Geological evolution of the basement rocks in the east-central part of the Rondônia Tin Province, SW Amazonian craton, Brazil: U–Pb and Sm–Nd isotopic constraints

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Geological evolution of the basement rocks in the east-central part of the Rondoˆnia Tin Province, SW Amazonian craton, Brazil: U
Pb and Sm
Nd isotopic constraints /

/

Bruno L. Payolla a,d,, Jorge S. Bettencourt b, Marianne Kozuch c, Washington B. Leite, Jr d, Allen H. Fetter d, W. Randall Van Schmus c a Centrais Ele´tricas do Norte do Brasil S/A, SQN 408, Bloco L, Apto 202, CEP 70856-120 Brasilia, Brazil Instituto de Geocieˆncias, Universidade de Sa˜o Paulo, Caixa Postal 11348, CEP 05422-970 Sa˜o Paulo, Brazil c Department of Geology, University of Kansas, Lawrence, KS 66045, USA d Instituto de Geocieˆncias e Cieˆncias Exatas, Universidade Estadual Paulista, Av. 24 A, no. 1515, CEP 13506-900 Rio Claro, Brazil b

Received 17 April 2001; received in revised form 5 December 2001; accepted 30 April 2002

Abstract On the basis of geologic, petrologic, and U
/Pb geochronologic data the basement rocks in the east-central part of the Rondoˆnia Tin Province (RTP, southwestern Amazonian craton) are grouped into five lithologic associations: (1) tonalitic gneiss (1.75 Ga); (2) enderbitic granulite (1.73 Ga); (3) paragneiss; (4) granitic and charnockitic augen gneisses (1.57
/1.53 Ga); and (5) fine-grained granitic gneiss and charnockitic granulite (1.43
/1.42 Ga). The first three are related to development of the Paleoproterozoic Rio Negro-Juruena Province and represent the oldest crust in the region. The tonalitic gneisses and enderbitic granulites show calc-alkaline affinities and Nd isotopic compositions (initial o Nd //0.1 to /1.5; TDM of 2.2
/2.1 Ga) that suggest a continental arc margin setting for the original magmas. The paragneisses yield TDM values of 2.2
/2.1 Ga suggesting that source material was primarily derived from the Ventuari-Tapajo´s and Rio Negro-Juruena crusts, but detrital zircon ages and an intrusive granitoid bracket deposition between 1.67 and 1.57 Ga. The granitic and charnockitic augen gneisses show predominantly A-type and within-plate granite affinities, but also some volcanic arc granite characteristics. The initial o Nd values (/0.6 to /2.0) indicate mixing of magmas derived from depleted mantle and older crustal sources. These rocks are correlated to the 1.60
/1.53 Ga Serra da Provideˆncia intrusive suite that reflects inboard magmatism coeval with the Cachoeirinha orogen located to the southeast. The fine-grained granitic gneiss and charnockitic granulites represent the first record of widespread magmatism at 1.43
/1.42 Ga in northern Rondoˆnia. Their geochemical signatures and the slightly positive initial o Nd values (/0.7 to /1.2) are very similar to those of the most evolved granites of the calc-alkaline Santa Helena batholith farther southeast. U
/Pb monazite and Sm
/Nd whole-rock-garnet ages demonstrate that a high-grade tectonometamorphic episode occurred in this region at 1.33
/1.30 Ga. This episode attained upper-amphibolite conditions and is interpreted as the peak of the Rondonian-San Ignacio orogeny. The U
/Pb and Sm
/Nd data presented here and data published on rapakivi granites elsewhere indicate that the east-central part of the RTP is a poly-orogenic region

 Corresponding author. Tel.: /55-61-429-6157; fax: /55-61-328-6019 E-mail address: [email protected] (B.L. Payolla). 0301-9268/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 1 2 1 - 3

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characterized by successive episodes of magmatism, metamorphism, and deformation between 1.75 and 0.97 Ga. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Amazonian craton; Rondoˆnia Tin Province; Mesoproterozoic; U
/Pb geochronology; Sm
/Nd isotopes

1. Introduction Limited geological and geochronological (mostly Rb
/Sr, Pb
/Pb and K
/Ar) studies in the last three decades have demonstrated that the western border of the Amazonian craton is made up of four major Proterozoic geochronological provinces (Cordani et al., 1979; Teixeira et al., 1989). These provinces are found as sub-parallel NW
/SE trending belts across the craton and decrease in age from the northeast to the southwest: Ventuari-Tapajo´s (1.95
/1.80 Ga), Rio Negro-Juruena (1.80
/1.55 Ga), Rondonian-San Ignacio (1.45
/1.30 Ga), and Sunsas (1.25
/1.00 Ga) (Fig. 1). Further U
/Pb zircon and Sm
/Nd geochronological studies (e.g. Cordani and Sato, 1999; Tassinari and Macambira, 1999) have confirmed this general crustal architecture, although others have proposed some modifications (e.g. Santos et al., 2000). Within the major provinces along the southwestern border of the Amazonian craton, Tassinari et al. (2000) recognized several subprovinces and terranes. In the Rio Negro-Juruena, Rondonian-San Ignacio, and Sunsas provinces the following components were identified: (i) the Alto Jauru terrane (1.79
/1.74 Ga) and the Cachoeirinha orogen (1.58
/1.54 Ga) within the Rio NegroJuruena Province; (ii) the Rio Alegre terrane (1.52
/1.47 Ga), the Santa Helena orogen (1.45
/ 1.42 Ga), and the Rondonian-San Ignacio orogen (1.40
/1.30 Ga) in the Rondonian-San Ignacio Province; and (iii) the Sunsas orogen (1.00
/0.95 Ga), the Nova Brasilaˆndia terrane (1.11
/1.00 Ga), and the Aguapeı´ thrust belt (1.0
/0.95 Ga) in the Sunsas Province. The Alto Jauru and Rio Alegre terranes consist of calc-alkaline rocks of island arc affinity, whereas the Cachoeirinha and Santa Helena orogens comprise continental arc affinity calc-alkaline rocks (Geraldes et al., 2001). The Rondonian-San Ignacio orogen involved crustal reworking in eastern Bolı´via and in northern

Rondoˆnia (Litherland et al., 1989; Tassinari et al., 2000). The Nova Brasilaˆndia terrane comprises volcanic and sedimentary rocks developed in an oceanic environment, whereas the Aguapeı´ thrust belt includes mainly sedimentary rocks deposited in an intracratonic setting. Both were deformed during the Sunsas orogeny (Rizzotto, 1999; Geraldes et al., 2001).

Fig. 1. Geological sketch map of the Amazonian craton. Modified after Tassinari and Macambira (1999).

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Fig. 2

143

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In the Rondoˆnia Tin Province (RTP) and vicinity (Fig. 2), various magmatic events have been recognized (Tassinari et al., 1996; Bettencourt et al., 1999; Santos et al., 2000). The oldest (
/1.75 Ga) event involves granodioritic to tonalitic rocks and is interpreted as a continental-arc system related to the Rio Negro-Juruena Province (Tassinari et al., 1996). The 1.60
/1.53 Ga granitoid rocks and related gneisses of the Serra da Provideˆncia intrusive suite (Bettencourt et al., 1999; Santos et al., 2000) are interpreted as inboard rapakivi magmatism related to the Cachoeirinha orogeny (Tassinari et al., 2000; Geraldes et al., 2001). Alternatively, however, a model of arc-related magmatism has been proposed (Tassinari et al., 1996). The rapakivi granites included in the Santo Antoˆnio intrusive suite (
/1.41 Ga), the Teotoˆnio intrusive suite (
/1.39 Ga), the Alto Candeias intrusive suite (
/1.34 Ga), and the Sa˜o Lourenc¸oCaripunas intrusive suite (
/1.31 Ga) are also interpreted as inboard magmatism, related to the development of the Rondonian-San Ignacio Province (Bettencourt et al., 1999). The rapakivi granites of the Santa Clara intrusive suite (1.08
/ 1.07 Ga) and the Younger Granites of Rondoˆnia (1.00
/0.97 Ga) are inboard magmatism related to the collisional stage of the Sunsas orogeny (Bettencourt et al., 1999). So far, only one high-grade metamorphic event, at
/1.33 Ga, has been recognized in northern Rondoˆnia, and is related to the metamorphic peak of the Rondonian-San Ignacio orogeny (Tassinari et al., 1999). 40K/40Ar ages (1.30
/0.97 Ga) and 40Ar/39Ar ages (1.30
/0.95 Ga) obtained by Teixeira and Tassinari (1984), Bettencourt et al. (1996), and Tohver et al. (2000), have been interpreted as cooling ages related to Rondonian-San Ignacio and Sunsas orogenies. This study presents U
/Pb zircon and monazite results and Nd whole-rock and garnet isotopic

data obtained from samples of five lithologic associations recognized in the medium- to highgrade crust of the east-central part of the RTP. These, together with geochemical data, place new constraints on the timing and nature of the magmatic and tectonometamorphic events that affected this part of the Amazonian craton. We also discuss the implications of these new data for the continuity of geologic events within the domains that constitute the craton.

2. Geologic setting of east-central part of the Rondoˆnia Tin Province The RTP, the second largest tin producer in Brazil, comprises mainly medium- to high grade metamorphic basement rocks of the Jamari and Jaru gneissic
/migmatitic complexes (Scandolara et al., 1999). These are locally overlain by lowgrade to undeformed supracrustal sequences and are intruded by at least six rapakivi granites suites ranging in age from 1.60 to 0.97 Ga (Bettencourt et al., 1999). The Jamari complex consists mainly of granitic, granodioritic and tonalitic orthogneisses, the Jaru gneissic
/migmatitic complex comprises paragneisses and orthogneisses (Scandolara et al., 1999). Reconnaissance Rb
/Sr geochronological results were initially interpreted to reflect either protolith crystallization (Teixeira and Tassinari, 1984) or regional metamorphism (Priem et al., 1989) at
/1.45 to 1.54 Ga. More recently, SHRIMP U
/Pb zircon ages have become available from a regional study of the Rio Negro-Juruena Province (Tassinari et al., 1996). Two orthogneisses from the east-central part of the RTP yielded crystallization ages of 17509/24 and 15709/17 Ma, and were interpreted as corresponding to two magmatic arcs related to tectonic evolution of the Rio Negro-Juruena

Fig. 2. Map of the east-central part of the RTP showing lithologic associations, locations of sampling sites, U
/Pb zircon ages determined in this and previous studies (this study: boxed; (a) Tassinari et al., 1996; (b) Bettencourt et al., 1999, (c) Payolla et al., 2001), and Nd depleted mantle model ages. Inset shows a geological sketch map of Rondoˆnia (Scandolara et al., 1999). JC: Jamari complex; JGMC: Jaru gneissic
/migmatitic complex; BG: Beneficente Group; NBG: Nova Brasilaˆndia Group; dotted: Rondoˆnia Proterozoic basins; squared: Phanerozoic Parecis basin; blank: Cenozoic sedimentary cover; dashed line: Boundary of the RTP; solid line: Eastcentral part of the RTP.

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Province (Tassinari et al., 1996). A high-grade paragneiss in the east-central part of the RTP yielded a SHRIMP U
/Pb metamorphic zircon age of 13319/8 Ma and was interpreted to represent the peak of the Rondonian-San Ignacio orogeny (Tassinari et al., 1999). As a result of a recent mapping project the basement rocks of the east-central part of the RTP were subdivided into five lithologic associations (Fig. 2): (1) tonalitic gneiss; (2) enderbitic granulite; (3) paragneiss; (4) granitic and charnockitic augen gneisses; and (5) fine-grained granitic gneiss and charnockitic granulite. Key features of the igneous rocks of these associations are described below. Geochemical data are presented in Table 1 and sample descriptions and analytical techniques are in Appendices A and B, respectively. 2.1. Tonalitic gneiss association The tonalitic gneiss association is found in the central sector of the area, and is the country rock of the Younger Granites of Rondoˆnia (Fig. 2). Three types of upper amphibolite facies tonalitic gneisses are recognized. The first is a gray, moderately to strongly foliated and homogeneous medium-grained tonalitic to granodioritic gneiss with sparse elongated amphibolitic enclaves. The minerals present are plagioclase, quartz, hornblende, biotite, and occasional K-feldspar and minor titanite, magnetite, and zircon. One sample of this gneiss (B-335; Fig. 2) yielded a SHRIMP U
/Pb zircon age of 17509/24 Ma (Tassinari et al., 1996). Toward the east, this gneiss grades into a banded migmatitic gneiss. Concordant, locally folded coarse-grained quartz-feldspatic veins define the banding. The third gneiss type is associated with the banded type, and is recognized by ubiquitous garnet in the matrix. The tonalitic gneisses fall into the calc-alkaline field (Fig. 3) and have narrow ranges of SiO2 (61
/ 65 wt.%) and mg # (42
/44; Table 1). They are medium- to high-K (Fig. 4) and metaluminous to marginally peraluminous (A/CNK /0.89
/1.01). In the Rb versus (Y/Nb) diagram, they fall into the field of volcanic arc granites (Fig. 5). On the ORG-normalized multi-element diagram (Fig. 6a), the tonalitic gneisses exhibit patterns enriched in

145

LILE (Rb, Ba, Th) relative to HFSE, with prominent negative Ta anomalies. These patterns are similar to those of granitoids emplaced in active continental margin (cf. Pearce et al., 1984). 2.2. Enderbitic granulite association The enderbitic granulite association is found in the southern part of the area, near Ariquemes (Fig. 2). The dominant rock types are grayish to greenish, homogeneous to banded, weakly to strongly foliated, granulite facies enderbitic gneisses. The banding is defined by variations in the proportions of the mafic and felsic minerals and enhanced by concordant, coarse-grained, quartz-feldspathic bands. Mineral assemblages present are quartz, plagioclase, hornblende, biotite, orthopyroxene, garnet, clinopyroxene, K-feldspar, and minor ilmenite, magnetite, apatite, and zircon. These are overprinted by symplectitic garnet-quartz coronas between pyroxenes and plagioclase and ilmenite/magnetite and plagioclase. The enderbitic gneisses contain disrupted, centimeter- to meterscale mafic granulite layers that are interpreted as deformed mafic dykes. Garnet amphibolite and clinopyroxene granulite units, several meters thick, also occur sporadically throughout the enderbitic granulite association. The rocks of the enderbitic granulite association are marginally calc-alkaline (Fig. 3) and show SiO2 from 48 to 64 wt.% and mg # from 31 to 64 (Table 1). They are metaluminous (A/CNK /0.64
/0.94) and medium- to high-K (Fig. 4). In the Rb versus (Y/Nb) diagram (Fig. 5), most plot into the volcanic-arc granite field. In general, the enderbitic granulites have the same trace element patterns as the tonalitic gneisses (Fig. 6a,b). 2.3. Paragneiss association The paragneiss association is found in the eastern part of the area and hosts the Santa Clara intrusive suite (Fig. 2). Minor discontinuous zones of paragneiss also are observed farther west, but cannot be correlated regionally. The association is dominated by banded, high-grade metapelitic migmatite, consisting of black or gray 1- to 10mm-wide layers of sillimanite, garnet, biotite,

146

Table 1 Major and trace element abundances of rocks from lithologic associations recognized in east-central part of the RTP Association Tonalitic gneiss association

Enderbitic granulite association

Sample

B-335

O-2594

Rock type

Tonalitic gneiss

Banded tonalitic Grt-tonalitic gneiss gneiss

A/CNKb mg #c

0.95 43.68

Trace elements (ppm) Rb 87 Sr 294 Y 21 Zr 176 Nb 6 Ba 607 La 28.00 Ce 58.10 Pr Nd 25.42 Sm 5.76 Eu 1.41 Gd Tb 0.73 Dy Ho Er Tm Yb 2.48

WB-223/B

B-3446/B

B-3446/A

WB-70

WB-17

Mafic granulite

Mafic granulite

Enderbitic granulite

Enderbitic gran- Enderbitic ulite granulite

WB-93/A Enderbitic granulite

62.71 0.72 15.02 7.06 0.11 2.66 5.42 2.54 2.67 0.17 1.06 100.14

64.53 0.58 16.25 5.96 0.09 2.06 4.79 2.84 2.52 0.12 0.68 100.15

48.18 1.24 15.25 12.74 0.18 7.08 9.38 3.05 1.62 0.18 0.77 99.67

49.66 0.67 18.22 9.76 0.14 8.86 9.73 2.18 0.72 0.12 0.42 100.48

57.03 0.94 16.12 10.27 0.15 3.93 7.27 2.40 1.25 0.24 0.38 99.98

59.65 1.25 15.41 9.41 0.16 2.14 6.28 2.83 2.12 0.41 0.23 99.89

61.86 0.74 15.95 7.03 0.11 2.98 5.74 2.60 2.12 0.15 0.17 99.45

64.42 0.69 14.81 6.54 0.11 2.58 5.01 2.38 3.23 0.16 0.59 100.52

0.89 42.73

1.01 41.75

0.64 52.39

0.83 64.25

0.87 43.11

0.84 31.07

0.94 45.63

0.90 43.86

94 293 35 166 17 626 36.96 74.44 7.08 32.21 6.52 1.48 6.34 0.91 5.05 1.09 3.22 0.50 3.18

88 248 24 159 11 562 39.40 72.80

72 189 26 79 6 224 12.80 26.31 3.55 15.49 3.88 1.52 4.74 0.86 4.92 1.02 3.20 0.47 3.04

9 172 21 54 11 243 12.23 26.98 2.89 13.75 3.09 0.89 3.17 0.52 3.30 0.71 2.23 0.31 1.98

19 347 34 174 24 501 26.43 67.70 6.44 30.45 6.38 1.64 5.84 0.84 4.92 0.99 3.02 0.41 2.73

57 287 56 305 24 428 55.49 115.9 11.45 52.31 11.22 2.50 11.29 1.64 9.36 1.92 5.77 0.79 5.42

99 318 24 178 12 606 31.92 62.88 6.24 27.51 6.32 1.38 5.37 0.74 4.21 0.81 2.57 0.34 2.20

104 262 41 169 19 778 51.66 112.3 11.43 48.89 9.19 1.48 8.23 1.19 6.65 1.28 4.09 0.56 3.60

29.50 5.84 1.30 0.69

2.43

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Major elements (wt.%) SiO2 61.25 TiO2 0.76 15.67 Al2O3 Fe2O3a 7.48 MnO 0.12 MgO 2.93 CaO 5.56 Na2O 2.52 K2O 2.03 P2O5 0.17 LOI 0.97 Total 99.46

B-250/A

Table 1 (Continued ) Association Tonalitic gneiss association

Enderbitic granulite association

Sample

B-335

O-2594

Rock type

Tonalitic gneiss

Banded tonalitic Grt-tonalitic gneiss gneiss

0.38 4.7 0.65 6.96

0.51 5.2 0.74 11.44

WB-223/B

B-3446/B

B-3446/A

WB-70

Mafic granulite

Mafic granulite

Enderbitic granulite

Enderbitic gran- Enderbitic ulite granulite

0.38 4.8 0.73 8.90

0.41 2.2 0.10 0.64

0.32 1.8 0.34 1.20

WB-17

0.44 4.0 0.65 1.93

0.89 15.3 1.62 7.83

0.36 4.6 0.78 5.02

WB-36

WB-175

Association Granitic and charnockitic augen gneisses association Sample

M-S-6030

WB-46/A

WB-46/C

AR-3/1

WB-44/A

WB-143

Rock type

Pink granitic gneiss

Opx-bearing granitic augen gneiss

Granitic gneiss

Grey granite

Pink granitic augen gneiss

Opx-bearing Pink Hbl-bearquartz monzonite ing Qtz syenite

Pink Hbl-bearing granite

68.02 0.64 14.00 5.14 0.07 0.49 2.52 2.93 5.04 0.16 0.42 99.43

68.97 0.53 14.48 3.72 0.04 0.90 2.14 2.98 5.49 0.15 0.47 99.87

68.62 0.48 15.39 2.93 0.05 0.58 2.16 3.47 5.60 0.14 0.63 100.05

77.54 0.13 11.67 1.50 0.01 0.12 1.11 2.95 5.10 0.02 0.09 100.24

62.99 1.16 15.34 5.94 0.08 1.19 3.71 3.22 5.07 0.43 0.75 99.88

67.89 0.44 15.41 3.31 0.06 0.32 2.10 3.20 6.65 0.12 0.54 100.04

69.53 0.76 14.19 3.90 0.05 0.68 2.37 3.38 5.20 0.23 0.37 100.66

A/CNKb mg #c

0.94 15.89

0.98 32.39

0.98 28.16

0.94 16.68

0.88 28.40

0.95 16.08

0.92 25.66

139 310 73 844 36 1566 90.32 192.3

167 242 59 507 28 1892 64.91 137.0

198 190 64 501 29 1095 88.40 185.1

0.91 20.10

Trace elements (ppm) Rb 260 Sr 83 Y 196 Zr 609 Nb 24 Ba 645 La 88.94 Ce 198.9

122 152 59 697 24 1020 78.26 161.5

179 239 44 411 14 1271 173.1 327.1

233 171 74 259 11 1008 47.18 94.47

307 43 117 204 23 184 139.0 257.0

Enderbitic granulite 0.58 4.6 0.82 16.19

147

Major elements (wt.%) SiO2 69.38 TiO2 0.65 Al2O3 13.88 4.33 Fe2O3a MnO 0.05 MgO 0.55 CaO 1.82 Na2O 4.24 K2O 4.59 P2O5 0.14 LOI 0.41 Total 100.04

WB-93/A

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Lu Hf Ta Th

B-250/A

148

Table 1 (Continued ) Association Granitic and charnockitic augen gneisses association Sample

M-S-6030

WB-46/A

WB-46/C

AR-3/1

WB-44/A

WB-143

Rock type

Pink granitic gneiss

Opx-bearing granitic augen gneiss

Granitic gneiss

Grey granite

Pink granitic augen gneiss

Opx-bearing Pink Hbl-bearquartz monzonite ing Qtz syenite

21.47 92.66 18.96 2.42 19.35 3.28 21.31 4.69 15.62 2.28 14.03 2.28 16.3 2.52 9.22

15.95 73.95 15.27 3.31 14.46 2.18 11.42 2.28 6.41 0.83 5.46 0.86 19.6 1.27 7.87

35.12 116.1 17.20 1.79 8.54 1.26 6.32 1.05 3.07 0.37 1.96 0.29 10.1 0.67 54.90

9.81 43.23 10.23 2.01 9.74 1.84 10.40 2.03 6.38 0.89 5.71 0.85 7.7 3.72 15.94

21.76 82.24 16.02 0.69 16.90 2.66 15.72 3.50 11.54 1.70 12.67 1.87 8.1 1.98 90.35

Association Fine-grained granitic gneiss and charnockitic granulite association Sample

WB-29/A

WB-51

WB-71

WB-223/A

WB-95

Rock type

Graniticgneiss

Grt-granitic gneiss

Charnockitic granulite

Charnockitic granulite

Granitic gneiss

Major elements (wt.%) SiO2 68.30 TiO2 0.68 Al2O3 13.68 5.58 Fe2O3a MnO 0.09 MgO 0.65 CaO 2.15 Na2O 3.44 K2O 5.04 P2O5 0.16 LOI 0.24 Total 100.01 A/CNKb mg #c

0.91 18.74

68.76 0.43 13.41 5.66 0.13 0.14 1.36 2.98 6.84 0.07 0.08 99.86

69.91 0.44 13.40 4.88 0.11 0.30 1.44 3.09 5.80 0.08 0.78 100.23

71.38 0.51 13.27 4.69 0.06 0.44 1.48 2.81 5.08 0.09 0.61 100.42

72.26 0.48 12.61 3.36 0.05 0.51 1.37 2.77 5.38 0.09 0.68 99.56

0.91 4.67

0.96 10.85

1.04 15.68

0.98 23.12

19.24 89.00 17.53 3.96 16.98 2.45 12.55 2.50 7.09 0.90 5.89 0.90 21.8 2.15 5.30

14.39 67.57 13.57 4.54 13.37 1.92 9.62 1.95 5.83 0.72 4.74 0.70 13.7 1.72 2.60

WB-175 Pink Hbl-bearing granite

21.99 78.51 14.92 2.71 11.30 1.92 10.60 2.05 5.88 0.86 5.14 0.75 12.1 1.45 9.53

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Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th

WB-36

Table 1 (Continued ) Association Fine-grained granitic gneiss and charnockitic granulite association Sample

WB-29/A

WB-51

WB-71

WB-223/A

WB-95

Rock type

Graniticgneiss

Grt-granitic gneiss

Charnockitic granulite

Charnockitic granulite

Granitic gneiss

a b c

173 40 100 981 15 536 80.38 163.7 16.09 74.73 16.19 2.93 17.61 2.90 15.85 3.54 10.85 1.58 10.62 1.64 24.0 1.6 20.45

157 81 99 679 19 874 88.70 180.5 17.63 77.40 16.11 2.82 17.10 2.70 15.29 3.26 10.38 1.48 10.28 1.58 16.4 0.91 22.33

142 73 79 693 23 722 157.0 296.0 34.70 123.0 22.20 2.50 21.70 3.14 15.90 2.98 8.90 1.32 8.26 1.15 16.4 0.60 28.70

177 90 70 372 15 691 76.89 146.3 13.68 58.57 11.72 1.66 12.53 1.83 10.49 2.22 6.76 1.00 6.46 0.99 11.4 1.03 23.38

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Trace elements (ppm) Rb 194 Sr 108 Y 104 Zr 508 Nb 31 Ba 689 La 82.38 Ce 171.2 Pr 16.57 Nd 73.79 Sm 15.72 Eu 2.52 Gd 15.95 Tb 2.56 Dy 15.12 Ho 3.28 Er 10.00 Tm 1.44 Yb 9.44 Lu 1.45 Hf 13.9 Ta 1.84 Th 18.25

Total Fe expressed as Fe2O3. Molecular Al2O3/(CaO/Na2O/K2O). mg number: 100/[Mg/(Mg/Fe)].

149

150

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Fig. 3. Chemical composition (Table 1) of the tonalitic gneiss, enderbitic granulite, granitic and charnockitic augen gneisses, and finegrained granitic gneiss and charnockitic granulite associations plotted in the AFM (Na2O/K2O/total FeO
/MgO; wt.%) diagram. Tholeiitic and calc-alkaline fields after Irvine and Baragar (1971).

Fig. 4. Chemical composition (Table 1) of the tonalitic gneiss, enderbitic granulite, granitic and charnockitic augen gneisses and fine-grained granitic gneiss and charnockitic granulite associations plotted in the K2O versus SiO2 diagram. Lines separating the low-K, medium-K, high-K and ultra-high-K fields are from Le Maitre et al. (1989) and Rickwood (1989). Symbols as in Fig. 3.

cordierite, ilmenite, magnetite, hercynite, and orthopyroxene melanosome (refractory restite) and 5- to 50-mm-wide irregular layers of creamypink K-feldspar, quartz, plagioclase, garnet, and

Fig. 5. Rb vs. (Y/Nb) discrimination diagram (Pearce et al., 1984) for the tonalitic gneiss, enderbitic granulite, granitic and charnockitic augen gneisses and fine-grained granitic gneiss and charnockitic granulite associations. ORG/ocean ridge granites, WPG/within plate granites, VAG/volcanic arc granites, syn-COLG/syn-collisional granites. Symbols as in Fig. 3.

cordierite leucosome. Metamorphic crystallization and anatexis took place under upper amphibolite to granulite facies conditions. Associated rocks include pink to creamy medium-grained garnetbearing leucogneisses, pink garnet-bearing augen gneisses, and rare calc-silicate gneisses. Mafic

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151

Fig. 6. Ocean ridge granite-normalized distribution patterns for: (a) tonalitic gneiss association; (b) enderbitic granulite association; (c) hornblende-bearing and orthopyroxene-bearing granitoids of the granitic and charnockitic augen gneisses association; (d) gray and pink granites of the granitic and charnockitic augen gneisses association; and (e) fine-grained granitic gneiss and charnockitic granulite association. Normalizing values from Pearce et al. (1984).

dikes intruded all rock types of the paragneiss association. 2.4. Granitic and charnockitic augen gneiss association This association comprises three groups of megacrystic and non-megacrystic igneous rocks variably affected by tectonometamorphic events: (1) pink granites; (2) charnockites (orthopyroxenebearing granitoids); and (3) gray granites. The degree of deformation varies from pristine, non-

foliated to gneissic and augen gneissic. The pink granites dominate in the Ariquemes and 58 BEC regions where they are associated with charnockites (Fig. 2). In the Ariquemes region, megacrystic hornblende-biotite monzogranites and syenogranites are found as the country rocks of the Younger Granites of Rondoˆnia. Rapakivi-textured K-feldspar megacrysts are relatively common both in the granites and the charnockites. In the 58 BEC region, non-megacrystic monzogranite to syenogranite and gneisses are present with hornblende, biotite, and magnetite as the main

152

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mafic minerals and titanite as a major accessory. One of the non-megacrystic granites (M-S-6030; Fig. 2) was dated by U
/Pb on zircon at 15709/17 Ma (Tassinari et al., 1996). The Unia˜o massif is the largest and least deformed granitic intrusion of the association in the studied area and consists of a composite body of pink megacrystic, hornblendebearing quartz monzonite, quartz syenite, and syenogranite, and a greenish megacrystic, pyroxene-bearing quartz monzonite (quartz mangerite). The hornblende-bearing quartz syenite (sample WB-36; Fig. 2) has an U
/Pb zircon age of 15329/5 Ma (Bettencourt et al., 1999). The gray granites dominate in the north, between the Samuel Reservoir and Jacunda´ River (Fig. 2). They are megacrystic to equigranular, hornblende and/or biotite syenogranite to monzogranite with titanite and allanite as major accessory minerals. Locally, they exhibit swarms of enclaves and dykes of mafic and hybrid rocks and rapakivi-texture is common in the granites near the margins of these mingled zones. The gray granites are generally undeformed and show a variable magmatic foliation. Augen gneisses with a S
/C structure are observed in mylonitic shear zones that crosscut the granites. The rocks of the granitic and charnockitic augen gneisses association have a high- to ultra-high-K character (Fig. 4), are metaluminous (A/CNK / 0.88
/0.98; Table 1), show strong iron enrichment (mg # between 16 and 28; Fig. 3), and have an Atype trace element signature. In the Rb versus (Y/ Nb) diagram (Fig. 5), they plot into the within plate granite field. On the ORG-normalized multielement diagram (Fig. 6c,d), they show patterns similar to those of crust-dominated, within plate granites (cf. Pearce et al., 1984). The orthopyroxene- and hornblende-bearing granitoids of the Unia˜o massif and Ariquemes region yield more homogeneous patterns, but do not have the Ba depletion typical of within plate granites (Fig. 6c). 2.5. Fine-grained granitic gneiss and charnockitic granulite association These rocks are pink or greenish, fine- to medium-grained, quartz-feldspathic banded amphibolite to granulite facies gneisses in the south-

ern part of the study area (Fig. 2). Banding is defined by alternating quartz, plagioclase and Kfeldspar layers and hornblende, garnet, orthopyroxene, magnetite, and clinopyroxene layers. Garnet occurs as subhedral crystals free from inclusions and as symplectic intergrowths with quartz between pyroxenes and plagioclase and hornblende and feldspars. Banding is enhanced by concordant, locally folded granitic veins. Metacharnockite and mafic granulites xenoliths support an intrusive origin for the protolith of the fine-grained gneisses and granulites. Amphibolites are important associated rock types, they are found as 30- to 100-cm-wide bands and rootless folds, some partially disaggregated. SiO2 content of the fine-grained granitic gneisses and charnockitic granulites ranges from 68 to 72 wt.% (Table 1). The rocks are characterized by strong iron enrichment (mg # between 5 and 23; Fig. 3), metaluminous to marginally peraluminous compositions (A/CNK /0.91
/1.04), and a highto ultra-high-K signature (Fig. 4). The trace element pattern is A-type, marked by high Rb, Zr, Y, Nb and REE and low Sr contents ( B/100 ppm), but high Ba (870
/540 ppm). In the Rb versus (Y/Nb) diagram (Fig. 5) these rocks plot into the within plate granite field. On the ORGnormalized multi-element diagram (Fig. 6e) they exhibit patterns enriched in LILE and with Nb
/Ta troughs. This is typical of granitoids emplaced in a crust-dominated, within plate setting (cf. Pearce et al., 1984). 2.6. Metamorphism Rocks with granulite facies assemblages are distributed along an
/E
/W trending belt extending from 58 BEC to Ariquemes (Fig. 2). North of this belt, only mineral assemblages within upperamphibolite conditions are observed. The regional structure of the east-central part of the RTP is dominated by pervasive NNE
/SSW to NNW
/ SSE trends. Metamorphic temperatures and pressures were estimated for the enderbitic granulites and garnet-bearing tonalitic gneiss by the senior author using the TWQ program of Berman (1991). The enderbitic granulites record temperatures of 770
/740 8C and pressures of 900
/700 MPa,

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which suggest that the peak granulite mineralogy was re-equilibrated during cooling or by a subsequent metamorphic event under upper-amphibolite conditions. A garnet-bearing tonalitic gneiss records a temperature of 720 8C and pressure of 840 MPa, compatible with upper-amphibolite facies.







3. U /Pb and Sm /Nd results Our U
/Pb zircon and whole-rock and garnet Nd isotopic data are shown in Tables 2 and 3 and Figs. 7
/9. Sample locations and the geochronological results are shown in Fig. 2. The analytical procedures are outlined in Appendix B.

153

migmatitic enderbitic gneiss (sample WB-70) were analyzed. The upper and lower intercepts of the six-point discordia are 17309/22 and 11879/86 Ma, respectively (Fig. 8a). The upper intercept is interpreted as the best estimate for the crystallization age of the tonalitic protolith, the lower intercept age may represent the effects of a metamorphic event around 1190 Ma. Nd isotopic data yield an initial o Nd of /0.1 to /0.6 and TDM of 2.06
/2.14 Ga (Table 2; Fig. 7), indicating that a significant older (Ventuari-Tapajo´s) crustal component. An associated mafic granulite (sample WB-223/B) has an initial o Nd of /3.0 and a lower TDM of 1.86 Ga (Table 2; Fig. 7) and presumably represents material derived from a depleted source with little contribution from older continental crust.

3.1. Tonalitic gneiss association 3.3. Paragneiss association One homogeneous, medium-grained tonalitic gneiss (sample B-335) with SHRIMP U
/Pb zircon age of 17509/24 Ma (Tassinari et al., 1996) and one garnet-bearing tonalitic gneiss (sample B-250/ A) were analysed for Nd isotopes. The samples have initial o Nd values of /1.5 and 0.0 and Nd model ages (TDM) of 2.20 and 2.06 Ga, respectively (Table 2; Fig. 7). The negative to zero o Nd(T) values indicate that the original tonalitic magma was derived from a source containing a significant older crustal component. The garnet-bearing tonalitic gneiss (sample B250/A) was also used to date the main metamorphic episode by constructing a two point whole rock-mineral isochron (cf. Mezger et al., 1992). Euhedral and inclusion-free garnets yielded a two-point garnet-whole rock chord suggesting an age on the order of
/1307 Ma (not illustrated). This age is similar to a SHRIMP U
/Pb metamorphic zircon age (Tassinari et al., 1999) and a U
/Pb monazite age (this study) from the region, and is interpreted to reflect the timing of the garnet growth during a high-grade metamorphic event at upper amphibolite conditions. 3.2. Enderbitic granulite association Six euhedral, colorless to pink zircon fractions from a homogeneous paleosome of a banded

Detrital small, euhedral, and pink to yellow zircons were selected from a gray, banded metapelitic migmatite (sample WB-152). Five singlegrain analyses yielded 207Pb/206Pb ages ranging from 1673 to 1808 Ma and plot roughly on a discordia with upper and lower intercepts at 19579/240 and 12549/270 Ma, respectively (Table 3; Fig. 8b). The scatter in the U
/Pb data is consistent with zircons derived from several different sources. As such, the intercept ages on the concordia diagram are neither precise, nor particularly useful. The individual data points obtained indicate, however, that the sources for this paragneiss ranged from 1808 to 1673 Ma, with the maximum depositional age constrained by the youngest age. Sm
/Nd analysis for two metapelitic migmatites (samples WB-140 and WB-152) indicate similar 147Sm/144Nd ratios (0.1088 and 0.1037) and TDM (2.15 Ga and 2.10 Ga) (Table 2; Fig. 7). A leucosome portion represented by a garnetbearing leucogneiss (sample WB-147) yields a high 147 Sm/144Nd ratio (0.1464) and an older TDM age of 2.40 Ga. Sm
/Nd garnet-whole-rock data for a garnetbearing leucosome portion of the paragneiss (sample WB-147) yields an age of
/1300 Ma (not illustrated). This age corresponds with a 1330
/1300 Ma high-grade metamorphic event,

154

Table 2 Sm
/Nd whole-rock and garnet analytical data Sample

Age (Ma)

Sm (ppm)

Nd (ppm)

147

Sm/144Nd

143

Nd/144Nd

Error (/10 6)

o Nd (0)f

o Nd (T)

f

TDM (Ga)g

5.92 4.03 3.04

30.81 21.26 1.82

0.1180 0.1146 1.0126

0.511656 0.511693 0.519392

6a 17a 27a

/19.2 /18.4

/1.5 0.0

2.20 2.06

Enderbitic granulite association WB-70a 17309/22 17309/22 WB-70b WB-223/Ba (1730)h

11.01 10.91 3.80

54.10 53.96 15.28

0.1230 0.1222 0.1505

0.511764 0.511798 0.512263

11a 15b 20a

/17.0 /16.4 /7.3

/0.6 0.1 3.0

2.14 2.06 1.86

3.85 6.81 3.75 6.24

21.42 28.11 4.93 36.37

0.1088 0.1464 0.4631 0.1037

0.511558 0.511982 0.514621 0.511517

13b 25a 11a 15b

/21.1 /12.8

2.15 2.40

/21.9

2.10

Granitic and charnockitic augen gneisses association M-S-6030a 15709/17c 19.10 WB-46/Ab 1560 15.55 1560 17.61 WB-46/Cb AR-3/1a 15449/05 11.06 WB-36b 15329/04d 14.76 15269/12 14.88 WB-44/Ab

92.40 78.32 125.8 45.97 74.10 77.66

0.1250 0.1200 0.0846 0.1450 0.1208 0.1158

0.511956 0.511891 0.511455 0.512101 0.511886 0.511853

5a 13b 10b 11a 6b 11b

/13.3 /14.6 /23.1 /10.4 /14.7 /15.3

1.1 0.8 /0.7 /0.2 0.2 0.5

1.85 1.86 1.86 2.07 1.88 1.84

Fine-grained granitic gneiss and charnockitic granulite WB-51b 14339/11 16.40 WB-71a (1433)h 16.51 WB-223/Aa 14249/10e 21.95

association 76.78 80.64 116.5

0.1291 0.1237 0.1139

0.512061 0.512011 0.511892

14b 15a 22a

/11.3 /12.2 /14.6

1.2 1.2 0.6

1.75 1.73 1.74

Paragneiss association WB-140b WB-147a WB-147 garneta WB-152b

a b c d e f g h

Determinations at the Geochronology Laboratory, University of Brasilia (143Nd/144Nd error at 9/1s ). Determinations at the Isotope Geochemistry Laboratory, University of Kansas (143Nd/144Nd error at 9/2s ). Tassinari et al. (1996). Bettencourt et al. (1999). Payolla et al. (2001). CHUR values used in o Nd calculations are 147Sm/144Nd/0.1966 and 143Nd/144Nd/0.512638. Depleted mantle model age according to DePaolo (1981). Age inferred from lithologic association.

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Tonalitic gneiss association 17509/24c B-335a a B-250/A (1750)h B-250/A garneta

Table 3 U
/Pb zircon and monazite analytical data Sample fractiona

Weight (mg)b

206 Pb U Pb/204Pb (ppm) (ppm) observed

Radiogenic ratiosc

Calculated ages (Ma)d

Error9/2s (%)

207

207

206

Error9/2s (%)

206

235

Error9/2s (%)

207

238

238

235

206

Pb/ U

Pb/ U

Pb/ Pb

Pb/ U

Pb/ U

207 Pb/206Pb 9/2s (Ma)

580 545 419 380 703 727

4438 2348 6770 3744 3601 23814

0.28704 0.26231 0.29720 0.28299 0.29064 0.29735

0.64 0.56 0.53 0.53 0.49 0.51

4.0500 3.5070 4.2717 3.9771 4.1190 4.2534

0.92 0.60 0.57 0.55 0.50 0.54

0.10233 0.09696 0.10424 0.10193 0.10279 0.10375

0.67 0.22 0.20 0.12 0.11 0.16

1627 1502 1677 1606 1645 1678

1644 1529 1688 1630 1658 1684

16679/12 15679/04 17019/04 16609/02 16759/02 16929/03

Paragneiss association Sample WB-152 NM(2) P, eu NM(2) P, eu, sm M(2) P, eu M(3) Y, eu M(5) Y, eu

131 237 240 302 287

454 824 791 1051 900

1488 1469 1872 2387 4676

0.27810 0.27269 0.28757 0.27286 0.30628

0.88 0.51 0.91 0.50 0.48

3.9394 3.8817 4.2720 3.8639 4.6676

0.90 0.51 0.94 0.50 0.49

0.10274 0.10324 0.10774 0.10270 0.11053

0.18 0.08 0.23 0.08 0.08

1582 1554 1629 1555 1722

1622 1610 1688 1606 1762

16749/03 16839/02 17619/04 16739/02 18089/02

Granitic and charnockitic augen gneisses association Sample WB-46/A NM(-2) Y, ac 0.005 501 1933 10073 M(-1) Y, eu 0.006 (1) 117 444 4818 M(0) Y, eu 0.023 145 548 5478 M(-1) pY, eu 0.007 114 433 5213 M(2) Y, fb 0.011 22 80 553 M(2) Y, ac 0.003 (1) 102 330 610 M(2) Y, ac 0.011 116 453 3611 M(3) Y, eu 0.006 154 591 1433 M(4) Y, eu 0.009 (1) 123 462 1665 M(5) Y, eu 0.005 (1) 132 506 896

0.25404 0.24907 0.25795 0.25044 0.25079 0.27623 0.25063 0.25360 0.25861 0.25132

0.47 0.69 0.49 0.55 0.82 0.70 0.48 0.70 0.65 0.89

3.2572 3.1367 3.3315 3.1889 3.1921 3.5551 3.1829 3.2511 3.3461 3.1919

0.48 0.79 0.49 0.59 0.86 0.77 0.49 0.71 0.65 0.89

0.09299 0.09134 0.09367 0.09235 0.09231 0.09334 0.09210 0.09298 0.09384 0.09211

0.08 0.38 0.08 0.22 0.23 0.34 0.10 0.09 0.09 0.10

1459 1434 1479 1441 1443 1572 1441 1457 1482 1445

1471 1442 1488 1455 1455 1540 1453 1469 1492 1455

14889/02 14549/07 15019/02 14759/04 14749/04 14949/06 14709/06 14879/02 15059/02 14709/02

Sample WB-46/C NM(-1) P, gr NM(-1) Y, gr M(-1) Y, gr M(-1) Y, gr M(-1) Y, gr M(1) Y, gr M(2) P, gr M(3) P, gr

0.25550 0.25600 0.25725 0.25430 0.25705 0.25825 0.25297 0.24859

0.60 0.47 0.56 0.51 0.52 0.49 0.54 0.60

3.3190 3.3030 3.3600 3.5265 3.3533 3.3388 3.2782 3.1832

0.63 0.47 0.59 0.52 0.53 0.49 0.56 0.61

0.09421 0.09358 0.09469 0.10058 0.09461 0.09377 0.09399 0.09287

0.18 0.06 0.09 0.10 0.09 0.09 0.12 0.11

1467 1469 1476 1461 1475 1481 1454 1431

1486 1482 1495 1533 1494 1490 1476 1453

15129/03 15009/02 15229/03 16359/02 15209/02 15039/03 15089/02 14859/02

0.012 0.012 0.008 0.012 0.019

0.006 0.033 0.008 0.013 0.012 0.012 0.012 0.007

257 262 288 226 307 304 264 259

876 898 949 756 1007 1023 926 913

1415 9172 2154 2236 3199 3914 3318 1897

155

175 148 133 115 212 224

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Enderbitic granulite association Sample WB-70 NM(-1) P, eu 0.017 M(-1) P, eu 0.008 (1) M(-1) C, eu 0.011 (1) M(0) C, eu 0.006 (1) M(1) 1P, eu 0.013 M(2) B, eu 0.016 (1)

156

Table 3 (Continued ) Sample fractiona

Weight (mg)b

206 Pb U Pb/204Pb (ppm) (ppm) observed

Radiogenic ratiosc

Calculated ages (Ma)d Error9/2s (%)

207

207

206

Error9/2s (%)

206

235

238

235

206

Pb/ U

Pb/ U

Pb/ Pb

Pb/ U

Pb/ U

207 Pb/206Pb 9/2s (Ma)

M(4) Y, gr M(5) Y, gr

0.003 (1) 0.003 (1)

262 316

814 1066

728 1159

0.25785 0.25402

0.89 0.76

3.3630 3.2856

0.90 0.77

0.09459 0.09381

0.12 0.13

1479 1459

1496 1478

15209/02 15049/03

Sample AR-3/1 NM(-1) B, eu M(-1) B, eu M(0) B, eu M(1) B, eu

0.003 0.009 0.009 0.009

250 286 459 546

928 1117 1862 1995

4763 7732 24388 30051

0.26537 0.24590 0.24053 0.26518

0.50 1.20 1.19 0.47

3.5131 3.2507 3.1765 3.4921

0.55 1.21 1.20 0.48

0.09602 0.09588 0.09578 0.09551

0.22 0.15 0.10 0.06

1517 1417 1389 1516

1530 1469 1452 1525

15489/04 15459/03 15439/02 15389/01

B/0.001 (1) 0.003 0.003 0.012 (1) 0.018 (1)


/ 449 245 321 282


/ 1710 945 1415 1215

576 6656 3069 19724 21495

0.17013 0.26175 0.25494 0.22241 0.21833

1.45 0.71 0.76 0.83 0.46

2.1649 3.4130 3.3364 2.7692 2.5728

1.49 0.73 0.76 0.99 0.47

0.09229 0.09456 0.09492 0.09030 0.08546

0.36 0.16 0.11 0.53 0.07

1013 1499 1464 1295 1273

1170 1507 1490 1347 1293

14739/07 15199/03 15269/02 14329/10 13269/01

Fine-grained granitic gneiss and charnockitic granulite association Sample WB-51 NM(-1) Y, eu 0.007 (1) 72 296 1344 0.24301 M(-1) Y, st 0.010 (1) 91 393 3041 0.23600

0.65 0.56

3.0131 2.9087

0.67 0.60

0.08993 0.08939

0.14 0.20

1402 1366

1411 1384

14249/03 14129/04

Sample WB-44/A NM(0) p NM(0) y, f NM(0) y, sm NM(0) y, lg Monazite

a

M, NM refer to magnetic, non-magnetic splits at Franz separator tilt (at 1.5 A) given in parentheses. Other abbreviations: ac, acicular; C, colorless; eu, euhedral; fb, football-shaped; lg, large; P, pink; p, pale; ro, rounded; st, stubby; sm, small; Y, yellow; B, brown. b (1) /single grain sample. c Corrected for analytical blank and original non-radiogenic Pb in zircon; see Appendix B.  Denotes radiogenic Pb isotopes (corrected for common Pb). d Based on the decay constants of Steiger and Ja¨ger (1977).

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Error9/2s (%)

207

238

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157

Fig. 7. o Nd versus age plot showing the initial o Nd values and evolution lines for rocks of the lithologic associations identified in the east-central part of the RTP. Depleted mantle model according to DePaolo (1981), CHUR is the chondritic uniform reservoir. Dotted lines: trajectories for paragneisses.

identified both by U
/Pb zircon and monazite data (Tassinari et al., 1999; this study) and the Sm
/Nd garnet-whole rock age obtained for the garnetbearing tonalitic gneiss (sample B-250/A; this study). 3.4. Granitic and charnockitic augen gneisses association A charnockitic augen gneiss (orthopyroxenebearing monzogranite; WB-46/A), a foliated monzogranitic dike (WB-46/C) that cuts the charnockitic augen gneiss, a gray biotite monzogranite (AR3/1), and a pink syenogranitic augen gneiss (WB44/A) were selected for U
/Pb studies (Table 3). These four samples, as well as the pink monzogranitic gneiss (M-S-6030) of Tassinari et al. (1996) and a hornblende-bearing quartz syenite of the Unia˜o massif (WB-36) from Bettencourt et al. (1999) were also analyzed for Nd isotopes (Table 2). Sample WB-46/A yielded yellow acicular to euhedral zircons, sample WB-46/C yellow to pink zircons. Multiple fractions analyzed for each yielded crystallization ages of
/1560 Ma but indicated significantly different lower intercepts (Fig. 8c). The lower intercept for the host charnockitic augen gneiss appears to be
/1200 Ma and may reflect the effects of a metamorphic event,

as in the case for the enderbitic gneiss (sample WB70; Fig. 8a). The lower intercept for the granitic dike (WB-46/C) indicates a substantially younger age, suggesting that other Pb-loss mechanisms may have affected these zircons. For the gray biotite monzogranite (sample AR3/1), four fractions of euhedral and brown zircons were analyzed. Two plots close to the concordia, and all four define a regression line with an upper intercept at 15449/5 Ma when forced through the origin (Fig. 8d). This age is interpreted as the best estimate for the crystallization age of the gray monzogranite. Four fractions of yellow zircons of variable size were analyzed from the augen gneiss (WB-44/A). The two most concordant fractions have 207 Pb/206Pb ages between 1519 and 1526 Ma; these are interpreted to delineate the probable range of crystallization ages for this unit. A three-point regression through the two most concordant and the most discordant fraction yields an upper intercept of 15269/12 Ma (Fig. 8e) and is considered as a reasonably good estimate of the crystallization age. Lack of collinearity of the U
/ Pb data for this unit may be due to cryptic inheritance as well as complex Pb-loss and possibly new zircon growth. Independent evidence for these is non-juvenile Nd signature (TDM of around 1.84 Ga) and evidence for high-grade metamorphism

158

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/169

Fig. 8. U
/Pb concordia diagrams for sample WB-70 from the enderbitic granulite association (a), WB-152 from the paragneiss association (b), WB-46/A and WB-46/C from the granitic and charnockitic augen gneisses association (c), AR-3/1 from the granitic and charnockitic augen gneisses association (d), WB-44/A from the granitic and charnockitic augen gneisses association (e), and WB-51 from the fine-grained granitic gneiss and charnockitic granulite association (f).

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around
/1330 Ma as indicated by the monazite in this rock (Fig. 8e). The monazite grain analyzed from this sample yielded slightly discordant 207 Pb/206Pb age of 13269/1 Ma (Table 3). Because of its relatively high closure temperature for Pb diffusion (
/725 8C; Parrish, 1990), the age of 13269/1 Ma is interpreted as the time of monazite growth or complete resetting of monazite at upper amphibolite conditions. The six samples analyzed for Nd isotopes yielded initial o Nd values of /0.6 to /1.2, and a narrow TDM range of 1.84
/2.07 Ga (Table 2). This indicates that either (1) the original magma was derived from an older crustal source with a Nd signature similar to the mafic granulite of the enderbitic gneiss association, or that (2) the original magma resulted from mixture of magmas derived from depleted mantle and older crustal sources (see Fig. 7).

159

Fig. 9. Four-point Sm
/Nd regression diagram for two wholerock-garnet pairs, tonalitic gneiss B-250/A and garnet-bearing leucogneiss WB-147.

a phenomenon commonly observed in continental arc rocks.

3.5. Fine-grained granitic gneiss and charnockitic granulite association 4. Discussion Two fractions of yellow euhedral and stubby zircons from a garnet-bearing granitic gneiss (sample WB-51) plot near the concordia, defining an upper intercept age of 14339/11 Ma and a lower intercept of 5489/230 Ma (Fig. 8f). The upper intercept is interpreted as the crystallization age of the protolith of the gneiss, the lower intercept is considered to have no geological significance. Three samples from the fine-grained granitic gneiss and charnockitic granulite association were analyzed for Nd isotopes (Table 2). They yielded slightly positive initial o Nd values of /1.2 to /0.7, and a narrow range of TDM between 1.75 and 1.73 Ga. This rock association displays positive initial o Nd values and the negative o Nd (at 1430 Ma) values of the surrounding older felsic basement (/ 4.9 to /2.9 for tonalitic gneisses and enderbitic granulites and /0.9 to /0.1 for granitic and charnockitic augen gneisses; Fig. 7) show that the fine-grained granitic gneiss and charnockitic granulites are clearly not a direct result of melting of the basement. They could represent a mixture of younger juvenile material and the older basement,

The 17309/22 Ma protolith age of the enderbitic granulites suggests that they formed during the same magmatic interval that produced the 17509/ 24 Ma tonalitic gneisses dated by Tassinari et al. (1996). These ages constrain the oldest magmatic episode recorded in the east-central part of the RTP that has been related to the development of a 1.80
/1.70 Ga magmatic arc in the Rio NegroJuruena Province (Tassinari et al., 1996). The geochemical signature and the Nd isotopic composition (initial o Nd //1.5 to /0.2; TDM /2.10
/ 2.20 Ga) of these rocks are interpreted to suggest that the parental calc-alkaline tonalitic magmas were derived from depleted sources with a significant contribution from an older crustal component. The tectonic setting that best accounts for these calc-alkaline magmas is an Andean-style continental margin with subduction toward the northeast (present coordinates) underneath Ventuari-Tapajo´s continental crust. The latter is characterized by arc-related igneous rocks with ages between 1.95 and 1.80 Ga and TDM between 2.1 and 2.0 Ga (Tassinari and Macambira, 1999).

160

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Paragneisses are recognized here as an important lithologic element in the east-central part of the RTP. Although the depositional age of the original sediment is not well established, a maximum can be inferred by the
/1674 Ma detrital zircon in the paragneisses and a minimum by a 1570 Ma foliated granite (Tassinari et al., 1996) that intrudes the gneisses. As such, the paragneisses probably do not represent high-grade metamorphic equivalents of the 1740
/1700 Ma volcano-sedimentary Beneficente and Roosevelt groups (Santos et al., 2000) exposed just to the southeast. Nonetheless, because these paragneisses contain detrital zircons of 1674
/1808 Ma and yield TDM between 2.2 and 2.1 Ga, the VentuariTapajo´s crust (1.95
/1.80 Ga) and the 1750
/1730 Ma magmatic arc tonalites and enderbitic granulites are postulated to have been a major source of detritus throughout the depositional history. The 1570
/1530 Ma granitic and charnockitic augen gneisses cover almost 50% of the study area (Fig. 2), and have been also identified farther south (Bettencourt et al., 1999; Santos et al., 2000). These rocks intruded the 1.75
/1.73 Ga arc-related tonalitic rocks and the 1.67
/1.57 Ga sediments, and are correlated with well-preserved megacrystic granites and charnockites of the Serra da Provideˆncia intrusive suite (U
/Pb ages of 1.60
/1.53 Ga, Bettencourt et al. 1999; TDM /1.87
/1.76 Ga; Bettencourt, Unpublished data). This suite comprises rapakivi granites, charnockites, and gabbros, and shows evidence of magma mingling and mixing (Bettencourt et al., 1997). A preferred interpretation for the observed geochemical signatures (A-type and within plate granites) and Nd isotopic compositions (initial o Nd //0.6 to /2.0; TDM /1.76
/2.07 Ga) of the megacrystic granites, charnockites, and related augen gneisses is a mixture of magmas derived from depleted mantle and older (1.95
/1.73 Ga) crustal sources. Integrated geologic, geochemical, and geochronologic data have led to recognition of four distinct intrusive episodes in the Serra da Provideˆncia intrusive suite: (i) 1590
/1570 Ma (megacrystic granites of the Serra da Provideˆncia batholith); (ii) 1570
/1560 Ma (charnockites of Ouro Preto/ Ariquemes region); (iii) 1550
/1540 Ma (gray granites of the Samuel region); and (iv) 1530 Ma

(pyroxene- and hornblende-bearing granitoids of Unia˜o massif and pink granites of Ariquemes region). In the southwestern Amazonian craton, the Serra da Provideˆncia intrusive suite has been interpreted as a probable inboard expression of the subduction-related magmatism of the Cachoeirinha orogeny, situated farther southeast in the state of Mato Grosso (Tassinari et al., 2000; Geraldes et al., 2001). The fine-grained granitic gneiss and charnockitic granulite association are voluminously minor within the study area (Fig. 2); however, these rocks are widespread farther to the south and west. The protolith crystallization age of 14339/11 Ma for the granitic gneiss overlaps the age of 14249/10 Ma obtained by Payolla et al. (2001) for a charnockitic granulite (sample WB-223/A; Fig. 2) and provide the first evidence of widespread Aand within-plate-type granitic magmatism during the Rondonian-San Ignacio time in Rondoˆnia. These geochemical features and the Nd isotopic compositions (initial o Nd //1.2 to /0.7; TDM / 1.75
/1.73 Ga) suggest that they may represent a mixture of younger juvenile material and an older basement comprising the tonalitic gneiss, enderbitic granulite, and granitic and charnockitic augen gneisses associations. Subduction-related magmatic events in the Rondonian-San Ignacio Province include 1.48
/1.42 Ga calc-alkaline granitoids of the Santa Helena batholith (Geraldes et al., 2001) and the
/1.4 Ga Pensamiento granitoid complex in Bolı´via (Litherland and Bloomfield, 1981; Litherland et al., 1986; Tassinari et al., 2001); both involved juvenile sources with variable contributions of older continental material (Geraldes et al., 1999; Darbyshire, 2000). The 1.43 Ga rocks recorded in the study area are correlated with the most evolved granites of the Santa Helena batholith farther to the southeast in Mato Grosso. This implies that an almost continuous magmatic arc existed along the southwestern margin of Amazonia at
/1.42 Ga. Our U
/Pb and Sm
/Nd data show that at least one high-grade tectonometamorphic event at upper amphibolite conditions affected the eastcentral part of the RTP. A Sm
/Nd age of 13039/ 39 Ma obtained from a four-point regression of two whole rock-garnet pairs (Fig. 9) and the 1330

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Ma monazite age reflect this pervasive recrystallization event that affected the basement rocks in the study area; the event has been identified in eastern Bolivia as the San Ignacio orogeny (Litherland et al., 1989). The P
/T conditions established for this metamorphism (770
/720 8C and 900
/700 MPa; Tohver et al., 2000; unpublished data by the senior author) are similar to the closure conditions of Pb in monazite (
/725 8C; Parrish, 1990). The crystallization or complete resetting of monazite at
/1330 Ma (Table 3) and the crystallization of garnets at
/1300 Ma (Fig. 9) and zircon at 1330 Ma (Tassinari et al., 1999) indicate a metamorphic peak at 1330
/1300 Ma; this is interpreted as the climax of the Rondonian-San Ignacio orogeny. Hornblende and biotite 40Ar/39Ar ages of 1200
/ 1100 Ma reported by Bettencourt et al. (1996) and Tohver et al. (2000) are interpreted to register cooling from a high-grade tectonometamorphic event related to the Sunsas orogeny (Tohver et al., 2001). This event affected the basement rocks of the Amazonian craton north of the Nova Brasilaˆndia belt (Tohver et al., 2001). The poorly constrained lower intercepts at 1200 Ma (Fig. 8a,c) may reflect this high-grade tectonometamorphic episode in our study area. A similar age (
/12119/18 Ma) has been reported in detrital zircons from a paragneiss in southern Rondoˆnia and is interpreted as the maximum age of the Nova Brasilaˆndia sedimentation (Santos et al., 2000). Biotite 40Ar/39Ar ages between 1001 and 912 Ma are also recorded in the basement of the eastcentral part of the RTP and are interpreted as a thermal effects related to the emplacement of the Younger Granites of Rondoˆnia (1.00
/0.97 Ga; Bettencourt et al., 1996). These granites are considered to be products of inboard magmatism related to the collisional stage of the Sunsas orogeny (Bettencourt et al., 1999).

5. Concluding remarks The east-central part of the RTP represents a southwestern extension of the Rio Negro-Juruena Province (cf. Cordani et al., 1979), which comprises basement rocks related to magmatic arc evolution at 1.8
/1.55 Ga (Tassinari et al., 1996).

161

The results presented here lead to the recognition of new lithologic elements, revision of others, and suggest some modifications on the magmatic, metamorphic, and deformational history of both Rondoˆnia and the southwestern Amazonian craton. The data lend additional support to the argument that the RTP is a multi-orogen region, characterized by sucessive episodes of magmatism, metamorphism, and deformation between 1.75 and 0.97 Ga. As crustal segments with similar tectonic pattern are recognized worldwide (e.g. Gaa´l and Gorbatschev, 1987; Rivers, 1997; Karlstrom et al., 2001), tectonic histories and patterns from the RTP may be useful for testing Rodinia supercontinent reconstructions also. The oldest rocks of the RTP are the 1.75
/1.73 Ga magmatic arc tonalites related to the evolution of the older magmatic arc system of the Rio Negro-Juruena Province (1.8
/1.7 Ga; Tassinari et al., 1996). In contrast to a juvenile origin for most of the Rio Negro-Juruena basement rocks (Tassinari et al., 1996; Tassinari and Macambira, 1999; Santos et al., 2000), the isotopic data presented here suggest significant contribution from an older crust (probably Ventuari-Tapajo´s). Available evidence suggests the existence of an active continental margin in southwestern Amazonia between 1.79 and 1.70 Ga, comprising Andean-style, calc-alkaline magmatic arcs in Rondoˆnia (this study) and calc-alkaline island arcs in Mato Grosso (Geraldes et al., 2001). The Yavapai Province in USA (Van Schmus et al., 1993) and the ˚ ha¨ll and Gower, Gothian terranes of Baltica (A 1997) represent coeval orogenic belts and provinces around the world. The paragneisses are recognized as an important lithologic element in the RTP. Detrital zircon ages and an intrusive granite bracket deposition of the original sediments between 1.67 and 1.57 Ga. This sedimentation is interpreted to reflect a tectonically quiet period that extended throughout Rondoˆnia between the end of the 1.75
/1.70 Ga calcalkaline magmatism and the beginning of the 1.60
/1.53 A-type, within-plate magmatism. A similar period of quiescense has been recognized throughout Laurentia, between the end of Labradorian orogenesis and the start of Pinwarian activity, and has been linked to a passive con-

162

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/169

tinental margin setting (Wakeham Group; Gower, 2001). Nd model ages of 2.2
/2.1 Ga and detrital zircon ages of 1.81
/1.67 Ga suggest that the paragneisses contain a significant component of Paleoproterozoic detritus, probably from the 1.95
/1.80 Ga Ventuari-Tapajo´s crust and the 1.75
/1.73 Ga tonalites. In the Mesoproterozoic, the tonalites and paragneisses were affected by four distinct magmatic events at 1.57
/1.53, 1.43
/1.42, 1.08
/1.07 and 1.00
/0.97 Ga, and probably by two further tectonometamorphic events at 1.33
/1.30 and 1.2
/1.1 Ga. These events are ascribed to propagation of the effects of orogenic processes in the southeast into foreland regions. The widespread granitic and charnockitic augen gneisses (1.57
/1.53 Ga) show A-type and withinplate geochemical signatures in contrast to the magmatic-arc interpretation of Tassinari et al. (1996). These rocks are correlated to the Serra da Provideˆncia intrusive suite (Bettencourt et al., 1999) and are interpreted as the products of inboard magmatism coeval with the calc-alkaline Cachoeirinha suite in the Mato Grosso area (Tassinari et al., 2000; Geraldes et al., 2001). The counterpart of this magmatism in the northwestern Amazonian craton are the 1.55
/1.52 Ga syntectonic granites of the Ic¸ana and Uaupe´s suites (Santos et al., 2000), the 1.56
/1.54 Ga rapakivi granites and charnockites of the Surucucus and Mucajaı´ intrusive suites (Gaudette et al., 1996; Santos et al., 2000), and the Parguaza granite (Gaudette et al., 1978). The interval of 1.6
/1.5 Ga, known as the ‘magmatic gap’ in Laurentia (Davidson, 2000; Van Schmus, 2001), represents an important period of calc-alkaline and rapakivi magmatism in the Fennoscandian (or ˚ ha¨ll et al., 2000), and in the Baltic) Shield (A Amazonian craton (Geraldes et al., 2001). Our results confirm the widespread distribution of this magmatic event in the western Amazonian craton and reinforce the hypothesis of an Amazonia
/ Baltica connection during the Mesoproterozoic, envisioned as a major, laterally continuous continental-margin at 1.6
/1.5 Ga (Fig. 10; see also Geraldes et al., 2001). The fine-grained granitic gneiss and charnockitic granulites represent the first record of wide-

spread 1.43
/1.42 Ga magmatism with A-type and within plate signature in northern Rondoˆnia and probably registers mixing of juvenile material and old basement. Similar-age rocks are found in Mato Grosso, including the calc-alkaline granites of the Santa Helena batholith and the juvenile, mafic volcanic and plutonic rocks of the Rio Alegre domain. These have been interpreted as products of 1.5
/1.4 Ga continental margin accretionary processes (Geraldes et al., 2001). The 1.43
/1.42 Ga magmatism of Rondoˆnia could represent inboard expressions of these accretionay process, and would imply some continuity of the southwest margin of Amazonia at
/1.42 Ga. Although
/1.45 Ga anorogenic magmatism is common in Laurentia (Van Schmus et al., 1993; Rivers, 1997), orogenic belts of that age are relatively uncom-

Fig. 10. Possible early to mid-Neoproterozoic (Rodinian) configuration of Laurentia, Baltica and Amazonia (modified from Fig. 10 in Dalziel, 1997; Fig. 1b in Karlstrom et al., 2001; and Fig. 16 in Geraldes et al., 2001), showing the proposed lateral continuity of 1.6
/1.5 Ga calc-alkaline and rapakivi suites and 1.47
/1.35 Ga juvenile crust and granite suites in Amazonia and Baltica. Laurentia geology is based on Rivers (1997), Van Schmus (2001), and Karlstrom et al. (2001). Baltica ˚ ha¨ll et al. (2000). Amazonia geology is geology is based on A based on Tassinari et al. (2000), Santos et al. (2000), Geraldes et al. (2001), and this study. G
/R/granite
/rhyolite provinces; RO/Rondoˆnia; MT /Mato Grosso.

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mon. Significant examples are the Pinwarian magmatism and orogeny (Wasteneys et al., 1997; Corrigan et al., 2000) and juvenile high-silica granites and rhyolites of the St. Francois Mountains (Van Schmus, 2001). The distribution of the 1.5
/1.4 Ga magmatism in the southwestern Amazonian craton suggests that the hypothetical lateral correlation of Laurentia and Amazonia can be extended to cover this time interval (Fig. 10; see also Geraldes et al., 2001). The new monazite and whole rock-garnet dates, combined with previously published data, demonstrate that a high-grade tectonometamorphic event affected the RTP at 1.33
/1.30 Ga. This event is interpreted as the peak metamorphism of the Rondonian-San Ignacio orogeny (Tassinari et al., 1999). A second tectonometamorphic event at 1.2
/1.1 Ga is constrained by hornblende 40 Ar/39Ar ages and zircon lower intercepts, and represents extensive reworking of the basement rocks during the Sunsas orogeny (Tohver et al., 2001). Finally, the 1.08
/0.97 Ga rapakivi granites of the Santa Clara intrusive suite and the Younger Granites of Rondoˆnia intruded the RTP as inboard manifestations of the collisional stage of the

163

Sunsas orogeny, developed farther south. These 1.3
/0.97 Ga tectonometamorphic and magmatic events also took place in Laurentia where they have been referred to as the Elzevirian (1.3
/1.2 Ga) and Grenvillian (
/1.2
/0.98 Ga) orogenies (Rivers, 1997).

Acknowledgements This study was supported by grants from the Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo-FAPESP (grants to B.L. Payolla, Proc. no. 95/0290-9) and PADCT-FINEP (research grants to J.S. Bettencourt, conveˆnio PADCT/ FINEP 64.99.027.00), which we acknowledge with appreciation. Analytical work at the University of Kansas was supported by NSF Grant to Van Schmus and by funds from the Department of Geology, University of Kansas. The content of this paper constitutes part of Payolla’s PhD thesis at the Universidade Estadual Paulista, and is a contribution to IGCP-426 (Granite Systems and Proterozoic Lithospheric Processes).

164

Appendix A: Sample locations with brief descriptions Sample

gneiss association 10 km NE from Cachoeirinha Upper Jacunda´ River Upper Jacunda´ River

Enderbitic granulite association WB-223/ Hill 8 km SW from Ariquemes B B-3446/B 18 km NE from Ariquemes B-3446/ 18 km NE from Ariquemes A WB-70 Sa˜o Paulo farm, 2.5 km SE from Ariquemes WB-17 Umuarama farm, 25 km NE from Ariquemes WB-93/ Bom Viver farm, 14 km NE from A Ariquemes

Location [long. (8W)/ Rock type lat. (8S)]

Mineral assemblagea

63800?05ƒ/9822?46ƒ 62850?34ƒ/9825?42ƒ 62850?46ƒ/9820?28ƒ

Homogeneous tonalitic gneiss Banded tonalitic gneiss Garnet-bearing tonalitic gneiss

Pl
/Qtz
/Hbl
/Bt
/Ksp
/Mg
/Tit Pl
/Qtz
/Hbl
/Bt
/Ksp
/Mg
/Tit Pl
/Qtz
/Hbl
/Bt
/Grt
/Ksp
/ Mag

63806?14ƒ/9856?56ƒ

Mafic granulite

Pl
/Opx
/Cpx
/Hbl
/Mag

62858?35ƒ/9845?05ƒ 62858?35ƒ/9845?05ƒ

Mafic granulite Enderbitic granulite

63800?18ƒ/9856?10ƒ

Enderbitic granulite

62852?52ƒ/9845?47ƒ

Enderbitic granulite

62858?05ƒ/9850?11ƒ

Enderbitic granulite

Pl
/Opx
/Cpx
/Hbl
/Mag Pl
/Qtz
/Hbl
/Bt
/Opx
/Grt
/ Ksp
/Mag
/Ilm Pl
/Qtz
/Hbl
/Bt
/Opx
/Grt
/ Ksp
/Mag
/Ilm Pl
/Qtz
/Hbl
/Bt
/Cpx
/Opx
/ Grt
/Ksp
/Mag Pl
/Qtz
/Hbl
/Bt
/Opx
/Grt
/ Ksp
/Mag
/Ilm

Paragneiss association WB-140 Bandeirantes farm, 35 km NE from 62838?23ƒ/9838?39ƒ Oriente Novo WB-147 Near 58 BEC village 62816?05ƒ/9843?13ƒ WB-152 20 km NE from Oriente Novo 62813?17ƒ/9828?48ƒ

Granitic and charnockitic augen gneisses association M-S15 km NE from 58 BEC village 62808?44ƒ/9839?14ƒ 6030

Metapelitic migmatite

Qtz
/Ksp
/Pl
/Bt
/Sil
/Grt

Garnet-bearing leucogneiss Metapelitic migmatite

Qtz
/Ksp
/Pl
/Grt Qtz
/Ksp
/Pl
/Bt
/Crd
/Sil
/ Grt
/Spl
/Ilm
/Mag

Pink monzogranitic gneiss

Ksp
/Pl
/Qtz
/Hbl
/Bt
/Tit
/ Mag

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Tonalitic B-335 O-2594 B-250/A

Locality

Appendix (Continued ) Locality

Location [long. (8W)/ Rock type lat. (8S)]

Mineral assemblagea

WB-46/ A WB-46/ C AR-3/1 WB-44/ A WB-143

Japoneˆs quarry, 8 km SE from Ariquemes Japoneˆs quarry, 8 km SE from Ariquemes Quarry near Samuel Reservoir Road cut, 38 km N from Ariquemes Bom Jardim farm, near 58 BEC

63802?21ƒ/9858?32ƒ

Ksp
/Pl
/Qtz
/Opx
/Hbl
/Bt
/ Grt
/Ilm Qtz
/Ksp
/Pl
/Bt
/Hbl
/All
/ Mag Ksp
/Pl
/Qtz
/Bt
/Tit
/Mag Ksp
/Qtz
/Pl
/Bt

WB-36

Sun Hill, 8 km W from 58 BEC

62816?25ƒ/9840?20ƒ

WB-175

Cascavel quarry, near 58 BEC

62811?50ƒ/9839?07ƒ

63802?21ƒ/9858?32ƒ 63827?55ƒ/8848?55ƒ 63804?11ƒ/9836?51ƒ 62811?11ƒ/9840?58ƒ

Fine-grained granitic gneiss and charnockitic granulite association WB-29/ 25 km NE from Oriente Novo 62858?51ƒ/9830?49ƒ A WB-51 Colombo farm, 2.2 km SE from 63801?42ƒ/9856?22ƒ Ariquemes WB-71 Hill 18 km E from Ariquemes 62855?09ƒ/9854?47ƒ WB-223/ Hill 8 km SW from Ariquemes A WB-95 17 km NE from Ariquemes a

63806?14ƒ/9856?56ƒ 62854?45ƒ/9850?19ƒ

Opx-bearing monzogranitic augen gneiss Monzogranitic gneiss Grey biotite monzogranite Pink syenogranitic augen gneiss Opx-bearing quartz monzonite Pink hornblende-bearing quartz syenite Pink hornblende-bearing syenogranite

Granitic gneiss

Ksp
/Pl
/Qtz
/Opx
/Hbl
/Ilm
/ Mag Ksp
/Pl
/Qtz
/Hbl
/Ilm
/Mag Ksp
/Pl
/Qtz
/Hbl
/Bt
/Ilm
/ Mag

Ksp
/Qtz
/Pl
/Hbl
/Bt
/Mag

Garnet-bearing granitic gneiss Ksp-Qtz
/Pl
/Hbl
/Mag
/Grt
/ Tit
/Ep Charnockitic granulite Ksp
/Qtz
/Pl
/Opx
/Cpx
/Hbl
/ Mag
/Grt Charnockitic granulite Ksp
/Qtz
/Pl
/Hbl
/Opx
/Grt
/ Mag Granitic gneiss Ksp
/Qtz
/Pl
/Hbl
/Bt
/Mag

B.L. Payolla et al. / Precambrian Research 119 (2002) 141
/169

Sample

Mineral abbreviations after Kretz (1983).

165

166

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Appendix B. Analytical techniques B.1. Geochemistry Samples for geochemical analysis were crushed and powdered in an agate shatter-box. Major elements and Rb, Sr, Y, Zr, and Nb were determined by X-ray fluorescence (XRF) on glass discs at the Geochemistry Laboratory, IGCE, Universidade Estadual Paulista, Rio Claro, Brazil. Rare earth elements (REEs) and Ba, Hf, Ta, and Th were determined by inductively coupled plasma emission mass spectrometry (ICP-MS) at the Actlabs, Ancaster, Canada, except two samples (B-335 and B-250/A) analyzed by instrumental neutron activation analysis (INAA) at the USGS, Menlo Park, USA. B.2. U
/Pb geochronology Seven samples of approximately 10 kg were crushed for geochronology using the mineral separation techniques at the IGCE-UNESP at Rio Claro, Sa˜o Paulo, Brazil. Final mineral separations and U
/Pb data were obtained at the Isotope Geochemistry Laboratory, Department of Geology, Kansas University Center for Research, University of Kansas, Lawrence, USA. Zircon fractions used for analyses were airabraded (Krogh, 1982) and individual grains carefully selected by hand prior to dissolution. Zircons were dissolved and Pb and U were separated using procedures modified after Krogh (1973) and Parrish (1987). For small samples and single crystal analyses samples were total-spiked with a mixed 205Pb
/235U tracer solution. Some of the Pb analyses were measured in static mode using a VG Sector multi-collector mass spectrometer equipped with a Daly detector for large samples. However, most of the U
/Pb analyses were done in single collector mode using an ion-counting Daly system. Pb was loaded on single Re filaments using silica gel and phosphoric acid. Uranium was also loaded with the Pb, phosphoric acid and silica gel on the same single rhenium filament and measured as UO2. Pb compositions were corrected for mass discrimination as determined by analysis of NBS

SRM-982 (equal-atom) Pb and monitored by analysis of NBS SRM-983 (radiogenic) Pb. Uranium fractionation was monitored by analyses of NBS SRM U-500. Uncertainties in Pb/U ratios due to uncertainties in fractination and mass spectrometry for typical analyses are 9/0.5%; in some instances weak signals (e.g. single crystals) caused uncertainties to range up to 9/1.5%. Radiogenic 208Pb, 207Pb and 206Pb were corrected for modern blank Pb and for nonradiogenic original Pb corresponding to Stacey and Kramers (1975) model Pb for the approximate age of the sample. Uncertainties in radiogenic Pb ratios are typically 9/0.1% unless the samples had an unusually low 206Pb/204Pb ratio, in which case uncertainties in the common Pb correction could cause greater uncertainties. Decay constants used were 0.155125 /109 year 1 for 238U and 0.98485 /10 9 year 1 for 235U. Blanks ranged from
/10 to B/2 pg total Pb; in most cases they do not contribute significantly to uncertainties in the ages of samples, although some of the singlecrystal analyses may show the effects of blank Pb as larger uncertainties in the calculated ages. U
/Pb data were regressed using the ISOPLOT program of Ludwig (1993). Model 1 regressions were accepted if probabilities of fit were better than 30%; model 2 regressions were used if probabilities of fit were less than 30%. Uncertainties in concordia intercept ages are given at the 2s level. B.3. Sm
/Nd isotopes The preparation of rock powders and separation of garnet concentrates for Sm
/Nd analyses were performed at the IGCE-UNESP at Rio Claro, Sa˜o Paulo, Brazil. Nine whole-rock Sm
/Nd analyses were performed at the Isotope Geochemistry Laboratory of the University of Kansas, Lawrence, USA. Eight other whole-rock and two garnet Sm
/Nd analyses were performed at the Geochronology Laboratory of the University of Brasilia, Distrito Federal, Brazil. In the Isotope Geochemistry Laboratory of the University of Kansas the whole-rock powders for Sm
/Nd analysis were dissolved and REE were extracted using the general methods of Patchett

B.L. Payolla et al. / Precambrian Research 119 (2002) 141
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and Ruiz (1987). Isotopic compositions were measured with a VG Sector multi-collector mass spectrometer. Sm was loaded with H3PO4 on a single Ta filament and typically analysed as Sm  in a static-multicollector or single-collector mode. Nd was loaded with phosphoric acid on a single Re filament having a thin layer of AGW-50 resin beads and analysed as Nd  using dynamic-multicollector mode. External precision based on repeated analyses of our internal standard is 9/40 ppm (2s) or better; all analyses are adjusted for instrumental bias determined by measurements of our internal standard for periodic adjustment of collector positions; Sm
/Nd ratios are correct to within 9/0.5%, based on analytical uncertainties. Replicate analyses of standard BCR-1 yielded Nd /29.449/0.70 ppm, Sm / 6.779/0.21 ppm, 147Sm/144Nd/0.13939/0.0007 and 143Nd/144Nd /0.5126419/0.000007, yielding o Nd(0) /0.19/0.1 (at 1s). At the Geochronology Laboratory of the University of Brası´lia the Sm and Nd separation methodology is basically that described in Richard et al. (1976), with the addition of some improvements (Gio´ia and Pimentel, 2000). The garnets, after being washed in warm 4 N HNO3, were dissolved in Teflon bombs and the whole-rock powders were dissolved in 15 or 30 ml Savillex capsules. In this technique the separation of the REE as a group using cation-exchange columns precedes reversed-phase chromatography for the separation of Sm and Nd using columns containing HDEHP (di-2-ethyl-hexil phosphoric acid) supported on PTFE powder. A mixed 149 Sm
/150Nd spike was used. Sm and Nd samples were loaded on the Re evaporation filament of a double filament assembly and the isotopic measurements were carried out on a multi-collector Finnigan MAT-262 mass spectrometer in static mode. Uncertainties on Sm/Nd and 143Nd/144Nd ratios are considered to be better than 9/0.05% (1s) and 9/0.000005 (1s), respectively, based on repeated analyses of international rock standards BCR-1 and BHVO-1. 143Nd/144Nd ratios were normalized to a 146Nd/144Nd of 0.7219. Nd procedure blanks were smaller than 100 pg. Replicate analyses of standard BCR-1 yielded Nd /28.77 ppm, Sm/6.66 ppm, 147Sm/144Nd/

167

0.1401 and 143Nd/144Nd /0.5126329/0.000002 (at 1s), yielding o Nd(0) //0.1. Replicate analyses from the same rock powder of sample WB-70 were used for interlaboratorial comparison. In the Isotope Geochemistry Laboratory of the University of Kansas this sample yielded Nd /53.96 ppm, Sm /10.91 ppm, 147 143 Sm/144Nd /0.1222 and Nd/144Nd / 0.511798. In the Geochronology Laboratory of the University of Brasilia replicate analysis yielded Nd/54.01 ppm, Sm /11.01 ppm, 147Sm/144Nd / 0.1230 and 143Nd/144Nd /0.511764.

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