Provenance and age delimitation of Quadrila´tero Ferrı´fero
sandstones based on zircon U–Pb isotopes
Le´o A. Hartmann
a,*, Issamu Endo
b, Marcos Tadeu F. Suita
b, Joa˜o Orestes S. Santos
c,
Jose´ Carlos Frantz
a, Maurı´cio A. Carneiro
b, Neal J. McNaughton
d, Mark E. Barley
daInstituto de Geocieˆncias, Universidade Federal do Rio Grande do Sul, Av. Bento Gonc¸alves, 9500, 91501-970 Porto Alegre, Rio Grande do Sul, Brazil bDepartamento de Geologia, Universidade Federal de Ouro Preto, Ouro Preto, Minas Gerais, Brazil
cPesquisador, Conselho Nacional do Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq), Porto Alegre, Rio Grande do Sul, Brazil dCentre for Global Metallogeny, The University of Western Australia, Crawley 6009, Western Australia
Received 1 August 2003; accepted 1 July 2005
Abstract
The Quadrila´tero Ferrı´fero has some of the largest iron and gold deposits in the world and is a major geotectonic unit of the Sa˜o Francisco Craton in Brazil. U–Pb zircon SHRIMP geochronology of six detrital sedimentary and metasedimentary rocks (114 zircon crystals, 118 spot analyses) has improved the understanding of the sedimentary processes and provenance ages of both rocks and the associated iron formation. The age of deposition of the iron formation is constrained between 2.58 and 2.42 Ga. The presence of an old Paleoarchean crust is dated in detrital zircon crystals, including the oldest zircon in South America (3809G3 Ma). Only high-Th/U, magmatic zircon crystals are present in the dated sedimentary rocks, and these indicate that the crust of the region was formed mostly during the Jequie´ cycle (six age peaks between 3055 and 2635 Ma). This time span ofw420 m.y. is similar to the duration of a long-lived Wilson cycle. Most of the Rio das Velhas Basin was filled during
approximately 30 m.y. between 2746 and 2717 Ma, though volcanism probably started earlier. The youngest detrital zircon age from the Minas Supergroup indicates that the intracratonic basin fill, including the iron formation, was deposited after 2580 Ma. Therefore, the crust was cratonized shortly after the intrusion of minor granitic bodies at around 2.62–2.58 Ga. A large gap in orogenic activity is indicated by the absence of zircon ages of 2580–2182 Ma.
q2006 Elsevier Ltd. All rights reserved.
Keywords:Quadrila´tero Ferrı´fero; Archean; Geochronology; SHRIMP; Zircon; Iron formation
1. Introduction
The Quadrila´tero Ferrı´fero is a major Precambrian geotectonic unit of the Sa˜o Francisco Craton (Fig. 1), contains giant iron and gold deposits, and has some of the oldest rocks in the Brazilian shield. Studies of detrital zircon crystals from these rocks by sensitive high resolution ion microprobe (SHRIMP II) have gone far to clarify their history.
The deposition of a giant iron formation is a major event in the evolution of the continental crust and intrinsically related to the changing composition of the atmosphere. The establish-ment of the age of deposition of Precambrian sediestablish-mentary rocks has many problems (Dodson et al., 1988; Rainbird et al., 2001; Nelson, 2001; Eriksson et al., 2001; Santos et al., 2003b;
Hartmann et al., 2003), particularly in the absence of interstratified volcanic flows or tuffs. The Quadrila´tero Ferrı´fero has giant iron ore deposits—29 billion tons contain-ing 50–65% Fe (Dardenne and Schobbenhaus, 2000)—but no volcanic beds have been identified in the ore or even immediately below or above it. Consequently, we used the SHRIMP II for spot (w25 to 30mm) determinations of U–Pb
isotopes in detrital zircon crystals from sedimentary beds lying immediately below and above the iron formation, as well as from other units, to improve understanding of the Archean and early Paleoproterozoic evolution of the Quadrila´tero Ferrı´fero. The age of iron formation deposition in the Minas Supergroup has been estimated as Neoarchean or Paleoproter-ozoic (Renger et al., 1994; Babinski et al., 1995; Teixeira et al., 2000). We present results that help evaluate the maximum age of ore deposition and thus contribute to the understanding of the evolution of the Rio das Velhas and Minas Supergroup basins, a major issue in the Precambrian geology of South America.
Journal of South American Earth Sciences 20 (2006) 273–285
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doi:10.1016/j.jsames.2005.07.015
All previous provenance investigations have considered the dated detrital zircon crystals as representative of the tectonic events in the source (Machado et al., 1996; Noce, 2000; Teixeira et al., 2000), but we test the proposal of Hartmann and Santos (2004) that only high-Th/U, magmatic zircon crystals are present in the sedimentary detritus. The identification of this tectonic bias in previous interpretations is essential for any future provenance investigations in the Quadrila´tero Ferrı´fero.
2. Geology and samples
The Quadrila´tero Ferrı´fero is located in the southern portion of the Sa˜o Francisco Craton (e.g. Dorr, 1969; Almeida and Hasui, 1984; Almeida et al., 2000; Hartmann and Delgado, 2001). The geology of the Quadrila´tero Ferrı´fero (Fig. 2) has been investigated for several decades (e.g.Dorr, 1969; Herz, 1970; Chemale et al., 1994; Carneiro et al., 1998), and intensive fieldwork in the past decade has led to an accurate
Fig. 1. Geotectonic setting of the Quadrila´tero Ferrı´fero in South America, with reference to the Late Paleoproterozoic–Mesoproterozoic supercontinent Columbia (Rogers and Santosh, 2002; Supercontinent Atlantica ofHartmann, 2002).
understanding of the main geological relationships in the province. The giant Lake Superior-type iron formations from the Quadrila´tero Ferrı´fero (Fig. 2) are in the middle portion of the Minas Supergroup (Table 1), which has quartzites at the base and iron formations and quartzo-feldspathic schists at the top. Volcanic rocks are not known in the Minas Supergroup.
The Minas Supergroup was deposited over the Rio das Velhas greenstone belt and granitic–gneissic basement. Deformation is complex, and the superposition of several tectonic events formed the dome and keel structure presently observed in this southern portion of the Sa˜o Francisco Craton (Marshak et al., 1997).
Fig. 2. Geological map displaying major units in the Quadrila´tero Ferrı´fero. The quadrangular shape of the province is defined by the synclines of the Minas Supergroup. Location of dated sedimentary samples indicated.
Table 1
Stratigraphic column of the Quadrila´tero Ferrı´fero (Machado et al., 1996)
Supergroup Group Formation Rocks
Itacolomi!2.15 Ga Quartzite, sandstone, conglomerate, phyllite Angular and profound erosional unconformity
Minas,!2.6 Ga Piracicaba Sabara´a Graywacke, schist, phyllite, metavolcanics, conglomerate, quartzite, minor iron-formation
Barreiro Phyllite and graphitic phyllite Taboo˜es Ortoquartzite
Fecho do Funil Quartzose and dolomitic phyllite, siliceous phyllite Cercadinho Ferruginous quartzite, phyllite, conglomerate, dolomite Erosional unconformity
Itabira Gandarela Dolomite and minor limestone, itabirite, phyllite
Caueˆ Itabirite (oxide-facies iron formation), phyllite and dolomite Carac¸a Batatal Phyllite, chert, oxide-facies iron formation
Moeda Quartzite, conglomerate, phyllite and graywacke Erosional and angular unconformity
Rio das Velhas, 2.78–2. 72 Ga
Maquine´ Casa Forte Quartzite, conglomerate, phyllite and graywacke Palmital Phyllite, quartzite, graywacke, basal conglomerate
Nova Lima Chloritic phyllite, graywacke, carbonate-facies iron formation, metavolcanics, quartzite, sandstone, tilloid, conglomerate and dolomite
Sampling of major units (Fig. 2) includes the Rio das Velhas greenstone belt and overlying sedimentary units. In the classical Serra da Moeda section of the Minas Supergroup, the sedimentary rocks were presumably deposited discordantly over the Mamona Granodiorite, because they are younger, but the contact is highly tectonized. Both the basement and the clastic basin were deformed in subvertical, transcurrent shear zones.
Samples 1 and 2 are sandstones from the Nova Lima Group, the basal unit of the Rio das Velhas greenstone belt, collected at UTM 7,769,623; 625,002 (sample 1) and UTM 7,770,065; 625,700 (sample 2). A Mamona granodiorite mylonite at the contact with the base of the sedimentary sequence, located at UTM 7,756,360; 608,678, has been dated (L.A. Hartmann, unpublished). Close to the Mamona Granodiorite, we studied sample 3, a quartzite from the Moeda Formation located at UTM 7,756,454; 609,047, a few tens of meters above the base of the quartzite. Sample 4, a Moeda Formation quartzite, is located at UTM 7,758,108; 608,640, at the top of the lower quartzite unit and 50 m below the contact with an iron formation. Sample 5 is a quartzo-feldspathic schist from the Sabara´ Group above the iron formation, collected at UTM 7,743,975; 653,225. Sample 6 is a sandstone from the Itacolomi Group, much higher in the stratigraphy, collected at UTM 7,742,347; 656,167. This sampling enables a tighter delimitation of the depositional ages of the sandstones from three sedimentary basins and the associated iron formation, including the magmatic provenance of detrital sedimentary rocks.
3. SHRIMP U–Pb zircon geochronology
Previous geochronological investigations have elucidated important parts of the evolution of the Quadrila´tero Ferrı´fero betweenw3.4 and 2.6 Ga in the Archean and 2.15–2.06 Ga in
the Paleoproterozoic (Machado et al., 1992, 1996; Machado and Carneiro, 1992; Teixeira et al., 1996; Endo, unpublished; Carneiro et al., 1998; Noce, 2000; Noce et al., 2000; Endo and Machado, 2002). Lead isotopic studies in carbonate rocks above the iron formation (Babinski et al., 1995) indicate a minimum depositional age of 2420G20 Ma for the ore, though Pb isotopic evidence indicates younger Paleoproter-ozoic tectonism in the Itabira District (Olivo et al., 1996). Detrital zircon crystals yield ages of around 2.15 Ga in the Itacolomi Group, higher up in the stratigraphy (Machado et al., 1992). Overall, the iron formations are considered Paleoproterozoic in age by most investigators (e.g. Teixeira et al., 2000).
The age of the major event that formed the Rio das Velhas greenstone belt is considered to be approximately 2.78– 2.72 Ga, was named the Rio das Velhas tectonothermal event byMachado and Carneiro (1992), and could be interpreted as an orogeny. The youngest zircon207Pb–206Pb ages (ID-TIMS) byMachado et al. (1996)from the Maquine´ Group (2877 Ma) were interpreted as the age of the source. Sm–Nd isotopic studies indicate the presence of older crust of about 3.2 Ga in the region (Carneiro et al., 1998).
The zircon crystals were separated from the rocks by crushing and milling 1 kg of rock, followed by heavy liquid and magnetic methods. The crystals were mounted on an epoxy disc, polished to half their thicknesses, and carbon coated for backscattered electron imaging. The mount was repolished and gold coated for SHRIMP II U–Pb isotopic determinations at Curtin University of Technology, Western Australia (Smith et al., 1998). Data reduction used the SQUID software (Ludwig, 2001), and plots were prepared with Isoplot/Ex (Ludwig, 1999).
Our geochronological investigation includes the study of the internal structure of the zircon crystals by electronic imaging (backscattered electrons) prior to isotopic study (Fig. 3). In a few crystals, the images were made after ion microprobe dating (Fig. 3A–C). Zircon crystals from sample 1, a sandstone from the Nova Lima Group, show little rounding and complex internal structure (Fig. 3A–C). The oldest zircon crystal in
Fig. 3C is the oldest in South America and has a complex, altered internal structure. Crystals from sample 2, another Nova Lima Group sandstone, are similar to sample 1 but less complex (Fig. 3D–F). The zircon crystals from sample 3, Moeda Formation (Fig. 3M–P), have some rounding but are overall prismatic and elongated 2:1. Few fractures are observed, but the internal structure is complex in all these crystals; some zircon crystals from the Moeda Formation have fine euhedral zoning of magmatic origin (Fig. 3G, J and K), whereas others are homogeneous along rims (e.g. lower 10mm
thick rim in Fig. 3H) or nearly throughout the entire crystal (Fig. 3L). Zircon crystals from sample 4 (Moeda Formation,
Fig. 3G–L) are similar to sample 3, because of their minimal rounding and fine euhedral zoning. Sample 5, a quartz-feldspar schist from the Sabara´ Group, has rounded, homogeneous zircon crystals (Fig. 3Q, R).
Crystals from sample 6, a sandstone from the Itacolomi Group (Fig. 3S, T), are also comparable to samples 3 and 4 because they are poorly rounded and have euhedral zoning. The youngest Archean zircon dated in this project (grain b36-1; 2580G7 Ma) is shown inFig. 3M, and the oldest Paleoproter-ozoic zircon (grain c65-1; 2185G15 Ma) is in Fig. 3T. Xenotime overgrowths are present on zircon crystals from the Itacolomi sandstone (Fig. 3S).
In sample 1, the sandstone from the Nova Lima Group, 11 spots in 10 crystals resulted in the identification of the oldest zircon in South America with an age of 3809G3 Ma (Fig. 4A). The oldest zircon previously dated in the Quadrila´tero Ferrı´fero is 3539G34 Ma old (207Pb/206Pb, ID-TIMS; Machado et al., 1996). Three age groups are present in the detrital zircon population: 3809G3 Ma (nZ1), 2953G3 Ma (nZ3), and
2759G10 Ma (nZ4). The youngest grain (d14b) is 2749G
7 Ma (1% discordant), which limits the deposition of the Nova Lima Group to!2749 Ma.
In sample 2, another sandstone from the Nova Lima Group, the youngest population (Fig. 4B) is 2774G13 Ma (nZ5), and the
youngest grain (b53) is 2745G6 Ma. This youngest age is 9% discordant; we use the age of grain d14 of sample 1 to constrain the maximum age for deposition of the Nova Lima Group. A detrital zircon from this unit was dated by (Machado et al., 1996)
at approximately 2770 Ma. The oldest age group is 2935G12 Ma (nZ6). Five of 24 crystals (25 analyses) from this sample are
older than 3050 Ma (3050, 3069, 3208, 3209, and 3307 Ma). The youngest age group (2774G13 Ma) was not identified in Nova Lima Group quartzites by Machado et al. (1996)because the zircon crystals in their dated samples yield a youngest age of about 2870 Ma. The age of 2774G13 Ma is better defined in this sample 2 than in sample 1 and coincides with the youngest age of
w2772 Ma, dated byMachado et al. (1996)in Rio das Velhas
greenstone belt zircon crystals.
The age of magmatic crystallization of the Mamona Granodiorite is 2708G5 Ma, as determined by five U–Pb SHRIMP analyses on five zircon crystals (L.A. Hartmann, unpublished). A207Pb/206Pb zircon age of 2721G3 Ma by ID-TIMS was previously obtained byMachado et al. (1992)from the same outcrop. The age of this rock is significant for this investigation, because it intrudes the Nova Lima Group and is covered by Moeda Formation quartzites.
A total of 27 isotopic analyses in 27 zircon crystals from sample 3, the basal quartzite of the Moeda Formation, yield a spread of provenance ages between 3583G8 Ma and
2708G5 Ma (Fig. 4C). Three pooled ages are defined at 2713G8 Ma (nZ4), 2869G4 Ma (nZ8), and 3010G18 Ma
(nZ3). The Mamona Granodiorite is well represented in the
detrital zircon population by the 2708G5 Ma ages. Other ages are 3583G8, 3348G5, 3338G6 and 3263G5 Ma.
The dating of 27 spots in 25 zircon crystals from sample 4, the upper Moeda Formation quartzite (Fig. 4D), shows a shift toward younger ages compared with sample 3; sample 4 has a well-represented population at 2646G15 Ma (nZ12) and a
small population at 2584G10 Ma (nZ3). This spread is the
result of erosion of varying provenance terrains. The youngest grain (b44) is 2524G5 Ma old, but the data are 22% discordant, so we use the age of 2580G7 Ma (grain b36, 100% concordant) to constrain the maximum age of the Moeda Formation and the Carac¸a Group.
In sample 5, the Sabara´ Group quartz-feldspar schist, 21 spots in 21 zircon crystals yield two main age groups, the oldest at 2896G4 Ma (Fig. 4E). The youngest pooled age is 2719G20 Ma (nZ12), representative of Mamona
Granodior-ite-type intrusions. Another possible age group is 3012G
12 Ma (nZ2).
Fig. 3. Backscattered electron images of zircon crystals showing analyzed spots and ages in Ma. (A–C) Sample 1, Nova Lima Group sandstone; (D–F) sample 2, Nova Lima Group sandstone; (G–L) sample 4, Moeda Formation quartzite; (M–P) sample 3, Moeda Formation quartzite; (Q, R) sample 5, Sabara´ Formation quartz-feldspar schist; (S, T) sample 6, Itacolomi Group sandstone. Oldest zircon in South America shown in (C). Scale bar is 10mm long. Xenotime overgrowths shown in (S). White circle around analyses pits in (A–C).
Reconnaissance dating of six zircon crystals (seven spots) from the Itacolomi Group sandstone, sample 6, yields three Archean ages (3236G18, 3046G9 and 2812G13 Ma) and four Paleoproterozoic ages pooled at 2173G8 Ma (nZ4;
Fig. 4F). The youngest grain (c62) is 2143G16 Ma.
4. Discussion and conclusions
The dating of a large number of detrital zircon crystals, possibly 117 (Vermeesch, 2004), is required to detect a source representing 5% of the population (95% confidence interval). In this reconnaissance study, the number of detrital grains investigated in each sample ranges between 6 (sample 6) and 27 (sample 3). Major sources for the deposition of the investigated units are thus detected, but minor sources, which contribute 10% or less to the filling of the basins, may remain undetected. However, this study is relevant for the under-standing of the evolution of the Quadrila´tero Ferrı´fero in the
Archean and Paleoproterozoic because the number of events and their ages are now better known.
The oldest known detrital zircon crystal of South America is from sample 1, the Nova Lima Group, and is 3809G3 Ma (Table 2, Fig. 5). The analysis is nearly concordant and offers a clue for the possible presence of Paleoarchean crust in the Quadrila´tero Ferrı´fero. Growth of the continental crust in the region was more voluminous in the Mesoarchean and Neoarchean (3055–2635 Ma) than in older times; this trend is worldwide, because crust 3.9 Ga or older is not abundant in the continents, as interpreted from the ages of detrital zircon crystals (e.g. Nutman, 2001). Older crust may have been recycled by younger events or not have been formed in large volumes; this hypothesis requires additional studies. Nevertheless, the beginning of the Jequie´ cycle of crust formation seems close to 3055 Ma in the Quadrila´tero Ferrı´fero, a novel contribution to Archean studies.
Table 2
U–Pb zircon SHRIMP isotopic data from Quadrila´tero Ferrı´fero samples
Isotopic ratios Ages
U (ppm) Th (ppm) Th 4f206 (%) 207Pb 208Pb 206Pb 207Pb 208Pb 207Pb 206Pb Disc. (%)
Spot U 206Pb 206Pb 238U 235U 232Th 206Pb 238U
Sample 1, Nova Lima Group sandstone
d.1-1 48 37 0.78 0.141 0.1916G0.70 0.2225G0.96 0.5518G1.36 14.5795G1.53 0.1542G1.71 2756G11 2833G31 K3 d.3-1 49 28 0.58 0.129 0.2073G0.80 0.1577G1.01 0.5606G1.30 16.0200G1.53 0.1495G1.86 2884G13 2869G30 1 d.6-1 103 32 0.32 0.077 0.2154G0.42 0.0891G0.87 0.5908G1.40 17.5453G1.46 0.1610G1.91 2946G7 2993G33 K2 d.8-1 121 89 0.76 0.051 0.2030G0.39 0.2110G0.63 0.5622G1.23 15.7334G1.29 0.1542G1.42 2850G6 2876G29 K1 d.9-1 89 47 0.54 0.003 0.2034G0.48 0.1495G0.77 0.5589G1.16 15.6772G1.25 0.1552G1.39 2854G8 2862G27 0 d.11-1 169 124 0.76 0.036 0.2165G0.31 0.2064G0.52 0.5912G1.06 17.6510G1.10 0.1608G1.20 2955G5 2994G25 K1 d.12-1 125 64 0.53 0.079 0.2166G0.41 0.1482G0.65 0.6263G2.88 18.7026G2.91 0.1716G2.98 2955G7 3135G72 K6 d.14-1 413 323 0.81 0.087 0.1887G0.26 0.2242G0.48 0.3817G1.14 9.9312G1.17 0.1048G1.26 2731G4 2084G20 24 d.14b1 190 107 0.58 0.036 0.1908G0.46 0.1657G3.10 0.5236G1.11 13.7772G1.20 0.1485G3.30 2749G7 2715G25 1 d.15-1 228 193 0.88 0.012 0.3744G0.20 0.2492G0.46 0.7735G1.23 39.9359G1.24 0.2194G1.31 3809G3 3694G35 3 d.16-1 138 104 0.78 0.016 0.1923G0.39 0.2120G1.54 0.4934G1.66 13.0813G1.71 0.1335G2.28 2762G6 2586G35 6 Sample 2, Nova Lima Group sandstone
b.17-1 122 19 0.17 0.082 0.2297G0.42 0.0479G1.65 0.5998G0.90 18.9950G1.00 0.1667G2.61 3050G7 3029G22 1 b.31-1 149 79 0.53 0.103 0.2218G0.66 0.1412G1.13 0.5739G1.20 17.5497G1.37 0.1496G1.86 2994G11 2924G28 2 b.32-1 209 106 0.51 0.098 0.2038G0.56 0.1369G0.96 0.5307G1.11 14.9133G1.24 0.1402G1.59 2857G9 2744G25 4 b.32-1 431 36 0.08 0.361 0.1939G0.60 0.0348G1.79 0.2444G0.96 6.5349G1.13 0.0772G6.99 2776G10 1410G12 49 b.33-1 37 13 0.35 K0.157 0.2154G1.40 0.0937G2.58 0.5781G1.91 17.1655G2.37 0.1605G5.68 2946G23 2941G45 0 b.35-1 272 206 0.78 0.021 0.2324G0.25 0.2133G0.39 0.6180G0.84 19.8043G0.88 0.1684G0.93 3069G4 3102G21 K1 b.36-1 259 377 1.45 0.118 0.1934G0.51 0.3872G0.56 0.5364G1.05 14.3013G1.17 0.1424G1.22 2771G8 2768G24 0 b.38-1 168 41 0.25 0.049 0.2126G0.59 0.0679G1.40 0.5758G1.14 16.8783G1.29 0.1556G2.34 2926G10 2931G27 0 b.39-1 98 74 0.75 0.170 0.2020G0.86 0.1974G1.13 0.5483G1.34 15.2708G1.59 0.1410G2.13 2842G14 2818G31 1 b.40-1 376 240 0.64 K0.016 0.2539G0.38 0.1668G0.62 0.6375G1.05 22.3189G1.12 0.1673G1.36 3209G6 3179G26 1 b.41-1 145 108 0.75 0.039 0.1935G0.67 0.1996G0.96 0.5343G1.21 14.2575G1.38 0.1421G1.60 2772G11 2760G27 0 b.42-1 43 24 0.55 0.067 0.2031G1.29 0.1463G2.00 0.5832G2.00 16.3339G2.38 0.1544G3.63 2851G21 2962G48 K4 b.43-1 206 157 0.77 0.035 0.2163G0.53 0.2028G0.80 0.5773G1.11 17.2124G1.23 0.1524G1.40 2953G9 2938G26 1 b.44-1 136 50 0.37 0.062 0.2054G0.65 0.0995G1.31 0.5698G1.21 16.1406G1.37 0.1522G1.87 2870G11 2907G28 K1 b.45-1 101 76 0.75 0.144 0.2701G0.65 0.1960G1.03 0.6661G1.31 24.8066G1.46 0.1703G1.81 3307G10 3291G34 0 b.46-1 101 82 0.82 0.091 0.2198G0.76 0.2174G1.08 0.5689G1.33 17.2419G1.53 0.1501G1.82 2979G12 2903G31 3 b.47-1 116 59 0.51 0.171 0.2031G0.87 0.1384G1.30 0.5600G1.38 15.6860G1.63 0.1489G2.85 2852G14 2867G32 K1 b.48-1 69 94 1.36 0.221 0.1918G1.02 0.3695G1.08 0.5353G1.50 14.1559G1.81 0.1436G1.94 2758G17 2764G34 0 b.49-1 155 76 0.50 0.027 0.2022G0.39 0.1393G0.67 0.5498G0.86 15.3287G0.94 0.1517G1.12 2844G6 2824G20 1 b.50-1 72 55 0.79 0.068 0.1943G0.62 0.2155G1.50 0.5160G1.04 13.8255G1.21 0.1401G1.89 2779G10 2682G23 3 b.51-1 124 96 0.79 0.027 0.2029G0.42 0.2211G0.60 0.5555G0.91 15.5401G1.00 0.1541G1.09 2850G7 2848G21 0 b.52-1 82 25 0.32 0.040 0.2149G0.50 0.0853G1.08 0.5762G1.00 17.0704G1.11 0.1539G1.65 2943G8 2933G23 0 b.53-1 262 161 0.63 0.033 0.1904G0.33 0.1823G1.00 0.4752G0.83 12.4769G0.89 0.1363G1.32 2746G5 2506G17 9 b.54-1 100 55 0.57 K0.035 0.2536G0.42 0.1572G0.75 0.6554G1.01 22.9187G1.10 0.1804G1.26 3208G7 3249G26 K1 b.55-1 289 97 0.35 0.057 0.2009G0.31 0.0889G2.34 0.4693G0.85 13.0016G0.90 0.1188G2.55 2834G5 2481G17 12 Sample 3, Moeda Formation quartzite
b.34-3 82 34 0.43 0.082 0.1859G0.61 0.1221G1.11 0.5231G3.64 13.4052G3.69 0.1489G3.84 2706G10 2712G81 0 b.34-4 191 126 0.68 0.008 0.1783G0.45 0.1773G0.68 0.4450G3.60 10.9423G3.62 0.1155G3.68 2637G7 2373G71 10 b.35-2 77 49 0.66 K0.051 0.1796G0.66 0.1851G0.90 0.5088G3.65 12.6010G3.71 0.1434G3.79 2649G11 2651G79 0 b.35-3 174 137 0.81 0.010 0.1733G0.44 0.2244G0.59 0.4821G3.60 11.5179G3.62 0.1329G3.66 2589G7 2537G75 2 b.36-3 99 85 0.89 0.039 0.1851G0.53 0.2492G0.94 0.5353G3.63 13.6587G3.67 0.1507G3.77 2699G9 2764G82 K2
(continued on next page)
L.A.
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American
Earth
Sciences
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Table 2 (continued)
Isotopic ratios Ages
U (ppm) Th (ppm) Th 4f206 (%) 207Pb 208Pb 206Pb 207Pb 208Pb 207Pb 206Pb Disc. (%)
Spot U 206Pb 206Pb 238U 235U 232Th 206Pb 238U
b.36-1 182 173 0.98 0.008 0.1723G0.40 0.2733G0.50 0.4937G3.61 11.7298G3.63 0.1373G3.68 2580G7 2587G77 0 b.36-4 138 107 0.80 K0.023 0.1792G0.45 0.2207G0.62 0.5153G3.60 12.7289G3.63 0.1417G3.67 2645G7 2679G79 K1 b.37-1 25 8 0.34 0.003 0.1891G1.08 0.0987G2.17 0.4831G3.97 12.5972G4.11 0.1386G4.63 2735G18 2541G83 7 b.37-3 178 78 0.45 0.028 0.1795G0.45 0.1255G0.79 0.4848G3.60 11.9976G3.63 0.1345G3.70 2648G7 2548G76 4 b.37-4 128 69 0.56 0.040 0.1841G0.87 0.1281G0.96 0.4542G3.64 11.5290G3.75 0.1042G3.79 2690G14 2414G73 10 b.38-1 103 74 0.75 K0.046 0.1779G0.63 0.2051G0.86 0.4708G3.64 11.5470G3.69 0.1290G3.76 2633G10 2487G75 6 b.38-3 236 79 0.35 0.025 0.2113G0.32 0.0945G0.65 0.5619G3.58 16.3677G3.59 0.1530G3.65 2915G5 2875G83 1 b.39-4 202 141 0.72 0.002 0.1777G0.41 0.1923G0.60 0.4522G3.59 11.0830G3.61 0.1206G3.66 2632G7 2405G72 9 b.39-2 148 78 0.55 0.022 0.2568G0.41 0.1457G0.75 0.6067G3.83 21.4849G3.86 0.1621G3.93 3227G7 3057G93 5 b.40-1 180 183 1.05 0.005 0.1866G0.38 0.2901G0.48 0.5255G3.59 13.5179G3.61 0.1447G3.64 2712G6 2723G80 0 b.41-1 57 82 1.47 0.034 0.2018G0.63 0.3965G0.72 0.5601G3.68 15.5836G3.73 0.1510G3.78 2841G10 2867G85 K1 b.41-2 306 145 0.49 0.004 0.1798G0.32 0.1364G0.54 0.4810G3.57 11.9254G3.59 0.1335G3.63 2651G5 2532G75 5 b.42-1 80 37 0.48 0.097 0.1781G0.67 0.1309G1.06 0.4858G3.65 11.9288G3.71 0.1339G3.83 2635G11 2553G77 3 b.43-2 175 159 0.94 0.023 0.1865G0.40 0.2623G0.51 0.5134G3.59 13.2008G3.62 0.1433G3.64 2711G7 2671G79 1 b.44-3 127 182 1.48 0.007 0.1776G0.49 0.4149G0.54 0.4807G3.61 11.7678G3.65 0.1346G3.67 2630G8 2530G76 4 b.44-2 426 474 1.15 0.013 0.1667G0.31 0.2996G0.37 0.3567G3.57 8.1976G3.58 0.0930G3.59 2524G5 1967G60 22 b4422 113 138 1.25 K0.036 0.1776G0.53 0.3446G0.66 0.4421G3.64 10.8251G3.68 0.1214G3.72 2630G9 2360G72 10 b.45-2 153 101 0.68 0.049 0.1865G0.42 0.1885G0.62 0.5335G3.60 13.7158G3.62 0.1466G3.70 2711G7 2756G81 K2 b.47-1 489 489 1.03 0.058 0.1737G0.33 0.2850G0.93 0.3134G3.57 7.5058G3.58 0.0865G3.69 2594G6 1757G55 32 b4712 276 154 0.58 0.386 0.1820G0.69 0.1541G0.88 0.4725G3.62 11.8597G3.68 0.1269G3.74 2671G11 2495G75 7 b.48-1 202 111 0.57 0.001 0.2657G0.29 0.1525G0.53 0.6684G3.59 24.4876G3.60 0.1789G3.64 3281G5 3300G93 K1 b.49-1 316 112 0.37 0.014 0.2695G0.23 0.0886G0.53 0.6333G3.57 23.5317G3.58 0.1535G3.63 3303G4 3162G89 4 Sample 4, Moeda Formation quartzite
a.9-1 245 195 0.82 0.015 0.2072G0.35 0.2448G0.48 0.4572G2.17 13.0624G2.20 0.1358G2.25 2884G6 2427G44 16 a.9-2 230 106 0.48 0.063 0.2048G0.33 0.1288G0.59 0.5497G2.18 15.5184G2.20 0.1483G2.28 2865G5 2824G50 1 a.10-1 57 22 0.40 K0.038 0.1871G0.70 0.1067G1.35 0.5145G2.39 13.2715G2.50 0.1360G2.81 2717G12 2676G52 2 a.10-2 48 18 0.39 0.013 0.3228G0.54 0.0955G1.27 0.7706G2.45 34.2992G2.51 0.1893G2.84 3583G8 3683G69 K3 a.11-2 199 99 0.51 0.019 0.2039G0.36 0.1406G0.63 0.5466G2.20 15.3645G2.23 0.1495G2.31 2857G6 2811G50 2 a.12-1 147 52 0.37 0.101 0.2256G0.38 0.1001G0.78 0.6008G2.21 18.6872G2.25 0.1633G2.38 3021G6 3033G54 0 a.12-2 98 60 0.63 0.021 0.1886G0.61 0.1832G0.84 0.4594G2.28 11.9485G2.36 0.1335G2.46 2730G10 2437G46 11 a.13-1 183 88 0.50 K0.009 0.2038G0.36 0.1360G0.65 0.5577G2.20 15.6754G2.23 0.1525G2.34 2857G6 2857G51 0 a.14-1 173 115 0.69 0.027 0.2627G0.32 0.1827G0.53 0.6587G2.20 23.8635G2.22 0.1752G2.28 3263G5 3262G56 0 a.14-2 113 53 0.49 K0.065 0.1865G0.51 0.1348G0.84 0.5375G2.26 13.8203G2.31 0.1485G2.44 2711G8 2773G51 K2 a.15-2 186 128 0.71 0.013 0.2056G0.36 0.1931G0.54 0.5697G2.20 16.1473G2.23 0.1553G2.28 2871G6 2907G51 K1 a.15-1 113 73 0.67 0.052 0.1871G0.50 0.1945G0.74 0.5078G2.25 13.1016G2.31 0.1479G2.43 2717G8 2647G49 3 a.16-2 70 39 0.57 0.052 0.2041G0.58 0.1612G0.96 0.5364G2.34 15.0930G2.41 0.1508G2.57 2859G9 2768G53 3 a.16-1 300 31 0.11 K0.004 0.2059G0.30 0.0408G0.87 0.5461G2.16 15.5038G2.18 0.2078G2.36 2874G5 2809G49 2 a.16-3 116 101 0.90 0.027 0.2083G0.46 0.2470G0.62 0.5868G2.25 16.8501G2.30 0.1613G2.37 2892G7 2976G54 K3 a.17-1 67 44 0.67 K0.040 0.2224G0.57 0.1849G0.90 0.5916G2.37 18.1395G2.43 0.1622G2.59 2998G9 2996G57 0 a.19-2 127 112 0.91 0.039 0.2774G0.35 0.2420G0.76 0.7132G2.29 27.2752G2.32 0.1894G2.63 3348G5 3470G61 K4 a.20-2 92 45 0.51 K0.016 0.2044G0.57 0.1376G0.89 0.5676G2.29 15.9993G2.36 0.1529G2.49 2862G9 2898G53 K1 a.20-1 89 38 0.44 0.077 0.2515G0.48 0.1186G0.89 0.6437G2.28 22.3261G2.33 0.1743G2.49 3194G8 3204G58 0 a.20-3 48 32 0.68 0.015 0.2240G0.65 0.1853G1.01 0.6130G2.43 18.9303G2.51 0.1661G2.69 3009G10 3082G59 K2 a.21-1 66 46 0.73 0.063 0.2057G0.61 0.1958G0.91 0.5582G2.34 15.8327G2.42 0.1504G2.56 2872G10 2859G54 0 a.21-3 277 112 0.42 K0.038 0.1862G0.32 0.1157G0.58 0.4916G2.16 12.6186G2.18 0.1366G2.26 2708G5 2578G46 5 a.22-1 268 138 0.53 0.013 0.2567G0.28 0.1423G0.52 0.5820G2.17 20.5982G2.19 0.1559G2.25 3226G4 2957G51 8
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a.22-3 72 49 0.71 0.019 0.1872G0.62 0.1965G0.90 0.5149G2.33 13.2895G2.41 0.1434G2.53 2718G10 2677G51 1 a.23-1 98 51 0.54 0.101 0.2755G0.41 0.1487G0.76 0.6750G2.26 25.6400G2.30 0.1843G2.42 3338G6 3325G59 0 a.24-1 177 104 0.60 0.021 0.2319G0.38 0.1627G0.62 0.5777G2.20 18.4705G2.24 0.1558G2.31 3065G6 2939G52 4 Sample 5, Sabara´ Formation quartz-feldspar schist
b.2-1 50 58 1.18 0.515 0.1875G1.31 0.3163G1.33 0.5307G2.27 13.7198G2.62 0.1432G2.72 2720G22 2744G51 K1 b.4-1 42 36 0.89 0.183 0.1877G1.36 0.2508G2.44 0.5278G2.04 13.6588G2.45 0.1498G3.27 2722G22 2732G46 9 b.8-1 547 21 0.04 0.042 0.2121G0.33 0.0105G1.93 0.5686G1.13 16.6302G1.18 0.1529G2.33 2922G5 2902G27 1 b.8-2 59 40 0.70 0.090 0.2075G1.00 0.1903G1.47 0.5542G1.77 15.8579G2.04 0.1510G2.40 2886G16 2843G41 3 b.9-1 40 56 1.45 0.638 0.1803G1.70 0.3987G1.43 0.5076G1.90 12.6182G2.55 0.1406G2.49 2656G28 2646G41 K1 b.11-1 66 33 0.51 0.337 0.1905G1.29 0.1423G1.92 0.5244G1.59 13.7721G2.05 0.1475G2.58 2746G21 2718G35 0 b.12-1 46 13 0.29 0.170 0.1890G1.32 0.0794G2.73 0.5289G1.87 13.7803G2.29 0.1466G3.47 2733G22 2737G42 0 b.13-1 130 29 0.23 0.147 0.1869G0.74 0.0631G1.70 0.5303G1.34 13.6681G1.53 0.1435G2.25 2715G12 2743G30 0 b.14-1 76 108 1.47 0.380 0.1817G1.22 0.3963G1.01 0.5457G1.67 13.6699G2.07 0.1480G2.03 2668G20 2807G38 0 b.16-1 245 218 0.92 0.015 0.2088G0.54 0.2463G0.64 0.5669G1.34 16.3178G1.45 0.1519G1.53 2896G9 2895G31 K1 b.17-1 55 22 0.40 K0.012 0.1875G1.10 0.1141G2.05 0.5156G1.70 13.3276G2.03 0.1457G2.79 2720G18 2680G37 K4 b.18-1 80 315 4.08 0.094 0.2250G1.11 1.0516G0.68 0.6201G1.65 19.2364G1.99 0.1600G1.87 3017G18 3110G41 1 b.19-1 30 35 1.22 K0.111 0.2155G1.34 0.3297G1.65 0.5797G2.08 17.2199G2.48 0.1565G2.81 2947G22 2947G49 1 b.20-1 58 72 1.28 0.243 0.1915G1.16 0.3477G1.52 0.5451G1.67 14.3964G2.03 0.1490G2.43 2756G19 2805G38 1 b.21-1 117 82 0.73 0.177 0.2081G0.75 0.1916G1.07 0.5779G1.37 16.5832G1.57 0.1528G1.81 2891G12 2940G32 0 b.22-1 260 130 0.52 0.023 0.2072G0.46 0.1362G1.02 0.5873G1.25 16.7753G1.33 0.1552G1.65 2884G7 2978G30 0 b.25-1 60 137 2.37 0.090 0.2020G2.25 0.6376G0.94 0.5702G1.80 15.8789G2.88 0.1536G2.12 2842G37 2909G42 1 b.26-1 9 2 0.22 K0.225 0.1921G4.18 0.0727G10.99 0.5155G3.62 13.6524G5.53 0.1726G11.81 2760G69 2680G79 3 b.29-1 44 50 1.18 0.397 0.1873G1.61 0.3240G1.48 0.5171G1.83 13.3511G2.44 0.1428G2.46 2718G27 2687G40 K1 b.30-1 65 67 1.06 0.138 0.2238G0.91 0.2847G1.18 0.5838G1.60 18.0129G1.84 0.1565G2.08 3008G15 2964G38 12 Sample 6, Itacolomi Group sandstone
c.61-1 148 39 0.27 0.119 0.2292G0.59 0.0734G2.72 0.6054G1.30 19.1303G1.43 0.1640G3.63 3046G9 3052G32 0 c.62-1 131 30 0.23 0.045 0.1334G0.89 0.0671G1.88 0.4004G1.46 7.3652G1.71 0.1149G2.46 2143G16 2171G27 K1 c.62-2 77 54 0.72 K0.034 0.1355G1.89 0.2075G1.47 0.3892G1.69 7.2698G2.53 0.1119G2.30 2170G33 2119G30 2 c.63-1 112 29 0.27 0.216 0.1983G0.81 0.0740G1.68 0.5439G1.38 14.8701G1.60 0.1490G2.27 2812G13 2800G31 0 c.64-1 122 71 0.60 0.070 0.1375G0.94 0.1684G1.29 0.4076G1.93 7.7246G2.14 0.1151G2.37 2195G16 2204G36 0 c.65-1 167 156 0.96 K0.064 0.1367G0.84 0.2734G0.89 0.4045G1.29 7.6223G1.54 0.1147G1.62 2185G15 2190G24 0 c.70-1 39 6 0.15 0.386 0.2583G1.13 0.04293.36 0.6645G1.90 23.6650G2.21 0.1849G4.08 3236G18 3284G49 K1
Notes. Isotopic ratio errors in %. All Pb in ratios are radiogenic components corrected for204Pb, except grain QF10-a. 24-1, corrected for208Pb. disc.
Zdiscordance, as 100K100{t[206Pb/238U]/t[207Pb/206Pb]}.
f206Z(common206Pb)/(total measured206Pb) based on measured204Pb. Uncertainties are 1s. n.a.Znot applicable.
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The isotopic results show that the Jequie´ cycle may have started about 100 m.y. (3055 Ma) earlier than previously considered (2960 Ma) and may encompass several granitoid-greenstone units formed during approximately 420 m.y. (3055–2635 Ma), which have not yet been discriminated in the Rio das Velhas greenstone belt. Older Paleoarchean magmatic activity is detected at 3809, 3582, and 3229 Ma (Fig. 5), suggesting the presence of pre-Rio das Velhas greenstone belts and granitoids in the southern Sa˜o Francisco Craton. The presence of an older Paleoarchean crust has been proposed by Carneiro et al. (1998) on the basis of Sm–Nd isotopic data. The Paleoarchean crust may be exposed at the surface of the shield but is now included in the Rio das Velhas unit, which requires a revision of concepts and geological maps to integrate new isotopic investigations.
It is remarkable that only 2 among 117 analyses have Th/U ratios less than 0.1. The two low ratios are compatible with metamorphic zircon and present in two preserved metamorphic rims (sample 2, grain b32; sample 5, grain b8;Table 2). Almost all dated zircon crystals are magmatic, because the Th/U ratios are greater than 0.1. The absence of metamorphic zircon crystals and portions of crystals in samples 5 and 6 (Sabara´ and Itacolomi groups) may be due to the low abundance of granulite facies rocks registered in most of the Trans-Amazonian orogen in South America (Santos et al., 2003a). Amphibolite facies and greenschist facies conditions cause less intense recrystallization of zircon. However, the predominance of high-Th/U, magmatic zircon compositions in the Quad-rila´tero Ferrı´fero sandstones agrees withHartmann and Santos (2004) and Hartmann et al. (2004), namely, that metamorphic source terrains are not usually dated by detrital zircon geochronology of mature sandstones. Metamorphic zircon crystals and portions of crystals are usually richer in U and therefore more defective, leading to comminution (i.e. grain size reduction) by physical and chemical processes during
sedimentation, and are washed from the sands to be incorporated in shales. The provenance investigation of events related to metamorphic terrains in the Quadrila´tero Ferrı´fero requires further studies, because they are not commonly detected in detrital zircon ages.
The time range of about 420 m.y. is much longer than the 140 m.y. estimated byMachado et al. (1996)for the formation of the Rio das Velhas greenstone belt but typical of a long-lived, complete Wilson cycle of oceanic crust formation, consumption, and collision (Condie, 1997; Hartmann, 2002). We suggest that the Archean evolution of the granite– greenstone terrain occurred by orogenic pulses, not continu-ously. Zircon age spectra from sedimentary rocks provide a record of magmatic events of an orogeny (e.g.Eriksson et al., 2003; Rino et al., 2004). Six main age peaks between 3055 and 2635 Ma occur at 3005, 2953, 2853, 2747, 2712, and 2635 Ma and are interpreted as the activity of six orogenic pulses during the Jequie´ cycle. A small peak at 2588 Ma corresponds to post-orogenic granitic rocks. The Rio das Velhas event (here interpreted as an orogeny) was proposed by Machado and Carneiro (1992) at around 2.78 Ga, but the SHRIMP U–Pb zircon data indicate two age peaks atw2759 andw2710 Ma,
so two orogenic events probably occurred around 2.75 Ga. The main dated volcanic event in the greenstone belt occurred at approximately 2.76 Ga (Machado et al., 1996), but these authors date an older rhyolite at 3029 Ma. Incorporating the present data set, we suggest the possible superposition or lateral manifestation of several greenstone belts in the province. Additional detailed geological mapping, integrated with robust geochronology, could elucidate the complexity of granite–greenstone generation in the Quadrila´tero Ferrı´fero.
The depositional age of the Nova Lima Group sandstones is bracketed between 2746G5 and 2717G8 Ma, on the basis of previous dating and the present data set (Fig. 6). According to these data, the youngest age of 2746G5 Ma for detrital zircon crystals in the Nova Lima Group sedimentary rocks means that the Rio das Velhas Basin did not collect zircon crystals from the 2708G5 Ma Mamona Granodiorite-type and is therefore older than this granodiorite. This 2708G5 Ma population is well represented in the Moeda Formation quartzites, which are part of the platform sequence deposited over the granite– greenstone terrain. This time span of about 30 m.y. is a tight constraint on the age of deposition of the Nova Lima Group sandstones. Further investigations are required, such as U–Pb dating of the xenotime overgrowths that may be present on zircon, because the dating of the xenotime may yield the age of diagenesis (McNaughton et al., 1999).
The youngest detrital zircon crystal dated in sandstone indicates the maximum possible age of the basin fill.Table 3
summarizes the data for the three sedimentary basins investigated in the Quadrila´tero Ferrı´fero and establishes an important parameter for understanding the evolution of the province.
Machado et al. (1996)estimate the age of deposition of the iron formation (Minas Supergroup) at 2612 (G5)–2420 Ma, a range of about 192 m.y. covering the late Neoarchean and early Siderian. The age of 2420 Ma corresponds to the deposition of
2500 3000 3500
Frequency 2000 3055 3229 3582 3809 2712 2853 2635 4000 0 5 10 15 pre-Jequié Cyclecrust 3810-3200 Ages,Ma Jequié Cyclecrust 3055-2635 Trans-Amazonian Cyclecrust Second orogeny 2182-2141 2953 2747 3005 2588
Fig. 5. Major events in the Quadrila´tero Ferrı´fero on a frequency histogram (probability curve shown) of 109 U–Pb SHRIMP analyses (out of 117 analyses) of zircon crystals from the six studied sedimentary rocks. Jequie´ cycle includes Rio das Velhas orogeny of 2780–2720 Ma (Machado and Carneiro, 1992). Six orogenic events of the Jequie´ cycle are indicated.
the Gandarela Formation dolomites (Babinski et al., 1995), which cover the Carac¸a Group. The youngest detrital zircon crystals detected by Machado et al. (1996) in the Moeda Formation are 2651G33, 2655G29, 2649G16, and 2610G
30 Ma old. The age of 2610 Ma, together with the ages of a pre-Moeda Formation granite of 2612G5 Ma (Noce, unpublished) and 2567G8 Ma (Endo, unpublished; Endo and Machado, 2002), have been used to delimit the maximum age for the Carac¸a Group Basin. We now know that the age of grain b36 (2580G7 Ma), obtained in the upper Moeda Formation quartzite, indicates that the maximum age for the Moeda Formation and Carac¸a Group deposition is approximately 30 m.y. younger than previously believed.
Sample 4, of the upper Moeda Formation, has a significant population (nZ12) of 2646G15 Ma, comparable to the
youngest grains found byMachado et al. (1996) in the same unit. Another and even younger population of 2584G10 Ma also is present, indicating that the iron formation above the Moeda Formation must be younger than 2584 Ma. A few, small granitoid bodies dated at around 2612 Ma (e.g. Noce, 2000) require basinal deposition of the ore after this age. A minimum age of deposition of 2420G19 Ma for the Gandarela Formation (Babinski et al., 1995), also above the iron formation, places the deposition age close to the boundary between the Late Neoarchean and Early Siderian.
Only Archean zircon crystals were found in our sample from the Sabara´ Group (nZ20), and the main population of 2719G
20 Ma (nZ12) may be derived from the Mamona Granite and
correlative bodies. The ages of all dated detrital zircon crystals
from the Sabara´ Group (sample 5) are older than 2.71 Ga, which is the magmatic age of the Mamona Granodiorite. Using a larger sample (nZ53), Machado et al. (1996) find two
Rhyacian ages in zircon crystals but with large uncertainties (2255G110 and 2122G140 Ma). These authors confirm the presence of Paleoproterozoic zircon in the Sabara´ Group using U–Pb dating (2131G5, 2125G4 and 2164G13 Ma).
All these Paleoproterozoic ages pertain to the Trans-Amazonian cycle (2.26–2.00 Ga), which is one of the most important crustal formation events in South America (Santos et al., 2003a). In addition to the precise time span of the orogenic cycle,Santos et al. (2003a)discover the cyclicity of crust formation and deformation within the cycle and identify four main orogenies, but 12 orogenic pulses may have occurred (Hartmann et al., 2004). Specifically in the Quadrila´tero Ferrı´fero region, Endo (unpublished) identifies two main Trans-Amazonian orogenies (2250–2100, 2059–2000 Ma). The Sabara´ Group Paleoproterozoic ages near 2150 Ma
Fig. 6. Frequency histogram (probability curve shown) of zircon ages from two samples from the Moeda Formation and two samples from the Nova Lima Group.
Table 3
Youngest zircon grains in Quadrila´tero Ferrı´fero quartzites, corresponding to the maximum possible ages of basins
Sample Grain Unit Basin Age (Ma) Disc. (%)
5 c62 Itacolomi Group Itacolomi 2143G16 1 3 b35 Moeda Formation Minas 2649G11 0 4 b14 Sabara´ Formation Minas 2668G20 0 6 d14b Nova Lima Group Rio das
Velhas
2749G07 1
Disc.Zdiscordance.
correspond to the first orogeny in the Quadrila´tero Ferrı´fero (e.g.Endo, unpublished) and correlate with the second main orogeny of the Trans-Amazonian cycle (Santos et al., 2003a). Unlike in the Itacolomi Group sample, the absence of late Trans-Amazonian cycle detrital zircon crystals suggests that the Sabara´ Group is not post-Trans-Amazonian cycle in age but is part of the Trans-Amazonian belt. The rock composition of the Sabara´ Group is dominated by immature greywacke but also includes mafic schists and metavolcanic rocks. Neither detrital zircon ages nor the rock composition favor a deposition in a post-Trans-Amazonian foreland basin, as was proposed by
Machado et al. (1996). The lack of late Trans-Amazonian detrital zircon (i.e. formed during 2080–2030 Ma) and the deposition during volcanic activity indicate the Sabara´ Basin is an orogenic basin, probably an intraarc basin.
This study confirms the presence of 2.17 Ga old zircon crystals in sandstones from the Itacolomi Group, high in the stratigraphy above the iron formation, as part of the Trans-Amazonian cycle (2.26–2.00 Ga, Santos et al., 2003a). The main population of 2173G8 Ma and the youngest grain of 2143G16 Ma correlate with the third main orogeny of that cycle (Santos et al., 2003a).Machado et al. (1996), studying a larger sample of 43 grains, found three even younger grains (2059G58, 2078G12, and 2073G30 Ma old) that are comparable to the third Trans-Amazonian orogeny, which is mainly composed of late- to post-tectonic granitoids (Santos et al., 2003a). The available isotopic data indicate that the Itacolomi Group is post-Trans-Amazonian in age and was derived both from Trans-Amazonian and Archean rocks. Its maximum possible age of 2059 and the fluvial origin of its deposits favor deposition in a foreland basin, as stated by
Machado et al. (1996). These authors propose that the Itacolomi Group deposition was associated with the Trans-Amazonian orogen, whereas we suggest a post-orogenic deposition with an Orosirian age (!2.03 Ga). The possible source of the Itacolomi Basin sedimentary rocks is the Trans-Amazonian belt to the east and south.
Mamona Granodiorite mylonite dating characterizes the magmatic age at 2708G5 Ma, whereas the zircon ages in the two Serra da Moeda quartzites show that an older basement was collected at the beginning of the sedimentation. Erosion of this older source exposed a deeper, younger (w2.62 Ga) granitic
source during the sedimentation of the top quartzite unit. The most important conclusions of this investigation are as follows:
1. The Archean Quadrila´tero Ferrı´fero province formed mostly during six orogenic pulses during the Jequie´ cycle of orogenies (3055–2635 Ma), but the Jequie´ cycle may be much longer (420 m.y.) than previously believed (140 m.y.). 2. The dated detrital zircon crystals have high-Th/U (O0.1), magmatic compositions, and the ages of metamorphic sources are extremely poorly represented in the detrital zircon crystals.
3. The maximum age of the Nova Lima Group is 2746G5 Ma, which is approximately 26 m.y. younger than the limit (2772 Ma) proposed byMachado et al. (1996). The Archean
history of the province was completed with the injection of small granitic bodies at 2.65 Ga. A detrital zircon population dated at 2580 Ma may be derived either from granitic intrusions or shear zones.
4. The maximum age of the Minas Supergroup is 2580 Ma (40 m.y. younger than previously believed), and its deposition is limited to 2580–2420 Ma (Neoarchean–Early Siderian).
5. A Paleoarchean basement is represented by detrital zircon crystals, including the oldest dated zircon in South America (3809G3 Ma) reported thus far.
6. The basal volcanics (2.76 Ga) of the Rio das Velhas greenstone belt and coeval basement granites were partly eroded to form the Nova Lima Group sandstones.
7. The main depositional stage of the Rio das Velhas Basin covered the time span 2746–2717 Ma.
8. The age of giant iron formation deposition is bracketed at 2580–2420 Ma.
9. Little orogenic activity occurred between 2580 and 2185 Ma, as suggested by the lack of detrital zircon U–Pb SHRIMP ages in this time interval.
10. The Sabara´ Group was probably deposited in an intraarc basin related to the second Trans-Amazonian orogeny (2.17–2.13 Ga), whereas the Itacolomi Group is post-Trans-Amazonian (!2.03 Ga) and possibly Orosirian in age. The deposition of part of the Sabara´ Group in the Serra do Curral probably was contemporaneous with the deposition of the Itacolomi Group.
Acknowledgements
This project was supported by Conselho Nacional do Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) and Departamento de Geologia, Universidade Federal de Ouro Preto, both maintained by the Brazilian Government. The sensitive high resolution ion microprobe (SHRIMP II) is operated jointly by Curtin University of Technology, The University of Western Australia, and the Geological Survey of Western Australia, with the support of the Australian Research Council. Paul Potter is acknowledged for suggestions on the manuscript. Two JSAES anonymous reviewers made some significant contributions to the improvement of the article.
References
Almeida, F.F.M., Hasui, Y. (coords.), 1984. O Pre´-Cambriano do Brasil. Edgard Blu¨cher, Sa˜o Paulo, 378 p.
Almeida, F.F.M., Brito Neves, B.B., Carneiro, C.D.R., 2000. The origin and evolution of the south American platform. Earth-Science Reviews 50, 77–111.
Babinski, M., Chemale Jr., F., Van Schmus, W.R., 1995. The Pb/Pb age of the minas supergroup carbonate rocks, Quadrila´tero Ferrı´fero, Brazil. Precambrian Research 72, 235–245.
Carneiro, M.A., Teixeira, W., Carvalho Jr., I.M., Fernandes, R.A., 1998. Ensialic tectonic setting of the Archean Rio das Velhas Greenstone Belt: Nd and Pb isotopic evidence from the Bonfim Metamorphic Complex, Quadrila´tero Ferrı´fero, Brazil. Revista Brasileira de Geocieˆncias 28, 189–200.
Chemale Jr., F., Rosie`re, C.A., Endo, I., 1994. The tectonic evolution of the Quadrila´tero Ferrı´fero, Minas Gerais, Brazil. Precambrian Research 65, 25–54.
Condie, K.C., 1997. Plate Tectonics and Crustal Evolution. Butternworth-Heinemann, Oxford. 282 p.
Dardenne, J.A., Schobbenhaus Filho, C., 2000. The metallogenesis of the South American Platform. In: Cordani, U.G., Milani, E.J., Thomaz Filho, A., Campos, D.A. (Eds.), Tectonic evolution of South America. 31st International Geological Congress, Rio de Janeiro, pp. 755–850. Dodson, M.H., Compston, W., Williams, I.S., Wilson, J.F., 1988. A search for
detrital zircons in Zimbabwean sediments. Journal of the Geological Society, London 145, 977–983.
Dorr II, J.V.N., 1969. Physiographic, stratigraphic and structural development of the Quadrila´tero Ferrı´fero, Minas Gerais, Brazil. USGS Prof. Paper, 641-A, 110 p.
Endo, I., unpublished. Regimes tectoˆnicos do Arqueano e Proterozo´ico no interior da Placa Sanfranciscana: Quadrila´tero Ferrı´fero e a´reas adjacentes, Minas Gerais. 243 p. (PhD Thesis, Instituto de Geocieˆncias, Universidade de Sa˜o Paulo).
Endo, I., Machado, R., 2002. Reavaliac¸a˜o e novos dados geocronolo´gicos (Pb/Pb e K/Ar) da regia˜o do Quadrila´tero Ferrı´fero e adjaceˆncias. Revista do Instituto de Geocieˆncias, Universidade de Sa˜o Paulo, Geologia, Se´rie Cientı´fica, Sa˜o Paulo, 2, 23–40.
Eriksson, P.G., Martins-Neto, M.A., Nelson, D.R., Aspler, L.B., Chiarenzelli, J.R., Catuneanu, O., Sarkar, S., Altermann, W., Rautenbach, C.J.W., 2001. An introduction to precambrian basins: their characteristics and genesis. Sedimentary Geology 141–142, 1–35. Eriksson, K.A., Campbell, I.H., Palin, J.M., Allen, C.M., 2003. Predominance
of grenvillian magmatism recorded in detrital zircon crystals from modern Appalachian rivers. The Journal of Geology 111, 707–717.
Hartmann, L.A., 2002. The mesoproterozoic supercontinent Atlantica in the Brazilian shield—review of geological and U–Pb zircon and Sm–Nd isotopic evidence. Gondwana Research 5, 157–163.
Hartmann, L.A., Delgado, I.M., 2001. Cratons and orogenic belts of the Brazilian shield and their contained gold deposits. Mineralium Deposita 36, 207–217.
Hartmann, L.A., Santos, J.O.S., 2004. Predominance of high Th/U, magmatic zircon in Brazilian shield sandstones. Geology 32, 73–76.
Hartmann, L.A., Bitencourt, M.F., Santos, J.O.S., McNaughton, N.J., Rivera, C.B., Betiollo, L., 2003. Prolonged paleoproterozoic magmatic participation in the Neoproterozoic Dom Feliciano Belt, Santa Catarina, Brazil, based on zircon U–Pb SHRIMP geochronology. Journal of South American Earth Sciences 16, 477–492.
Hartmann, L.A., Philipp, R.P., Liu, D., Wan, Y., Wang, Y., Santos, J.O.S., Vasconcellos, M.A.Z., 2004. Paleoproterozoic magmatic provenance of detrital zircon crystals, Porongos complex quartzites, southern Brazilian shield. International Geology Review 46, 127–157.
Herz, N., 1970. Gneissic and igneous rocks of the Quadrila´tero Ferrı´fero, Minas Gerais, Brazil. USGS Prof. Paper 641-B, 1–58.
Ludwig, K.R., 1999. Using Isoplot/Ex, version 2, a geochronological Toolkit for microsoft Excel. Berkeley Geochronological Center Special Publication 1a, 47 p.
Ludwig, K.R., 2001. Squid 1.02: a user manual. Berkeley Geochronological Center Special Publication 2, 19 p.
Machado, N., Carneiro, M.A., 1992. A major archean tectonothermal event in the Sa˜o Francisco shield, Brazil: U/Pb evidence from the Quadrila´tero Ferrı´fero, Minas Gerais. Canadian Journal of Earth Sciences 29, 2341–2346. Machado, N., Noce, C.M., Ladeira, E.A., Belo de Oliveira, O.A., 1992. U–Pb
geochronology of archean magmatism and proterozoic metamorphism in the Quadrila´tero Ferrı´fero, southern Sa˜o Francisco Craton, Brazil. Geological Society of America Bulletin 104, 1221–1227.
Machado, N., Schrank, A., Noce, C.M., Gauthier, G., 1996. Ages of detrital zircon from archean-proterozoic sequences: implications for greenstone belt setting and evolution of a transamazonian foreland basin in Quadrila´tero Ferrı´fero, southeast Brazil. Earth and Planetary Science Letters 141, 259–276.
Marshak, S., Tinkham, D., Alkmim, F.F., Brueckner, H., Bornhorst, T., 1997. Dome-and-keel provinces formed during Paleoproterozoic orogenic col-lapse—diapir clusters of core complexes? Examples from the Quadrila´tero Ferrı´fero (Brazil) and the Penokean Orogen (USA). Geology 25, 415–418. McNaughton, N.J., Rasmussen, B., Fletcher, I.R., 1999. SHRIMP uranium–
lead dating of diagenetic xenotime in siliciclastic sedimentary rocks. Science 285, 78–80.
Nelson, D.R., 2001. Determination of depositional age and provenance of clastic sedimentary rocks. Sedimentary Geology 141, 37–60.
Noce, C.M., unpublished. Geocronologia dos eventos magma´ticos, sedimen-tares e metamo´rficos na regia˜o do Quadrila´tero Ferrı´fero, Minas Gerais. 128 p. (PhD Thesis, Instituto de Geocieˆncias, Universidade de Sa˜o Paulo). Noce, C.M., 2000. Geochronology of the Quadrila´tero Ferrı´fero: a review.
Geonomos (Ouro Preto) 8, 15–23.
Noce, C.M., Teixeira, W., Que´me´neur, J.J.G., Martins, V.T.S., Bolzachini, E., 2000. Isotopic signatures of paleoproterozoic granitoids from the southern Sa˜o Francisco Craton and implications for the evolution of the transamazonian orogeny. Journal of South American Earth Sciences 13, 225–239.
Nutman, A.P., 2001. On the scarcity ofO3900 Ma detrital zircon crystals in R3500 Ma metasediments. Precambrian Research 105, 93–114. Olivo, G.R., Gauthier, M., Garie´py, C., Carignan, J., 1996. Transamazonian
tectonism and Au–Pd mineralization at the Caueˆ Mine, Itabira District, Brazil: Pb isotopic evidence. Journal of South American Earth Sciences 9, 273–279.
Rainbird, R.H., Hamilton, M.A., Young, G.M., 2001. Detrital zircon geochronology and provenance of the Torridonian, NW Scotland. Journal of the Geological Society of London 158, 15–27.
Renger, F.E., Noce, C.M., Romano, A.W., Machado, N., 1994. Evoluc¸a˜o sedimentar do Supergrupo Minas: 500 m.a. de registro geolo´gico no Quadrila´tero Ferrı´fero, Minas Gerais, Brasil. Geonomos (Ouro Preto) 2, 1–11. Rino, S., Komiya, T., Windley, B.F., Katayama, I., Motoki, A., Hirata, T., 2004. Major episodic increases of continental crustal growth determined from zircon ages of river sands; implications for mantle overturns in the early precambrian. Physics of the Earth and Planetary Interiors 146 (1–2), 369–394.
Rogers, J.J.W., Santosh, M., 2002. Configuration of Columbia, a mesoproter-ozoic supercontinent. Gondwana Research 5, 5–22.
Santos, J.O.S., Hartmann, L.A., Bossi, J., Campal, N., Schipilov, A., Pin˜eyro, D., McNaughton, N.J., 2003a. Duration of the trans-amazonian cycle and its correlation within south America based on U–Pb SHRIMP geochronology of La Plata Craton, Uruguay. International Geology Review 45, 27–48.
Santos, J.O.S., Potter, P.E., Reis, N.J., Hartmann, L.A., McNaughton, N.J., 2003b. Age, source and regional stratigraphy of the roraima supergroup and roraima-like outliers in northern south America based on U–Pb geochronology. Geological Society of America Bulletin 115, 331–348. Smith, J.B., Barley, M.E., Groves, D.I., Krapez, B., McNaughton, N.J.,
Bickle, M.J., Chapman, H.J., 1998. The Scholl shear zone, west Pilbara: evidence for a terrane boundary structure from integrated tectonic analyses, SHRIMP U/Pb dating and isotopic and geochemical data of granitoids. Precambrian Research 88, 143–171.
Teixeira, W., Carneiro, M.A., Noce, C.A., Machado, N., Sato, K., Taylor, P.N., 1996. Pb, Sr and Nd isotope constraints on the archean evolution of gneissic granitoid complexes in the southern Sa˜o Francisco craton, Brazil. Precambrian Research 78, 151–164.
Teixeira, W., Sabate´, P., Barbosa, J., Noce, C.M., Carneiro, M.A., 2000. Archean and Paleoproterozoic tectonic evolution of the Sa˜o Francisco Craton. In: Cordani, U.G., Milani, E.J., Thomaz Filho, A., Campos, D.A. (Eds.), Tectonic evolution of South America. Ed. 31st International Geological Congress, Rio de Janeiro, 101–137.
Vermeesch, P., 2004. How many grains are needed for a provenance study? Earth and Planetary Science Letters 224, 441–451.
