Tectonic evolution of the southern margin of the Amazonian craton in the late Mesoproterozoic ...

28 Pages·2014·2.07 MB·English

Anais da Academia Brasileira de Ciências (2014) 86(1):57-84
(Annals of the Brazilian Academy of Sciences)
Printed version ISSN 0001-3765 / Online version ISSN 1678-2690
http://dx.doi.org/10.1590/0001-37652014104212
www.scielo.br/aabc
Tectonic evolution of the southern margin of the Amazonian craton in the late
Mesoproterozoic based on field relationships and zircon U-Pb geochronology
1 2 3 4
GILMAR J. RIZZOTTO , LÉO A. HARTMANN , JOÃO O.S. SANTOS and NEAL J. MCNAUGHTON
1
Serviço Geológico do Brasil (CPRM), Rua 148, nº 485, Setor Marista, 74170-110 Goiânia, GO, Brasil
2
Instituto de Geociências, Universidade Federal do Rio Grande do Sul,
Avenida Bento Gonçalves, 9500, 91501-970 Porto Alegre, RS, Brasil
3
Redstone Resources, Suite 3, 110 East Parade, 6004, East Perth, WA, Australia
4
Curtin University of Technology, GPO Box U1987, 6845, Bentley, WA, Australia
Manuscript received on May 31, 2012; accepted for publication on November 6, 2012
ABSTRACT
New U-Pb zircon geochronological data integrated with field relationships and an airborne geophysical
survey suggest that the Nova Brasilândia and Aguapeí belts are part of the same monocyclic, metaigneous
and metasedimentary belt formed in the late Mesoproterozoic (1150 Ma-1110 Ma). This geological
history is very similar to the within-plate origin of the Sunsás belt, in eastern Bolivia. Thus, we propose
that the Nova Brasilândia, Aguapeí and Sunsás belts represent a unique geotectonic unit (here termed
the Western Amazon belt) that became amalgamated at the end of the Mesoproterozoic and originated
through the reactivation of a paleo-suture (Guaporé suture zone) in an intracontinental rift environment.
Therefore, its geological history involves a short, complete Wilson cycle of ca. 40 Ma. Globally, this
tectonic evolution may be related with the final breakup of the supercontinent Columbia. Mafic rocks
and trondhjemites in the northernmost portion of the belt yielded U-Pb zircon ages ca. 1110 Ma, which
dates the high-grade metamorphism and the closure of the rift. This indicates that the breakup of
supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia
at ca. 1.1-1.0 Ga and that the Western Amazon belt was formed during the accretion of the Arequipa-
Antofalla basement to the Amazonian craton.
Key words: Amazonian craton, intracontinental rift, Mesoproterozoic, Western Amazon belt, U-Pb
geochronology.
INTRODUCTION landmass has participated in major paleogeo-
The Amazonian craton constitutes a central core graphic reconstructions, such as the formation of
of the South American continent and it was formed the Columbia and Rodinia supercontinents (e.g.,
by the amalgamation of Proterozoic accretionary Rogers and Santosh 2003, Li et al. 2008). The
belts that are successively younger toward the Rodinia supercontinent was formed in the late
southwestern border of the craton (Cordani et al. Mesoproterozoic to early Neoproterozoic by the
2000, Santos et al. 2000, 2008). This continental agglutination of the existing cratonic fragments,
such as Amazonia and Laurentia. In Amazonia,
Correspondence to: Gilmar José Rizzotto
the records of this agglutination are present in the
E-mail: [email protected]
An Acad Bras Cienc (2014) 86 (1)
58 GILMAR J. RIZZOTTO, LÉO A. HARTMANN, JOÃO O.S. SANTOS and NEAL J. MCNAUGHTON
southwesternmost part of the Amazonian craton. (ca. 1.09 Ga). These researchers also suggest the
This region is composed of rocks belonging to existence of a suture belt (Nova Brasilândia belt)
the Sunsás Orogen, according to Santos et al. indicating that the initial docking of the Amazonian
(2008), or the Rondonian-San Ignacio Province craton with southern Laurentia (Llano uplift) at 1.2
(Tassinari et al. 1996). The first authors proposed Ga was followed by strike-slip motion, allowing
that the Sunsás Orogen is characterized by an for later suturing of the Paraguá craton in the final
autochthonous evolution that lasted for 360 m.y. stages of the amalgamation of Rodinia. They also
(from 1465 to 1110 Ma) and was formed by suggest that the Nova Brasilândia sedimentation
four orogenies: Santa Helena (1460-1420 Ma), occurred during a transtensional phase at < 1.2 Ga.
Candeias (1370-1320 Ma), San Andrés (1275 Ma) In contrast, alternative positions of the
and Nova Brasilândia (1180-1110 Ma). They also Amazonian craton in Mesoproterozoic are envi-
postulated the nonexistence of the Paraguá craton, saged to accommodate smaller crustal fragments,
a cratonic fragment that consists of metamorphic such as the Arequipa-Antofalla (i.e., Keppie and
basement rocks in the eastern Bolivia Precambrian Ortega-Gutierrez 1999, Loewy et al. 2003, 2004,
shield, which was first defined by Litherland et Casquet et al. 2006). In this way, Boger et al. (2005)
al. (1986), and argued that Paraguá “craton” and present three possible scenarios for the accretion
Amazonia were already close together, as parts of of the Paraguá Block and the Arequipa-Antofalla
the same continental masses in the middle to late basement to the western margin of the Amazonian
Mesoproterozoic. In the same way, Sadowski and craton. Nevertheless, latest authors favor the tectonic
Bettencourt (1996) argued that there is no clear model by which the Paraguá Block was accreted to
evidence that the Paraguá craton is a unit that is the margin of a coherent Amazonian craton during
independent from the Amazonian craton, and they the Sunsás Orogeny.
consider it to be part of the Amazonian craton that The tectonic framework of the Amazon
is defined as the Paraguá Block. basement (Rondônia-Juruena Province) is of a
Additionally, Bettencourt et al. (2010), in a multi-orogenic nature, whereas the adjacent Nova
recent review of the evolutionary history of the Brasilândia Group in the Nova Brasilândia belt
Rondonian-San Ignacio Province, characterized the most likely originated in an intracontinental rift
Province as a composite orogen, consisting of an setting. Rifting was followed by the opening of a
older, complex accretionary orogen (1556-1430 Ma) proto-ocean at <1215 Ma as part of a single Wilson
that was followed by the collision of the Paraguá cycle (Rizzotto 1999).
Block with the southern part of the cratonized With a similar geochronological and tectonic
Amazonian craton at 1340-1320 Ma. evolution, the Sunsás and Aguapeí belts occur
The proposal for the tectonic framework of along the southeastern rim of the Paraguá Block.
the Amazon made by Tohver et al. (2004, 2005a, b, These belts are most likely related to the timing of
2006) draws a comparison between the Grenville an intraplate extension episode, a stage equivalent
history recorded in the SW Amazonian craton with to the Nova Brasilândia belt passive margin. The
that observed in southernmost Laurentia. They deposition age of the Aguapeí Group occurred
propose that during the late Mesoproterozoic, the between 1167 ± 27 Ma and 1149 ± 7 Ma (Santos
SW Amazonian craton was marked by two separate et al. 2005). However, Tohver et al. (2004, 2005a)
tectonic events, the first related to collision with and Teixeira et al. (2010) suggest the absence of
southern Laurentia (ca. 1.2-1.15 Ga) and the physical and structural continuity between the
second caused by suturing of the Paraguá craton northernmost extent of the Aguapeí belt and the
An Acad Bras Cienc (2014) 86 (1)
TECTONIC EVOLUTION OF THE SOUTHEASTERN AMAZONIAN CRATON 59
E–W trending Nova Brasilândia belt. Teixeira et the Jamari magmatic arc, the Alto Jauru Terrane,
al. (2010) suggest that the Aguapeí belt is a late the Quatro Cachoeiras orogeny, the Cachoerinha
tectonic component of the Sunsás province as a orogen and the Anorogenic magmatism (Fig. 1).
result of the Sunsás collision. The Paleoproterozoic basement rocks of the
The estratigraphic correlation between the Sunsás Province (Paraguá Block) (Fig. 1) comprise
Aguapeí and Sunsás Group is in agreement with the three main units: the Lomas Manechi Complex
idea that late Mesoproterozoic extensional dynamics display granulitic metamorphism generated during
produced a system of continental, aborted rifts over the Mesoproterozoic, at ca. 1330-1350 Ma (Santos et
the cratonized SW margin of the Amazonian craton, al. 2008). The Chiquitania Complex and San Ignacio
evolving to a wide intracratonic basin (e.g., Leite Group subsequently deformed and metamorphosed
and Saes 2003). during the San Ignacio Orogeny by means of three
Therefore, the inconsistencies regarding the successive events of deformation accompanied
collided Paraguá Block-Amazonian craton and its by voluminous syn-tectonic granite magmatism
geodynamic behavior during the Mesoproterozoic (Litherland et al. 1986).
are under evaluation. However, Rizzotto and This Paleoproterozoic basement was amalga-
Hartmann (2012) suggested a new scenario for mated along WNW-trending Mesoproterozoic
the operation of Mesoproterozoic plate tectonics, belts, described here from oldest to youngest. The
with the definition of the Guaporé suture zone, a Alto Guaporé belt (Rizzotto and Dehler 2007) is
mega-structure that marks the suturing of the proto- a accretionary orogen formed by oceanic plate
Amazonian craton and the Paraguá Block in the late subduction between 1.47-1.43 Ga (Rizzotto and
Calymmian to middle Ectasian, ca. 1470-1320 Ma. Hartmann 2012). This constitutes the Guaporé suture
The Guaporé suture is a zone of crustal weakness zone between the proto-Amazonian craton and
reactivated during sedimentation and the proto- the Paraguá Block. The rocks of the Alto Guaporé
oceanic opening of the Nova Brasilândia basin. belt are exposed from the northwestern sector
In this paper, we present new zircon of Rondônia state to the southeastern and to the
U-Pb geochronological results from the two southwestern portion of Mato Grosso state. They are
Mesoproterozoic Nova Brasilândia and Aguapeí largely covered by the Cenozoic sedimentary rocks
orogenies of the southwestern Amazonian craton, of the Parecis and Guaporé Basins, in an area of 200
coupled with field relationships, in a contribution to x 60 km, with WNW-ESE trend marked by strong
resolved some of these issues. These results place magnetic anomalies. This belt consists predominantly
new delimitations on the geologic evolution of the of, plutonic, sedimentary and mafic-ultramafic
southwestern margin of the Amazonian craton, and volcanic rocks of the Trincheira Complex and calc-
this sheds light on paleogeographic correlations alkaline granitic rocks. The Colorado Complex is a
between Amazonia and Laurentia. meta-sedimentary sequence occurring in the Guaporé
suture and is composed of paragneiss, pelitic schist,
GEOLOGICAL OVERVIEW
calc-silicate gneiss, para-amphibolite and banded iron
The studied area is located between the Sunsás and formation. The protolith of the paragneiss and pelitic
Rondônia-Juruena Provinces (Santos et al. 2008), schist is interpreted as a turbiditic sequence deposited
southwestern Amazonian craton (Fig. 1). The in a passive margin (Rizzotto and Quadros 2007).
basement rocks of the Rondônia-Juruena Province The Nova Brasilândia and Aguapeí belts (ca.
are included in six major Paleo-Mesoproterozoic 1150-950 Ma) are exposed along the Bolivia-
terranes and events: the Juruena magmatic arc, Brazil border (Fig. 2). The Nova Brasilândia belt
An Acad Bras Cienc (2014) 86 (1)
60 GILMAR J. RIZZOTTO, LÉO A. HARTMANN, JOÃO O.S. SANTOS and NEAL J. MCNAUGHTON
Figure 1 - Simplified map of the southwestern Amazonian craton showing the approximate boundaries of the main Provinces,
belts, tectonic elements, lithologic units, and location of dated samples.
An Acad Bras Cienc (2014) 86 (1)
TECTONIC EVOLUTION OF THE SOUTHEASTERN AMAZONIAN CRATON 61
Figure 2 - Geotectonic map of the southwestern Amazonian craton highlighting
the Nova Brasilândia, Aguapeí and Sunsás rifts.
originated from the opening and closing of an Amazonian craton was first estimated by Rizzotto
ocean, whereas the Aguapeí belt is an aborted rift, (1999) using U-Pb geochronology in the Nova
both generated by reactivation along the Guaporé Brasilândia belt. To the north of this belt, the
suture zone at ca. 1110-950 Ma. granitoid basement rocks of the Rondônia-Juruena
The region being examined comprises the Nova Province record a polycyclic evolution charac-
Brasilândia and Aguapeí belts, with the integration of terized by the complex juxtaposition of distinct
new field and geochronological data. The Sunsás belt units and widespread shear zone structures.
is also described for comparison. The importance of The granitoid basement rocks were sheared
the theme is centered on the register of the last orogenic by a wide network of sinistral, amphibolite-grade,
phase in the southwestern border of the Amazonian strike-slip shear zones defined as the Ji-Paraná
craton and its correlation with the Grenville province shear zone (Scandolara et al. 1999) which extends
in southern Laurentia. over hundreds of kilometers and cross-cut all
preexisting rock fabrics. According to Tohver et
Grenvillian-age deformation in the basement of
al. (2005a), metamorphism along the shear zone
the Rondônia-Juruena Province
occurred at temperatures of 450-550°C, and the
The timing of the deformation and generation of timing of this deformation varies from ca. 1.18 to
orogenic rocks of Stenian age in the southwestern 1.15 Ga. Consistent with these findings, recent data
An Acad Bras Cienc (2014) 86 (1)
62 GILMAR J. RIZZOTTO, LÉO A. HARTMANN, JOÃO O.S. SANTOS and NEAL J. MCNAUGHTON
from field work in central Rondônia by Quadros of voluminous plutonic mafic rocks with a strong,
et al. (2012) defined the Ji-Paraná-Cujubim positive magnetic signature (Fig. 3). This magmatism
transpressional sinistral system as extending to is related to the successful initial rifting that formed
the northwest of the Ji-Paraná shear zone. Within a narrow proto-ocean, currently with approximately
this zone, the Igarapé Quinze Formation consists 12 x 110 km, during the evolution of continental
of paragneisses and schists that trend N40W. The to oceanic rift phase. These mafic rocks are here
dating of detrital zircon from the schist yielded considered to record an extensional anorogenic
U-Pb ages in the range of 1940 Ma and 1367 Ma. magmatism associated with fragmentation of a pre-
The younger age is interpreted as the maximum age Rodinia supercontinent.
of sedimentation of the sedimentary protoliths of the This mafic unit is bordered in the north by a
Igarapé Quinze Formation (Quadros et al. 2012). transcurrent sinistral shear zone, which consists of
This formation is interpreted as turbiditic deposits mafic granulites (norite and gabbro as protoliths),
and the tectonic structures allow its correlation with rare acid granulites, banded amphibolites,
the Nova Brasilândia Group (Quadros et al. 2012). porphyroblastic amphibolites, and interbedded
According to these authors, lenses and pods of trondhjemites, diabase dikes and lenses of calc-
anatectic granite were generated by partial melting silicate gneisses. Rare and scattered basalts,
of metapelites. Zircons grains from a locally- metasedimentary rocks and banded iron formation
derived anatetic melt yield a U-Pb age of 1138 ± indicate a high degree of exhumation of the terrain
8 Ma. This age represents the peak metamorphic with the accompanying erosion of the top portion
conditions experencied by the sedimentary protolith of the volcano-sedimentary rocks. The Rio Branco
along the Ji-Paraná-Cujubim belt. monzogranitic orthogneiss was emplaced under a
transpressive regime in the banded amphibolite at
NOVA BRASILÂNDIA BELT
ca. 1113 Ma, with T =1630 Ma (Rizzotto 1999).
DM
The Nova Brasilândia Group in the Nova Brasilândia The transtensional stage was succeeded by
belt consisted originally of a supracrustal unit of deep- a transpressional regime accompanied by crustal
sea turbidites with a siliceous-clastic-to-carbonatic shortening, as evidenced by regional EW- and WNW-
composition. The unit was metamorphosed in upper- ESE-trending structures, dominated by oblique thrust
amphibolite to granulite facies to produce quartz- faults (D1). The northeast vergence and medium-to
feldspar gneisses, mica schists and minor calc- high-grade metamorphism correspond to a progressive
silicate gneisses. The metasedimentary package also thrust to transcurrent shear zones with tectonic
includes metagabbro sills, amphibolites and scarce transport from SSW to NNE. The main extensional
metabasalts. SHRIMP U-Pb analyses of detrital structures are oriented in the N45°E direction, while
zircons grains from the Nova Brasilândia pelitic the structures with a direction of N45°W correspond
rocks yield ages that range from ca. 2090 ± 17 Ma to to a frontal thrust. The synkinematic emplacement of
1122 ± 12 Ma. The main population of zircon has an the Rio Branco granite and anatectic leucogranite
age of 1215 Ma, interpreted by Rizzotto (1999) as the (1110 ± 8 Ma) was contemporaneous with this
maximum deposition age for the sequence. Sample compressional stage. These are the first Grenvillian-
reprocessing shows a younger group of zircon grains age orogenic rocks within the Nova Brasilândia belt
with an age of 1122 ± 12 Ma, interpreted as the age described by Rizzotto (1999) within the southwestern
of the basin opening (see section 4). margin of the Amazonian craton.
The other unit consists of mafic sills that The geological history of the Nova Brasilândia
outcrop in a narrow and elongated WNW-ESSE belt belt is supplemented with the emplacement of the
An Acad Bras Cienc (2014) 86 (1)
TECTONIC EVOLUTION OF THE SOUTHEASTERN AMAZONIAN CRATON 63
Figure 3 - Magnetometric data (analytical signal amplitude map) of southwestern margin of the Amazonian craton. The
highlighted (white line) magnetic lineaments indicate the rift zones and areas with mafic rocks outcrops of the Nova
Brasilândia proto-oceanic rift phase.
Rio Pardo granite, which comprises a kilometer- Thus, the Nova Brasilândia Group originated
sized syn- to late transcurrent body with an age of in an intracontinental rift setting followed by the
the 1005 ± 41 Ma, injected during the development opening of a proto-ocean and was further subjected
of transcurrent shear zones. to crustal shortening, deformation and associated
An Acad Bras Cienc (2014) 86 (1)
64 GILMAR J. RIZZOTTO, LÉO A. HARTMANN, JOÃO O.S. SANTOS and NEAL J. MCNAUGHTON
magmatism. This is suggested by detailed structural AGUAPEÍ BELT
studies and metamorphic petrology coupled with
The Aguapeí belt consists of a narrow metasedimen-
chemical and petrogenetic constraints determined
tary belt that borders the eastern margin of the
from the sedimentary and igneous rocks (Rizzotto
Paraguá Block in the southwestern Mato Grosso
1999). This scenario is supported by recent
state. It comprises a folded metasedimentary unit
paleomagnetic results reported by D’Agrella-Filho
(Aguapeí Group) deposited in an NNW-SSE trending
et al. (2012) where the 1420 Ma Indiavaí and Nova
aulacogen (Saes 1999) or in a rift environment (Saes
Guarita poles suggest that the Nova Brasilândia
et al. 1992). Souza and Hildred (1980) characterized
belt does not represent a suture zone, but rather an
the Aguapeí Group as a thick sequence of siliciclastic
intracratonic structure.
rocks composed, from bottom to top, of the Fortuna,
In contrast, Tohver et al. (2004, 2005a) inter-
Vale da Promissão and Morro Cristalina formations.
preted the Nova Brasilândia belt as the result of the
The Fortuna Formation shows thick packages of
crustal thickening through imbrication caused by
oligomictic conglomerates and quartz sandstones
the transpressive suturing of the Amazonian and with interbedded metasiltite/metapelite deposited in
Paraguá cratons at ca. 1.09 Ga. They used detailed a shallow sea environment. The Vale da Promissão
structural data, supported by thermobarometric Formation displays a transitional interfingered contact
calculations and geochronological information. with the unit underlying and comprises a sequence
Metamorphic ages (U-Pb in titanite and monazite; dominated by metapsamites, metapelites and rare
40Ar/39Ar in amphibole and biotite) range from metarenites deposited in a deep marine environment.
1.09-1.06 Ga to 0.97- 0.91 Ga throughout the belt The Morro Cristalina Formation is composed of
(e.g., Tohver et al. 2004, 2006). This age range fluvial sandstones and siltstones. The depositional
reveals a long history of metamorphism, tectonic age of the Aguapeí Group is estimated between 1167
exhumation and cooling. ± 27 Ma and 1149 ± 7 Ma (Santos et al. 2005).
According to Rizzotto (2001), the distensional The Aguapeí Group overlies the Rio Alegre
phase and final orogenic collapse (1005-970 Ma) of plutono-volcanic rocks, occurring discontinuously
the Nova Brasilândia belt was accompanied by late along more than 500 km between the northeastern/
to post-tectonic granites dated at 1.05 Ga (εNd(T) eastern margins of the Paraguá Block. The
= +0.5), with the generation of the foreland basins depocenter of the Aguapeí Group coincides with
(Palmeiral Formation) in a cratonic stabilization the paleo-suture previously established in the Rio
phase accompanied by bimodal intraplate Alegre terrane (Guaporé suture zone-Rizzotto
magmatism. This stage was accompanied by and Hartmann 2012). The pronounced magnetic
lateral movements of crustal blocks that generated low that characterizes the Aguapeí Group may
large strike-slip zones (Rio Branco shear zone). be partially related to a failed rift that is filled
This is in agreement with the 40Ar/39Ar and K-Ar with Mesoproterozoic sediments (Fig. 3). We
analyses on amphibole from amphibolite (Tohver suggest that the tectonic evolution of the Aguapeí
et al. 2004) and muscovite from granitic mylonite Group was initiated by rifting that subsequently
(Tassinari 1993), respectively, along this strike-slip failed, allowing the development of a long-lived
zone. These analyses yielded a minimum age of intracratonic sedimentary basin.
metamorphism and deformation (D ) at 970-966 The Aguapeí Group exhibits pervasive
2
Ma and 965 ±23 Ma. Similar ages were obtained by deformation and low-grade metamorphism (e.g.,
Fernandes et al. (2006) in sericite of the metarenites Geraldes et al. 2001, Ruiz 2005) within a regional
and metaconglomerates of the Aguapeí Group. synform structure indicated by gentle folds
An Acad Bras Cienc (2014) 86 (1)
TECTONIC EVOLUTION OF THE SOUTHEASTERN AMAZONIAN CRATON 65
(upright to northeast-dipping) and NW-trending Sunsás granites (Litherland et al. 1986).
shears. Sinistral shearing and thrusts deformed The Sunsás belt is structurally marked by
this sequence. Recrystallized mylonites yielded upright NW-trending folds formed contempora-
K/Ar and 40Ar/39Ar mica ages between 960 and neously with extensive mylonitic shear zones
910 Ma (e.g., Geraldes et al. 1997, Fernandes that developed during low-to-medium grade
et al. 2006, Ruiz et al. 2007). Because of the metamorphism, active at ca. 1080-1050 Ma (Boger
occurrence side by side of folded and sheared rocks et al. 2005).
(Sunsás and Aguapeí Groups) and flat-lying and
MATERIALS AND METHODS
unmetamorphosed rocks (Huanchaca and Ricardo
Franco ridges), Santos et al. (2002) prefer the usage The samples were crushed, milled and split into
of Aguapeí I for the folded and metamorphosed fractions for whole rock isotope geochemistry,
unit and Aguapeí II for the flat-lying cover. zircon, rutile and titanite dating. Zircon, titanite and
rutile were separated from twelve samples using
SUNSÁS BELT heavy liquid and magnetic separation techniques.
The sedimentary units of the Sunsás Group All rocks were investigated in thin section, and
were deposited between 1300 and 1000 Ma and their chemical compositions were determined.
unconformably overlie the metamorphic rocks Nine rock samples were analyzed by SHRIMP at
of the basement. They were deformed together the Curtin University of Technology-Australia,
with the basement during the subsequent Sunsás and three samples were analyzed by LA-MC-ICP-
Orogeny at ca. 1000 Ma (Litherland et al. 1986). MS. All grains used for zircon, titanite and rutile
The Sunsás Group comprises a sedimentary dating were imaged with backscattered electrons
sequence up to 6 km thick, found mainly in the to determine their internal structure and the nature
Serrania Sunsás, Bolivia (Litherland and Bloomfield (igneous or metamorphic) of their growth zones.
1981). In general, this group comprises a basal The sampling sites are shown in Figure 1, and
conglomerate overlain by psammitic units (arkoses, general data for each sample (coordinates, rock
sandstones, quartzites), pelitic units (mudstones name, and stratigraphic unit) are described in the
and siltstones), and a combination of feldspathic next item.
sandstones, pelites and quartzites (Berrrangé and The analytical procedures are described in
Litherland 1982). Sedimentary structures, such the online edition (Appendix SA) (Supplementary
as cross-beds and ripple marks, are common in material), and the isotopic data are presented in Table I.
the psammitic units, mainly in quartzites of the
RESULTS
Serrania Los Tajibos. Berrrangé and Litherland
(1982) suggested that the sedimentary rocks of NOVA BRASILÂNDIA BELT
the Sunsás Group were deposited in an alluvial The samples were collected along an EW belt
to deltaic environment. Later, Saes et al. (1992) that contains primarily mafic rocks belonging
referred to the sediments as deposited in an intra- to proto-ocean, the Nova Brasilândia rift. These
continental rift. lithotectonic units were deformed by NW-SE (D )
1
The upper psammitic units (Vibosi Group) of the to E-W (D ) transpressional phases, corresponding
2
Sunsás Group were intruded by the layered Rincón to a progressive thrust to transcurrent shear
del Tigre igneous complex (Prendergast 2000), which zones. Generally, all localities preserve evidence
yielded a Rb–Sr age of ca. 990 Ma. This age is similar of granulite facies metamorphism associated
to that obtained for the late syn- to post-tectonic with ductile deformation; orthopyroxene often
An Acad Bras Cienc (2014) 86 (1)
66 GILMAR J. RIZZOTTO, LÉO A. HARTMANN, JOÃO O.S. SANTOS and NEAL J. MCNAUGHTON
Disc. % 4 7 3 1 0 2 -1 1 -1 0 -5 -6 -2 -4 -2 -6 -3 -1 5 3 4 7 10 6 4
18 10 6 12 9 13 15 15 14 16 17 31 11 34 18 9 10 9 84 8 8 36 44 14 37
b b ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
P P
s206 207 1081 1120 1014 1113 1113 994 972 991 983 995 1031 1087 1078 1089 1097 1086 1093 1094 1229 1101 1108 1134 1128 1121 1101
e
g
A 4 3 3 5 5 0 0 0 0 1 5 8 5 7 5 5 4 4 6 3 3 7 0 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
d. Pb U ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
studie 206 238 1035 1042 984 1099 1111 976 982 981 995 994 1084 1148 1098 1131 1124 1151 1124 1107 1170 1069 1069 1054 1010 1053 1058
mples 1.76 1.48 2.81 1.60 1.58 1.23 2.42 1.21 1.25 1.29 2.55 2.79 2.42 2.51 1.82 1.48 1.67 1.99 8.28 2.83 2.60 3.45 1.82 1.27 1.86
or the sa 208Pb232Th 0.0523± 0.0512± 0.0504± 0.0551± 0.0551± 0.2735± 0.1574± 0.1723± 0.1672± 0.1663± 0.0537± 0.0590± 0.0581± 0.0575± 0.0590± 0.0575± 0.0565± 0.0569± 0.0603± 0.0514± 0.0531± 0.0553± 0.1179± 0.1152± 0.0754±
esults f err. corr. 0.851 0.942 0.978 0.920 0.954 0.860 0.834 0.830 0.852 0.823 0.864 0.740 0.940 0.699 0.859 0.956 0.939 0.949 0.335 0.959 0.963 0.700 0.449 0.835 0.513
r
al 3 0 7 7 9 0 9 9 1 6 7 3 4 8 0 9 9 8 3 6 6 6 1 0 0
c 4 4 4 4 4 1 0 0 1 1 4 7 4 6 5 3 3 3 5 3 3 7 1 1 1
alyti Pb U ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1. ±1.
E IU-Pb an Ratios206 238 0.1741 0.1754 0.1648 0.1859 0.1881 0.1636 0.1646 0.1644 0.1670 0.1667 0.1832 0.1950 0.1857 0.1918 0.1905 0.1955 0.1904 0.1873 0.1989 0.1804 0.1804 0.1776 0.1696 0.1775 0.1784
Le 8 8 0 9 6 8 0 2 0 1 0 4 4 0 4 5 8 6 7 2 2 2 8 1 5
ABnit 1.6 1.4 1.5 1.5 1.5 1.2 1.3 1.3 1.3 1.4 1.7 2.3 1.5 2.4 1.7 1.4 1.4 1.4 4.5 1.4 1.4 2.5 2.4 1.3 2.1
Tnd tita 207Pb235U 18± 13± 93± 54± 86± 02± 31± 58± 53± 19± 97± 54± 95± 36± 73± 91± 36± 21± 08± 54± 32± 78± 64± 37± 48±
a 1 6 5 6 8 3 2 3 5 6 5 3 2 0 9 3 9 6 3 9 0 9 0 8 7
n 8 8 6 9 9 6 6 6 6 6 8 0 9 0 9 0 9 9 2 8 9 8 8 8 8
o 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 2. 1. 2. 1. 2. 1. 1. 2. 1. 1. 1. 1. 1. 1.
c
r
MS zi 0.88 0.50 0.32 0.63 0.47 0.65 0.72 0.74 0.68 0.80 0.86 1.57 0.52 1.72 0.89 0.43 0.51 0.46 4.30 0.40 0.38 1.80 2.22 0.72 1.84
P- Pb Pb ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
C-IC 207 206 7548 7696 7301 7667 7669 7229 7152 7218 7191 7231 7363 7571 7537 7575 7606 7565 7593 7596 8133 7621 7650 7748 7726 7698 7621
M 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
A-
nd L 2064f % 0.10 -0.01 0.00 0.00 0.00 0.02 0.08 -0.05 0.13 0.00 0.01 0.07 -0.01 0.00 0.06 0.02 0.01 0.01 0.68 0.03 0.00 0.26 0.18 -0.01 0.92
a
RIMP 206Pb ppm 105.7 159.1 319.2 322.8 125.8 35.7 37.9 33.1 26.8 30.8 80.2 18.8 191.5 23.3 65.1 245.4 169.5 198.2 37.3 199.0 199.0 11.6 33.0 27.5 32.4
SH Th U 0.37 0.80 0.23 1.16 0.57 0.99 1.01 0.99 1.00 1.02 0.13 0.37 0.03 0.37 0.53 0.53 0.52 0.56 0.37 0.08 0.08 0.31 0.93 0.92 0.96
Th ppm 254 821 501 2266 426 244 260 225 180 211 66 40 40 50 202 753 521 668 77 100 100 23 203 160 195
U spotppm GRT1, zircon h.1-1706 h.1-21056 h.1-32254 h.2-12021 h.3-1779 GRT2, titanite a.1-1254 a.1-2268 a.2-1234 a.3-1186 a.3-2215 GRT2, zircon a.5-1509 a.5-2112 a.9-11201 a.10-1141 a.10-2398 a.11-11461 a.11-21036 a.13-11231 GRT3, zircon g.1-1217 g.2-1a1284 g.2-1b1284 g.2-276 GRT3, titanite d.4-1226 d.4-2180 d.4-3210
An Acad Bras Cienc (2014) 86 (1)