Structural Analysis of the Itapucumi Group In

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1 56 2 Structural analysis of the Itapucumí Group in the Vallemí region, northern 57 3 58 4 Paraguay: Evidence of a new Brasiliano/Pan-African mobile belt 59 5 60 6 Ginaldo Ademar da Cruz Campanha a,*, Lucas Warren b, Paulo César Boggiani a, 61 7 Carlos Henrique Grohmann a, Alberto Arias Cáceres c 62 8 63 9 a Instituto de Geociências da USP, 05508-080 Sao Paulo, SP, Brazil 64 b 10 Programa de Pós-Graduação em Geoquímica e Geotectônica do Instituto de Geociências da USP, Brazil 65 c 11 Industria Nacional de Cemento, INC, Paraguay 66 12 67 13 68 article info abstract 14 69 15 70 Article history: 16 The Neoproterozoic (Ediacaran) Itapucumí Group in northern Paraguay is composed of carbonate and 71 Received 21 October 2009 siliciclastic rocks, including ooid grainstones, marls, shales and sandstones, containing Cloudina fossils in 17 72 Accepted 20 April 2010 the eastern region. It is almost undeformed over the Rio Apa Cratonic Block but shows a strong defor- 18 mational pattern at its western edge. A detailed structural analysis of the Itapucumí Group was con- 73 19 Keywords: ducted in the Vallemí Mine, along with a regional survey in other outcrops downstream in the Paraguay 74 20 Western Gondwana River and in the San Alfredo, Cerro Paiva and Sgt. Lopez regions. In the main Vallemí quarry, the 75 Neoproterozoic 21 structural style is characterized by an axial-plane slaty cleavage in open to isoclinal folds, sometimes 76 Ediacaran e 22 Paraguay belt overturned, associated with N S trending thrust faults and shear zones of E-vergence and with a low- 77 23 Rio Apa block grade chlorite zone metamorphism. The structural data presented here are compatible with the 78 24 hypothesis of a newly recognized mobile belt on the western side of the Rio Apa Cratonic Block, with 79 25 Palavras-chave: opposite vergence to that of the Paraguay Mobile Belt in Brazil. Both belts are related to the Late Bra- 80 26 Gondwana Ocidental siliano/Pan-African tectonic cycle with a Lower Cambrian deformation and metamorphism age. The 81 Neoproterozóico deformation could be due to the late collision of the Amazonian Craton with the remainder of Western 27 82 Ediacarano Gondwana or to the western active plate boundary related to the Pampean Belt. The structural and 28 83 Faixa Paraguai lithologic differences between the western Itapucumí Group in the Vallemí and Paraguay River region 29 Bloco Rio Apa and the eastern region, near San Alfredo and Cerro Paiva, suggest that this group could be divided into 84 30 two lithostratigraphic units, but more stratigraphic and geochronological analyses are required to 85 31 confirm this possibility. 86 32 Ó 2010 Published by Elsevier Ltd. 87 33 88 34 89 35 resumo 90 36 91 37 O Grupo Itapucumí é constituído por rochas carbonáticas e siliciclásticas, caracterizadas pela presença de 92 38 grainstones com oóides, margas, folhelhos e arenitos de idade neoproterozóica (ediacarana), devido à 93 39 ocorrência do fóssil Cloudina na sua porção leste. Apresenta-se quase indeformado quando recobre o 94 40 bloco cratônico do Rio Apa, porém mostra um intenso padrão deformacional na sua porção ocidental. 95 41 Análise estrutural detalhada foi realizada na mina Vallemí, norte do Paraguai, bem como reconhecimento 96 de outros afloramentos ao longo do rio Paraguai e na região entre San Alfredo, Cerro Paiva e Sargento 42 97 Lopez. Na mina principal em Vallemí, o estilo estrutural caracteriza-se pelo desenvolvimento de clivagem 43 ardosiana plano-axial de direção aproximada NeS em dobras abertas a isoclinais, por vezes com flanco 98 44 inverso e associadas a empurrões com vergência para E com desenvolvimento de metamorfismo de baixo 99 45 grau, na zona da clorita. Os dados estruturais levantados reforçam a hipótese da existência de uma nova 100 46 faixa móvel na margem oeste do Bloco Rio Apa, com vergência oposta à da Faixa Paraguai Meridional, 101 47 relacionada ao final do evento Brasiliano/Pan-Africano, com deformação e metamorfismo no Cambriano 102 48 Inferior. A deformação pode ser atribuída à colisão tardia entre o craton Amazônico e o restante do 103 49 Gondwana Ocidental, ou à borda de placa ativa associada à Faixa Pampeana. As diferenças estruturais e 104 50 em parte litológicas, entre as exposições da parte ocidental do Grupo Itapucumí, na região de Vallemí, das 105 51 106 52 * Corresponding author. Tel.: þ55 11 3091 3946; fax: þ55 11 3091 4258. Q1 107 53 E-mail address: [email protected] (G.A.daC. Campanha). 108 54 109 e Ó 55 0895-9811/$ see front matter 2010 Published by Elsevier Ltd. 110 doi:10.1016/j.jsames.2010.04.001

Please cite this article in press as: Campanha, et al., Structural analysis of the Itapucumí Group in the Vallemí region, northern..., Journal of South American Earth Sciences (2010), doi:10.1016/j.jsames.2010.04.001 SAMES895_proof ■ 8 September 2010 ■ 2/11

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111 da parte oriental, nos arredores de San Alfredo e Cerro Paiva, conduzem à possibilidade do Grupo Ita- 176 fi 112 pucumí vir a ser dividido no futuro em duas unidades estratigrá cas distintas, o que requer estudos 177 fi fi 113 estratigrá cos e geocronológicos mais pormenorizados para uma conclusão de nitiva. 178 Ó 2010 Published by Elsevier Ltd. 114 179 115 180 116 resumen 181 117 182 118 Un análisis estructural detallado fue realizado en la Mina Vallemí en el norte de Paraguay. Afloran allí 183 119 litotipos del Grupo Itapucumí, de edad neoproterozoica, constituído por calcarenitas oolíticas, margas, 184 120 lutitas y areniscas. En la mina principal, la sucesión presenta deformación relativamente intensa, con 185 121 desarrollo de clivaje de plano axial y rumbo NeS. Se observan pliegues abiertos a isoclinales, a veces con 186 122 flanco invertido y asociados a cabalgamientos con vergencia al NE y E. Se registra asimismo meta- 187 fi 123 mor smo de grado bajo (zona de clorita). Los datos estructurales refuerzan la hipótesis de una nueva faja 188 124 móvil en el margen oeste del Bloque Río Apa, con vergencia opuesta a la del Cinturón Paraguay 189 meridional, este último relacionado al evento Brasiliano/Pan-Africano. Las diferencias estructurales y en 125 190 parte litológicas entre las exposiciones de la parte occidental del Grupo Vallemí y las de la parte oriental, 126 191 en los alrededores de Colonia San Alfredo, sugieren la posibilidad de que el Grupo Itapucumí pueda ser 127 subdividido en dos unidades litoestratigráficas distintas. Para llegar a una conclusión definitiva son 192 128 necesarios estudios estratigráficos y geocronológicos más pormenorizados. 193 129 Ó 2010 Published by Elsevier Ltd. 194 130 195 131 196 132 197 133 1. Introduction proto-Pacific margin. The main cratonic blocks involved are the 198 134 Amazonian, Rio Apa, São Francisco-Congo, Paranapanema (or Par- 199 135 The geology of northern Paraguay is dominated by extensive aná), Rio de La Plata, Pampia and Arequipa-Antofalla (Fig. 1). Recent 200 136 Phanerozoic sedimentary covers, including the Cenozoic Chaco review articles about this sector of Gondwana were published by 201 137 Basin and the Paleozoic Paraná Basin. The southern portion of the Rapela et al. (1998), Kröner and Cordani (2003), Rapela et al. (2007), 202 138 Rio Apa Cratonic Block outcrops between these sedimentary covers Ramos and Coira (2008), Cordani et al. (2009) and Cordani (2009). 203 139 in continuity with the exposures in the state of Mato Grosso do Sul, The Paraguay Fold Belt (Almeida, 1968; Alvarenga et al., 2000), 204 140 Brazil. The eastern side of the Rio Apa Block is flanked by the sometimes referred to as the ParaguayeAraguaia Belt or the 205 141 Paraguay Fold Belt and its correlated cratonic covers, represented Paraguay-Pampean Belt, is a curved belt around the Amazonian and 206 142 by the Corumbá and Cuiabá groups, while its southern region is Rio Apa cratonic blocks. It has undeformed or slightly deformed 207 143 covered by the Itapucumí Group. sedimentary covers over the cratons, passing to a marginal fold- 208 144 The Late Neoproterozoic to Cambrian tectonic evolution of this and-thrust belt with tectonic vergence towards the cratonic areas, 209 145 region is related to the amalgamation of Western Gondwana and its and it is characterized by low-grade metamorphism (Fig. 1). The 210 146 211 147 212 148 213 149 214 150 215 151 216 152 217 153 218 154 219 155 220 156 221 157 222 158 223 159 224 160 225 161 226 162 227 163 228 164 229 165 230 166 231 167 232 168 233 169 234 170 235 171 236 172 Fig. 1. Geotectonic context of the study area. (1) Phanerozoic covers; (2) cratons and cratonic blocks: AM e Amazonian, AA e Arequipa/Antofalla, LA e Luís Alves, RP e Rio de La 237 e e e e e e e 173 Plata, PA Pampia, PR Paraná, RA Rio Apa, SF São Francisco; (3) Neoproterozoic cratonic covers; (4) Neoproterozoic fold belts: Rb Ribeira, Ct Chiquitos-Tucavaca; Aç 238 Araçuaí; Br e Brasília, Df e Dom Feliciano, Py e Paraguay, Ar e Araguaia, WP e Western Pampean; EP e Eastern Pampean; (5) Transbrasiliano lineament; (6) strike-slip fault; (7) 174 reverse fault; (8) tectonic vergence; (9) international borders (modified from Alvarenga and Trompette, 1993; Kröner and Cordani, 2003; Rapela et al., 2007; Ramos and Coira, 2008; 239 175 Cordani, 2009; Ramos et al., in press). 240

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241 Chiquitos-Tucavaca Belt in Bolivia (Litherland et al., 1986) forms Plata Cratons and the Brasilia Belt (Ramos and Coira, 2008; Cordani, 306 242 a branch at an angle to the northern and southern Paraguay belts, 2009). 307 243 separating the Amazonian Craton from the southern Rio Apa, The western and southwestern sides of the Rio Apa Block in 308 244 Pampia, and Arequipa-Antofalla blocks. Jones (1985) interpreted northern Paraguay are poorly understood due to widespread 309 245 the combined system of the northern Paraguay, Chiquitos-Tucavaca Cenozoic sedimentary covers and difficult accessibility. The Paleo- 310 246 and southern Paraguay belts as a triple (RRR) plate junction. proterozoic and Mesoproterozoic gneissic-granitic basement (Rio 311 247 The eastern side of the Paraguay Belt in Brazil is covered by the Apa Complex, São Luís Group, and San Ramón Suite) is exposed 312 248 Paleozoic Paraná sedimentary basin, making it difficult to establish mainly northeast of the San Alfredo region (Araújo et al., 1982; 313 249 its relationship to the Paranapanema Block and the Brasília Belt. Cordani et al., 2005). 314 250 Some authors have suggested that the Transbrasiliano Lineament In northern Paraguay, terrigenous and carbonate metasedi- 315 251 separates the large Amazonian Craton, the Rio Apa Block and the mentary successions were identified as the Itapucumí Group 316 252 Paraguay and Araguaia Belts from the Paranapanema and Rio de La (Fig. 2), representing a Precambrian cratonic cover over the 317 253 318 254 319 255 320 256 321 257 322 258 323 259 324 260 325 261 326 262 327 263 328 264 329 265 330 266 331 267 332 268 333 269 334 270 335 271 336 272 337 273 338 274 339 275 340 276 341 277 342 278 343 279 344 280 345 281 346 282 347 283 348 284 349 285 350 286 351 287 352 288 353 289 354 290 355 291 356 292 357 293 358 294 359 295 360 296 361 297 362 298 363 299 364 300 365 301 366 302 367 303 368 304 369 305 Fig. 2. Location and geologic map of the study area (based on Schobbenhaus et al., 1981; Araújo et al., 1982; Clerici, 1986; Wiens, 1986; DSGMP, 1993; Lacerda Filho et al., 2004). 370

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371 southern portion of the Rio Apa Cratonic Block. The Itapucumí 436 372 Group has been bio- and chrono-correlated with the Ediacaran 437 373 Tamengo Formation (Corumbá Group, Almeida, 1965) of the 438 374 Paraguay Belt in Brazil based on the discovery of a Cloudina fossil in 439 375 the San Alfredo region (Boggiani and Gaucher, 2004). 440 376 In preliminary studies of the Vallemí region of northern 441 377 Paraguay, Boggiani (1998) identified systems of overturned folds- 442 378 and-thrust faults with east vergences, contrary to those identified 443 379 in the Paraguay Belt. 444 380 The analysis of this region suggests that there are two distinct 445 381 lithostratigraphic and structural domains for the Precambrian 446 382 carbonates that outcrop in central-northern Paraguay, which are 447 383 usually attributed to the Itapucumí Group. The first domain is 448 384 located in the Vallemí region and the surrounding hills along the 449 385 Paraguay River in the southwestern edge of the Rio Apa Cratonic 450 386 Block. It is composed of siliciclastic rocks (red sandstones, silt- 451 387 stones, and claystones) with intercalations of basic rocks in the 452 Fig. 3. Stratigraphic column of the Itapucumí Group. 388 basal portion, overlaid by carbonate rocks (ooid grainstones) and 453 389 marls that have been partially affected by low-grade meta- 454 390 morphism (chlorite zone) and intense deformation. The second calcareous rocks intercalated with marly rhythmites at the top. The 455 391 domain is an extensive horizontal or sub-horizontal sedimentary calcareous rocks, predominantly ooid grainstones, were formed in 456 392 cover over the southern portion of the Rio Apa Cratonic Block (San oolitic sandbanks and barrier islands that had undergone post- 457 393 Alfredo e Cerro Paiva region) with shallow water sedimentary depositional dolomitization under evaporitic conditions. 458 394 facies (breccias, thrombolites, stromatolites, grainstones with tidal Isotopic data for the Itapucumí Group are not conclusive. 459 395 bundle laminations, and ooid grainstone layers with cross-stratifi- Kawashita (1996), Oliveira (2004), and Cordani et al. (2005) 460 396 cation, Fig. 6). To date, the Cloudina has been found only in this analyzed 87Sr/86Sr isotopes in carbonates from the Vallemí Mine 461 397 second domain. and compared them with the Proterozoic seawater variation curve 462 398 The work presented here is the result of a structural character- (Gorokov et al., 1995). The results suggested a sedimentation age of 463 399 ization carried out at the Vallemí Mine (Industria Nacional de about 560 Ma (Ediacaran). An Rb/Sr isochron obtained from 464 400 Cemento e INC, Paraguay) and the neighboring Tres Cerros area, as calcareous samples of the Vallemí Mine (Oliveira, 2004) yielded an 465 401 well as a reconnaissance along the Paraguay River in the San age of 517 24 Ma, which was interpreted as an isotopic homog- 466 402 Alfredo, Cerro Paiva and Sargento José E. López regions. Recent enization event and attributed by the author to diagenesis. 467 403 advances in the exploitation of the Vallemí Mine have allowed 468 404 a better understanding of the deformation pattern of these rocks. 469 3. Local geology 405 In this article, we analyze the hypothesis that the Itapucumí 470 406 Group at the southwest edge of the Rio Apa Cratonic Block (Vallemí 471 The current study was carried out in the westernmost Itapucumí 407 and other exposures along the Paraguay River) represents the 472 Group outcrops of the Vallemí region. The primary study site was 408 outcropping vestiges of a fold-and-thrust belt with a vergence 473 the INC Mine, but additional work took place in small calcareous 409 opposite to that of the Southern Paraguay Belt. We also address the 474 rock quarries in the neighboring Tres Cerros region and in outcrops 410 idea that these belts form a system of belts with centripetal ver- 475 following the Paraguay River southwards (Fig. 2). 411 gences directed towards the Rio Apa Cratonic Block. Detailed 476 The Itapucumí Group comprises the lower Vallemí Formation 412 geological and structural data are presented and compared to the 477 and uppermost Camba Jhopo Formation (Wiens, 1986; Fig. 3). 413 regional geologic knowledge currently available. 478 414 The lower Vallemí Formation comprises meter-scale layers of 479 massive and cross-stratified arkoses as well as a decameter-scale 415 2. The Itapucumí group 480 416 body of basic rock, possibly effusive, overlain by intercalations of 481 fine-grained massive red sandstones and pelites. 417 The units currently known as the Itapucumí Group in Paraguay 482 The basal unit is overlaid by the Camba Jhopo Formation, 418 were preliminarily described in regional geological reconnaissance 483 possibly through an unconformity. The upper unit begins with 419 studies (DuGraty, 1865; Boettner, 1947) and were named the “Ita- 484 a 130-m-thick succession of massive to stratified ooilitic calcareous 420 pucumi Series” by Harrington (1950), who thought that they were 485 rocks, sometimes with trough and low-angle cross-stratifications. 421 possibly of Ordovician age. Eckel (1959) described the succession as 486 At the top, the unit changes to a 90-m-thick succession comprised 422 being about 300e400 m thick, composed of light to dark gray 487 of marly rhythmite intercalations with terrigenous pelites, dark 423 carbonates (locally oolitic) in erosive contact over the basement 488 mudstones and meter- to decameter-scale layers of massive, 424 and culminating at the top with marls and pelitic rocks. The lith- 489 partially dolomitized calcareous rock (Fig. 3). 425 ologic similarity of the Itapucumí Series to successions described in 490 426 Brazil allowed Putzer (1962) to suggest a Cambrian or Precambrian 491 427 age and a correlation with the Tamengo Formation of the Corumbá 3.1. Structures 492 428 Group. 493 429 Wiens (1986) carried out the first detailed stratigraphic study of The rocks in the main exploitation area of the Vallemí Mine are 494 430 the Itapucumí Series, in which he reclassified the unit as a group. It affected by a relatively intense deformation pattern, marked by the 495 431 was proposed that this group be divided into two distinct forma- development of axial-plane slaty cleavage and open to isoclinal 496 432 tions: the predominantly terrigenous Vallemí at the bottom and the folds. Some of these folds are overturned and are associated with 497 433 carbonatic Camba Jhopo at the top (Fig. 3). thrust faults (Fig. 4). 498 434 According to Boggiani (1998), the Vallemí quarry section is The foliation that developed under metamorphic conditions is 499 435 a succession with siliciclastic rocks (arkoses) at the bottom and characterized as a fine slaty cleavage in the pelitic layers (S1; Fig. 5A, 500

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501 566 502 567 503 568 504 569 505 570 506 571 507 572 508 573 509 574 510 575 511 576 512 577 513 578 514 579 515 580 516 581 517 582 518 583 519 584 520 585 521 586 522 587 523 588 524 589 525 590 526 591 527 592 528 593 529 594 530 595 531 596 532 597 533 598 534 599 535 600 536 601 537 602 538 603 539 604 540 605 541 606 542 607 543 608 544 609 545 610 546 611 547 612 548 613 549 614 550 615 551 616 552 617 553 618 554 619 555 620 556 621 557 622 558 623 559 624 560 625 561 626 562 627 563 Fig. 4. Structural-geologic map and cross-section of Vallemí Mine. 628 564 629 565 630

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631 696 632 697 633 698 634 699 635 700 636 701 637 702 638 703 639 704 640 705 641 706 642 707 643 708 644 709 645 710 646 711 647 712 648 713 649 714 650 715 651 716 652 717 653 718 654 719 655 720 656 721 657 722 658 723 659 724 660 725 661 726 662 727 663 728 664 729 665 730 666 731 667 732 668 733 669 734 670 735 671 736 672 737 673 738 674 739 675 740 676 741 677 742 678 743 679 744 680 745 681 746 682 747 683 748 684 749 685 750 686 751 687 752 688 Fig. 5. Structural features of the Camba Jhopo Formation in the Vallemí Mine: (A) photomicrograph showing microfolded sedimentary bedding with axial-plane slaty cleavage 753 689 (plane parallel light); (B) photomicrograph showing clastic sedimentary texture cut by slaty cleavage (crossed polarized light); (C) impure carbonate levels (marls) with well- 754 developed rhythmic bedding and perpendicular slaty cleavage; (D) inverted limb of fold with upright sedimentary stratiﬁcation and tectonic cleavage with lower dip angle; (E) 690 southern portion of the Vallemí Mine with decameter-scale open folds affecting alternating layers of marly rhythmites and grainstones; (F) ductile-brittle shear zone of low angle 755 691 (thrust fault) with pelitic marly rhythmites in the hanging wall and grainstones in the footwall; (G) detail of previous photo showing S/C structures indicating transport towards 756

692 ESE; (H) conjugate kink bands indicating horizontal maximum compression (s1) WNW-ESE; (I) strike-slip fault in the southern portion of the Vallemí Mine, with the slip direction 757 693 indicated by the attitude of the observed slickenside striae (350/10); (J) detail of the previous photo showing cataclastic breccia associated with the fault plane. 758 694 759 695 760

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761 826 762 827 763 828 764 829 765 830 766 831 767 832 768 833 769 834 770 835 771 836 772 837 773 838 774 839 775 840 776 841 777 842 778 843 779 844 780 845 781 846 782 847 783 848 784 849 785 850 786 851 787 852 788 853 789 854 790 855 791 856 792 857 793 858 794 859 795 860 796 861 797 Fig. 5. (continued). 862 798 863 799 B, C and D) formed by the orientation of very fine sericite-chlorite The thrust faults (Fig. 5F and G) have sub-horizontal undulating 864 800 grains. This mineral association and the absence of other meta- planes, which in certain locations make the faults appear exten- 865 801 morphic minerals (e.g., biotite) characterize a low metamorphic sional, but the continuity of the faults is evident in several places. 866 802 grade (greenschist facies, chlorite zone). This slaty cleavage has an The ductile-brittle character of these thrusts and the associated S/C 867 803 average dip of 56 at an azimuth of 290 (Fig. 7B) and frequently structures are consistent with the low metamorphic grade 868 804 cuts the sedimentary stratification (S0) at an angle. (greenschist facies, chlorite zone) observed in the pelitic rocks. 869 805 The sedimentary stratification (S0) is more prominent in the They display attitude (Fig. 7D and E) and kinematic indicators 870 806 impure carbonates, where it is marked by alternating layers with compatible with a sub-horizontal WNW-ESE maximum compres- 871 807 variable siliciclastic and carbonate contents (Fig. 5A). The bedding sive stress (s1) orientation. They further display a transport direc- 872 808 poles fit to a girdle in the stereonet, indicating a mean fold axis tion towards ESE compatible with the observed eastward vergences 873 809 plunging 14 at an azimuth of 358 (Fig. 7A). The S1 cleavage is in of the D1 folds and S1 foliation. The presence of sub-horizontal 874 810 the axial-plane position. The intersection of lineations between S0 tension gashes and kink bands with approximately NeS upright 875 811 and S1 confirms this pattern, with an average plunge of 28 at an axial planes (Fig. 5H) corroborates this interpretation of a sub- 876 812 azimuth of 352 (Fig. 7C). horizontal WNW-ESE s1 orientation. Brittle normal and trans- 877 813 The northwest and central-eastern portions of the mine expose current faults (Fig. 5I and J) cut these structures, but without more 878 814 tight overturned folds with an axial plane that dips to the west precise stratigraphic or chronological data, it is difficult to precisely 879 815 (Fig. 5D). Most of this area is located within an inverted fold limb on determine the age of these younger brittle deformation events. 880 816 top of sub-horizontal basal thrust faults. This structural pattern contrasts with that found in other nearby 881 817 In the southern portion of the mine, the fold pattern changes to areas. In Tres Cerros and some outcrops along the Paraguay River, 882 818 a succession of more open antiforms and synforms (Fig. 5E). The such as the Fonciere and Guyrati Ports, the bedding is tilted with 883 819 fold axis and axial-plane foliation gradually pass from NeS in the dips ranging from horizontal to almost 35 (Fig. 6F) but without 884 820 northern portion of the mine to NNE in the southern portion apparent evidence of metamorphism or tectonic cleavage 885 821 (Fig. 4), which could be explained by a second-generation fold with development. 886 822 an approximately EeW trending axial plane and an NW plunging However, at the Itapucumí Port located downstream of the 887 823 axis, as suggested by the dispersion of S1 poles and L1 lineations Paraguay River, the westernmost quarries show a deformation 888 824 (Fig. 7B and C). However, there is no additional evidence supporting pattern similar to that found in the Vallemí area. In particular, the 889 825 this interpretation, e.g., axial-plane cleavages with EeW strike. deformation includes open to tight asymmetrical folds with upright 890

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891 956 892 957 893 958 894 959 895 960 896 961 897 962 898 963 899 964 900 965 901 966 902 967 903 968 904 969 905 970 906 971 907 972 908 973 909 974 910 975 911 976 912 977 913 978 914 979 915 980 916 981 917 982 918 983 919 984 920 985 921 986 922 987 923 988 924 989 925 990 926 991 927 992 928 993 929 994 930 995 931 996 932 997 933 998 934 999 935 1000 936 1001 937 1002 938 1003 939 1004 940 1005 941 1006 942 1007 943 1008 944 1009 945 1010 Fig. 6. Field and petrographic features of the undeformed Itapucumí Group domain in the southern Rio Apa Cratonic Block (San Alfredo and Sgt. Lopez colonies, Cerro Paiva, Bello 946 Horizonte and Primavera Farms): (A) laminated microbialitic facies composed of laterally continuous and disrupted laminae, occasionally with curls and desiccation cracks; (B) fine- 1011 947 grained grainstone with trough cross-stratification; (C) thrombolitic domes; Cloudina occurs in the troughs; (D) lenticular concentration of Cloudina shells; (E) photomicrograph of 1012 948 ooid grainstone (small dark circles are bubbles in the mounting medium); and (F) transverse section typical of Cloudina shells (note the excentric emplacement of the inner shell). 1013 949 1014 950 1015 951 short limbs, slaty cleavage in the pelitic rocks and en-echelon Outcrops west of the Paraguay River are rare and very difficult to 1016 952 carbonate-filled tension gashes. It is thus possible that the limit reach, thus impeding a full understanding of the development of 1017 953 between the western deformed domain and the eastern unde- deformation in the western area. 1018 954 formed domain is nearly NeS where it passes the Vallemí and On the southeast side of the Rio Apa Cratonic Block, the limit 1019 955 ItapucumíPorts (Fig. 2). with the Paraguay Belt (the limit between the eastern deformed 1020

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1021 1086 1022 1087 1023 1088 1024 1089 1025 1090 1026 1091 1027 1092 1028 1093 1029 1094 1030 1095 1031 1096 1032 1097 1033 1098 1034 1099 1035 1100 1036 1101 1037 1102 1038 1103 1039 1104 1040 1105 1041 1106 1042 1107 1043 1108 Fig. 7. Stereonets of structural data (SchmidteLambert diagram, lower hemisphere), Vallemí Mine: (A) poles of sedimentary stratification (S0) with the best-fit girdle and pi axis 1044 indicated; (B) poles of slaty cleavage (S1) with average pole and plane indicated; (C) intersection lineations between bedding and slaty cleavage (L1) with average attitude indicated; 1109 1045 (D) measured faults with slickensides represented as great circles and striae represented as points with the following kinematic indications: r: reverse, n: normal, d: dextral, s: 1110 1046 sinistral; (E) the reverse faults only; (F) poles of sedimentary stratification in the Tres Cerros region with the average plane indicated. 1111 1047 1112 1048 1113 1049 1114 1050 and western undeformed domains) is roughly located between Reliable radiometric ages from Vallemí are still precarious but 1115 1051 Sargento José E. López (Paraguay) and Bela Vista (Brazil). are also suggestive of an Ediacaran age. 1116 1052 On the south-central portion of the Rio Apa Cratonic Block near Isotopic analyses of the carbonate rocks from the Vallemí Mine 1117 1053 San Alfredo, Cerro Paiva, Primavera Farm and the Apa River, the (Kawashita, 1996; Cordani et al., 2005) yielded an average 87Sr/86Sr 1118 1054 Itapucumí Group (in some places the carbonatic Camba Jhopo value of about 0.7085, which is compatible with the end of the 1119 1055 Formation) rests unconformably on the basement (Fig. 6), with sub- global increase in these values in marine water that occurred in the 1120 1056 horizontal to low-dipping bedding (up to 15). Ediacaran age (see Halverson et al., 2007). However, the low-grade 1121 1057 metamorphism in Vallemí rocks could have affected this result. 1122 1058 4. Discussion The only direct radiometric age available from the Vallemi Mine 1123 1059 was a Rb/Sr isocron age obtained by Oliveira (2004) of 517 24 Ma, 1124 1060 The attitudes of fold axes, slaty cleavage (S1), and axial planes in which was interpreted as the age of an isotopic homogenization 1125 1061 the Vallemí Mine are similar to those observed in the southern event during diagenesis. However, it is also possible to attribute this 1126 1062 Paraguay Belt (e.g., in the Serra da Bodoquena on the Brazilian side). age to the deformation and metamorphism that these rocks have 1127 1063 The most conspicuous difference is the structural vergence shown undergone. If this were the case, the deformation and meta- 1128 1064 by the dip of the axial-plane cleavage, the fold asymmetry, and the morphism of the Vallemí area would be synchronous with those of 1129 1065 kinematic indicators that indicate a westward vergence in the the southern Paraguay Belt, with a lower Cambrian age. 1130 1066 Paraguay Belt and an eastward vergence in Vallemí. Thus, the available age information favors the hypothesis that 1131 1067 The Vallemí region and the southern Paraguay Belt are sepa- the Itapucumí carbonate sequences in Vallemí (deformed rocks), 1132 1068 rated by the Rio Apa Cratonic Block (Figs. 1 and 2), which is overlain San Alfredo (cratonic cover) and the Tamengo Formation in Brazil 1133 1069 in its southern part by sedimentary carbonates of the undeformed are chrono-correlated, with the lithologic differences arising as the 1134 1070 domain of the Itapucumí Group. These rocks have been correlated result of distinct sedimentary settings. 1135 1071 with the Ediacaran Corumbá Group in the southern Paraguay Belt In this context, the existence of a newly recognized Brasiliano 1136 1072 in Brazil based on the occurrence of Cloudina fossils in both regions mobile belt in the Vallemí region is a reasonable hypothesis. This 1137 1073 (Boggiani and Gaucher, 2004). belt would be parallel and symmetric to the southern Paraguay 1138 1074 Cloudina is an important index fossil associated with the Belt, both showing centripetal vergence towards the Rio Apa Block. 1139 1075 Precambrian-Cambrian limit. In Oman (Ara Group, Huqf Super- Much discussion has focused on whether the Rio Apa Block is 1140 1076 group), Cloudina occurs beneath an ash bed, with a UePb zircon linked to the Amazonian Craton (e.g., Ruiz et al., 2005). If a new fold 1141 1077 concordia age of 542.0 0.3 Ma (Amthor et al., 2003). Similar ages belt did exist in the Vallemi region, the Rio Apa Block would 1142 1078 have been obtained for Cloudina-bearing carbonates in Namibia represent either a relatively small cratonic fragment in the context 1143 1079 (Grotzinger et al., 1995). Fike et al. (2006) consider the first of the Western Gondwana orogeny or a narrow promontory of the 1144 1080 appearance of Cloudina to have occurred about 548 Ma. Thus, 542 to southern portion of the Amazonian Craton, flanked by two mobile 1145 1081 548 Ma can be considered as the age range for this fossil. In Brazil, belts (Fig. 1). 1146 1082 an average SHRIMP age of 543 3 Ma for zircon in volcanic ash in The Ediacaran sedimentation age and the Lower Cambrian 1147 1083 the Tamengo Formation near Corumbá (Boggiani et al., 2005; deformation age of this region are markedly younger than the 1148 1084 Babinski et al., 2008) is consistent with the 542 to 548 Ma range orogenic ages from the eastern Brasília and Ribeira Belts, with the 1149 1085 suggested by Fike et al. (2006). main peak having an age of about 630 Ma (Trompette et al., 1998; 1150

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1151 Brito Neves et al., 1999; Alkmim et al., 2001; Valeriano et al., 2004, Uncited reference 1216 1152 2008). It has been suggested that the ParaguayeAraguaia basin was 1217 1153 related to the Brasiliano Orogenic Chain of the Brasilia and Ribeira Grant, 1990. 1218 1154 Belts as a foreland basin (Trompette, 1994; Trompette et al., 1998). 1219 1155 Another interpretation postulates that the sedimentation of the 1220 Acknowledgements 1156 Paraguay Belt units evolved from a rift to a passive margin 1221 1157 (Boggiani, 1998; Gaucher et al., 2003). In this model, an ocean 1222 The authors would like to thank Victor A. Ramos and the 1158 should have existed to the east, but evidence of this ocean would be 1223 anonymous referees for their reviews and suggestions. Financial 1159 buried under the Paraná sedimentary basin. However, 1224 support for this research came from the CNPq (Conselho Nacional de 1160 Woldemichael (2003) suggested the presence of a collision margin 1225 Pesquisa do Brasil; PROSUL Project e Grant 490234/2005-4 to P.C.B.) 1161 between the Rio Apa and Paranapanema Cratons based on mag- 1226 and FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo; 1162 netothelluric and gravimetric data. 1227 Grant 04/012330 to P.C.B.). The authors are indebted to geologist 1163 Paleomagnetic data (Trindade et al., 2003; Tohver et al., 2006; 1228 Mr. José Paradeda of INC, PY (National Industry of Cement, 1164 Cordani et al., 2009) suggest that the Amazonian Craton was not 1229 Paraguay) for logistic support and geologist Júlio César Galeano 1165 linked to Western Gondwana before Early Cambrian times. In this 1230 Inchausti of the Ministry of Mines and Energy of Paraguay. This 1166 scenario, the collision between the Amazonian and the remainder 1231 paper represents a contribution to IGCP project 478. 1167 of Western Gondwana (e.g., the São Francisco, Congo and Para- 1232 1168 napanema Cratons) would be responsible for the Para- 1233 1169 guayeAraguaia orogeny in the Lower Cambrian. Campanha and References Q2 1234 1170 Brito Neves (2004) proposed that a WNW-ESE to NW-SE general 1235 1171 compression was responsible for the final amalgamation of Alkmim, F.F., Marshak, S., Fonseca, M.A., 2001. Assembling west Gondwana in the 1236 1172 Neoproterozoic: clues from the São Francisco Cráton region, Brazil. Geology 29, 1237 Western Gondwana during its late stages of convergence. 319e322. 1173 On the other hand, the sedimentation and deformation ages of Almeida, F.F.M.de., 1965. Geologia da Serra da Bodoquena (Mato Grosso), vol. 219. 1238 1174 the Vallemí region and the Paraguay Belt are similar to the ages of Boletim da Divisão de Geologia e Mineralogia, Departamento Nacional da 1239 e 1175 the Pampean (Puncoviscan) Belt in Argentina. This belt is also Produção Mineral, DNPM. 1 96. 1240 Almeida, F.F.M.de., 1968. Evolução tectônica do Centro-Oeste Brasileiro no Pro- 1176 characterized by Ediacaran sedimentation followed by late Neo- terozóico superior. Anais da Academia Brasileira de Ciências, (Suplemento 1241 1177 proterozoic to Cambrian deformation, magmatism, and meta- Simpósio de Manto Superior) 40, 285e296. 1242 1178 morphism, with Ordovician and Devonian post-orogenic Alvarenga, C.J.S., Trompette, R., 1993. Evolução Tectônica Brasiliana da Faixa Para- 1243 guai: a Estruturação da Região de Cuiabá. Revista Brasileira de Geociências 23 1179 magmatism (Rapela et al., 2007; Ramos and Coira, 2008; (1), 18e30. 1244 1180 Siegesmund et al., 2009). A western and an eastern branches of Alvarenga, C.J.S.de, Moura, C.A.V, Gorayeb, P.S.S., de Abreu, F.A.M., 2000. Paraguay 1245 1181 the Pampean Belt are recognized, limiting the Pampia Cratonic and Araguaia belts. In: Cordani, U.G., Milani, E.J., Thomaz Filho, A. and Campos, 1246 D.A. (Eds.), Tectonic Evolution of South America, Rio de Janeiro, 31st Interna- 1182 Block (Fig. 1). The eastern Pampean Belt is a magmatic arc devel- tional Geological Congress, pp. 183e193. 1247 1183 oped over the western border of the Rio de La Plata Craton (Rapela Amthor, J.E., Grotzinger, J.P., Schröder, S., Bowring, S.A., Ramezani, J., Martin, M.W., 1248 1184 et al., 2007; Ramos and Coira, 2008; Cordani, 2009; Ramos et al., in Matter, A., 2003. Extinction of Cloudina and Namacalathus at the Precambrian- 1249 e e Cambrian boundary in Oman. Geology 31, 431 434. 1185 press). Thus, E W compression in the Vallemí region could be Araújo, H.J.T., Santos, Neto A., Trindade, C.A.H., Pinto, J.C.A., Montalvão, R.M.G., 1250 1186 related to a western active plate boundary. The deformation of the Dourado, T.D.C., Palmeira, R.C.B., Tassinari, C.C.G. 1982. Geologia. In: Projeto 1251 1187 Vallemí Belt could be the distant expression of the Pampean RADAMBRASIL, Folha SF 21, Rio de Janeiro, vol. 28, 23e124. 1252 e 1188 Babinski, M.; Boggiani, P.C.; Fanning, M.; Simon, C.M.; Sial, A.N., 2008. U Pb 1253 orogeny and represent the closure of the westernmost Gondwana SHRIMP geochronology and isotope chemostratigraphy (C, O, Sr) of the 1189 boundary, including the Pampia, Rio Apa, Rio de La Plata and Par- Tamengo Formation, southern Paraguay belt, Brazil. In: VI South American 1254 1190 anapanema Cratonic Blocks. Symposium on Isotope Geology, San Carlos de Bariloche. Proceedings. 1255 1191 Boettner, R., 1947. Estúdio geológico desde Puerto Fonciére hasta Toldo-Cué. Revista 1256 de la Facultad de Química y Farmacia de la Universidad Nacional del Paraguay 3, 1192 9e14. 1257 1193 5. Conclusions Boggiani, P.C.,1998. Análise Estratigráfica da Bacia Corumbá (Neoproterozóico) e Mato 1258 1194 Grosso do Sul. Unpubl. Ph.D. Thesis, University of São Paulo, Brazil, 181 pp. 1259 Boggiani, P.C., Gaucher, C., 2004. Cloudina from the Itapucumí Group (Vendian, 1195 New structural data from the Itapucumí Group in northern Paraguay): Age and Correlations. In: Symposium on Neoproterozoic-Early 1260 1196 Paraguay strengthen the hypothesis of a new Brasiliano/Pan- Paleozoic Events in SW-Gondwana, 1, Extended Abstracts, IGCP Project 478, 1261 1197 African mobile belt on the western margin of the Rio Apa Cratonic Second Meeting, Brazil, pp. 13e15. 1262 Boggiani, P.C., Babinski, M., Yamamoto, J.K., Fairchild, T.R., Ricomini, C., Diratgitch, 1198 Block, with a vergence opposite to that of the southern Paraguay A.A., Liu, D., 2005. U-Pb SHRIMP investigation of ash beds in the Corumbá 1263 1199 Belt. The confirmation of this idea would have implications for the Group (Ediacaran), Paraguay Belt, Brazil. In: 2nd Symposium on Neoproterozoic 1264 1200 regional tectonic framework of the region. In particular, the Rio Apa e Early Paleozoic Events in southwestern Gondwana, 2, Windhoek e Namibia, 1265 Abstracts, IGCP-478, pp. 8e9. 1201 Block would consist of a smaller cratonic fragment surrounded by Brito Neves, B.B., Campos Neto, M.C., Fuck, R.A., 1999. From Rodinia to western 1266 1202 the southern Paraguay Belt to the east and by the newly proposed Gondwana; an approach to the Brasiliano-Pan African cycle and orogenic 1267 1203 mobile belt to the west. collage. Episodes 22 (3), 155e166. 1268 1204 Campanha, G.A.C., Brito Neves, B.B., 2004. Frontal and oblique tectonics in the 1269 The deformation and metamorphism of the Vallemí area would Brazilian Shield. 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