Gondwana Research, V 7, No. 4, pp. 913-921. 0 2004 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Ages of Detrital Zircon from Siliciclastic Successions South of the S5o Francisco Craton, Brazil: Implications for the Evolution of Proterozoic Basins

Claudia Sayso Valladares1t2", Nuno Machado3, Monica Heilbron1r2 and Gilles GauthielP

' Tektos - Geotectonic Research Group of the Faculty of Geology, Rio de Janeiro State University (UERJ), Brazil ' Rina S6o Francisco Xavier, 524 - sala A 4006, Maracanz, Rio de Janeiro, RJ, Brazil, CEP: 20550-023, E-mail: heilbron@uerj.br

* Dept. Gtologie, Universitt de Montrial, Montrial, Canada ' Corresponding author: E-mail: vallada@uerj.br

GEOTOP-Sc. de la Terre, UQAM - Canada, E-mail: r30404@er,uqarn.ca

(Manuscript received November 12,2003; accepted May 10,2004)

Abstract

In this work we report 207Pb/Z06Pb LA-ICPMS ages of 152 detrital zircons from lower greenschist facies quartzites from Proterozoic basin successions of the southern border of the ,550 Francisco Craton, southern Minas Gerais State, Brazil. These are the intracratonic SCo Jo50 del Rei basin, the intraplate continental margin Andreldndia basin, and the Serra do Our0 Grosso sequence, developed on a crystalline basement older than 1.8 Ga, and deformed and metamorphosed during the Brasiliano Orogeny, ca. 0.59-0.50 Ga. The data constrain both the ages of the sources and the interval of sedimentation. The detrital zircons of the Serra do Ouro Grosso sequcnce were derived predominantly from the erosion of a Neoarchean crust, 2.5-2.8 Ga old, with only one grain showing a Paleoproterozoic age (2,245283 Ma) older than the Transamazonian event. Zircons extracted from a shelf quartzite of thc lowermost sequence of the SCo JoCo del Rei basin indicate derivation from the 1.8-2.2 Ga Transamazonian crust, with subordinate contribution from the 2.5-2.9 Ga Archean crust. The 1,809+41 Ma age is interpreted as the maximum limit for sedimentation in this basin. The results confirm the regional correlation with the Espinhaqo Rift successions. The zircons extracted from an autochthonous quartzite of the Andreldndia sequence yielded ages in the 1.0-2.2 Ga range, with a modal class at 1.2-1.3 Ga. Only two of the forty analyzed zircons yield Archean ages. The youngest zircon yields 1,086285 Ma. The zircons from the alloehthonous quartzite yield ages between 1.0-2.7 Ga, with a modal class at 2.1-2.2 Ga. Only five of 45 analyzed grains yield Archean ages. The youngest zircon has an age of 1,047-1-77 Ma. The results indicate that the detrital sediments deposited during the second marine flooding event of the Andrelindia sedimentation were mainly derived from the erosion of Mesoproterozoic and Paleoproterozic rocks. The 1,047277 Ma age is interpreted as the maximum depositional age for the described association.

K e y words: Detrital zircon 207Pb/206Pb ages, laser ablation ICPMS, Proterozoic basins, southern S5o Francisco Craton, Brazil.

Introduction and Objectives

The determination of detrital zircon populations present in metasedimentary successions can be used to constrain both the provenance and minimum age for deposition. One possible methodology, useful for rocks ranging from greenschist to amphibolite facies, is the determination of 207Pb/206Pb ages by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The methodology and results were discussed by Feng et al. (1993) and Machado and Gauthier (1996), and in principle the results can be interpreted as indicative of minimum ages. Nevertheless, Machado and Gauthier (1996) showed that

for rocks of low to medium metamorphic grades, the obtained data are identical within error to ages obtained with the U-Pb/TIMS methodology. The advantage of the method for sedimentary provenance studies is the number of grains that can be analyzed per day (ca. SO) on the same sample, providing a statistically meaningful age distribution. Compared to conventional U-Pb dating by isotope dilution- thermal ionization mass spectrometry, the LA-ICPMS method offers the advantages of minimal sample preparation and high analytical throughput. The most significant limitations of the method used in this work are the inability to yield reliable U-Pb values and the large analytical error of, at least 1-10%.

914 C.S. VALLADARES ET AL.

The paper reports ages of detrital zircon from lower greenschist quartzites of different Proterozoic basin successions (Ribeiro et al., 1995; Pacciullo et al., 2000; Trouw et al., 2000), located south of the SZo Francisco Craton: (a) the SSo JoSo del Rei basin, (b) the Andrelindia basin, and (c) The Serra do Ouro Grosso sequence. Constraints of the probable source rocks as well as the sedimentation ages of these successions are the main objectives of this contribution.

Tectonic Setting and Select Samples The studied metasedimentary successions crop out

within the Neoproterozoic Ribeira and Brasilia Belts, located respectively east/southeast and west/southwest of the S5o Francisco Craton, southeastern Brazil (Fig. 1).

Stratigraphic, metamorphic and structural analysis of Proterozoic sedimentary successions occurring in the southernmost SZo Francisco Craton and in its southern border led to the recognition of several depositional sequences separated by unconformities. One of them is the Serra do Our0 Grosso sequence, that comprises quartzite and conglomeratic quartzite deposited on an Archaean to Paleoproterozoic basement (Fig. 2). The age of deposition of this sequence is still a matter of debate.

The others successions are located in the autochthonous and allochthonous domains of the external portion of the Brasilia and Ribeira Belts. Recently, Ribeiro et al. (1995) and Trouw et al. (2000) recognized the sedimentary record of three superposed basins named Siio JoZo del Rei, Carandai and Andrelindia Basins. The first two are considered to be related to an intracratonic setting, whilst

. '4 A

Ib

Fig. 1. la-Tectonic subdivision of Brazil showing the Neoproterozoic belts, cratonic blocks and major Phanerozoic intracratonic basins adapted from Almeida et al. (2000). Legend: 1-Cratonic blocks (I-Amazonian; 11-SBo Francisco; 111-Rio de la Plata); 2-Neoproterozoic/Cambrian belts (Bbb-Brasilia belt; Rbb-Ribeira belt; Arb-AraGuai belt). %-Major tectonic units of southeastern Brazil adapted from Trouw et al. (2000). The outlined rectangle indicates the area detailed in figure 2. Legend: 1-Phanerozoic cover; 2-Cretaceous/Tertiary alkaline rocks; 3-SSo Francisco Craton and foreland domains; 4-Nappes of Brasilia belt; 5-Interference zone between Brasilia and Ribeira belts; 6 to 10: Ribeira belt units; 6 J u i z de Fora domain; 7-Paraiba do Sul Klippe; 8 and 9-Costeiro domain with the arc-related granitoids of the Rio Negro Complex; 10-Cab0 Frio domain.

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AGES OF DETRITAL ZIRCON FROM SAO FRANCISCO CRATON, BRAZIL 915

Fig. 2. Simplified geological map of the Precambrian units south of the SZo Francisco Craton adapted from Trouw et al. (2000). Legend: 1-Basement units. 2 to 6-Proterozoic successions: 2, Quartzite, metapelite and metaconglomerate of the S6o JoZo del Rei basin; 3-Metapelite and metalimestone of the Carandai basin; 4 to 6: Andrelhdia basin units: 4-A1 and A2- paragneiss, quartzite, schist and amphibolite; 5-A3- quartzite and A4- phyllite/schist and quartzite intercalations; 6-A5- biotite schist/gneiss with local dropstones and A6- biotite schist/ gneiss and calcsilicate, metachert and amphibolite intercalations. 7-Interference zone between Brasilia and Ribeira belts. SJ-SHo JoHo del Rei, Ba-Barbacena, Lv-Lavras, Car-Carrancas. LuN-LuminBrias Nappe; Ck-Carrancas Klippe.

the AndrelAndia Basin is thought to represent a continental margin (Ribeiro et al., 1995; Paciullo et al., 1998). All three sequences lie on poorly dated basement (greenstone belts and gneiss complexes) which is the southern extension of an Archean terrain intruded by Paleoproterozoic granitoids with ages of as 1.98 Ga (Rb-Sr age; Heilbron et al., 1989) and 2.22-2.12 Ga (zo7Pb-206Pb, Avila, 2000). The Proterozoic successions were deformed and metamorphosed during the Brasiliano Orogeny, in a ductile nappe system with top-to- east tectonic transport. The associated metamorphism varies from middle greenschist to granulite facies. The extent to which the 750-450 Ma Brasiliano Orogeny affected the different sequences is poorly constrained. Sm-Nd, Rb-Sr and K-Ar ages indicate that the AndrelAndia sequence was metamorphosed at ca. 0.50-0.66 Ga (Heilbron et al., 1989; Sollner and Trouw, 1997). Therefore, the deposition in the three basins must have occurred between ca. 1.98 Ga and 0.5-0.7 Ga.

Analytical Methods

Sample crushing was done at the Laboratorio Geologico de Processamento de Amostras (LGPA) of the Universidade do Estado do Rio de Janeiro (UERJ), Brazil, following the standard clean procedures described in Machado et al. (1996). Heavy minerals were concentrated by panning

at the Laboratorio de Preparaqgo de Amostras para Geocronologia - LOPAG, Universidade Federal de Our0 Preto -UFOP, Brazil, instead of the traditional procedures of Wilfley table separation, sieving and concentration by heavy liquids. Zircon concentrates were separated into four magnetic and two diamagnetic fractions at GEOTOP- UQAM-Canada, the latter being preferred for hand picking. For each sample, 40-50 grains, devoid of alteration, inclusions, and fractures, and without visible core or overgrowths, were selected for analysis. When the overgrowths were large enough to be analyzed it was possible to obtain metamorphic ages. All the chromatic and morphological grain types present in each sample were selected.

The laser ablation-ICPMS method (LA-ICPMS) has been used since 1993 (Feng et al., 1993, Machado and Gauthier, 1996) and yields reliable zo7Pb/206Pb ages on zircon and monazite on a routine basis with a precision better than lo%, generally ca. 5%. The current limitations of the method are the relatively low precision and the inability to yield reproducible U/Pb values. Previous work used the laser in Q-switched mode in which the large size of the sample pit (40-50pm) precludes the analysis of grains smaller than 80pm (Feng et al., 1993; Machado and Gauthier, 1996). In this work the laser beam was used to raster the sample in continuous mode which allows the

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916 C.S. VALLADARES ET AL.

analysis of grains as small as 40pm. This procedure also avoided shattering or perforating zircons during analysis.

A VG LaserProbe coupled to a VG PQII+ ICP-MS instrument equipped with an SX-300 type quadropole and a Galileo discrete dynode channeltron electron multiplier was used in this study. The LaserProbe uses a Spectron SL-282 Nd:YAG 280 mJ pulsed laser operating at 1064 nm. The instrument is tuned daily on z08Pb by ablating NIST 610 glass standard in order to obtain maximum intensity, minimum background, and optimum resolution for Pb peaks, in particular 207Pb. The operating conditions are given on table 1. The details of the analytical procedure can be found in Machado and Gauthier (1996). The correction of mass bias is made relative to an in-house zircon standard (UQ-Zl) analyzed in the same conditions as those for the samples. This standard has been precisely

Table 1. LA-ICPMS operating conditions.

Laser probe: Fisions VG Nd:YAG at 1064 nm Laser mode: Flash lamp voltage: Frequency: Output power: Pulse width:

Forward power: Refleted power: Coll gas: Aux. Gas: Carrier gas:

Dwell time: Quad settle time: Points/peak: DAC/step: Acquisition time: No. of isotopes: No. of repeats:

ICP-MS: Fisions VG PQII +

Acquisition parametres - Peak Jumping mode

Gas blank Mass 202 204 206 207 208 232 235 238

cps (1 sigma) 730t150 1 8 0 t 6 0 70230 130260 153270 8 2 8 11210 9 2 9

'Qpical net intensities on the standard UQ-Zl

202Hg 204Pb 206Pb 207Pb 208Pb 232Th 235U 238U

(CPS) 5-50 < 15 20000-30000 1800-2200 1800-2200 10000-12000 800-1200 10000-160000

Continuous 600V 5HZ 2-3 mJ 5 ns

1350W ow 14 l/m 0.9 l /m 1.1 l /m

10.24 ms lOms 3 1 7 s

5 -7

Sensitivity (cps/ppm) not determined CLOD 530-798 620-760 581-710 158-190 444-667 400-625

dated by U-Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and yielded an age of 1143 t 1 Ma (Machado and Gauthier, 1996). LA-ICPMS results are listed (Table 2) and quoted at the 70% confidence level.

Geology and Results Serra do Our0 Grosso sequence

An isolated body composed of quartzites with approximately 200 m of thickness crops out at Serra do Ouro Grosso ridge. The overall structure is defined by a homocline of low-grade sedimentary rocks that lie on Archean/Paleoproterozoic basement. Migmatitic gneisses, greenstone meta-volcanic successions, and meta-granitoids comprise the still poorly dated basement. The Serra do Ouro Grosso sequence has been correlated to the basal quartzites of the Tiradentes Platform of the Sgo Jog0 del Rei Basin (Trouw et al., 2000).

We selected for sampling a green-mica-quartzite (outcrop ITA-04, Fig. 2) from a quarry located south of Itutinga town. The zircons are mainly colorless and yellowish and a few are of pale brown color. Their size ranges from smaller crystals of ca. 40 pm to prismatic bigger ones that reach 200pm. They are elongated 1:2 and euhedral, and the majority show one visible overgrowth that was not large enough to be analyzed by this method. Both the core as the overgrowth have the characteristic percussion marks, showing that the source of the zircons was submitted to a geologic event with temperature high enough to produce an overgrowth before the depositional event that causes the percussion marks. Based on regional geology this overgrowth is interpreted as the result of overgrows either during a magmatic event (crustal refusion) or a metamorphic event. The metamorphic record of the basement rocks, reported by Cherman (1999) indicates two episodes. The oldest is of amphibolite facies and the youngest varies from greenschist to transition between greenschist and amphibolite facies.

The 207Pb-206Pb analyses (Fig. 3a) yielded ages from 3.22 to 2.4 Ga (sample ITA-04, Table 2), defining two modal classes at 2.8-2.7 and 2.6-2.5 Ga. A single grain yielded an age of 2 ,245t85 Ma which is interpreted as the maximum age for sedimentation.

Siio Joiio Del Rei basin

The sedimentary record in this basin consists mainly of a quartzitic succession up to 1000 m thick unconformably overlain by the Carandai basin (Ribeiro, 1997). Four depositional sequences cut by mafic dykes are preserved. According to Ribeiro et al. (1995), the lower three sequences consist of quartzitic units and record foreshore to lower shoreface facies deposited on an epicontinental shelf with

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AGES OF DETRITAL ZIRCON FROM SAO FRANCISCO CRATON, BRAZIL 917

a NE-SW coastline and NE longshore currents. The fourth and uppermost sequence, interpreted as a braided plain delta, is an upward coarsening and thickening succession consisting of metapelites, quartzites and meta-conglomerates. The northward directed paleo-currents indicate the progressive emergence of continental blocks to the south of the NE-SW coastline. This paleoslope reversal marks the onset of rifting and culminated with the intrusion of mafic dykes, tilting and uplift (Ribeiro, 1997).

The previous estimation for basin filling was based on Nd T,,, model ages varying from 1.8 to 1.3 Ga for five metamafic dykes intruded in SSo JoSo del Rei units (Ribeiro et al., 1990).

Zircons extracted from one of the lower basal quartzite units, Tiradentes sequence, (outcrop TIR-01, Fig. 2) are all rounded and mostly spherical, but a minor proportion are elongated 1:2; they vary in color from colorless to pink to dark brown and all have characteristic percussion

Table 2. LA-ICPMS zo7Pb/206Pb ages for detrital zircons.

Serra do Ouro Grosso S l o Jolo del Rei Andrelhndia autochthonous Andrelhndia allochthonous

Sample ITA-03 (43 grains) -

Sample ITA-04 (30 grains) Sample TIR-01 (43 grains) Sample ITA-01 (36 grains) Grain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Ane (Ma) + /- I

2245 2618 2504 2813 2871 2711 2782 2580 2640 2760 2810 3141 2429 3219 2778 2676 2812 2759 2702 2526 2503 3022 2999 3024 2577 2923 2699 2579 2703 2434

83 36 51 61 64 43 71 82 60 41 85 34 56 20 42 30 68 68 73 43 36 92 36 87

9 51 19 21 37 56

Grain 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16a 16b 17 18 19 20 21 22a 22b 23 24 25 26 27 28 29 30 31 32a 32b 33 34 35 36 37 38a 38b 39 40 41 42 43 a 43b

Age (Ma) 2449 2806 1994 1939 2857 2103 2069 2422 2207 2205 2143 2432 1886 1892 1809 2167 2129 1853 2440 2538 1982 2148 2097 2105 2039 1922 2060 2160 2513 2206 2409 2076 1966 2143 2146 2063 2663 2160 2576 1938 2026 2054 2038 2183 2026 2085 2152 2171

+/- 25 21 26 22 31 74 35 99 22 38 35 30 40 67 41 83 86 45 13 27 28 38 93 89 34 83 24 66 73 50 30 46 32 36 42 34 17 43 20 25 44 34 46 80 65 50 78 79

Grain 1 2 3a 3b 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

I Age (Ma) 2020 1779 1327 1257 1275 1469 1277 1905 1475 1276 1652 1808 1580 1520 1224 1144 1645 1436 1287 1368 1891 1739 1560 1242 2157 3089 2150 2083 2643 1473 1714 1968 2040 1958 1985 1622 1086

+ /- 26 27 94 99 47 52 70 31

121 59 73 48 88

113 57 88 48 92 78

151 40

185 72 49 72 50 41 81 17 73 54 20 74

119 41 30 85

Grain 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33a 33b 34 35 36 37 38 39 40 41 42 43

Age (Ma) 2049 1142 1969 2733 1173 1055 2151 1251 2102 2748 2054 1394 2210 2079 1565 1176 1154 1994 1282 1921 2130 2171 1514 1343 1047 2184 1274 1824 2715 1921 2184 1466 1992 1922 1432 2618 1840 2125 2024 2716 2024 2107 1798 1801

+/- 40 78

110 36 83 79 34 39 24 58 16

118 42 54 19 74 63 21 74 19 32 41 94 40 77 54 68 70 43 41 80 12 53 53 63 23 58 41 8

36 37

111 79 35

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918 C.S. VALLADARES ET AL.

8

marks. Five grains from this sample were large enough (120-200 pm) to be analyzed on two different trails and the ages obtained were identical within error (grains 16, 22, 32, 38 and 43, sample TIR-01, Table 2). The z"7Pb/206Pb ages (Fig. 3b) vary between 2.86 and 1.81 Ga (Table 2) and define a modal class at 2.2-2.1 Ga followed by the 2.1-2.0 Ga class as the next most abundant. A smaller group of zircons yield ages in the 2.4-2.9 Ga interval. The age of the youngest zircon, 1,809541 Ma (Table 2) provides the best estimate for the maximum age of deposition in this basin.

AndrelBndia basin

The metasediments of the Andrelhdia sequence are interpreted as deposited on a Neoproterozoic intraplate continental margin that surrounded the southern Siio Francisco paleocontinent (Trouw et al., 2000). Following the Brasiliano Orogeny, the successions are found far away from the foreland, in the internal zones of the Brasilia and Ribeira Belts located respectively southwestward and southeastward of the S5o Francisco Craton (Fig. lb). The regional distribution of the main lithofacies associations suggests, for the Brasilia Belt side, a NW-SE coastline and a depocenter to the WSW. From base to top and basinward the main lithofacies associations are (Paciullo et al., 1988): Al-paragneiss, interpreted as feldspathic sandstone; with amphibolite and ultramafic lenses; A2-paragneiss, quartzite and schist; A3-quartzite; All-gray phyllite/schist and quartzite; AS-biotite schist/gneiss; A6-schist, paragneiss, quartzite, metachert and calc-silicate rocks. The A1 association is interpreted as coastal plain deposits of the rift stage whereas the A2 association represents the first marine incursion into the basin. The A3 quartzites constitute an aggradational parasequence set of the spreading phase that was abruptly covered by the A4 pelites during a marine-flooding event. The biotite schist and gneiss of the A5 association locally contain dropstones and are interpreted as turbidites related to glaciation (Paciullo, 1997). The A6 association comprises pelagic, hemipelagic and turbiditic deposits developed after the proto-oceanic stage was attained. Amphibolite bodies are common in the A l , A2 and A6 associations, to the southeast of the study area (interference zone between Brasilia and Ribeira Belts, Fig. l b ) and represent continental and oceanic tholeiitic magmatism associated with the basin evolution (Gonqalves and Figueiredo, 1992; Paciullo, 1997). They record Nd model ages (T,,) of ca. 1.2 and 1.05 Ga, and E,, 1o valuesof +4.8 and +3.1, supporting their derivation from a depleted mantle reservoir and a short crustal residence time.

Two samples of quartzite beds from the A4 association were analyzed: one from the autochthonous domain (outcrop ITA-01, Fig. 2), and the other one (outcrop ITA-03,

c)

12 14] a)

$ 8 -

ITA-04 N=30

TIR-0 1 N=43

1C

LL i:; ~ ,~ 4

2

0

r

ITA-01 N=36

I dl ITA-03

Fig. 3. Histogram for ages of detrital zircon from quartzites of (a) Serra do Our0 Grosso sequence; (b) SLo JoHo del Rei sequence; (c) and (d) Andreliindia A4 association from autochthonous and allocthonous domains, respectively. N = total number of grains analyzed.

Fig. 2) from the basal thrust sheet of an allochthonous domain of the Brasilia Belt. As mentioned before, the thrust sheets related to the Brasilia Belt are vergent to the east, onto the western margin of the S5o Francisco paleocontinent.

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Detrital zircons extracted from both quartzites (samples ITA-01 and ITA-03) display low to medium sphericity and are mainly rounded and sub-rounded, although some preserve prismatic faces. They are mainly pinkish and yellowish, although a few are colorless. They are variable in size, between 40 and 200 pm. All have characteristic percussion marks.

The zircons from the autochthonous quartzite yielded ages in the range 1.0-2.2 Ga, with a modal class at 1.2-1.3 Ga (ITA-01, Fig. 3c). Grain 3 was large enough (200mm) to be analyzed on two different trails and the ages obtained were identical within error (Table 2). Only two out of thirty-six grains yielded Archean ages, one 2,643 -I- 17 Ma and the other 3,089 250 Ma (Table 2). The youngest zircon yielded an age of 1,086k85 Ma. Apart from the 1.2-1.3 Ga ages, the other ages are relatively uniformly distributed between 1.0 Ga and 2.2 Ga. This distribution indicates the presence of Mesoproterozoic zircon-bearing source rocks not well documented from the southern Siio Francisco Craton.

The zircons from the allochthonous quartzite yielded ages in the range 1.0-2.7 Ga, with a 2.1-2.2 Ga modal class Ga (ITA-03, Fig. 3d). Many zircons have ages in the range 1.0-1.5 Ga and only five of forty three grains yielded Archean ages, being 2,733236 Ma, 2,748258 Ma, 2,715243 Ma, 2,618223 Ma, 2,716536 Ma. Grain 33 was large enough (120-200pm) to be analyzed on two different trails and the ages obtained were identical within error (Table 2). The youngest zircon yielded an age of 1,047-+77 Ma (Table 2).

Discussion: Proterozoic Basin Evolution

The new 207Pb/206Pb LA-ICPMS data combined with the detailed geological data (Trouw et al., 2000; Paciullo, 1997; Ribeiro, 1997; Avila, 2000; and Teixeira et al., 2000) reported for the southernmost region of the SBo Francisco Craton support the proposed Proterozoic basin evolution:

Episode 1 (ca. 2.25 Ga): The Serra do Our0 Grosso intracratonic basin

The Serra do Our0 Grosso sequence was derived predominantly from the erosion of a 2.5-2.8 Ga Neoarchean crust with subordinate contribution of Paleoproterozoic rocks. The existence of overgrowths indicates that the source area underwent some kind of metamorphism or magmatic overgrowth during the Paleoproterozoic. The ca. 2.25 Ga age is the best estimate for the beginning of the sedimentation for this sequence. It is also interesting to stress that, until now, only U-Pb and 207Pb-206Pb ages around 2.2 to 2.0 Ga (Avila, 2000; Teixeira et al., 2000) were reported for the “Transamazonian”

granitoids of this sector along the southern border of the Siio Francisco Craton.

Episode 2 (<ca. 1.8 Ga): Siio Jo60 del Rei basin

Data of the SBo JoBo del Rei basin successions indicate derivation from a 2.2-2.0 Ga crust, with subordinate contribution from Neoarchean sources. The age of the youngest zircon, 1,809+41 Ma (Table 2) provides the best estimate for the maximum age of deposition in this basin. Ages between 2.2 and 2.0 Ga are regarded as the signature of the so-called Transamazonian orogeny in the SBo Francisco Craton (Machado et al., 1992; Noce, 1995; Teixeira and Figueiredo, 1991), and represent arc-related rocks, collisional granitoids and metamorphism. These considerations may be taken to suggest that the formation of the SBo Joiio del Rei basin may post-date a period of erosion of continental crust stabilized after the Transamazonian orogeny

The Serra do Our0 Grosso sequence was tentatively correlated with basal quartzites of the Tiradentes Platform of the ,550 Joiio del Rei basin (Trouw et al., 2000), but the new data do not corroborate this assumption because: (a) the maximum age of deposition of the S5o Jo50 del Rei basin (1,809241 Ma) is younger than that of the Serra do Our0 Grosso sequence (2,245285 Ma); (b) the Serra do Our0 Grosso sequence was derived predominantly from the erosion of a 2.5-2.8 Ga Neoarchaean crust with no signal of the “Transamazonian” zircons, whilst the SBo JoBo del Rei basin successions were derived predominantly from the erosion of 2.0-2.2 Ga crust formed during the Transamazonian orogeny; (c) only the SBo Jo50 del Rei basin records detrital zircon ages younger than the supposed Transamazonian event (ca 2.07-2.04). Based on these new data, we suggest that the Serra do Our0 Grosso succession is older than the S2o Jo50 del Rei successions, unless a unique provenance pattern reflects different source areas.

Episode 3 (ca. 1.3-1.05 Ga): The Carandai basin

As suggested by Ribeiro et al. (1990) and Trouw et al. (2000), renewed rifting promoted the opening of the Carandai Basin. This basin includes two sequences separed by a paleokarst surface: the lower carbonate rich Barroso sequence and the upper pelitic Prados sequence (Ribeiro et al., 2003).

Until now no direct geochronological data for the metasediments of the Carandai basin successions are available. The time interval of 1.3-1.0 Ga proposed by Ribeiro et al. (1995) is based on Sm/Nd model ages of mafic dykes and sills of the SBo JoBo del Rei and Andrelhdia units. The maximum age of deposition (1.3 Ga) corresponds to the younger T,, model age obtained for mafic dykes which crosscut the underlying S5o JoBo del

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920 C.S. VALLADARES ET AL.

Rei units, but do not cut the Carandai metasediments, suggest the minimum. The age of termination of the sedimentation in the Carandai basin is taken from the maximum age of deposition of the overlying Andreldndia basin succession, proposed above, based on the age of the youngest detrital zircon (1,047+-77 Ma).

Episode 4 (ca. 1.05-0.61 Ga): TheAndrel6ndiapassive margin

The detrital sediments of the Andreldndia basin (A4 association) were derived mainly from the erosion of Mesoproterozoic and Paleoproterozoic rocks. The Archean contribution is minimal, indicating that in the internal sector of the Transamazonian Orogen juvenile rocks predominated. The age of the youngest zircon (1,047+77 Ma) provides the best estimate for the maximum age of deposition of the Andreldndia basin A4 association. The ages of 604?:19 Ma (Sm/Nd in garnets of retro-eclogites from mafic intrusive rocks in the Andreldndia sequence) and 567+11 Ma (zircon inferior intercept from a quartzite of the Andrelbdia sequence), obtained by Trouw and Pankhurst (1993) and Sollner and Trouw (1997) respectively, provide the best estimate for the minimum age of the sedimentation. The data permit proposing the time interval between 1,047 Ma and 604-567 Ma for the deposition in the Andreldndia Basin and are in accordance with the time interval of 1.0-0.6 Ga proposed by Trouw et al. (20001, based on Nd T,, model ages obtained from amphibolites occurring to the southeast of the study area (Heilbron et al., 1989). The presence of several Mesoproterozoic source rocks to the north-northeast of the present location, to the interior of the SZo Francisco Craton, as indicated by provenance indicators, are of particular importance since igneous rocks in the 1.0-1.6 Ga range are not known from the Siio Francisco Craton, with the possible exception of mafic dykes.

Regional Correlations We tentatively discuss the significance of obtained data

in the context of the regional geology of the southern Siio Fancisco Craton, including the Iron Quadrangle, where substantial geochronological and tectonic data are available in the literature.

Combining the data reported by Machado et al. (19961, Teixeira et al. (2000) and Alkmin and Marshack (1998) a simplified tectonic evolution of the Minas Supergroup during the Paleoproterozoic can be summarized as follows: (a) deposition of the Caraqa Group between 2.6 and 2.4 Ga; (b) deposition of Itabira Group ca. 2.4 Ga; (c) deposition of the Piracicaba Group between 2.4 and 2.2 Ga, and finally; (d) deposition of Sabari Formation and Itacolomi Group around 2.13- 2.10 and 2.06 Ga. The first three sedimentary successions are interpreted as part

of a passive margin prior to the Transamazonian Orogeny, while the Sabari and Itacolomi successions are regarded as syn to late orogenic deposits deposited in a foreland tectonic setting.

The pre to syn Transamazonian orogenic granitoids are well studied in the Mineiro Belt close to Siio JoZo del Rei town (Avila, 2000; Teixeira et al., 2000). The reported U-Pb and Pb-Pb data fall in the 2.22 to 2.12 Ga interval. Metamorphic ages reported for both the Iron Quadrangle and the SBo Joiio del Rei area indicate that the main Transamazonian metamorphic episode occurred in the 2.07 to 2.02 Ga period (Noce, 1995; Machado et al., 1992).

The new zo7Pb-206Pb data presented here indicate that the Serra do Our0 Grosso succession probably represents the oldest Proterozoic siliciclastic succession of the investigated area. The ca. 2.25 Ga youngest zircon found in this sequence is immediately older than the above- mentioned orogenic rocks, suggesting deposition prior to the Transamazonian Orogeny. Combining the geochronological data with the mature composition of the sequence, we suggest that the Serra do Our0 Grosso sequence could be chronocorrelated with the Piracicaba Group of the Minas Supergroup. It is still possible that the deposition of the Serra do Our0 Grosso succession is related to an intracratonic basin.

The SZo JoBo del Rei basal unit presents similar age distribution with the Itacolomi Group, with zircon populations coeval with the main phases of the Transamazonian Orogeny (2.22 to 2.04 Ga). The youngest age obtained, around 1.8 Ga, corresponds to post-tectonic rocks described in the southern border of the SBo Francisco Craton by Heilbron et al. (1989) and A d a (2000). Therefore, the results indicate erosion of a stable crust after the Transamazonian Orogeny. Previous regional correlation with the Espinhaqo Rift successions, as suggested by Ribeiro et al. (1990) and Trouw et al. (2000) based on Nd T,,, model ages for metamafic dykes that crosscut the Siio Jog0 del Rei units, are also corroborated by the presented data.

The two samples of the Andreldndia successions display identical detrital signatures, with populations derived from Archean, Tranzamazonian and Mesoproterozoic sources. The first two populations are well constrained in the southern segment of the Siio Francisco Craton and discussed above. The Mesoproterozoic source is poorly defined in the cratonic region, but as we investigated both autochthonous and allochthonous settings, one might speculate that the zircons could be derived from eroded volcanic successions, only preserved as subvertical feeder dykes that crosscut the Siio Jo50 del Rei successions. As stated befcre, T,, model ages of 1.8 to 1.3 Ga are in agreement with this interpretation. The youngest detrital zircon age of 1.05 Ga constrains the beginning of the sedimentation of the Andreldndia successions.

Gondwana Research, K 7, NO. 4, 2004

AGES OF DETRITAL ZIRCON FROM SAO FRANCISCO CRATON, BRAZIL 92 1

Acknowledgments C. Valladares acknowledges financial support from CAPES

for her stay at UQAM, CNPq (Process: 46817/2000) and FAPERJ (Process: E-26/170.649/ 99), Brazil. The ICPMS regional facility at the UM is financed and supported through NSERC grants to N. Machado. The authors acknowledge A. Ribeiro and E Paciullo for suggestions and A. Ribeiro and R. Van Schmus for the critical review of the manuscript.

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