Sampling and analytical methods

1.77 Ga and prior to diamictites Jequitaı´ For- mation at the base the Bambuı´ Group Pimentel et al., 1991. After revising the available geochronological and microfossils data from the Paranoa´ Group, Fairchild et al. 1996 concluded that the best estimate of the age of this unit is 1170 – 950 Ma. This unit has been interpreted as a passive margin sequence that was deposited at the western border of the Sa˜o Francisco craton Dar- denne, 1979; Pimentel et al., 1999. Petrographic, chemical and isotopic 147 Sm 144 Nd studies indi- cate that the Paranoa´ sediments were derived from a Paleoproterozoic sialic basement located in the Sa˜o Francisco craton Guimara˜es 1997; Pimentel et al. 1999. The Jequitaı´ diamictites consist of clasts sup- ported by a clay-rich matrix with siltstone and sandstone lenses. The clasts include quartz, quartzite, granite, limestone and siltstone. The age of this unit is poorly constrained and falls between 688 9 69 and 900 9 2 Ma. While the lower age limit is based on Pb – Pb dating of the Sete Lagoas SLA formation limestones Babinski et al. 1999, the upper age limit is based on U – Pb dating of zircon from basic dikes that cut the underlying siliciclastic sequence Es- pinhac¸o Supergroup Uhlein et al. 1999. Hence, the available data indicate that the Jequitaı´ glaci- ation event is probably Sturtian. This unit crops out intermittently at the base of the Bambuı´ group and, depending of the region, overlies the Paranoa´ sediments or the granite – gneiss base- ment Karfunkel and Hoppe, 1988; Uhlein, 1991; Uhlein et al. 1999. The Bambuı´ group was de- posited during a transgressive phase and after the deposition of the Jequitaı´ Formation Dardenne 1979. The Bambuı´ Group covers an area of more than 300 000 km 2 over the Sa˜o Francisco craton. These sediments were deposited in an epiconti- nental sea, commencing with a carbonate – pelite facies that was followed by siliciclastic sedimenta- tion characterized by immature sediments Dard- enne, 1978; Dardenne 1979; Misi and Kyle, 1994. The base of this unit consists of laminated argillaceous limestones, dolostones, siltstones, stromatolitic dolomite and marlstone SLA for- mation. Overlying are rhythmic interbeds of mudstones, siltstones and very fine-grained sand- stones of the Serra de Santa Helena Formation, followed by a second carbonate unit with argilla- ceous limestones, marlstones, shales and siltstones with dark colored lime grainstone lenses Lagoa do Jacare´ formation. The upper portion of the Bambuı´ group consists of siliciclastic rocks, mainly greenish siltstones and arkoses Serra da Saudade and Treˆs Marias formations. In con- trast to the Paranoa´ group sediments described above, the Bambuı´ group siliciclastic rocks are mineralogically and texturally immature sedi- ments, which have been accumulated during the tectonic inversion of a foreland basin Guimara˜es 1997. Pb – Pb isochron ages obtained for un- deformed carbonate rocks from the SLA forma- tion is 688 9 69 Ma and is considered as the minimum depositional age of this formation Babinski et al., 1999. Evidence of the inversion also occurs in the southwestern part of the basin west of SLA, Fig. 1 and Fig. 3, where conglom- erates outcrop associated to a fan delta system that was also related to the tectonic evolution of the foreland basin Castro 1997. According to Guimara˜es 1997 and Castro 1997 findings, the tectonic inversion of the basin was related to eastward thrusting movements that, as has been pointed out, affected significantly the sedi- mentological evolution of the basin. Nd isotope studies also give support for the inversion of the Bambuı´ basin. They show that the Bambuı´ group pelites present Nd model ages distinctively younger than those of the Paranoa´ group sedi- ments, and thus indicate that its sediments were derived from a younger source region Pimentel et al. 1999.

3. Sampling and analytical methods

We have analyzed oxygen and carbon isotopes along three stratigraphic profiles in central Brazil Fig. 1: SSD and SLA, both in the State of Minas Gerais, and Sa˜o Domingos SD, in the State of Goia´s. Samples from SSD are fresh limestones, dolo- stones and marlstones, sampled along a 2000 m profile that includes both the Paranoa´ and Bam- buı´ sediments. The Paranoa´ sediments consist Fig. 3. Geological cross section of southwestern border of the Bambuı´ foreland basin during the tectonic inversion after Castro, 1997. mainly of siliciclastic sediments and minor dolo- stones with up to 30 m thick stromatolite-bearing dolomite lenses Fig. 2A. These rocks are over- lain by sediments of the Bambuı´ group, which includes from base to top: limestones, dolostones with stromatolites and marlstones of the Sete Lagoas formation, siltstones of the Serra de Santa Helena formation, siltstones, marlstones and fetid and dark colored limestones of the Lagoa do Jacare´ formation. The redish-lime- stones and dolostones from the base of the SLA formation cap carbonate have also been de- scribed in other parts of the Bambuı´ basin and are believed to represent a lithostratigraphic marker which lies directly above glaciogenic di- amictites or a narrow karstified and brecciated carbonate horizon Dardenne 1979; Montes et al. 1981; Guimara˜es 1996; Alvarenga et al. 1998. In SSD the sediments are progressively more de- formed from the top to the base eastwards of the sequence due to the SD fault Alvarenga, 1978. The rocks exhibit no sign of metamor- phism or fluid percolation Fig. 2. Limestones and dolostones of the SD region overly unconformably Archean ? granitic and gneissic rocks, as well as Paleoproterozoic ? metapelites and intrusive rocks of the SD volcano-sedimentary sequence Fig. 2B. In this area, the base of the Bambuı´ consists of stroma- tolitic dolostones and limestones SLA forma- tion superposed by siltstone with minor limestone lenses Serra de Santa Helena forma- tion. Higher up in the stratigraphy are fetid and dark colored limestones of the Lagoa do Jacare´ formation. Samples were collected along a 140 m-thick sedimentary sequence that dips 30° westward and presents no signs of metamor- phism. Samples from SLA consist of dolostones, lime- stones and pelitic rocks that overly granite and gneiss basement rocks. In contrast to previous areas, the lower contact of the Bambuı´ group is tectonic and marked by intense shearing close to the fault zone, the latter with centimetric veins of calcite and quartz that indicate local carbonate remobilization. R .V . Santos et al . Precambrian Research 104 2000 107 – 122 113 Table 1 Carbon and oxygen isotopes and elemental composition of samples from the SSD profile a d 18 O PDB MgCa100 Sr ppm Fe ppm Mn ppm Ba ppm Si ppm Height m Sample d 13 C PDB Lithology Nd Nd SD-73 Nd Dolostone Nd Nd 0.00 0.9 − 5.6 Nd Nd Nd Nd Nd Nd Nd Dolostone SD-75 − 7.8 0.4 13.00 Nd Dolostone Nd Nd Nd Nd Nd 20.00 0.4 − 6.7 SD-76 − 8.8 Nd Nd Nd Nd Nd Nd SD-78 30.00 Dolostone 1.3 Nd Nd Nd Nd Nd Nd 1.0 − 9.9 SD-80 Dolostone 40.00 Nd Dolostone Nd Nd Nd Nd Nd 50.00 0.1 − 10.4 SD-81 Nd Dolostone Nd Nd Nd Nd Nd 55.00 0.8 − 11.3 SD-82 Nd Nd Nd Nd Nd Nd SD-83 − 10.3 0.9 65.00 Dolostone Nd Nd SD-86 Nd Dolostone Nd Nd 905.00 0.6 − 4.3 Nd Nd Nd Nd Nd Nd Nd Dolostone SD-87 − 4.3 1.4 915.00 Nd Dolostone Nd Nd Nd Nd Nd 925.00 1.4 − 5.5 SD-88 14 1299 204 30 125 SD-17 Dolostone 1096.00 2.7 − 2.6 28 19 1505 203 31 87 28 Dolostone SD-19 − 2.7 2.9 1105.70 28 Dolostone 30 814 107 32 110 1115.40 4.1 − 1.5 SD-23 28 Dolostone 22 996 113 37 118 1125.10 4.0 − 2.2 SD-24 21 1212 121 39 159 27 2.3 − 2.5 SD-25 Dolostone 1134.80 28 Dolostone 52 596 78 43 99 1144.50 1.9 − 0.2 SD-26 2 Limestone 183 1592 140 59 425 1154.20 0.8 − 7.5 SD-27 Nd Nd Nd Nd Nd Nd MG98-14H − 7.7 − 0.6 1155.00 Limestone − 7.7 Nd Nd Nd Nd Nd Nd MG98-14G 1155.80 Limestone − 0.6 Nd Nd Nd Nd Nd Nd − 0.7 − 7.7 MG98-14F Limestone 1156.60 Nd Limestone Nd Nd Nd Nd Nd 1157.40 − 2.8 − 6.5 MG98-14E Nd Nd Nd Nd Nd MG98-14D Limestone 1158.20 − 3.7 − 6.4 Nd Nd Nd Nd Nd Nd Nd Limestone MG98-14C − 2.4 2.3 1159.80 Nd Limestone Nd Nd Nd Nd Nd 1160.60 2.2 − 2.9 MG98-14B Nd Limestone Nd Nd Nd Nd Nd 1161.40 − 2.4 − 5.7 MG98-14A 18 8828 594 31 519 27 0.9 − 5.9 SD-08 Dolostone 1163.90 27 Dolostone 11 7418 460 29 470 1173.60 1.2 − 7.5 SD-09 27 Dolostone 12 8820 578 31 537 1183.30 1.2 − 8.6 SD-10 187 1026 155 43 120 1 SD-11 − 6.6 1.7 1193.00 Limestone 3 Limestone 312 851 87 51 327 1202.70 2.4 − 7.6 SD-02 188 1449 71 39 966 16 Dol-limestone SD-03 − 5.6 1.6 1212.40 7 Limestone 417 764 52 46 341 1222.10 2.5 − 6.3 SD-04 4 Limestone 427 350 31 36 159 1231.80 2.2 − 6.7 SD-O5 216 1438 70 53 1753 19 SD-06 − 2.5 2.7 1241.50 Dol-siltstone 26 973 52 33 184 SD-07 Dolostone 1251.20 2.8 − 2.7 28 26 1003 52 39 1027 27 3.1 − 3.9 SD-13B Dolostone 1260.90 27 Dolostone 41 1058 51 32 495 1270.60 2.4 − 2.3 SD-14 28 Dolostone 23 1246 59 34 256 1280.30 2.1 − 1.9 SE-15 17 805 50 32 99 28 SE-16 − 1.7 3.3 1290.00 Dolostone R .V . Santos et al . Precambrian Research 104 2000 107 – 122 Table 1 Continued Lithology MgCa100 Sr ppm Fe ppm Mn ppm Ba ppm Si ppm Height m d 13 C PDB Sample d 18 O PDB − 11.3 4 557 6241 223 107 3417 1300.00 SD-62 7.9 Cal-siltstone 425 7597 398 154 3476 3 SD-61 − 9.4 10.4 1310.00 Cal-siltstone − 6.9 1 897 999 74 50 240 1390.00 SD-59A 10.7 Limestone 1553 2357 158 83 949 1 − 7.0 SD-58 Cal-siltstone 1395.00 12.1 1921 969 69 85 648 SD-56 Limestone 1405.00 13.5 − 4.6 347 5991 588 34 2101 2 SD-31A − 10.1 8.3 1630.00 Cal-siltstone 954 4423 1190 62 SD-33 1609 Cal-siltstone 1665.00 9.9 − 8.8 1 1139 559 29 52 157 1 Limestone SD-36 − 6.9 10.6 1700.00 1 Limestone 1677 1454 217 69 149 1735.00 10.5 − 7.9 SD-38 1 Limestone 1370 491 43 77 173 1770.00 9.3 − 7.9 SD-39 1173 4376 1234 56 910 1 SD-43 − 7.3 9.9 1805.00 Limestone SD-46 1358 Limestone 657 51 63 188 1840.00 10.5 − 5.2 1557 4141 203 62 1174 8.9 − 5.4 SD-49 Cal-siltstone 1875.00 Limestone 1573 1313 87 60 509 1910.00 9.5 − 4.5 SD-52 − 7.3 1398 1109 107 67 182 SD-54B Limestone 1945.00 6.9 a Nd = not determined. Table 2 Carbon and oxygen isotopes of samples from the SD profile Lithology Sample Height m d 13 C PDB d 18 O PDB 5 DG-1A − 2.6 Dolostone − 7.2 7 DG-1B − 1.5 Dolostone − 4.0 10 − 2.2 Dolostone − 4.4 DG-02 Dolostone DG-19 20 − 1.0 − 4.6 Pinkish DG-20B 24 0.2 − 2.9 dolomite 38 DG-21 10.2 Limestone − 10.1 DG-22 45 Limestone 9.8 − 2.7 125 11.6 Limestone − 7.5 DG-27 Limestone DG-28B 130 12.8 − 4.6 factors 1.01025 and 1.01111. The uncertainties of the isotope measurements were 0.2‰ during the period of analyses. Trace element data were ob- tained after partially reacting the samples with HCl 10 for 24 h and then analyzing the filtered solution by ICP-AES at the University of Brası´lia. These analyses are qualitative considering that only a fraction of the samples was dissolved by the acid solution.

4. Results