic
granodiorite with an age of 2215 ± 2 Ma obtained by Pb-evaporation on zircon. The granodiorite is exposed south of Brasil Novo
on the western bank of the Xingu River (Fig. 2). Additionally, Santos (2003) presented an age of 2491 ± 7 Ma (U–Pb SHRIMP) for
inherited zircon crystals included in the Brasil Novo Tonalite
(2182 ± 7 Ma) (Fig. 2).
241
Zircon crystals from sample MJ36 selected for analysis have
rounded edges, few inclusions and fractures, and are pale brown
to colorless, transparent to translucent and show faint oscillatory
zoning. From seven grains analyzed, only three crystals emitted enough Pb for isotopic measurements to be useful in the age calculation (Table 1, Fig. 3). They yielded a mean age of 2191 ± 2 Ma from
104 isotopic ratios distributed in 15 blocks.
Santos (2003) obtained a similar age of 2182 ± 6 Ma (zircon U–
Pb SHRIMP) for a tonalite exposed near Brasil Novo, on the left
margin of the Xingu River in the northwestern part of the study region (Fig. 2).
5.5. 2.15 Ga old granodiorite
In Belo Monte, at the eastern margin of the Xingu River (Fig. 2),
granodiorites are common. They host E–W quartz-feldspathic
veins, which are folded, giving them a gneissic structure. Sample
MCM18 is a medium- to fine-grained leucogranodiorite having
granular to granoblastic, locally cataclastic textures and incipient
foliation. Biotite and hornblende are the main mafic minerals,
and titanite, apatite, zircon and allanite are accessory minerals.
Zircon crystals show rounded edges, light pink color, and are
translucent to transparent, with few inclusions and fractures. They
have oscillatory zoning suggesting an igneous origin. The isotopic
results for six grains yielded the mean age of 2154 ± 4 Ma calculated from 264 ratios in 39 blocks (Table 1, Fig. 3).
Vasquez et al. (2008) presented a U–Pb SHRIMP zircon age of
2133 ± 10 Ma for a sheared tonalite from the northwestern part
of the region, on the east bank of the Xingu River (Fig. 2). This rock
contains 2340 Ma-old inherited zircon crystals. Additionally, a
quartz monzodiorite intruded into the Três Palmeiras greenstone
belt at the Galo gold mine was dated at 2160 ± 3 Ma.
5.3. Três Palmeiras metandesite
5.6. 2.08 Ga old monzogranite and granodiorite
Mafic to intermediate metavolcanic rocks, metatuffs, and associated tonalites and diorites are exposed in the eastern part of the
Três Palmeiras greenstone belt. These rocks are cut by gold-bearing
quartz veins with arsenopyrite, pyrite and chalcopyrite related to
NW–SE shear zones. Sample MDM07C, from the Zé Meneses gold
mine (Fig. 2), is a metandesite with porphyroclasts of plagioclase
and hornblende, in a microgranular biotite and quartz groundmass.
Epidote, zircon, apatite and titanite are accessory minerals.
Zircon crystals from sample MDM07C are prismatic, bipyramidal, but with slightly rounded edges. They are pale brown color,
translucent to transparent, and have few inclusions and fractures.
Radial cracks and long inclusions parallel to the c-axis are observed
in some grains. Of the crystals selected for isotopic analysis, eight
were used to calculate an age of 2359 ± 2 Ma (Table 1, Fig. 3),
which was obtained from 220 isotopic ratios of 33 blocks.
Two other rocks from Bacajá domain furnished similar ages by
U–Pb SHRIMP on zircon (Fig. 2): a tonalite (2313 ± 9 Ma, Faraco
et al., 2005), intruded into the Jacaré Complex and located close
to the town of Novo Repartimento, and a metatonalite
(2338 ± 5 Ma, Vasquez et al., 2008), intruded into the Três Palmeiras greenstone belt.
5.4. 2.19 Ga old monzogranite
Sample MJ36 was also collected at Belmonte (Fig. 2) and is a
monzogranite of medium grain size and pale gray to pink color,
which cuts the MDM01 metaquartz-monzodiorite gneiss. The rock
shows hipidiomorphic granular to granoblastic texture with moderate to incipient mylonitic foliation, and is locally banded. Antiperthite is present and amphibole and biotite are the mafic
minerals. Accessory minerals are allanite and zircon; muscovite
seems to be an alteration product.
A coarse-grained biotite leucogranodiorite (sample MCM58)
showing N70 W sub-horizontal magmatic foliation is exposed
approximately 15 km NE from Novo Repartimento (Fig. 2).
Zircon grains from sample MCM58 are euhedral, pale to dark
brown, forming short prisms with few inclusions or cracks, and
having weak oscillatory zoning. From seven grains selected for isotopic analysis, only three emitted enough Pb to be considered in
the calculation of a mean age of 2077 ± 3 Ma (Table 1, Fig. 3), obtained from 19 blocks encompassing 140 isotopic ratios.
Barros et al. (2007) analyzed a sample (MCM55b) of titanite–
biotite granodiorite, collected 8 km NW from Novo Repartimento
(Fig. 2). This granodiorite is considered to be a variety of the
MCM58 leucogranodiorite. Four zircon crystals yielded an age of
2076 ± 6 Ma by the Pb-evaporation method. A fifth grain indicated
an age of 2110 ± 11 Ma, which ‘‘probably corresponds to an inherited grain from an early stage of the long magmatic event associated with the Maroni-Itacaiúnas Province”.
A foliated, medium-grained, pale pink monzongranite (sample
MDM02) is exposed between Belmonte and the Manelão gold mine
(Fig. 2). In this monzogranite, varietal minerals are biotite and
muscovite, while zircon, allanite and apatite are accessory
minerals.
Zircon crystals are frequently euhedral, form long, colorless to
pale brown prisms with few inclusions and fractures, and faint
oscillatory zoning. Some grains have rounded edges, radial cracks
and metamictization features. Four zircon grains provided isotopic
results suitable for age calculation and yielded a mean age of
2085 ± 4 Ma from 72 isotopic ratios divided into 10 blocks (Table
1, Fig. 3).
The petrographic features and age of sample MDM02 are very
similar to those described by Souza and Kotschoubey (2005) for
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M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
the Felício Turvo Granite (2069 ± 6 Ma, Pb-evaporation method), at
the Manelão gold mine. In light of additional mapping undertaken
by Faraco et al. (2005), it is possible to confirm that both samples
belong to the same elongated igneous body.
Similar ages to those discussed above were obtained for granitic
rocks from the Bacajá domain, on the left margin of the Xingu River
(Fig. 2); for example, the Belo Monte monzogranite yielded a U–Pb
SHRIMP age for zircon of 2086 ± 6 Ma (Santos, 2003). To the southwest, Vasquez et al. (2005) described an inequigranular biotite
monzogranite with an age of 2077 ± 2 Ma (Pb-evaporation on zircon) crosscutting a 2215 ± 2 Ma old porphyroclastic granodiorite.
At the eastern end of the Bacajá domain (Fig. 2), Faraco et al.
(2005) reported a U–Pb SHRIMP zircon age of 2114 + 35/ 33 Ma
for a granodiorite of the Valentim Complex.
6. Whole-rock Sm–Nd isotopic data
Except for sample MCD58, all samples dated by the Pb-evaporation technique were also analyzed by the Sm–Nd method. Analyses
on eight additional samples were also carried out. The results are
given in Table 2 and Fig. 4. There is a direct relationship between
the number of samples analyzed and the estimated area of exposure of each unit in the study area. In consequence, there are more
results for the younger groups compared with the older ones. Sample MDM07A, from the same gold mine where metandesite
MDM07C was collected, corresponds to a metadiorite with amphibole and biotite. Taking into consideration the field relationships,
and that it is isotopically similar to the metandesite MDM07C
(see Table 2), it was considered to be coeval with the metandesite,
and also to belong to the Três Palmeiras greenstone belt.
The crustal residence ages were calculated using the De Paolo
(1988) model for the depleted mantle (TDM), whereas eNd(T)(CHUR)
values were calculated using the zircon ages obtained in this study.
Where the ages are not available, the sample was correlated with
one of the lithological units of the Bacajá domain, taking into account similarities of petrographic features and geographical distribution. Its age was assumed to be that of the unit.
Since metandesite MDM07C is composed of different fragments
from drill-core samples, two of them (C1 and C2) were analyzed in
order to check heterogeneity in the sequence. The Sm and Nd contents of the sample set range from 2 to 9 ppm and 12 to 71 ppm,
respectively, with the lower contents in the mafic to intermediate
rocks from the Três Palmeiras greenstone belt. An exception is
sample MCM56 (hornblende-biotite granodiorite), with 25 ppm
Sm and 171 ppm Nd.
Nd TDM model ages for the sample set range from 2.25 to
2.93 Ga and can be divided into two groups with Paleoproterozoic
and Archean model ages. The samples with Paleoproterozoic model
ages are dominated by 2.08 Ga monzogranite to granodiorite. A
particular aspect of this group is that all the samples were collected
in the northern part of the area, especially along the BR-230 road.
Samples from all other lithological units belong to the group with
Archean model ages. The higher eNd(T) values are close to zero and
correspond to samples of the group with Paleoproterozoic model
ages (eNd(T) from 0.60 to +0.83) and also to the mafic to intermediate rocks of the Três Palmeiras greenstone belt (eNd(T) from –0.87
to +0.78). A special case is represented by sample MDM03 (Archean tonalitic gneiss), with one sample showing the highest eNd(T)
value (+2.7). Apart from these samples, the others have negative
eNd(T) values ranging from 2.9 to 8.3, typical of rocks with a
crustal origin.
In summary, taking into account the division into lithological
units and the Nd isotopic data, the samples from the study area
can be divided into the following geochronological and isotopic
groups:
1. Paleoproterozoic monzogranite and granodiorite (2.08 and
2.15 Ga) with Paleoproterozoic Nd TDM ages (2.25–2.47 Ga)
and eNd(T) close to zero, between 0.60 and +0.83;
2. Paleoproterozoic monzogranite to granodiorite and quartzmonzodioritic gneiss (2.08–2.44 Ga) with Archean Nd TDM ages
(2.57–2.93 Ga) and negative eNd(T), between 8.33 and 2.9;
3. Paleoproterozoic mafic to intermediate rock (2.36 Ga) with
Archean Nd TDM ages (2.56–2.71 Ga) and eNd(T) close to zero,
between 0.87 and +0.78, and
4. Archean tonalitic gneiss (2.67 Ga) with Archean Nd TDM age
(2.65 Ga) and positive eNd(T) (+2,66).
7. Discussion
Table 3 summarizes the geochronological data available for
igneous and metaigneous rocks from central-eastern Bacajá
Table 2
Whole-rock Sm–Nd isotopic results from the central-eastern part of the Bacajá domain.
Sample
Sm (ppm)
Nd (ppm)
147
Sm/144Nd
2.67 Ga Archean tonalitic gneiss
MDM03
3.47
16.32
0.12845
2.44 Ga Siderian gneisses
MDM01
13.05
71.42
0.11048
2.36 and 2.31 Ga Siderian metandesites and metadiorites
MDM07C1
2.50
12.19
0.12412
MDM07C2
2.84
13.76
0.12485
MDM07A
2.47
11.95
0.12490
2.22–2.18 Ga Rhyacian granitoids
MJ36
8.16
52.20
0.09448
MJ37
7.08
45.07
0.09492
2.16 to 2.13 Ga Rhyacian granitoids
MCM18
2.37
17.60
0.08140
2.09 to 2.07 Ga Rhyacian granitoids
MCM27
9.04
53.42
0.10234
MCM54
1.95
12.63
0.09319
MCM55
6.13
44.93
0.08248
MCM56
25.19
171.07
0.08903
MDM02
3.90
25.49
0.09257
MDM04
7.44
38.16
0.11787
MDM08
5.32
23.51
0.13691
MDM09
2.44
19.61
0.07506
*
Estimated age.
143
Nd/144Nd (±2r)
f
(Sm/Nd)
eNd(0)
t(zircon) (Ga)
eNd(t)
TDM (Ga)
2.66
2.65
0.511571 (22)
0.3470
20.81
2.67
0.511104 (17)
0.4383
29.92
2.44
2.90
2.89
0.511549 (23)
0.511476 (31)
0.511549 (16)
0.3690
0.3653
0.3650
21.24
22.67
21.24
2.36
2.36
2.36
0.78
0.87
0.55
2.56
2.71
2.58
0.510774 (15)
0.510912 (12)
0.5197
0.5174
36.36
33.67
2.19
2.19
7.63
5.04
2.93
2.76
0.511013 (32)
0.5862
31.70
2.15
0.21
2.35
0.511132 (10)
0.511266 (26)
0.511047(22)
0.510957 (08)
0.510806 (14)
0.511376 (16)
0.511804 (13)
0.510642 (28)
0.4797
0.5262
0.5807
0.5474
0.5294
0.4008
0.3040
0.6184
29.38
26.76
31.04
32.79
35.74
24.62
16.27
38.94
2.08
2.08
2.08
2.08
2.08
2.08
2.08
2.08
4.25
0.83
0.60
4.12
7.89
3.63
0.34
6.55
2.63
2.25
2.33
2.57
2.84
2.67
2.47
2.66
*
*
*
*
*
*
*
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M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
15
-5
-10
-10
-15
-15
-20
-20
Pre-Trans-Amazonian rocks
-25
-35
-40
Carajás Block
Trans-Amazonian rocks
2.19 Ga monzogranite
MJ36
MJ37
2.15 Ga granodiorite
MCM18
2.08 Ga monzogranite to granodiorite
MDM02
MCM27
MDM04
MCM54
MDM08
MCM55
MDM09
MCM56
-25
2.67 Ga tonalitic gneiss
MDM03
2.44 Ga quartz-monzodioritic gneiss
MDM01
2.36 Ga metandesite and metadiorite
MDM07C1
MDM07C2
MDM07A
-30
-30
-35
-40
-45
-50
1.0
CHUR
0
-5
-45
DM
5
CHUR
0
B
10
DM
5
ε (Nd)
15
A
10
Carajás Block
-50
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
1.0
1.2
1.4
1.6
1.8
2.0
T(Ga)
2.2
2.4
2.6
2.8
3.0
3.2
3.4
T(Ga)
Fig. 4. eNd vs. time diagram from the central-eastern part of Bacajá domain: A – Pre-Trans-Amazonian rocks, and B – Trans-Amazonian rocks. Field of Archean rocks from
Carajás block is also plotted (see text for references).
domain, including those obtained in this work. Only zircon analyses (U–Pb SHRIMP and Pb-evaporation) were considered in order
to better constrain the timing of the magmatic events. The rocks
are Neoarchean and Paleoproterozoic (Siderian and Rhyacian periods). Taking into consideration the ages of the rocks and their geological setting, the following magmatic events, and their most
probable tectonic settings may be recognized in the Bacajá
domain:
1. Archean tonalitic gneiss, whose protolith was formed at ca.
2.67 Ga, occurs at the Manelão gold mine, and is included in
the Xingu Complex (Souza and Kotschoubey, 2005). At the sampling site, evidence that clarifies the origin and the genetic relationship of this gneiss with the surrounding rocks was not
observed. Since it is the oldest rock in the domain, adjacent
rocks intrude or cover it (Souza and Kotschoubey, 2005). Correlation with Archean rocks in the adjacent Carajás block in the
Central Amazonian Province seems unlikely. In the Carajás
block, the rocks are older (3.00–2.76 Ga), and have generally
higher Nd TDM ages, mainly 2.9–3.20 Ga (e.g. Olszewski et al.,
1989; Sato and Tassinari, 1997; Dall’Agnol et al., 1999b; Teixeira et al., 2002; Rämö et al., 2002; Galarza, 2002; Rolando and
Macambira, 2003; Barros et al., 2004), whereas the Manelão
mine gneiss is clearly juvenile. Unless new results show that
these rocks played a significant role in the evolution of the Bacajá domain, a possible working hypothesis is that this gneiss is
just a small fragment of older crust, trapped during the accretion of arcs which probably generated the Bacajá domain during
the Paleoproterozoic. Its isotopic characteristics suggest that
the protolith was an island-arc or TTG suite. Another Archean
Table 3
Geochronological data available for zircon from igneous and metaigneous rocks of the central-eastern of the Bacajá domain.
Rock type/lithological unit/sample
Area
2.67 Ga Archean tonalitic gneiss
Tonalitic gneiss/MDM03A
Manelão gold mine
Siderian gneisses of 2.44 Ga
Quartz-dioritic gneiss
Brasil Novo
Quartz-monzodioritic gneiss/MDM01
Belmonte village
2.36 and 2.31 Ga Siderian metatonalites and metandesites
Metandesite/Três Palmeiras/MDM07C
Zé Meneses gold mine
Porphyroclastic metatonalite
Bacajá River
Tonalite/Jacaré Complex
Novo Repartimento
2.22–2.18 Ga Rhyacian granitoids
Porphyroclastic granodiorite
Brasil Novo
Monzogranite/MJ36
Belmonte
Tonalite
Brasil Novo
2.16–2.13 Ga Rhyacian granitoids
Quartz monzodiorite
Galo gold mine
Leucogranodiorite/MCM18
Belo Monte
Sheared tonalite
Xingu River-Brasil Novo
Granodiorite/Valentim Complex
Novo Repartimento
2.09–2.07 Ga Rhyacian granitoids
Belo Monte Monzogranite
Belo Monte
Leucogranodiorite/MCM58
Novo Repartimento
Granodiorite
Novo Repartimento
Felício Turvo Granite/MDM02
Manelão gold mine
Felício Turvo Granite
Manelão gold mine
Monzogranite
Xingu River-Brasil Novo
Zircon age (Ma) magmatic/inherited
Method
Ref.
2671 ± 3
Pb-evaporation
1
2440 ± 7
2439 ± 4
Pb-evaporation
Pb-evaporation
3
1
2359 ± 2
2338 ± 5
2314 ± 9
Pb-evaporation
U–Pb SHRIMP
U–Pb SHRIMP
1
4
5
2215 ± 2/2524 ± 5
2191 ± 2
2182 ± 6/2491 ± 7
Pb-evaporation
Pb-evaporation
U–Pb SHRIMP
3
1
2
2160 ± 3
2154 ± 4
2133 ± 10/2340
2114 +35/-33
U–Pb SHRIMP
Pb-evaporation
U–Pb SHRIMP
U–Pb SHRIMP
4
1
4
5
2086 ± 6
2077 ± 3
2076 ± 6/2110 ± 11
2085 ± 4
2069 ± 6
2077 ± 2
U–Pb SHRIMP
Pb-evaporation
Pb-evaporation
Pb-evaporation
Pb-evaporation
Pb-evaporation
2
1
6
1
7
3
References: 1 – this work; 2 – Santos (2003); 3 – Vasquez et al. (2005); 4 – Vasquez et al. (I 2008); 5 – Faraco et al. (2005); 6 – Barros et al. (2007); 7 – Souza et al. (2003).
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M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
gneiss is exposed at the western end of the Bacajá domain,
south of Uruará, outside the area showing in Fig. 2. It is a
2503 ± 10 Ma old tonalitic gneiss with inherited crystals dated
at 2581 ± 6 Ma (U–Pb SHRIMP on zircon, Santos, 2003).
2. Siderian granitoids crystallized at ca. 2.44 Ga and were later
transformed into quartz-dioritic gneiss (Brasil Novo area) and
quartz-monzodioritic gneiss (Belmonte area). At the Brasil Novo
area, the quartz-dioritic gneiss is a xenolith included in Rhyacian granitoids. Apart from the Nd results for the Belmonte
gneiss, which suggest an origin from melting of older Archean
crustal rocks (Table 2), their situation is similar to that of the
Archean gneiss of Bacajá domain, i.e., there is no clear evidence
to speculate about the origin of these rocks. The hypothesis suggested here that these Siderian gneisses are simply small remnants of older continental crust (in a continental arc?) needs
confirmation.
3. Siderian metatonalites, metadiorites and metandesites (2.36–
2.31 Ga, Table 3) are associated with the Três Palmeiras greenstone belt and with the Jacaré Complex, in the northwestern
and northeastern parts of the study area (Fig. 2), respectively.
According to Jorge João et al. (1987), the compositions of the
Três Palmeiras mafic rocks range from island-arc tholeiite to
MORB. On the other hand, the Nd isotopes (Table 2) denote a
common mantle source, and no or very little crustal contamination for both metandesite and metadiorite. This, added to characteristics previously described, is indicative of an island-arc
environment. In spite of the absence of conclusive evidence to
characterize the tectonic setting of the Três Palmeiras greenstone belt and surrounding gneiss and metagranitoid units, it
is thought that remnants of an oceanic floor are present, as
has been suggested for the northern part of Maroni-Itacaiúna
Province – Guiana shield (Vanderhaeghe et al., 1998). These
supracrustal rocks in the Bacajá domain, together with the
2.44 Ga gneisses, are the first Siderian rocks reported in the
Amazonian craton, and represent a unique feature that contrasts with other domains of the Maroni-Itacaiúnas Province.
The Jacaré Complex (Novo Repartimento area) was described
by Faraco et al. (2005) as a 2313 ± 9 Ma old (SHRIMP age for a
tonalite) association of protomylonitic monzogranite, metamonzogranite, metatonalite, metagranogranodite, tonalite
and metasienogranite. However, Barros et al. (2007) reported
the age of 2076 ± 6 Ma for a granodiorite from the same area,
in agreement, therefore, with the age of 2077 ± 3 Ma reported
here for another granodiorite sample (Fig. 2, Table 3). Our
Sm–Nd results (Table 2) suggest the participation of a juvenile
component in the northern part of the region, corroborating
the homogeneity of the rock types as suggested by Barros
et al. (2007). Since tonalite is a subordinate rock type in the
area, the hypothesis that Siderian rocks have restricted occurrences remains to be tested; they might represent remnants
of plutonic rocks associated with the Três Palmeiras greenstone
belt.
4. Rhyacian granitoids are widespread in the northern part of the
Bacajá domain and are represented by different rocks types.
Tonalite, granodiorite and monzogranite were emplaced
roughly in this sequence during an igneous event which lasted
for approximately 140 My. Few detailed petrographic and
structural studies have been carried out on the Bacajá domain
(Vasquez et al., 2005; Barros et al., 2007), and the observation
that the older rocks are more deformed than the younger ones
needs confirmation.
Despite the scarcity of data, it is possible to trace a parallel with
other better studied domains of the Maroni-Itacaiúnas Province. In
this way, the Rhyacian granitoids can be separated into subgroups, which may correspond to different stages (or orogenies:
Santos, 2003) of the tectono-magmatic evolution of the TransAmazonian cycle, as already suggested for the northern part of
the province, the Guiana shield (e.g. Vanderhaeghe et al., 1998; Delor et al., 2003; Santos, 2003; Rosa-Costa et al., 2006).
Monzogranite, tonalite and quartz monzodiorite showing hipidiomorphic granular to granoblastic texture with moderate to incipient mylonitic foliation, locally banded, are recognized in the Brasil
Novo and Belmonte areas. They were intruded between 2.22 and
2.18 Ga into older continental crust, as indicated by their Nd isotopic compositions (Nd TDM ages = 2.9 and 2.8 Ga; eNd(T) = 5.0 and
8.3) and by the presence of inherited zircon grains (2491 ± 7 Ma
and 2524 ± 5 Ma, see Table 3). The evidence corroborates the tectonic setting as a continental arc at the margin of an Archean continent, representing, therefore, the fist stage of the TransAmazonian cycle.
The period between 2.16 and 2.13 Ga is characterized by the
emplacement of tonalite, quartz monzodiorite and granodiorite
in the northwestern part of the region. They cross cut the Três
Palmeiras greenstone belt, which had been already accreted to
the continental margin. A particular feature of this phase, which
contrasts with the previous, is the presence of juvenile material
(Table 2). In fact, this period corresponds to the main granitogenesis in the Guiana shield, similarly correlated with the evolution of a
continental arc (e.g. Vanderhaeghe et al., 1998; Delor et al., 2003;
Rosa-Costa et al., 2006).
The rocks formed in the central-eastern Bacajá domain during
the short period of time from 2.09 to 2.07 Ga mainly comprise
granodiorites, monzogranites with subordinate syenogranites
(Felício Turvo Granite), and charnockites with preserved igneous
textures. They predominate in the northern part of the study region, where Barros et al. (2007) reported a belt of calc-alkaline Itype granitoids. Nd isotopes allowed the classification of these
rocks into two groups: granitoids with Paleoproterozoic Nd TDM
ages (2.25–2.47 Ga) and eNd(T) close to zero, between 0.60 and
+0.83, and granitoids with Archean Nd TDM ages (2.57–2.84 Ga)
and eNd(T) essentially negative, between 7.9 and 3.6. These data
lead to the proposal that both juvenile and Archean reworked
crusts are the sources for the last magmatic products of the
Trans-Amazonian cycle. Some degree of mixing generated intermediate Nd TDM ages values.
8. Conclusions
Although a Paleoproterozoic evolution for the Bacajá domain
was proposed several decades ago (Amaral, 1974; Cordani et al.,
1979), its geology is still very poorly known, especially when compared with that of the adjacent Carajás block. Over the last years,
efforts of teams from the Federal University of Pará and CPRM-Brazilian Geological Survey have improved the knowledge about the
domain, allowing the tracing of parallels with other areas of the
Maroni-Itacaiúnas Province, especially those in the Guiana shield.
Our new geological and isotopic results combined with previous
data lead to the proposal of a comprehensive multi-stage evolution
for the eastern part of the Bacajá domain, starting during the Neoarchean and ending at the Rhyacian.
Neoarchean tonalitic gneiss (2671 ± 3 Ma) included into the
Xingu Complex is the oldest rock recorded in the Bacajá domain.
Due to its composition and juvenile nature, it probably represents
a remnant of an island arc or TTG suite and marks an early stage of
crust formation. Another probable remnant is represented by Siderian gneisses crystallized at ca. 2.44 Ga. Contrasting with the Neoarchean gneiss, protholiths of these rocks represent reworked
continental crust, most likely in a continental arc. Despite the
uncertainty about the origin of these gneisses, it is evident that
these rocks cannot be correlated with the Archean rocks of the
M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246
Carajás block, which are older and originated from reworking of a
ca. 2.9–3.2 Ga crust.
The Três Palmeiras greenstone succession forms NW–SE belts
affected by shear zones and hosting gold deposits. Its volcano-plutonic association, emplaced between 2.36 and 2.34 Ga, represents
the first Siderian supracrustal rocks recorded in the Amazonian
craton. This succession is surrounded by younger continental
rocks, suggesting that it was part of a probable island arc/oceanic
floor accreted to the continental margin, a hypothesis corroborated
by its chemical composition and juvenile origin indicated by Nd
isotopes.
Rhyacian granitoids were intruded at different times during an
interval of ca. 140 My (2.22–2.08 Ga), marking at least three stages
of the Trans-Amazonian cycle. In general, the granitoids related to
the younger stages are chemically more evolved and less deformed. The first stage is represented by granitoids produced at
2.22–2.18 Ga by melting of the Archean crust in a probable continental arc setting. The second encompasses 2.16–2.13 Ga old
granitoids which display a larger juvenile component in their original magmas. Finally, the third stage (2.09–2.07 Ga) was mainly
characterized by the emplacement of granodiorites, monzogranites
(Felício Turvo Granite), and charnockites produced from melting of
either juvenile or reworked crust during soft collisions.
The present-day configuration of the lithological units of the
area investigated suggests that the collision of the Bacajá domain
against the Archean Carajás block occurred during the Trans-Amazonian cycle. Although only a few Nd analyses are available, especially in the northern part of the domain, the Nd TDM ages and
eNd(T) reveal that juvenile rocks dominated in the north, whereas
in the south, approaching the boundary with the Carajás block, a
reworked crust predominates. No clear contribution of the Carajás
block was observed on the rocks of the Bacajá domain, although
mixing processes could mask the evidences of this inheritance.
Acknowledgments
This work was supported by CNPq (Grant 467104/00-0), CTMineral/FINEP 01/2001 project, CPRM–Geological Survey of Brazil and
Pará-Iso Laboratory/UFPA. R. Florencio is acknowledged for technical support during analytical work at UFPA. The manuscript was
substantially improved with the constructive contributions of the
reviewers, as well as the English language review of I. McReath.
This paper is a contribution to PRONEX/CNPq (Proj. 103/98, Grant
662103/1998-0).
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