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Precambrian Research xxx (2006) xxx–xxx
Timing of crust formation, deposition of supracrustal sequences,and Transamazonian and Brasiliano metamorphism in the East
Pernambuco belt (Borborema Province, NE Brazil):Implications for western Gondwana assembly
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Sergio P. Neves a,∗, Olivier Bruguier b, Alain Vauchez c, Delphine Bosch c,Jose Maurıcio Rangel da Silva a, Gorki Mariano a
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a Departamento de Geologia, Universidade Federal de Pernambuco, 50740-530 Recife, Brazil9b ISTEEM, Service ICP-MS, Universite de Montpellier II, 34095 Montpellier, France10
c Laboratoire de Tectonophysique, Universite de Montpellier II, 34095 Montpellier, France11
Received 21 July 2005; received in revised form 10 January 2006; accepted 21 June 2006
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Abstract13
The main structural feature of the central domain of Borborema Province (NE Brazil) is a network of dextral and sinistralshear zones. These shear zones rework an older, regionally developed, flat-lying foliation in orthogneisses and supracrustal belts,which in the East Pernambuco belt was formed under amphibolite facies conditions. This study reports LA-ICP-MS U–Pb zirconages of metaigneous and metasedimentary rocks aiming to constraint the pre-transcurrent tectonothermal evolution in the EasternPernambuco domain. Ages of 2125 ± 7 and 2044 ± 5 Ma in a mafic layer of banded orthogneiss are interpreted as the age of theprotolith of the orthogneiss and of high-grade Transamazonian metamorphism, respectively. The latter age is consistent with theoccurrence of low Th/U, metamorphic zircon xenocrysts, dated at 2041 ± 15 Ma, in the leucosome of a migmatitic paragneiss. Agranitic orthogneiss dated at 1991 ± 5 Ma reflects late to post-Transamazonian magmatic event. A similar age (1972 ± 8 Ma) wasfound in rounded zircon grains from a leucocratic layer of banded orthogneiss. Ages of detrital zircons in a paragneiss sample indicatederivation from sources with ages varying from the Archean to Neoproterozoic, with peak ages at ca. 2220, 2060–1940, 1200–1150and 870–760 Ma. Detrital zircons constrain the deposition of the supracrustal sequence to be younger than 665 Ma. Magmaticzircons with the age of 626 ± 15 Ma are found in the leucosome of a migmatitic paragneiss and constrain the age of the Brasilianohigh-temperature metamorphism. A lower intercept age of 619 ± 36 Ma from a deformed granodiorite dated at 2097 ± 5 Ma and thecrystallization age of 625 ± 24 Ma of the felsic layer of banded orthogneiss also confirm the late Neoproterozoic metamorphism.These results show that the present fabric in basement and supracrustal rocks was produced during the Brasiliano orogeny.
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Paleoproterozoic ages reported in this study are similar to those found in other sectors of the Borborema Province, the Cameroonand Nigeria provinces, and the Sao Francisco/Congo craton. They show the importance of the Transamazonian/Eburnean eventand suggest that these tectonic units may have been part of a larger, single continental landmass. Likewise, similarities in post-Transamazonian metamorphic and magmatic events in the Borborema, Nigeria and Cameroon provinces suggest that they shared acommon evolution and remained in close proximity until the opening of the Atlantic Ocean.
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© 2006 Elsevier B.V. All rights reserved.34
Keywords: Laser ablation ICP-MS; Zircon U–Pb geochronology; Neoproterozoic belts; Transamazonian orogeny; Brasiliano orogeny35
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∗ Corresponding author. Tel.: +55 81 2126 8240; fax: +55 81 2126 8236.E-mail address: [email protected] (S.P. Neves).
1 0301-9268/$ – see front matter © 2006 Elsevier B.V. All rights reserved.2 doi:10.1016/j.precamres.2006.06.005
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1. Introduction37
There is broad consensus that most of western38
Gondwana was already formed by 600 Ma. Continen-39
tal reconstructions for this period (e.g., Caby et al.,40
1991; Castaing et al., 1994; Trompette, 1997) show that41
the Brasiliano/Pan-African Borborema, Cameroon and42
Nigeria provinces occupied a central position in relation43
to the Amazonian and West Africa cratons, to the west,44
the Sao Francisco/Congo craton, to the south, and the45
Saharan metacraton (Abdelsalam et al., 2002), to the east46
(Fig. 1). In the lack of paleomagnetic data, understand-47
ing how and when this configuration was reached rely on48
geological and geochronological grounds. Knowledge49
of the tectonothermal history of the late Neoproterozoic50
belts is thus essential to evaluate possible correlations51
between adjacent (within individual provinces) and dis-52
tant (transcontinental) units and, therefore, to provide53
insights into the dynamics of amalgamation of western54
Gondwana.55
The Precambrian crustal evolution of the Borborema56
Province has been much debated in recent years. Resolv-57
ing some critical pending issues is necessary to elabo-58
rate continental reconstructions for the Neoproterozoic.59
In the central domain, comprised between the Patos60
and Pernambuco shear zone systems (Fig. 1), the most61
controversial issues are (1) the existence of a contrac-62
tional event in the early Neoproterozoic (Cariris Vel-63
hos orogeny, ∼1 Ga; Brito Neves et al., 1995), and64
(2) whether or not terranes accretion took place dur-65
ing this proposed orogeny. The suggestion of an early66
Neoproterozoic orogeny resulted from the discovery of67
1000–900 Ma-old intermediate to felsic metavolcanic68
rocks and orthogneisses in the Alto Pajeu belt (Fig. 1;69
Brito Neves et al., 1995; Van Schmus et al., 1995;70
Kozuch et al., 1997; Brito Neves et al., 2000, 2001a;71
Kozuch, 2003). Peraluminous orthogneisses intercalated72
in the supracrustal sequence were interpreted as syncol-73
lisional granites. Santos and Medeiros (1999) proposed74
that the Alto Pajeu belt is one of four tectonostrati-75
graphic terranes that amalgamated during the Cariris76
Velhos and Brasiliano orogenies to constitute the cen-77
tral domain. Several authors (Mariano et al., 2001;78
Guimaraes and Brito Neves, 2004; Neves, 2003 and ref-79
erences therein) have, however, questioned the existence80
of the Cariris Velhos orogeny and the terrane accre-81
tion model, suggesting, instead, continuity between the82
proposed terranes since the Paleoproterozoic Transama-83
zonian orogeny. Therefore, in this paper, the following84
non-genetic terms will be used to describe supracrustal85
successions and orthogneisses occurring from west to86
east in the central domain: Cachoeirinha belt, Alto87
Pajeu belt, Alto Moxoto belt and East Pernambuco belt 88
(Fig. 1). 89
To improve knowledge and address the controversial 90
points above, zircon grains from samples from the East 91
Pernambuco belt were dated by laser ablation inductively 92
coupled plasma-mass spectrometry (LA-ICP-MS). The 93
aim of this study is threefold: (1) constrain the timing 94
of magmatic and metamorphic events and of deposition 95
of supracrustal sequences, (2) compare its geological 96
evolution with other regions in northeastern Brazil and 97
with the Pan-African belts of Nigeria and Cameroon, and 98
(3) assess how these domains and surrounding cratons 99
pulled together to make up western Gondwana. 100
2. Geological setting 101
2.1. Regional geology 102
The Borborema Province is characterized by a com- 103
plex network of large transcurrent shear zones (Vauchez 104
et al., 1995; Fig. 1). In the central domain, a linked system 105
of E–W- to ENE–WSW-striking dextral and NNE–SSW- 106
to NE–SW-striking sinistral shear zones is spatially 107
associated with abundant granitic and syenitic plutons 108
(Fig. 1B; Vauchez and Egydio-Silva, 1992; Guimaraes 109
and Da Silva Filho, 1998; Ferreira et al., 1998; Neves and 110
Mariano, 1999; Neves et al., 2000; Silva and Mariano, 111
2000). A former shallow-dipping regional foliation is 112
preserved in orthogneisses and metasediments outcrop- 113
ping between the strike and slip-related steeply dipping 114
to vertical mylonitic zones. The metamorphic grade 115
under which this foliation was developed differs between 116
the Cachoeirinha belt and the Alto Pajeu, Alto Moxoto 117
and East Pernambuco belts. The Cachoeirinha belt con- 118
sists of greenschist facies metapelites, metagreywackes 119
and bimodal metavolcanics (Bittar and Campos Neto, 120
2000; Kozuch, 2003; Medeiros, 2004) deformed at rel- 121
atively high pressures (6–9 kbar; Sial, 1993; Caby and 122
Sial, 1997). Its low metamorphic grade stands in contrast 123
with that of the other three belts, which were regionally 124
heated above 500 ◦C under low- to medium-pressures 125
metamorphic conditions (Vauchez and Egydio-Silva, 126
1992; Bittar and Campos Neto, 2000; Leite et al., 2000a; 127
Neves et al., 2000). 128
Orthogneiss complexes underlie large areas of the 129
Alto Pajeu, Alto Moxoto and East Pernambuco belts. 130
They yielded U–Pb and Pb–Pb evaporation ages mostly 131
varying from 2.2 to 2.0 Ga (Santos, 1995; Van Schmus 132
et al., 1995; Leite et al., 2000b; Brito Neves et al., 133
2001b; Melo et al., 2002; Kozuch, 2003; Neves et al., 134
2004; Santos et al., 2004a), and Sm–Nd data indicate the 135
existence of Archean protoliths for some of these Pale- 136
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Fig. 1. (A) South America–Africa fit showing cratons and Neoproterozoic provinces of western Gondwana, and sketch highlighting main shear zonesin Borborema Province. (B) Schematic geological map of eastern Borborema Province showing location of the studied area in the East Pernambucobelt (EPB) of central domain. Dashed lines highlight boundaries between the central and northern domains, and between the Cachoeirinha (CB),Alto Pajeu (APB) and Alto Moxoto (AMB) belts. PaSZ, Patos Shear Zone system; EPSZ, East Pernambuco Shear Zone system; WPSZ, WestPernambuco Shear Zone system.
oproterozoic orthogneisses (Van Schmus et al., 1995;137
Brito Neves et al., 2001b; Melo et al., 2002). Domi-138
nance of Paleoproterozoic to Archean Sm–Nd model139
ages in granitic and syenitic plutons (Ferreira et al., 1998;140
Mariano et al., 2001; Guimaraes et al., 2004) suggests141
that Paleoproterozoic to Archean basement constitute142
most of the central domain.143
In the Alto Pajeu belt, metavolcanic rocks have U–Pb144
zircon ages mainly comprised between 1000 and 970 Ma145
(Brito Neves et al., 1995; Van Schmus et al., 1995;146
Kozuch et al., 1997; Brito Neves et al., 2000; Kozuch,147
2003). Van Schmus et al. (1995) and Kozuch et al. (1997)148
report U–Pb ages for metavolcanic rocks in the Cachoeir-149
inha belt in the interval 810–720 Ma. Refinement of150
these data due to the presence of inherited zircons and151
new age determinations indicate a younger depositional 152
age (660–620 Ma; Kozuch, 2003; Medeiros, 2004). In 153
these two belts, Sm–Nd ages range from 1.8 to 1.2 Ga 154
(Brito Neves et al., 2001a; Kozuch, 2003; Archanjo and 155
Fetter, 2004). The oldest Nd model ages suggest that 156
Paleoproterozoic or older sources provided important 157
contribution for detritus that filled their precursor sed- 158
imentary basins. In the Cachoeirinha belt, this inference 159
is further supported by the occurrence of zircons with 160
ages up to 3278 Ma in a quartzite sample (Silva et al., 161
1997) and of Paleoproterozoic zircons in a metarhyolite 162
(Kozuch, 2003). The Sertania metasedimentary complex 163
in the Alto Moxoto belt yielded zircon grains with ages 164
around 2.0 Ga (Santos et al., 2004a) and Sm–Nd ages 165
varying from 2.0 to 3.0 Ga (Brito Neves et al., 2001b). 166
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These data indicate its provenance mainly from Pale-167
oproterozoic and Archean sources, but only places an168
upper bound on the age of deposition. The age of depo-169
sition of supracrustal sequences in the East Pernambuco170
belt is still unknown.171
2.2. Study area172
The study area is located in the northwestern part173
of the East Pernambuco belt (Fig. 1). It comprises174
banded orthogneisses, granitic augen gneisses, metased-175
imentary rocks and igneous intrusions (Fig. 2). Banded176
orthogneisses are characterized by alternating bands of177
dioritic and granitic compositions. Zircon U–Pb dat-178
ing from a monzodioritic orthogneiss and a granitic179
augen gneiss (Taquaritinga orthogneiss) in the southern180
part of the study area yielded ages of 1974 ± 32 and181
1521 ± 6 Ma, respectively (Sa et al., 2002).182
In the geological map of the state of Pernambuco183
(Gomes, 2001), Surubim and Vertentes complexes are184
recognized as two distinct supracrustal units, mainly185
based on the occurrence of metavolcanic rocks in the186
latter. Metavolcanic rocks were not identified by us in187
the study area nor in other localities of the East Pernam-188
buco belt. Metasedimentary rocks are indistinguishable189
in terms of rock association, structure or metamorphic190
grade between the Surubim and Vertentes complexes.191
Furthermore, our mapping shows that basement gneisses192
were misinterpreted as belonging to the Vertentes com-193
plex. Therefore, this complex is not considered here194
as a valid tectonostratigraphic unit. In consequence,195
metasedimentary rocks in the study are attributed to the196
Surubim complex. The main lithotypes are biotite gneiss,197
biotite schist, quartz-feldspar paragneiss, quartzite and198
marble, locally with small lenses of para-amphibolite199
and calc-silicate rock. Sillimanite and garnet are ubiqui-200
tous accessory phases, which together with local migma-201
tization attest high-temperature metamorphism.202
From the structural point of view, the study area203
is characterized by flat-lying gneissic foliation in204
orthogneisses and supracrustal rocks. This early fabric is205
deformed by recumbent to upright folds and transcurrent206
shear zones (Neves et al., 2005). Stretching lineations207
associated with the flat-lying foliation have ESE–WNW208
trend in supracrustal rocks and NE–SW trend in209
banded orthogneiss and Taquaritinga orthogneiss. In the210
metasedimentary sequence, numerous kinematic indica-211
tors showing a top-to-the-west/northwest sense of shear212
denote a well-developed non-coaxial deformation. These213
oblique lineations were interpreted (Neves et al., 2005) as214
the result of extension oblique to the transport direction215
in the deeper orthogneisses during progressive defor-216
mation. A deformed epidote-bearing biotite granodior- 217
ite (Alcantil pluton; Fig. 2) displays a flat-lying mag- 218
matic/gneissic foliation crosscut by subvertical shear 219
bands. This pluton was previously regarded as a Neo- 220
proterozoic intrusion emplaced during the top-to-the- 221
northwest tectonics (Neves et al., 2005). However, data 222
acquired in the present study favor its intrusion during 223
the Paleoproterozoic, followed by solid-state deforma- 224
tion during the Brasiliano orogeny (see below). Two 225
plutons partially outcrop in the southern part of the study 226
area (Fig. 2). The ca. 585 Ma-old, syenitic Toritama plu- 227
ton (Guimaraes and Da Silva Filho, 1998) is interpreted 228
as early kinematic with respect to strike-slip shearing 229
(Neves et al., 2000). The Santa Cruz do Capibaribe plu- 230
ton is a composite intrusion containing gabbronorites 231
and diorites in the core and monzonites at the margins, 232
displaying only local solid-state deformation. 233
3. Studied samples 234
Samples for this study represent the main lithological 235
units and key relations between age and deformation in 236
the study area. Six samples weighting 8–12 kg each were 237
collected from four localities (Fig. 2B). Samples SCC1A 238
and SCC1B are, respectively, mafic and felsic layers 239
of banded orthogneiss. SCC1A is a medium-grained, 240
dark-colored biotite amphibole gneiss with quartz mon- 241
zodioritic composition. SCC1B is a medium-grained, 242
leucocratic granitic gneiss containing less than 10% 243
biotite. The gneissic banding dips 36◦ towards N104◦E 244
and a strong stretching lineation plunging gently to 245
northeast (21◦, N47◦E) is present in both lithologies. 246
Sample SCC9 is a medium to coarse-grained sillimanite 247
biotite paragneiss containing garnet porphyroblasts up 248
to 1 cm in diameter. Sample SCC12 is the leucosome of 249
a migmatitic paragneiss, and SCC2 is a granitic gneiss. 250
Since contact relationships are not exposed, it is not pos- 251
sible to determine whether the granitic gneiss is a sheet 252
intercalated in metasedimentary sequence or whether it 253
underlies it. Quartz ribbons in sample SCC2 attest strong 254
solid-state deformation and define a lineation plunging 255
7◦, N150◦E. Sample SSC5 is from the Alcantil pluton, 256
showing foliation dipping 36◦ towards N24◦E. 257
4. Analytical techniques 258
Zircons were separated using conventional tech- 259
niques. After crushing and sieving of the powdered sam- 260
ples, heavy minerals were concentrated by panning and 261
then by heavy liquids. The heavy mineral concentrates 262
were subsequently processed by magnetic separation 263
using a Frantz separator. Zircon grains were hand picked 264
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Fig. 2. (A) Simplified geological map of the East Pernambuco belt showing location of studied area. Modified from Neves and Mariano (1999),Neves et al. (2000) and Gomes (2001). (B) Geological map of the studied area (modified from Neves et al., 2005) showing location of samplesanalysed by LA-ICP-MS, and existing TIMS U–Pb ages (Sa et al., 2002).
from the non-magnetic fraction at 1.5 A intensity and 1◦265
or 2◦ side tilt (Samples SCC1A and SCC9), 2◦ side tilt266
(samples SCC1B and SCC2), and 4◦ side tilt (samples267
SCC5 and SCC12). The grains were then mounted on268
adhesive tape, enclosed in epoxy resin with chips of a269
standard material (G91500; Wiedenbeck et al., 1995) 270
and polished to about half of their thickness. Internal 271
structure and morphology were subsequently observed 272
by Scanning Electron Microscopy (SEM) using a JEOL 273
1200 EX II operating at 120 kV. After BSE imaging, car- 274
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bon coating was removed by using alcohol and the resin275
grain mount was subsequently slightly repolished to get276
rid of any residual carbon which can potentially con-277
tain significant amount of 204Pb (see Hirata and Nesbitt,278
1995). The mount was then cleaned in ultra-pure MQ279
water and dried before its introduction in the ablation280
cell.281
Data were acquired at the University of Montpel-282
lier II using a 1991 vintage VG Plasmaquad II turbo283
ICP-MS coupled with a Geolas (Microlas) automated284
platform housing a 193 nm Compex 102 laser from285
LambdaPhysik. Analyses were conducted using an in-286
house modified ablation cell of ca. 5 cm3 which resulted287
in a shorter washout time and an improved sensitivity288
compared to the initial larger ablation cell (ca. 30 cm3).289
Ablation experiments were conducted in a He atmo-290
sphere to enhance sensitivity and reduce inter-element291
fractionation (Gunther and Heinrich, 1999). Data were292
acquired in the peak jumping mode in a series of five293
repeats of 10 s each, measuring the 202Hg, 204(Pb + Hg),294
206Pb, 207Pb, 208Pb and 238U isotopes similarly to the295
procedure described in Bruguier et al. (2001). Signal was296
acquired after a 10 s period of pre-ablation to allow for297
crater stabilization and to remove surface contamination298
as well as fall-out from previous analyses. The laser was299
fired using an energy density of 20 J cm−2 at a frequency300
of 3 or 4 Hz. The laser spot size was of 52 and 26 �m in301
samples SCC1A, SCC1B and SCC9, and 26 �m in sam-302
ples SCC2, SCC5 and SCC12. Some additional analyses303
using a spot size of 15 �m were further made in the rims304
of zircon grains from sample SCC1A.305
The Pb/Pb and U/Pb isotopic ratios of unknowns were306
calibrated against the G91500 zircon crystal as an exter-307
nal ablation standard, which was measured four times308
each five unknowns using the bracketing technique. Data309
were reduced using a calculation spreadsheet, which310
allows correction for instrumental mass bias and inter-311
element fractionation. Accurate common lead correction312
in zircon is difficult to achieve, mainly because of the313
isobaric interference of 204Hg on 204Pb. The contribu-314
tion of 204Hg on 204Pb was estimated by measuring the315
202Hg and assuming a 204Hg/202Hg natural isotopic com-316
position of 0.2298. This allows to monitor the common317
lead content of the analysed grain, but corrections often318
result in spurious ages. Analyses yielding 204Pb close319
to, or above the limit of detection were then rejected.320
Table 1 thus presents only analyses for which 204Pb was321
below detection limit. For instrumental mass bias, all322
measured standards were averaged to give a mean mass323
bias factor and its associated error. This mass bias fac-324
tor and associated error were then propagated with the325
measured analytical errors of each individual sample.326
Inter-element fractionation for Pb and U are much more 327
sensitive to analytical conditions and a bias factor was 328
thus calculated using the four standard measurements 329
bracketing each five unknowns. These four measure- 330
ments were then averaged to calculate a U–Pb bias factor 331
and its associated error, which were added in quadra- 332
ture to the individual error measured on each 206Pb/238U 333
unknown. This typically resulted in a 2–5% precision 334
(1σ R.S.D.%) after all corrections have been made (see 335
Table 1). Ages quoted below were calculated using the 336
Isoplot program of Ludwig (2000). 337
5. Zircon morphology and internal structure 338
Zircon grains from the mafic and felsic layers of 339
banded orthogneiss have distinct morphologies and 340
internal structures. In sample SCC1A (mafic band), most 341
grains are elongated (aspect ratios varying from 2:1 to 342
4:1), ranging from 150 to 400 �m in length. In spite 343
of rounded terminations, the original euhedral to sub- 344
hedral shape can still be recognized in many grains. 345
Oscillatory zoning, typical of magmatic growth, is com- 346
mon (Fig. 3A) although it is faint and partially obliter- 347
ated in many grains, suggesting local redistribution of 348
elements during metamorphism. Dissolution and repre- 349
cipitation in some grains is indicated by embayments 350
cutting the concentric zoning (Fig. 3B). Overgrowth 351
rims, where present, are usually thin (<20 �m), and 352
some grains exhibit structureless domains (Fig. 3B). All 353
these features are interpreted as representing a mag- 354
matic zircon population affected by a metamorphic 355
event. Inherited cores were not observed in the analyzed 356
grains. 357
In contrast with zircon grains from Sample SCC1A, 358
those from sample SCC1B have aspect ratio normally 359
between 1:1 and 2:1 and are shorter (less than 300 �m 360
long). Their main characteristic is the presence of over- 361
growths with thin oscillatory zoning, suggesting mag- 362
matic growth over preexisting crystals (Fig. 3C). Some 363
crystals have subhedral to euhedral shapes (Fig. 3D) and 364
oscillatory zoning typical of magmatic zircons. 365
In sample SCC2 (granitic orthogneiss) the dominant 366
zircon population consists of clear, subhedral to euhe- 367
dral grains with faint oscillatory or no apparent zoning, 368
sometimes with inherited cores (Fig. 3E and F). 369
The most common population of zircons in the 370
paragneiss sample SCC9 comprises rounded to slightly 371
elongated (aspect ratios up to 2.5:1) grains with clear 372
oscillatory zoning (Fig. 4A). Some grains also have 373
bright, high-U, overgrowth rims (Fig. 4A), preferentially 374
located at the terminations of the crystals and responsi- 375
ble for rounding of the original euhedral shape. A few 376
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Tabl
e1
LA
-IC
P-M
SU
–Th–
Pbre
sults
for
zirc
ons
from
rock
sof
the
Bor
bore
ma
Prov
ince
(Bra
zil)
Sam
ple
Pb*
(ppm
)U
(ppm
)T
h(p
pm)
Th/
U20
4Pb/
206P
b20
8Pb/
206P
b20
7Pb/
206P
b±1
σ20
7Pb/
235U
±1σ
206P
b/23
8U±1
σρ
App
aren
tA
ges
(Ma)
Dis
c(%
)
206P
b/23
8U±1
σ20
7Pb/
206P
b±1
σ
SCC
1A#1
156
580
110
0.19
3.66
E−0
6–
0.11
920.
0006
4.65
830.
2623
0.28
340.
0159
1.00
1608
7919
458
17.3
#2*
9927
485
0.31
4.87
E−0
6–
0.13
250.
0019
6.84
230.
2023
0.37
450.
0097
0.88
2050
4521
3225
3.8
#3*
9224
168
0.28
4.70
E−0
6–
0.12
680.
0005
6.51
620.
2461
0.37
280.
0140
0.99
2042
6520
547
0.5
#4*
103
274
118
0.43
4.41
E−0
6–
0.12
970.
0005
6.58
330.
1586
0.36
810.
0087
0.99
2020
4120
947
3.5
#5*
8321
561
0.29
6.63
E−0
6–
0.12
590.
0005
6.57
670.
1872
0.37
880.
0107
0.99
2071
5020
426
−1.4
#689
259
700.
276.
55E−0
6–
0.13
020.
0013
6.15
690.
1260
0.34
290.
0062
0.88
1901
3021
0117
9.5
#7*
131
419
158
0.38
4.19
E−0
6–
0.12
680.
0004
6.46
440.
1580
0.36
950.
0089
0.99
2027
4220
546
1.3
#8*
104
271
500.
194.
78E−0
6–
0.12
640.
0010
6.50
180.
2790
0.37
300.
0157
0.98
2044
7320
4914
0.2
#9*
3711
059
0.53
1.51
E−0
5–
0.12
610.
0005
6.14
780.
0812
0.35
360.
0044
0.95
1952
2120
448
4.5
#10
194
723
475
0.66
3.00
E−0
6–
0.12
310.
0009
4.70
080.
0492
0.27
700.
0022
0.75
1576
1120
0112
21.3
#11
216
672
472
0.70
2.67
E−0
6–
0.12
400.
0008
5.78
770.
0604
0.33
840.
0028
0.79
1879
1320
1511
6.8
#12
8728
160
0.21
6.57
E−0
6–
0.12
230.
0005
5.51
010.
2605
0.32
670.
0154
1.00
1822
7419
917
8.5
#13
287
795
639
0.80
2.16
E−0
6–
0.12
690.
0004
5.41
980.
1006
0.30
990.
0057
0.98
1740
2820
566
15.3
#14*
138
394
188
0.48
3.94
E−0
6–
0.13
100.
0007
6.81
170.
1324
0.37
710.
0070
0.96
2063
3321
1110
2.3
#15
137
477
131
0.27
4.37
E−0
6–
0.12
010.
0008
4.85
320.
0830
0.29
310.
0046
0.91
1657
2319
5712
15.3
#16*
8723
010
50.
466.
18E−0
6–
0.13
200.
0010
7.08
090.
0903
0.38
920.
0040
0.80
2119
1921
2413
0.2
#17*
7620
517
80.
879.
50E−0
6–
0.13
220.
0005
6.84
810.
1425
0.37
560.
0077
0.98
2056
3621
287
3.4
#18*
9631
111
40.
365.
23E−0
6–
0.12
530.
0004
6.15
400.
2326
0.35
620.
0134
1.00
1964
6320
336
3.4
#19
135
525
247
0.47
3.91
E−0
6–
0.13
190.
0010
4.63
410.
0921
0.25
490.
0047
0.93
1464
2421
2313
31.1
#20*
9728
110
30.
375.
51E−0
6–
0.12
660.
0006
6.25
220.
2710
0.35
820.
0154
0.99
1974
7320
518
3.8
#21
152
754
258
0.34
4.36
E−0
6–
0.12
520.
0004
3.49
460.
0773
0.20
250.
0044
0.99
1189
2420
316
41.5
#22*
155
352
201
0.57
4.70
E−0
60.
170
0.13
150.
0011
7.32
060.
2538
0.40
370.
0136
0.97
2186
6221
1815
−3.2
#23*
169
257
800.
315.
45E−0
60.
118
0.13
170.
0020
7.11
340.
2963
0.39
170.
0151
0.93
2130
7021
2127
−0.4
#24*
269
507
298
0.59
3.20
E−0
60.
178
0.13
150.
0010
6.77
040.
2441
0.37
350.
0132
0.98
2046
6121
1814
3.4
#25
236
587
330
0.56
3.89
E−0
60.
147
0.13
100.
0011
6.46
220.
1260
0.35
790.
0062
0.89
1972
3021
1115
6.6
#26
277
654
408
0.62
2.95
E−0
60.
185
0.13
190.
0011
6.46
480.
0618
0.35
540.
0014
0.42
1960
721
2415
7.7
#27
6113
676
0.56
6.14
E−0
60.
192
0.13
370.
0010
6.82
950.
1082
0.37
050.
0052
0.89
2032
2521
4713
5.4
#28*
106
258
710.
277.
25E−0
60.
076
0.12
570.
0009
6.42
350.
1082
0.37
070.
0057
0.91
2033
2720
3813
0.3
#29*
130
278
108
0.39
6.81
E−0
60.
122
0.13
170.
0014
7.17
600.
1942
0.39
520.
0098
0.91
2147
4521
2119
−1.2
#30*
258
631
281
0.45
3.28
E−0
60.
105
0.12
550.
0009
6.13
520.
2052
0.35
450.
0116
0.98
1956
5520
3613
3.9
#31*
288
489
231
0.47
3.44
E−0
60.
172
0.13
140.
0016
6.74
140.
1936
0.37
210.
0096
0.90
2039
4521
1722
3.7
#32*
147
323
118
0.37
5.18
E−0
60.
103
0.12
590.
0011
6.21
790.
1529
0.35
830.
0083
0.94
1974
3920
4115
3.3
#33
240
717
225
0.31
3.51
E−0
60.
107
0.12
590.
0012
5.98
110.
1053
0.34
460.
0051
0.84
1909
2420
4117
6.5
#34
6124
930
0.12
9.82
E−0
60.
053
0.12
210.
0014
4.56
980.
0872
0.27
150.
0041
0.80
1548
2119
8721
22.1
#35*
207
619
750.
123.
82E−0
60.
054
0.13
250.
0013
6.75
130.
1612
0.37
400.
0081
0.92
2048
3821
3217
3.9
#36*
144
436
209
0.48
5.34
E−0
60.
096
0.12
630.
0007
6.17
880.
1137
0.35
490.
0062
0.96
1958
3020
479
4.3
#37*
101
285
950.
336.
62E−0
60.
098
0.13
140.
0011
6.78
880.
0749
0.37
470.
0028
0.69
2052
1321
1714
3.1
#38
122
340
101
0.30
5.13
E−0
60.
080
0.12
550.
0013
5.79
870.
1460
0.33
520.
0077
0.92
1863
3720
3618
8.5
#39
189
540
142
0.26
3.19
E−0
60.
078
0.12
550.
0019
5.79
080.
1221
0.33
460.
0049
0.69
1861
2320
3627
8.6
#40
184
473
269
0.57
3.73
E−0
60.
158
0.13
130.
0007
6.32
510.
0620
0.34
950.
0028
0.81
1932
1321
1510
8.6
#41*
199
472
351
0.74
3.46
E−0
60.
193
0.13
120.
0017
6.64
350.
0893
0.36
740.
0013
0.26
2017
621
1423
4.6
#42*
135
335
127
0.38
4.43
E−0
60.
117
0.12
510.
0009
6.32
930.
1032
0.36
700.
0054
0.90
2015
2520
3012
0.7
#43*
188
459
208
0.45
3.64
E−0
60.
129
0.13
190.
0017
6.89
900.
1266
0.37
930.
0049
0.71
2073
2321
2423
2.4
#44*
228
566
271
0.48
2.59
E−0
60.
153
0.13
200.
0014
6.77
670.
1754
0.37
690.
0088
0.91
2062
4121
2419
2.9
UN
CO
RR
EC
TED
PR
OO
F
PRECAM 2699 1–20
8 S.P. Neves et al. / Precambrian Research xxx (2006) xxx–xxx
Tabl
e1
(Con
tinu
ed)
Sam
ple
Pb*
(ppm
)U
(ppm
)T
h(p
pm)
Th/
U20
4Pb/
206P
b20
8Pb/
206P
b20
7Pb/
206P
b±1
σ20
7Pb/
235U
±1σ
206P
b/23
8U±1
σρ
App
aren
tA
ges
(Ma)
Dis
c(%
)
206P
b/23
8U±1
σ20
7Pb/
206P
b±1
σ
#45*
230
548
267
0.49
2.87
E−0
60.
156
0.13
170.
0017
6.76
590.
1210
0.37
270.
0046
0.69
2042
2221
2023
3.7
#46*
7316
977
0.46
8.04
E−0
60.
136
0.13
390.
0008
7.18
340.
0931
0.38
920.
0045
0.90
2119
2121
4910
1.4
#47
145
370
162
0.44
3.58
E−0
60.
137
0.13
180.
0015
6.42
890.
0827
0.35
390.
0020
0.45
1953
1021
2120
7.9
#48*
193
468
114
0.24
2.67
E−0
50.
076
0.13
180.
0029
7.16
830.
3275
0.39
440.
0158
0.88
2143
7321
2239
−1.0
#49*
211
625
190
0.30
2.06
E−0
50.
087
0.13
190.
0019
6.95
190.
2354
0.38
230.
0117
0.91
2087
5521
2325
1.7
#50*
111
306
970.
324.
29E−0
50.
092
0.13
130.
0029
7.02
910.
2867
0.38
820.
0133
0.84
2114
6121
1639
0.1
#51
114
313
620.
203.
97E−0
50.
071
0.12
570.
0020
5.97
340.
1163
0.34
470.
0040
0.60
1909
1920
3828
6.3
#52
201
573
178
0.31
2.93
E−0
50.
104
0.12
590.
0026
5.81
830.
1714
0.33
520.
0070
0.71
1864
3420
4137
8.7
#53*
282
725
258
0.36
1.98
E−0
50.
104
0.12
630.
0020
6.22
920.
1175
0.35
770.
0038
0.56
1971
1820
4727
3.7
SCC
1B#1
*11
933
090
0.27
4.95
E−0
6–
0.12
170.
0013
5.89
860.
3450
0.35
150.
0202
0.98
1942
9619
8119
2.0
#2*
123
368
830.
234.
39E−0
6–
0.12
170.
0019
5.77
130.
1889
0.34
390.
0099
0.88
1906
4719
8128
3.8
#3*
8023
114
80.
646.
20E−0
6–
0.12
000.
0010
5.83
750.
1012
0.35
270.
0053
0.87
1948
2519
5715
0.5
#452
136
730.
531.
19E−0
5–
0.12
980.
0005
7.15
660.
3326
0.39
970.
0185
1.00
2168
8520
967
−3.4
#555
563
376
0.67
1.02
E−0
5–
0.06
580.
0013
0.92
310.
0268
0.10
180.
0021
0.72
625
1280
042
21.9
#6*
7922
512
20.
547.
05E−0
6–
0.12
000.
0012
6.03
050.
1294
0.36
450.
0069
0.89
2003
3319
5618
−2.4
#7*
6117
760
0.34
1.04
E−0
5–
0.12
090.
0016
5.86
200.
3088
0.35
170.
0179
0.97
1943
8519
6924
1.3
#8*
145
425
117
0.27
3.34
E−0
6–
0.12
160.
0005
5.69
620.
0993
0.33
970.
0058
0.98
1885
2819
807
4.8
#9*
115
346
117
0.34
5.55
E−0
6–
0.12
040.
0007
5.69
630.
1392
0.34
300.
0081
0.97
1901
3919
6311
3.1
#10*
154
463
135
0.29
3.15
E−0
6–
0.12
190.
0009
5.74
350.
0860
0.34
170.
0044
0.85
1895
2119
8414
4.5
#11*
143
424
225
0.53
4.13
E−0
6–
0.12
040.
0007
5.79
240.
1409
0.34
890.
0083
0.97
1929
3919
6310
1.7
#12*
112
325
860.
266.
10E−0
6–
0.12
130.
0007
5.98
610.
2552
0.35
790.
0151
0.99
1972
7119
7611
0.2
SCC
2#1
*37
106
24.3
80.
235.
14E−0
6–
0.12
130.
0008
6.12
940.
1257
0.36
640.
0072
0.95
2012
3419
7611
−1.8
#2*
3610
927
.31
0.25
5.37
E−0
6–
0.12
150.
0007
5.80
410.
0725
0.34
660.
0038
0.88
1918
1819
7810
3.0
#382
227
48.2
30.
212.
55E−0
6–
0.13
190.
0009
6.78
730.
1094
0.37
330.
0055
0.91
2045
2621
2312
3.7
#410
736
330
.69
0.08
2.12
E−0
6–
0.11
900.
0004
5.03
730.
0602
0.30
700.
0035
0.95
1726
1719
416
11.1
#5*
6619
229
.91
0.16
3.05
E−0
6–
0.12
140.
0007
5.94
310.
1193
0.35
510.
0068
0.96
1959
3219
7710
0.9
#6*
3595
19.3
60.
205.
92E−0
6–
0.12
280.
0017
6.26
700.
0937
0.37
020.
0022
0.40
2030
1019
9724
−1.7
#7*
2881
16.5
90.
208.
61E−0
6–
0.12
170.
0008
5.92
900.
1596
0.35
330.
0092
0.97
1951
4419
8112
1.6
#8*
3610
017
.84
0.18
5.44
E−0
6–
0.12
330.
0007
6.26
430.
0721
0.36
860.
0036
0.85
2023
1720
0411
−0.9
#9*
3698
15.8
50.
165.
46E−0
6–
0.12
360.
0009
6.30
460.
0935
0.37
000.
0048
0.88
2029
2320
0912
−1.0
#10*
3799
43.5
70.
446.
15E−0
6–
0.12
330.
0014
6.28
310.
1439
0.36
960.
0073
0.86
2027
3420
0521
−1.1
#11
5417
547
.22
0.27
4.25
E−0
6–
0.12
650.
0008
5.72
640.
2653
0.32
830.
0151
0.99
1830
7320
5011
10.7
#12*
5114
125
.59
0.18
4.31
E−0
6–
0.12
200.
0008
6.08
190.
1097
0.36
140.
0061
0.94
1989
2919
8611
−0.1
#13
3589
40.1
30.
455.
28E−0
6–
0.13
750.
0006
7.60
260.
0691
0.40
090.
0032
0.89
2173
1521
967
1.0
#14*
5919
339
.36
0.20
4.51
E−0
6–
0.12
280.
0008
6.08
070.
0679
0.35
920.
0035
0.87
1978
1719
9711
1.0
#15*
2883
21.8
20.
269.
70E−0
6–
0.12
220.
0011
6.02
800.
1212
0.35
770.
0065
0.90
1971
3119
8916
0.9
#16*
4914
417
.45
0.12
5.48
E−0
6–
0.12
320.
0012
6.14
140.
3689
0.36
150.
0205
0.94
1989
9620
0318
0.7
#17*
1438
5.61
0.15
1.32
E−0
5–
0.12
330.
0014
6.13
710.
0988
0.36
110.
0036
0.63
1987
1720
0420
0.8
#18*
4111
821
.83
0.19
4.91
E−0
6–
0.12
230.
0003
6.08
170.
1179
0.36
060.
0069
0.99
1985
3319
905
0.3
#19*
130
373
79.5
60.
211.
77E−0
6–
0.12
190.
0005
6.03
120.
0604
0.36
180.
0033
0.90
1990
1519
858
−0.3
#20*
3292
21.6
30.
238.
13E−0
6–
0.12
360.
0007
6.05
030.
2258
0.35
510.
0131
0.99
1959
6220
0810
2.5
#21
5916
224
.23
0.15
3.47
E−0
6–
0.12
820.
0007
6.70
440.
0816
0.37
920.
0041
0.88
2073
1920
7410
0.1
UN
CO
RR
EC
TED
PR
OO
F
PRECAM 2699 1–20
S.P. Neves et al. / Precambrian Research xxx (2006) xxx–xxx 9
SCC
5#1
4713
474
0.55
1.15
E−0
5–
0.12
830.
0006
5.82
960.
2949
0.32
950.
0156
0.94
1836
7520
758
11.5
#262
188
168
0.89
8.33
E−0
6–
0.12
670.
0006
5.92
250.
1740
0.33
890.
0099
0.99
1881
4820
538
8.4
#347
171
129
0.76
1.16
E−0
5–
0.12
190.
0015
4.92
530.
2436
0.29
300.
0132
0.91
1657
6519
8422
16.5
#43
311
0.04
2.44
E−0
4–
0.06
150.
0009
0.84
510.
0713
0.09
960.
0046
0.94
612
2765
830
7.1
#5*
4412
484
0.68
1.14
E−0
5–
0.13
000.
0010
6.59
890.
1624
0.36
800.
0081
0.90
2020
3820
9914
3.7
#6*
4211
578
0.68
1.48
E−0
5–
0.12
950.
0005
6.67
900.
3127
0.37
410.
0173
0.99
2048
8120
916
2.0
#712
034
138
31.
128.
01E−0
6–
0.12
910.
0007
6.28
200.
3132
0.35
290.
0172
0.98
1949
8120
8610
6.6
#869
291
295
1.01
1.32
E−0
5–
0.12
190.
0004
4.34
120.
2316
0.25
830.
0136
0.98
1481
6919
846
25.4
#9*
3391
580.
632.
38E−0
5–
0.12
990.
0005
6.53
890.
1601
0.36
510.
0088
0.99
2006
4220
977
4.3
#10
6220
515
60.
761.
51E−0
5–
0.12
350.
0014
5.30
410.
2700
0.31
160.
0155
0.98
1748
7620
0720
12.9
#11
5532
612
50.
391.
40E−0
5–
0.10
040.
0008
2.53
160.
1016
0.18
290.
0072
0.98
1083
3916
3115
33.6
#12*
6617
399
0.57
1.93
E−0
5–
0.12
990.
0004
6.83
230.
3240
0.38
140.
0179
0.99
2083
8320
976
0.7
#13
3310
364
0.62
2.57
E−0
5–
0.12
940.
0011
6.34
560.
1257
0.35
570.
0065
0.92
1962
3120
9015
6.1
#14*
7018
614
60.
781.
30E−0
5–
0.13
040.
0006
7.09
250.
3184
0.39
450.
0174
0.98
2144
8021
038
−1.9
#15
4613
684
0.62
2.26
E−0
5–
0.12
460.
0015
5.51
760.
1346
0.32
450.
0065
0.83
1812
3220
2321
10.4
#16
5923
198
0.42
1.59
E−0
5–
0.11
960.
0012
4.38
370.
2096
0.26
580.
0124
0.98
1519
6319
5018
22.1
#17
6216
311
80.
721.
11E−0
5–
0.12
780.
0003
6.44
290.
3484
0.36
570.
0196
0.99
2009
9220
684
2.8
#18*
6318
312
50.
681.
45E−0
5–
0.13
000.
0005
6.67
700.
3456
0.37
250.
0188
0.92
2041
8820
986
2.7
#19
2834
820
50.
593.
09E−0
5–
0.07
620.
0024
1.17
060.
2002
0.11
140.
0077
0.98
681
4511
0161
38.2
#20*
5214
912
70.
851.
48E−0
5–
0.13
050.
0010
6.91
600.
1202
0.38
430.
0058
0.81
2096
2721
0513
0.4
#21*
3610
286
0.85
2.31
E−0
5–
0.12
970.
0006
6.77
630.
2768
0.37
900.
0148
0.96
2072
6920
949
1.0
#22
6722
112
10.
551.
21E−0
5–
0.12
430.
0004
5.50
190.
1117
0.32
090.
0065
0.99
1794
3120
206
11.2
SCC
9#1
4522
675
0.33
2.21
E−0
50.
136
0.08
280.
0014
2.15
170.
0465
0.18
850.
0026
0.65
1113
1412
6432
11.9
#253
309
280.
091.
57E−0
50.
096
0.07
890.
0011
1.83
760.
0329
0.16
880.
0024
0.80
1006
1311
7127
14.1
#362
150
110
0.73
1.47
E−0
50.
219
0.11
910.
0002
5.91
870.
2116
0.36
030.
0117
0.91
1984
5519
434
−2.1
#474
556
365
0.66
1.04
E−0
50.
208
0.06
460.
0001
1.10
370.
0340
0.12
380.
0038
1.00
753
2276
35
1.3
#583
566
162
0.29
8.74
E−0
60.
142
0.07
980.
0012
1.55
530.
0482
0.14
140.
0038
0.86
853
2111
9131
28.4
#621
104
780.
743.
12E−0
50.
239
0.07
860.
0011
1.98
760.
0737
0.18
350.
0063
0.92
1086
3411
6128
6.5
#734
9252
0.57
2.21
E−0
50.
198
0.13
220.
0018
5.94
140.
2243
0.32
590.
0115
0.93
1818
5621
2824
14.5
#829
276
429
80.
394.
39E−0
60.
117
0.12
340.
0002
6.21
610.
2047
0.36
530.
0105
0.94
2007
4920
063
−0.1
#929
229
720.
311.
74E−0
50.
089
0.06
580.
0003
1.13
280.
0484
0.12
490.
0053
0.99
759
3079
911
5.0
#10
5613
960
0.44
1.47
E−0
50.
110
0.12
730.
0009
6.71
260.
2754
0.38
260.
0154
0.98
2088
7220
6013
−1.3
#11
1811
587
0.75
3.09
E−0
50.
265
0.06
690.
0004
1.20
970.
0333
0.13
110.
0035
0.98
794
2083
512
4.9
#12
2014
92
0.01
3.40
E−0
50.
261
0.06
780.
0006
1.09
540.
0308
0.11
710.
0032
0.96
714
1886
317
17.3
#13
3826
771
0.27
1.94
E−0
50.
098
0.07
190.
0007
1.38
310.
0179
0.13
940.
0011
0.63
842
698
421
14.5
#14
2722
016
70.
763.
00E−0
50.
225
0.06
770.
0010
1.10
120.
0319
0.11
800.
0032
0.93
719
1885
929
16.3
#15
2216
210
90.
674.
12E−0
50.
203
0.10
860.
0016
1.76
730.
0339
0.11
810.
0015
0.65
719
817
7627
59.5
#16
1613
569
0.51
3.52
E−0
50.
112
0.06
430.
0010
1.02
840.
0266
0.11
590.
0024
0.80
707
1475
233
6.0
#17
6418
159
0.32
1.40
E−0
50.
129
0.12
020.
0004
5.77
760.
2325
0.34
870.
0131
0.93
1928
6219
596
1.6
#18
5826
513
60.
511.
17E−0
50.
160
0.08
030.
0006
2.14
970.
0673
0.19
430.
0059
0.97
1144
3212
0316
4.9
#19
1399
650.
665.
95E−0
50.
203
0.06
470.
0020
1.06
370.
0546
0.11
930.
0048
0.79
727
2876
367
4.8
#20
131
394
670.
175.
95E−0
60.
087
0.11
830.
0003
5.08
700.
1057
0.31
180.
0064
0.99
1750
3119
315
9.4
#21
4334
910
00.
291.
51E−0
50.
123
0.07
180.
0005
1.10
830.
0480
0.11
200.
0048
0.99
684
2898
014
30.2
#22
1375
350.
464.
34E−0
50.
201
0.06
770.
0005
1.32
350.
0516
0.14
180.
0054
0.98
855
3086
017
0.6
#23
1490
390.
433.
41E−0
50.
145
0.07
100.
0010
1.38
020.
0252
0.14
110.
0016
0.62
851
995
629
11.1
#24
2613
145
0.34
2.61
E−0
50.
342
0.12
200.
0010
2.54
330.
0257
0.15
120.
0009
0.62
908
519
8614
54.3
UN
CO
RR
EC
TED
PR
OO
F
PRECAM 2699 1–20
10 S.P. Neves et al. / Precambrian Research xxx (2006) xxx–xxx
Tabl
e1
(Con
tinu
ed)
Sam
ple
Pb*
(ppm
)U
(ppm
)T
h(p
pm)
Th/
U20
4Pb/
206P
b20
8Pb/
206P
b20
7Pb/
206P
b±1
σ20
7Pb/
235U
±1σ
206P
b/23
8U±1
σρ
App
aren
tA
ges
(Ma)
Dis
c(%
)
206P
b/23
8U±1
σ20
7Pb/
206P
b±1
σ
#25
4734
077
0.23
1.43
E−0
50.
139
0.06
690.
0012
1.17
660.
0228
0.12
750.
0009
0.35
774
583
538
7.4
#26
2718
769
0.37
2.23
E−0
50.
174
0.07
800.
0002
1.46
170.
0354
0.13
590.
0033
1.00
821
1911
474
28.4
#27
2820
120
31.
012.
70E−0
50.
353
0.06
170.
0005
0.93
210.
0143
0.10
950.
0014
0.86
670
866
517
−3.3
#28
2214
942
0.28
2.39
E−0
50.
119
0.06
590.
0014
1.29
070.
0294
0.14
200.
0012
0.37
856
780
444
−6.5
#29
2112
272
0.59
3.85
E−0
50.
180
0.07
340.
0007
1.56
320.
0481
0.15
450.
0046
0.96
926
2610
2519
9.6
#30
9172
517
10.
247.
39E−0
60.
088
0.07
030.
0004
1.24
210.
0274
0.12
820.
0027
0.97
778
1693
611
17.0
#31
6330
121
40.
711.
17E−0
50.
238
0.07
800.
0005
2.03
700.
0388
0.18
940.
0034
0.94
1118
1811
4713
2.5
#32
8744
233
30.
759.
36E−0
60.
215
0.07
870.
0004
1.91
940.
0303
0.17
680.
0026
0.93
1049
1411
6611
10.0
#33
1712
870
0.55
3.21
E−0
50.
178
0.07
440.
0016
1.23
130.
0417
0.12
010.
0031
0.77
731
1810
5244
30.5
#34
7416
011
20.
701.
16E−0
50.
203
0.13
870.
0006
7.54
960.
1135
0.39
470.
0057
0.96
2145
2622
117
3.0
#35
3624
011
00.
461.
56E−0
50.
173
0.07
410.
0006
1.43
890.
0325
0.14
080.
0029
0.93
849
1710
4517
18.8
#36
4431
211
20.
361.
90E−0
50.
138
0.06
820.
0005
1.28
660.
0258
0.13
690.
0026
0.93
827
1487
415
5.4
#37
121
176
740.
423.
78E−0
60.
127
0.27
210.
0020
20.9
677
0.70
870.
5589
0.01
840.
9828
6276
3318
1113
.7#3
844
333
156
0.47
1.79
E−0
50.
207
0.06
850.
0009
1.13
440.
0242
0.12
020.
0020
0.76
731
1188
328
17.1
SCC
12#1
*25
242
130.
052.
16E−0
5–
0.06
250.
0008
0.89
130.
0227
0.10
350.
0023
0.87
635
1369
026
7.9
#2*
1213
327
42.
064.
15E−0
5–
0.06
190.
0006
0.83
320.
0305
0.09
760.
0034
0.96
600
2067
122
10.5
#3*
332
260.
801.
42E−0
4–
0.06
090.
0018
0.89
680.
0314
0.10
670.
0020
0.55
654
1263
762
−2.6
#4*
442
601.
441.
35E−0
4–
0.06
010.
0010
0.87
200.
0214
0.10
530.
0018
0.71
645
1160
537
−6.6
#5*
6881
767
0.08
1.10
E−0
5–
0.06
020.
0004
0.82
690.
0673
0.09
960.
0081
1.00
612
4761
115
−0.2
#6*
2021
830
71.
411.
77E−0
5–
0.06
040.
0012
0.81
450.
0272
0.09
780.
0026
0.81
601
1661
941
2.8
#710
131
513
0.04
5.88
E−0
6–
0.11
900.
0008
5.35
090.
1082
0.32
610.
0062
0.94
1819
3019
4212
6.3
#8*
4748
452
0.11
2.34
E−0
5–
0.06
030.
0006
0.82
470.
0275
0.09
920.
0031
0.95
610
1861
323
0.5
#957
185
180.
101.
22E−0
5–
0.12
250.
0010
5.43
070.
0908
0.32
140.
0047
0.88
1797
2319
9414
9.9
#10
137
441
260.
065.
25E−0
6–
0.12
160.
0010
5.37
390.
0755
0.32
040.
0037
0.81
1792
1819
8015
9.5
#11
125
360
280.
085.
56E−0
6–
0.12
290.
0005
5.79
220.
2003
0.34
180.
0117
0.99
1895
5619
997
5.2
#12
117
313
130.
044.
51E−0
6–
0.12
520.
0011
6.40
450.
0931
0.37
100.
0044
0.81
2034
2120
3215
−0.1
#13*
659
200.
341.
07E−0
4–
0.06
070.
0007
0.85
850.
0278
0.10
260.
0031
0.93
630
1862
825
−0.3
#14
119
355
120.
036.
08E−0
6–
0.12
370.
0006
5.75
740.
0909
0.33
760.
0051
0.96
1875
2420
108
6.7
Err
ors
are
1σan
dre
fer
tola
stdi
gits
.The
righ
than
dco
lum
nis
perc
enta
gedi
scor
danc
eas
sum
ing
rece
ntle
adlo
sses
.For
each
stud
ied
rock
,ana
lyse
sla
belle
d*
wer
ein
clud
edin
the
age
calc
ulat
ion,
whe
reas
othe
rsw
ere
omitt
ed.
UN
CO
RR
EC
TED
PR
OO
F
PRECAM 2699 1–20
S.P. Neves et al. / Precambrian Research xxx (2006) xxx–xxx 11
Fig. 3. SEM images of selected dated zircon grains in orthogneiss samples showing position of the LA-ICP-MS spot and corresponding age (errosquoted at the 1σ level). Sample SCC1A (mafic layer of banded orthogneiss). (A) Oscillatory-zoned zircon with rim overgrowth at the right side.(B) Rounded grain with irregular zoning. Sample SCC1B (felsic layer of banded orthogneiss). (C) Fragment of zircon grain containing largeelliptical core with coarse oscillatory zoning surrounded by overgrowth rim with thin oscillatory zoning. (D). Euhedral zircon grain with thinlyoscillatory-zoned overgrowth at upper and right side. Sample SCC2 (granitic orthogneiss). (E) Fragment of large, homogeneous euhedral grain withoscillatory-zoned core. (F) Grain with broadly elliptical form that yielded the oldest age of all analyzed zircons in orthogneiss samples.
elongated grains preserve subhedral shapes typical of377
magmatic zircon, suggesting transport over short dis-378
tances (Fig. 4B).379
Two zircon populations are observed in sample380
SCC12 (migmatitic paragneiss leucosome). One con-381
tains elongated (aspect ratio up to 4:1), subhedral to382
euhedral zircon grains with faint oscillatory zoning and383
thin or absent overgrowth rims (Fig. 4C). The other con-384
sists of rounded (Fig. 4D) to slightly elongated grains385
with overgrowths that may truncate internal oscillatory386
zoning. Inherited cores are present in some grains of the387
latter population.388
Finally, the deformed granodiorite sample SCC5 from389
the Alcantil pluton contains a homogeneous population390
of small (∼100 �m long), subhedral to anhedral, slightly391
elongated grains.
6. U–Pb zircon data 392
Table 1 shows the results of analytical data for the 393
studied samples. In the following, ages of zircons are 394
expressed in terms of either their 207Pb/206Pb ratios 395
(grains older than 1 Ga) or their 206Pb/238U ratios (grains 396
with Neoproterozoic ages). Errors for single analysis and 397
mean ages are quoted at the 2σ level. 398
6.1. Sample SCC1 (1A and 1B) 399
Analyses of zircons from sample SCC1A (mafic layer 400
of banded orthogneiss) fall into two age groups that 401
define two Pb loss trends in the concordia diagram 402
(Fig. 5a). For each population, analyses showing dis- 403
cordance smaller than 5% can be pooled together to 404
UN
CO
RR
EC
TED
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PRECAM 2699 1–20
12 S.P. Neves et al. / Precambrian Research xxx (2006) xxx–xxx
Fig. 4. SEM images of selected zircon grains in sample SCC9 (pelitic gneiss). (A) Rounded zircon grain with overgrowth rims at upper left andlower right sides truncating oscillatory-zoned core. (B) Elongated zircon grain with no apparent zoning. SEM images of selected zircon grains insample SSC12 (leucosome from migmatitic paragneiss) showing position of the LA-ICP-MS spot and corresponding age (errors quoted at the 1σ
level). (C) Subhedral grain with thin overgrowth rim. (D) Rounded grain with thin overgrowth rims at the left and right sides.
define 207Pb/206Pb weighted means of 2125 ± 7 and405
2044 ± 5 Ma (Fig. 5b). The clear distinction of these406
two age groups strongly suggests that they correspond407
to two different events. The lack of inherited cores in408
most zircon grains suggests that the group with the older409
age represents igneous crystallization of the protolith.410
This is consistent with well preserved oscillatory zon-411
ing in the grains where ca. 2125 Ma ages were obtained412
(Fig. 3A). Truncation of oscillatory zoning, recrystal-413
lized zones or regions with fading oscillatory zoning414
observed in some grains (Figs. 3A and B) are typi-415
cal of magmatic zircons modified by high-grade meta-416
morphism (e.g. Corfu et al., 2003). The youngest age417
of ca. 2044 Ma is thus interpreted as representing the418
Transamazonian metamorphic event. Because there is419
no discernable difference in the Th/U ratios between zir-420
cons of the two age groups (Table 1), local redistribution421
by recrystallization processes without new metamorphic422
growth is the most likely explanation for the igneous-like423
high Th/U (>0.1; Williams and Claesson, 1987) ratio of424
the zircon domains with ca. 2044 Ma ages. Overgrowth425
rims that clearly represent new zircon growth revealed426
to be too thin to be accurately dated Analyses showing427
high discordance indicate Pb losses that could be related428
either to a young (e.g. Brasiliano) event or to recent,429
zero age, disturbances, or even a combination of both430
(Fig. 5a).431
Analyses of zircons from the leucocratic band SCC1B 432
display a very different distribution when compared with 433
sample SCC1A. Most grains plot close to Concordia 434
(see Fig. 6a) at about 1.98 Ga, and, together with anal- 435
ysis #5 (Table 1), define a discordia line with upper 436
and lower intercepts of 1985 ± 12 and 578 ± 37 Ma 437
(MSWD = 1.2). The upper intercept is well constrained 438
by concordant analyses and ten highly concordant grains 439
give a 207Pb/206Pb weighted mean of 1972 ± 8 Ma 440
(Fig. 6b), in agreement with the upper intercept age. 441
The Th/U ratio of these grains (ranging from 0.2 to 0.7; 442
Table 1) is typical of magmatic zircons (Williams and 443
Claesson, 1987), which suggests that the 1972 Ma age 444
corresponds to crystallization of the zircons. Since these 445
analyses were obtained from large rounded cores sur- 446
rounded by a thin oscillatory zoned rim (see Fig. 3C), it 447
is concluded that the age of 1972 Ma corresponds to that 448
of the source rocks that underwent anatexis to produce 449
the leucocratic band. It is probably noteworthy that this 450
age is similar to the U–Pb age of 1974 Ma obtained by Sa 451
et al. (2002) from an orthogneiss some kilometers to the 452
southeast (Fig. 2), suggesting that the orthogneiss was 453
the main source component for the melt. One grain is 454
concordant at 2096 ± 14 Ma, indicating the source also 455
included a ca. 2.1 Ga old component. One euhedral zir- 456
con grain (see Fig. 3D) yielded a 206Pb/238U apparent age 457
of 625 ± 24 Ma (Fig. 6a). The high Th/U ratio (0.67) of 458
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Fig. 5. (a) U–Pb concordia diagram for zircons from sample SCC1A(mafic layer of banded orthogneiss). (b) Zoom showing the twoweighted mean ages of Paeloproterozoic zircons.
this grain (Table 2), its euhedral shape and the magmatic459
oscillatory zoning of overgrowths (Fig. 3D) are inter-460
preted as indicating growth from a magma. Therefore,461
this age most likely corresponds to the crystallization of462
the leucocratic band, implying that the mesoscopic struc-463
ture of the banded orthogneiss is a late Neoproterozoic464
feature, resulting from intrusion of syntectonic granitic465
melts in a preexisting protolith.466
6.2. Sample SCC2467
Sixteen near concordant analyses of zircon grains468
from the orthogneiss sample SCC2 yielded a well-469
constrained 207Pb/206Pb weighted mean age of470
1991 ± 5 Ma (Fig. 7). Some of these grains still preserve471
euhedral shapes (Fig. 3E), which together with high472
Th/U ratios (see Table 1) indicates crystallization from473
a magma. The 1991 ± 5 Ma age is thus interpreted as474
Fig. 6. (a) Concordia diagram showing discordia line for zircons fromsample SCC1B (felsic layer of banded orthogneiss). (b) Zoom showingthe 206Pb/207Pb weighted mean age of concordant Paleoproterozoiczircons.
corresponding to crystallization of the granitic pro- 475
tolith. Three other grains yielded older ages indicating 476
inherited source components of 2196 ± 14 (Fig. 3F), 477
2123 ± 24 and 2074 ± 20 Ma. The two latter roughly 478
correspond to the two mean ages obtained in sample 479
SCC1A. These results are interpreted as indicating 480
that the granitic orthogneiss is a late-Transamazonian 481
intrusion containing a small proportion of inherited 482
zircon grains. 483
6.3. Sample SCC9 484
U–Pb data for detrital zircons from paragneiss sam- 485
ple SCC9 exhibit ages ranging from more than 3320 to 486
ca. 665 Ma (Table 1). Data is reported in the concor- 487
dia diagram (Fig. 8a) and in a cumulative probability 488
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14 S.P. Neves et al. / Precambrian Research xxx (2006) xxx–xxx
Fig. 7. U–Pb concordia diagram for zircons from sample SCC2(granitic orthogneiss).
plot (Fig. 8b). Most analysis fall on or near the con-489
cordia curve and those with less than 5% discordance490
show age peaks at ca. 2220, 2060–1940, 1200–1150,491
860–760 and 665 ± 34 Ma. Several discordant grains492
have ages between 1100 and 900 Ma and a small peak493
is observed around 1690 Ma. Grains from all age groups494
have high Th/U ratios. Together with the oscillatory zon-495
ing observed in most grains, this indicates provenance of496
grains from igneous protoliths (Williams and Claesson,497
1987), which constrain the deposition of the supracrustal498
sequence to be younger than the youngest grain (ca.499
665 Ma) in the zircon population.500
6.4. Sample SCC12501
On a concordia plot (Fig. 9A), analyses of zircons502
from the leucosome of a paragneiss, except for one (#7;503
Table 1) define a discordia line (MSWD = 1.1) with upper504
and lower intercepts at 2041 ± 15 and 626 ± 15 Ma,505
respectively. Paleoproterozoic ages were obtained from506
rounded zircons grains (Fig. 4D) that have low Th/U507
ratios (0.04–0.1; Table 5), typical of metamorphic zir-508
cons. These grains are interpreted as inherited from509
a protolith metamorphosed at ca. 2040 Ma. This is in510
agreement with analysis #12 (Table 1), which is concor-511
dant at 2032 ± 30 Ma and reinforces the interpretation of512
the data for sample SCC1A that the peak of Transamazo-513
nian metamorphism occurred around this time. Zircons514
with Neoproterozoic ages plot near the concordia and515
have a 206Pb/238U weighted mean age of 632 ± 17 Ma516
(Fig. 9b) overlapping the lower intercept of the discordia517
line. These grains yield both high and low Th/U ratios518
(Table 5) typical of magmatic and metamorphic zircons,519
respectively. The high Th/U ratios of some grains (up520
Fig. 8. (a) U–Pb concordia diagram for zircons from sample SCC9(pelitic gneiss). Inset: zoom at the Neoproterzoic showing the U–Pbage of the youngest grain in the zircon population. Green, concor-dant grains; red, discordant grains. (b) Histogram plot for 206Pb/207Pbages of the analyzed zircons. Green, concordant grains; red, discor-dant grains. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of the article.)
to 2.06) suggest that the laser beam struck a Th-rich 521
inclusion, whereas the euhedral shape (Fig. 4C) and low 522
Th/U ratio of other grains is typical of zircons grown 523
under high grade conditions. The most precise lower 524
intercept age of 626 ± 15 Ma is therefore interpreted as 525
dating crystallization of the leucosome, and is thus taken 526
as our best estimate for the high-grade metamorphism of 527
the supracrustal sequence during the Brasiliano orogeny. 528
6.5. Sample SCC5 529
Analyses of zircon from the Alcantil pluton (SCC5; 530
Table 1) define a discordia line (Fig. 10a) with upper 531
and lower intercepts of 2103 ± 11 and 619 ± 36 Ma, 532
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Fig. 9. (a) Concordia diagram showing discordia line for zircons fromsample SCC12 (leucosome of migmatitic paragneiss). (b) Zoom show-ing the 206Pb/238U weighted mean age of concordant Neoproterozoiczircons.
respectively. The lower intercept is constrained by anal-533
ysis #4 (Table 1), which yielded a 206Pb/238U age of534
612 ± 54 Ma and a low Th/U ratio of 0.04, typical of535
growth in the solid state. This indicates that the gran-536
odiorite was metamorphosed at 619 ± 36 Ma, the more537
precise lower intercept of the discordia line. Most grains538
have older, mainly Paleoproterozoic ages, and a batch of539
eight concordant analyses yields a 207Pb/206Pb weighted540
mean age of 2097 ± 5 Ma (Fig. 10b). One grain (#17;541
Table 1) has a low discordance degree, but yields a sig-542
nificantly younger age (2068 ± 8 Ma) suggesting it has543
undergone disturbances, possibly during the ca. 2044 Ma544
Transamazonian event.545
The above results could be interpreted in two ways.546
First, that intrusion occurred during the Brasiliano547
orogeny and that temperature remained high enough548
after emplacement to allow growth of metamorphic zir-549
Fig. 10. (a) Concordia diagram showing discordia line for zircons fromsample SCC5 (Alcantil pluton). (b) Zoom showing the 206Pb/207Pbweighted mean age of concordant Paleoproterozoic zircons.
con. In this hypothesis, the zircon population would con- 550
sist almost entirely of xenocrystic grains inherited from a 551
homogeneous Paleoproterozoic source. Because this is a 552
rather unusual situation for granitic magmas, the second 553
possibility, that emplacement took place at 2097 ± 5 Ma 554
during the Transamazonian orogeny, is considered more 555
likely. The emplacement age of ca. 2100 Ma is younger 556
but comparable to that of the older age found in the 557
orthogneiss sample SCC1A (ca. 2125 Ma), suggesting 558
that the Alcantil pluton could represent less strained por- 559
tions of basement orthogneisses in the region. 560
7. Discussion 561
7.1. Tectonothermal evolution of the study area 562
This work clearly reveals that two main tectonother- 563
mal events affected the study area, one in the Pale- 564
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16 S.P. Neves et al. / Precambrian Research xxx (2006) xxx–xxx
oproterozoic (Transamazonian orogeny) and the other565
at the end of the Neoproterozoic (Brasiliano orogeny).566
The age pattern of sample SCC1A (mafic layer of567
banded orthogneiss) allows placing tight constraints568
on the events associated with the Transamazonian569
orogeny. The lack of inherited cores, as revealed by BSE570
images, suggests that the age cluster of 2125 ± 7 Ma571
corresponds to the crystallization age of the banded572
orthogneiss protolith. Six whole-rock samples of banded573
orthogneiss display geochemical characteristics simi-574
lar to calc-alkaline magmas, suggesting generation in575
a volcanic arc setting (Sa et al., 2002). Considering576
this, the age reported here could correspond to juve-577
nile crustal accretion. The younger age (2044 ± 5 Ma)578
found in sample SCC1A is associated with metamor-579
phic features observed in the analyzed zircon grains580
and is interpreted as dating the peak of Transamazo-581
nian metamorphism, possibly marking a major colli-582
sional event. This is corroborated by the occurrence583
of metamorphic zircons with this age in the paragneiss584
leucosome sample SCC12. The age of 1992 ± 7 Ma of585
sample SCC2 (granitic orthogneiss), and the mean age586
of 1972 ± 8 Ma for xenocrystic zircons from sample587
SCC1B (felsic layer of banded orthogneiss) are inter-588
preted as reflecting a stage of late to post-orogenic589
magmatism.590
The age pattern of the paragneiss sample SCC9591
reveals provenance of its protolith mainly from Paleo-592
proterozoic and mid-Neoproterozoic sources, and con-593
strains the deposition of the supracrustal sequence to594
be younger than 665 Ma (Fig. 8a and b). The Paleo-595
proterozoic ages correspond closely to the Transama-596
zonian event and may represent derivation of detrital597
grains from nearby orthogneisses, although more dis-598
tal sources cannot be excluded. Proximal sources with599
Archean ages that could provide the oldest analyzed600
zircon grain (>3320 Ma) have not yet been directly601
dated in the central domain, but their existence is602
suggested by Sm–Nd model ages of Paleoproterozoic603
orthogneisses (Van Schmus et al., 1995; Brito Neves604
et al., 2001b). However, even the oldest Sm–Nd ages605
are generally younger than 3300 Ma, which favors a606
more distal source. This source may be located either607
within an Archean nucleus identified in the northeast-608
ernmost part of the Borborema Province (Dantas et609
al., 1998, 2004), ∼250 km to the north of the study610
area, or within the Sao Francisco craton. Grains with611
late Paleoproterozoic ages of ca. 1690 Ma may have612
their source in augen gneisses/meta-anorthositic com-613
plexes (Accioly et al., 2000), which occur to the east614
of the study area (Fig. 2A). The abundance of zir-615
con grains with ages in the interval 1200–1150 Ma is616
intriguing, as rocks with these ages have not yet been 617
identified anywhere in the Borborema Province. It is 618
tentatively attributed to late Mesoproterozoic extension 619
and intraplate magmatism preceding the more exten- 620
sive Cariris Velhos rifting event. Felsic volcanic rocks 621
and granites related to the Cariris Velhos event (now 622
metavolcanics and orthogneisses) in the Alto Pajeu belt 623
constitute the most likely source for zircons with ca. 624
950–1050 Ma ages. A source for the abundant zircon 625
grains with mid-Neoproterozoic ages might be related 626
to magmatic episodes preceding and coeval with basin 627
formation. 628
The Neoproterozoic age of one magmatic zircon in 629
the felsic layer of banded orthogneiss (625 ± 24 Ma), 630
the maximum deposition age of the Surubim sequence 631
(665 Ma), the crystallization age of the leucosome from a 632
migmatitic paragneiss (626 ± 15 Ma), and the metamor- 633
phic age of the Alcantil pluton (619 ± 36 Ma) show that 634
high-temperature metamorphism was coeval with forma- 635
tion of a flat-lying foliation in basement and supracrustal 636
rocks. This metamorphism is clearly separated from 637
transcurrent shear zone development because the oldest 638
plutons deformed in the magmatic stage by strike-slip 639
shearing are younger than 592 Ma (Guimaraes and Da 640
Silva Filho, 1998; Neves et al., 2004). Although the 641
importance of the Transamazonian event in the study 642
area is obvious, fieldwork (Neves et al., 2000, 2005) and 643
the geochronological results from this study indicate that 644
the dominant mesoscopic ductile fabric in Paleoprotero- 645
zoic orthogneisses was produced during the Brasiliano 646
orogeny. 647
7.2. Regional correlations 648
7.2.1. Basement gneisses 649
The two age groups in sample SCC1A are similar 650
to those found in samples from the eastern portion of 651
the Sao Francisco craton, where recent SHRIMP U–Pb 652
data indicate magmatic crystallization at 2.2–2.1 Ga and 653
high-grade metamorphism at 2.08–2.05 Ga (Silva et al., 654
2002). In the Borborema Province, most zircon grains 655
that yielded Paleoproterozoic U–Pb ages were analyzed 656
by conventional methods (see Brito Neves et al., 2000, 657
and Neves, 2003, for a review of available data). The 658
spread of ages, mainly from 2.25 to 2.0 Ga, may in part 659
reflect mixed ages resulting from a combination of inher- 660
ited zircon cores, primary igneous zircon crystalliza- 661
tion, and metamorphic recrystallization. Nevertheless, 662
the existing data point out to an important period of crust 663
generation at 2.2–2.1 Ga, followed by deformation and 664
metamorphism, and then by intrusion of late- to post- 665
tectonic plutons. 666
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7.2.2. Supracrustal sequences667
The maximum deposition age of the Surubim com-668
plex is similar to that of the Cachoeirinha Group in the669
Cachoeirinha belt (Kozuch, 2003; Medeiros, 2004), and670
its zircon age pattern is remarkably similar to that found671
(Van Schmus et al., 2003) in the Serido belt (Fig. 1B).672
Several observations also suggest that the Surubim com-673
plex and the Sertania complex in the Alto Moxoto belt are674
correlated. Both complexes consist of the same rock type675
association, have similar metamorphic grade (although676
migmatization is more frequent in the Sertania complex),677
and display comparable carbon isotope signature in mar-678
bles (Santos et al., 2002). Although eight zircon grains679
from two samples of the Sertania complex had yielded680
U–Pb SHRIMP ages around 2.0 Ga and interpreted as681
indicating Paleoproterozoic sedimentation (Santos et al.,682
2004a), this only represents the maximum age of depo-683
sition.684
The probable connection between supracrustal suc-685
cessions in the East Pernambuco, Alto Moxoto,686
Cachoeirinha and Serido belts are consistent with depo-687
sition in a regionally extensive basin formed dur-688
ing broad-scale lithospheric extension. The small time689
span between deposition and deformation can explain690
the overall high-temperature metamorphism, as high691
thermal gradients resulting from crustal thinning can692
be maintained in the subsequent contractional phase693
(Thompson, 1989; De Yoreo et al., 1991; Thompson et694
al., 2001).695
7.2.3. Tectonothermal events696
Evidence for a metamorphic event in the early Neo-697
proterozoic was not found in this study and in all studies698
conducted so far in the central domain (Van Schmus699
et al., 1995; Leite et al., 2000b; Brito Neves et al.,700
2001a,b; Kozuch, 2003; Medeiros, 2004). Contractional701
deformation of this age during the proposed Cariris702
Velhos orogeny (Brito Neves et al., 1995) has been703
based on the interpretation that the early Neoprotero-704
zoic metaigneous and metasedimentary succession of705
the Alto Pajeu belt represents a subduction arc assem-706
blage intruded by syncollisional granites (Santos and707
Medeiros, 1999; Kozuch, 2003). However, the same708
top-to-the-WNW/NW tectonic transport is found in the709
supracrustal succession and augen gneisses of the Alto710
Pajeu belt (Medeiros, 2004), and in the Surubim com-711
plex (Neves et al., 2005; this study), the Sertania com-712
plex (Santos et al., 2004a), and the Cachoeirinha Group713
(Medeiros, 2004). Identical kinematics in these four714
belts strongly indicates deformation during the Brasil-715
iano orogeny. Furthermore, the geochemical character-716
istics of the metavolcanic and metaplutonic rocks of the717
Alto Pajeu belt are typical of intraplate magmas, not of 718
subduction-related ones (Bittar and Campos Neto, 2000; 719
Bittar et al., 2001; Neves, 2003; Guimaraes and Brito 720
Neves, 2004). These observations seriously cast in doubt 721
the existence of the Cariris Velhos event as an important 722
orogeny. 723
The Neoproterozoic age of deposition of supracrustal 724
sequences and a common flat-lying foliation in basement 725
gneiss and metasedimentary belts is observed throughout 726
the Borborema Province (Caby and Arthaud, 1986; Caby 727
et al., 1995; Neves et al., 2000, 2005). It is no longer 728
possible to claim that the Brasiliano orogeny was only 729
responsible for granite intrusion and strike-slip shearing, 730
as still advocated in several recent studies (Jardim de Sa 731
et al., 1995; Sa et al., 2002; Araujo et al., 2003; Santos et 732
al., 2004b). The present architecture of the Borborema 733
Province is a product of the Brasiliano orogeny, although 734
it is clear the importance of the Transamazonian orogeny 735
as a crust-forming event. 736
7.3. Implications for western Gondwana 737
The results of this study and the recent synthesis 738
by Ferre et al. (2002) and Toteu et al. (2004) on the 739
geodynamic evolution of Nigeria and Cameroon, respec- 740
tively, strengthen the earlier suggestion (Neves, 2003; 741
Neves et al., 2004) that these belts shared a common 742
evolution throughout most of the Proterozoic. Common 743
features include (1) extensive (ca. 2.1 Ga) Paleoprotero- 744
zoic crust, (2) dominance of metasedimentary sequences 745
with Neoproterozoic deposition ages, (3) ubiquitous 746
presence of flat-lying fabrics of late Neoproterozoic 747
age (∼640–600 Ma), and (4) dominance of transcur- 748
rent/transpressional deformation after 600 Ma. The lack 749
of evidence for closure of large oceanic domains in 750
all these regions does not support the interpretation of 751
the Borborema Province as a series of amalgamated 752
terranes (e.g. Santos and Medeiros, 1999; Santos et 753
al., 2004a,b). Destabilization of a preexisting conti- 754
nent formed at the end of the Transamazonian/Eburnean 755
orogeny (the Atlantica supercontinent of Rogers, 1996) 756
provides the simplest explanation to the above find- 757
ings. Several attempts to fragment this supercontinent 758
are recorded by late Paleoproterozoic to Neoprotero- 759
zoic intraplate extensional and magmatic events repre- 760
sented by failed rifts and A-type granites and related 761
rocks. A final period of plate-wide extension occurred 762
in the mid/late Neoproterozoic. This was immediately 763
followed by convergence and contractional deformation 764
marking the beginning of the Brasiliano/Pan-African 765
orogeny, which essentially occurred in an intracontinen- 766
tal setting.
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18 S.P. Neves et al. / Precambrian Research xxx (2006) xxx–xxx
7.4. Summary and conclusions767
The main conclusions of this study concerning the768
Precambrian tectonic and geochronological evolution769
of the study area in the East Pernambuco belt can be770
summarized as follows: (1) 2.15–2.10 Ga: generation of771
juvenile crust, (2) 2.05–2.03 Ga: peak Transamazonian772
metamorphism, (3) 1.99–1.97 Ga: intrusion of late773
orogenic magmas, (4) after 665 Ma: deposition of774
supracrustal sequences and (5) 630–610 Ma: develop-775
ment of flat-lying fabrics and Brasiliano high-grade776
metamorphism. Available data from the literature, in777
addition, support the intrusion of anorogenic plutons at778
1.7–1.5 Ga (Accioly et al., 2000; Sa et al., 2002), and the779
development of transcurrent shear zones and abundant780
magmatism at 590–580 Ma (Neves et al., 2000, 2004).781
Most of these features are found in other sectors of the782
Borborema Province (Neves, 2003) and in the Nigeria783
and Cameroon provinces (Ferre et al., 2002; Toteu et784
al., 2004; Njiosseu et al., 2005), suggesting a shared785
evolution during most of the Proterorozoic.786
Uncited reference787
Murphy et al. (2004).788
Acknowledgments789
LA-ICP-MS analyses were conducted as part of post-790
doctoral studies by SPN financed by the Brazilian agency791
Conselho Nacional de Desenvolvimento Cientıfico e Tec-792
nologico (CNPq). Samples were collected during field-793
work funded by the Fundacao de Amparo a Ciencia794
e Tecnologia do Estado de Pernambuco (FACEPE).795
The comments from two anonymous reviewers helped796
improving the manuscript.797
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