Electron microprobe dating of monazite substantiates ages of major geological events in the southern Brazilian shield

Journal of South American Earth Sciences 16 (2004) 699–713 www.elsevier.com/locate/jsames

Electron microprobe dating of monazite substantiates ages of major geological events in the southern Brazilian shield Hugo Tickyja, Le´o A. Hartmanna,*, Marcos A.Z. Vasconcellosb, Ruy P. Philippa, Marcus V.D. Remusa a

Instituto de Geocieˆncias, Universidade Federal do Rio Grande do Sul, Av. Bento Gonc¸alves, 9500, 91501-970 Porto Alegre, Rio Grande do Sul, Brazil b Instituto de Fı´sica, Universidade Federal do Rio Grande do Sul, Av. Bento Gonc¸alves, 9500, 91501-970 Porto Alegre, Rio Grande do Sul, Brazil Received 1 January 2004; accepted 31 January 2004

Abstract To obtain the chemical Thp –Pb isochron ages and surface maps of monazite crystals in igneous and metamorphic rocks from the southern Brazilian Shield, we employ Th– U-total Pb dating by an electron probe microanalyzer. The ages of two Trans-Amazonian metamorphic events are given by a felsic, garnet-bearing granulite from the Santa Maria Chico granulitic complex. The age of the first event, at approximately 2.35 Ga, was obtained by surface mapping in a grain included in garnet. The dating of the second event, 1899 ^ 43 Ma, is in agreement with previous data obtained in zircon crystals with sensitive high-resolution ion microprobe. Other determinations belong to the Brasiliano cycle. In the Sa˜o Gabriel block, an age of 643 ^ 129 Ma was obtained on monazite from a staurolite-garnet schist of the Cambaizinho Formation, whereas a staurolite-bearing schist from the Passo Feio complex yielded a 510 ^ 68 Ma age. Several units in the Dom Feliciano belt were dated, including the biotite-sillimanite gneisses of the Va´rzea do Capivarita complex (552 ^ 90 Ma), the sillimanite-garnet gneisses of Camboriu´ complex (565 ^ 77 Ma), the Treˆs Figueiras granite (558 ^ 57 Ma), and the Plaza Itapema granite (545 ^ 55 Ma). The ages presented in this study, obtained through monazite chemical dating, are confirmed through comparison with previous data regarding zircon crystals from the same geological units. q 2004 Elsevier Ltd. All rights reserved. Keywords: Brasiliano cycle; Electron microprobe; Monazite; Southern Brazilian Shield; Th–U-total Pb dating; Trans-Amazonian cycle

1. Introduction The chemical Th –U-total Pb method for single-grain dating of monazite by an electron probe microanalyzer (EPMA) is a powerful technique for determining reliable ages at moderate cost (Suzuki and Adachi, 1991; Montel et al., 1994, 1996; Cocherie et al., 1998; Catlos et al., 2002). The technique’s ability to make nondestructive, in situ measurements provides valuable geochronological information about the textural position of the analyzed mineral. The high spatial resolution of the electron beam (1 – 2 mm), combined with back-scattered electron (BSE) images, helps clarify complex mineral zonation patterns generated by different geological events (Cocherie et al., 1998; Zhu and O’Nions, 1999), which makes the compositional and age * Corresponding author. Fax: þ 55-51-33167302. E-mail address: [email protected] (L.A. Hartmann). 0895-9811/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2004.01.001

maps of monazite useful for tectonic analysis of metamorphic rocks (Cocherie et al., 1998; Williams et al., 1999). In addition, monazite included in garnet of rocks with complex metamorphic histories can preserve ages related to previous metamorphic events (Foster et al., 2000, 2002; Montel et al., 2000), and multiple tectonothermal events thus can be unveiled (Santosh et al., 2003). In South America, this technique has been successfully developed at Universidade de Sa˜o Paulo (Vlach and Gualda, 2000). The objective of this study is to present the results obtained from igneous and metamorphic rocks of the southern Brazilian Shield by the chemical Th –U-total Pb dating technique developed at Universidade Federal do Rio Grande do Sul. In some cases, we compare the results with zircon U –Pb isotopic ages determined by sensitive highresolution ion microprobe (SHRIMP II) for rocks from the same geological unit. We also interpret the geological significance of the analytical results.

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2. Geology and samples Igneous and metamorphic rocks analyzed in this study occur in the southern Brazilian Shield (Fig. 1) and include the Santa Maria Chico granulitic complex and the Cambaizinho Formation and Passo Feio complex situated in the Sa˜o Gabriel block. Another set of samples

was collected from the Plaza Itapema granite of the Brusque complex and the Va´rzea do Capivarita and Camboriu´ complexes, all of which are within the Dom Feliciano belt (Fig. 1). Hartmann et al. (2000) and Hartmann and Delgado (2001) present reviews of the regional geology and the Brazilian Shield, respectively.

Fig. 1. Geological sketch map of southern Brazil. Numbers indicate sample location. 1, 2: Santa Maria Chico granulitic complex; 3: Cambaizinho Formation; 4– 8: Passo Feio complex; 9: Va´rzea do Capivarita complex; 10, 11: Treˆs Figueiras granite; 12 –14: Camboriu´ complex; and 15: Plaza Itapema granite.

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Table 2 Analytical results of monazite from matrix and garnet rims, Santa Maria Chico granulitic complex

Fig. 2. Back-scattered electron image of felsic garnet-bearing granulite from Santa Maria Chico granulitic complex.

The Santa Maria Chico granulitic complex is situated in the western portion of the exposed Precambrian shield in Rı´o Grande do Sul (Fig. 1), part of the Sa˜o Gabriel block (Jost and Hartmann, 1984; Babinski et al., 1996; Chemale et al., 1997). To the west, this complex is covered by sediments and basalts of the Parana´ basin, whereas to the east, it is limited by the Neoproterozoic Dom Feliciano belt

Table 1 Representative analytical results of monazite using experimental EPMA conditions to quantify major and minor elements Sample

1

2

3

4

5

6

7

SiO2 FeO La2O3 Ce2O3 Pr2O3 Nd2O3 Sm2O3 Gd2O3 Tb2O Dy2O3 Ho2O3 Er2O3 Yb2O3 Tm2O3 P2O5 CaO ThO2 U2O3 Y2O3 PbO Total

0.21 0.06 15.00 32.62 3.45 12.67 1.97 0.76 NA – ,0.1 NA NA ,0.1 29.38 0.62 2.66 0.30 0.07 0.32 100.09

0.10 0.06 11.90 29.06 3.69 14.55 2.95 2.40 NA 0.63 ,0.1 NA NA ,0.1 30.28 0.56 1.98 0.27 1.86 – 100.29

0.24 0.16 15.05 30.75 3.02 11.36 1.74 1.29 NA 0.14 ,0.1 NA NA ,0.1 30.18 1.17 4.50 0.69 0.15 – 100.44

0.10 – 13.95 29.00 3.27 12.80 1.89 1.46 NA 0.59 ,0.1 NA NA ,0.1 30.69 0.83 3.44 0.47 1.54 – 100.20

0.43 – 14.50 29.92 3.43 11.79 2.12 1.32 NA 0.42 ,0.1 NA NA ,0.1 29.89 0.52 3.74 0.21 1.55 – 99.84

0.69 0.51 14.95 31.27 3.36 11.82 1.60 1.16 NA 0.16 ,0.1 NA NA ,0.1 29.17 0.44 4.42 0.18 0.26 – 99.99

0.22 – 13.30 29.78 3.40 11.92 2.22 1.08 NA 0.10 ,0.1 NA NA ,0.1 29.76 0.66 5.21 0.23 0.47 0.16 98.51

1. Santa Maria Chico granulitic complex. 2. Cambaizinho Formation. 3. Passo Feio complex. 4. Va´rzea do Capivarita complex. 5. Treˆs Figueiras granite. 6. Camboriu´ complex. 7. Plaza Itapema granite. Notes: Al (and-) below detection limit. NA, not analyzed. The contents of Tm2O3 and Ho2O3 are below detection limits but results around 0.2% may indicate interference with other elements.

Sample

U (ppm)

Th (ppm)

Pb (ppm)

Age (Ma)

1-A1 1-A2 1-A3 1-A4 1-A5 1-A6 1-A7 1-A8 1-A9 1-A10 1-B1 1-B2 1-B3 1-B4 1-B5 1-B6 1-B7 1-B8 1-B9 1-B10 1-C1 1-C2 1-C3 1-C4 1-C5 1-C6 1-C7 1-C8 1-C9 1-C10 2-E1 2-E2 2-E3 2-E4 2-E5 2-E6 2-E7 2-E8 2-E9 2-E10

– 742 ^ 181 2296 ^ 233 1883 ^ 229 2378 ^ 233 1969 ^ 288 830 ^ 192 1422 ^ 221 1564 ^ 224 – 411 ^ 178 643 ^ 188 712 ^ 191 397 ^ 158 547 ^ 169 353 ^ 143 237 ^ 114 399 ^ 145 452 ^ 151 209 ^ 103 1656 ^ 223 1278 ^ 215 1660 ^ 226 1758 ^ 225 1765 ^ 225 1626 ^ 224 1848 ^ 226 1957 ^ 225 1482 ^ 222 1767 ^ 226 – 1510 ^ 220 2992 ^ 243 – – 2484 ^ 236 1047 ^ 206 1095 ^ 208 1495 ^ 221 2918 ^ 240

106540 ^ 762 55370 ^ 513 31470 ^ 395 32200 ^ 399 36830 ^ 422 38620 ^ 431 48060 ^ 477 32240 ^ 401 35800 ^ 419 107100 ^ 766 28590 ^ 381 37820 ^ 427 41930 ^ 450 44770 ^ 463 50620 ^ 492 51260 ^ 496 56440 ^ 520 57960 ^ 528 59300 ^ 535 59830 ^ 538 20050 ^ 331 23450 ^ 350 25000 ^ 362 25380 ^ 363 26020 ^ 368 25740 ^ 364 25310 ^ 365 25570 ^ 364 26540 ^ 370 23730 ^ 356 77480 ^ 624 42330 ^ 451 45730 ^ 469 104590 ^ 755 98210 ^ 723 35750 ^ 419 44800 ^ 464 43290 ^ 455 43360 ^ 457 40760 ^ 443

9300 ^ 295 5080 ^ 249 3490 ^ 236 3530 ^ 238 3990 ^ 241 4180 ^ 242 4440 ^ 245 3090 ^ 235 3770 ^ 238 9510 ^ 298 2260 ^ 228 3380 ^ 240 3810 ^ 242 3960 ^ 244 4310 ^ 250 4460 ^ 251 5040 ^ 255 5290 ^ 256 5310 ^ 260 5440 ^ 257 2280 ^ 255 2760 ^ 232 2730 ^ 231 2700 ^ 231 2880 ^ 232 2770 ^ 231 2790 ^ 230 2740 ^ 232 2850 ^ 232 2600 ^ 230 6820 ^ 274 4189 ^ 257 5356 ^ 266 9000 ^ 294 8650 ^ 291 3977 ^ 257 4299 ^ 259 4119 ^ 257 4209 ^ ^ 259 4417 ^ 261

1878 ^ 58 1880 ^ 91 1877 ^ 124 1933 ^ 127 1879 ^ 111 1951 ^ 111 1868 ^ 102 1779 ^ 133 1944 ^ 121 1905 ^ 58 1627 ^ 161 1812 ^ 127 1841 ^ 115 1844 ^ 112 1767 ^ 101 1828 ^ 101 1891 ^ 94 1913 ^ 91 1874 ^ 90 1928 ^ 89 1872 ^ 179 2085 ^ 170 1884 ^ 155 1825 ^ 152 1900 ^ 149 1875 ^ 152 1866 ^ 150 1802 ^ 148 1910 ^ 152 1849 ^ 159 1893 ^ 74 1880 ^ 108 2014 ^ 93 1852 ^ 59 1894 ^ 62 1900 ^ 113 1898 ^ 107 1872 ^ 109 1853 ^ 107 1844 ^ 100

(Fernandes et al., 1992; Babinski et al., 1996, 1997; Remus et al., 1996). The complex has a bimodal composition with mafic garnet granulites and acid tonalite/trondhjemites intercalated on meter to kilometer scales. Other rocks include pyroxenites, anorthosites, a 500 m body of spinel lherzolite, and supracrustal relicts of quartzite, marble, and sillimanite gneiss. Three folding events have been recognized, but a subvertical banding oriented EW in the southern part and WNW in the northern part is the most prominent structure. A younger subhorizontal structure is present as 1– 10-mm thick seams of hornblende and minor quartz spaced 1– 20 cm apart (Hartmann, 1991). Rocks were modified by two metamorphic events (M1 and M2) in the granulite facies. M1 is recognized in large

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Table 3 Analytical results from grain D, Santa Maria Chico granulitic complex

Table 4 Analytical results from grain F, Santa Maria Chico granulitic complex

Sample

U (ppm)

Th (ppm)

Pb (ppm)

Age (Ma)

Sample

U (ppm)

Th (ppm)

Pb (ppm)

Age (Ma)

1-D1 1-D2 1-D3 1-D4 1-D5 1-D6 1-D7 1-D8 1-D9 1-D10 1-D11 1-D15 1-D20 1-D21 1-D22 1-D23 1-D24 1-D25 1-D26 1-D27 1-D28 1-D29 1-D30 1-D31 1-D33 1-D34 1-D35 1-D36 1-D37 1-D38 1-D39 1-D40 1-D41 1-D42 1-D44 1-D45 1-D46 1-D47 1-D48 1-D49 1-D50 1-D53 1-D54 1-D55 1-D56 1-D57 1-D59 1-D60

8951 ^ 229 9124 ^ 240 8938 ^ 240 8902 ^ 245 9016 ^ 239 10194 ^ 232 9413 ^ 246 9287 ^ 246 6215 ^ 297 9205 ^ 247 9138 ^ 203 9581 ^ 248 11571 ^ 180 10010 ^ 247 9465 ^ 252 9413 ^ 255 9206 ^ 255 9650 ^ 242 9703 ^ 219 10300 ^ 235 8500 ^ 244 7629 ^ 262 8158 ^ 258 6339 ^ 288 7596 ^ 257 9161 ^ 217 9498 ^ 208 9919 ^ 225 9867 ^ 207 9498 ^ 202 10176 ^ 199 9637 ^ 206 9463 ^ 212 8291 ^ 200 10373 ^ 284 10005 ^ 282 9837 ^ 284 9876 ^ 284 9966 ^ 285 10042 ^ 283 9776 ^ 284 8634 ^ 276 8761 ^ 280 9804 ^ 282 8130 ^ 275 8670 ^ 277 9496 ^ 282 9724 ^ 284

38200 ^ 440 37630 ^ 437 37010 ^ 435 38160 ^ 439 37380 ^ 435 39620 ^ 447 37830 ^ 438 37050 ^ 437 37590 ^ 436 37470 ^ 436 40730 ^ 450 38220 ^ 443 42140 ^ 460 40430 ^ 453 37420 ^ 437 37800 ^ 441 37360 ^ 437 38450 ^ 444 39870 ^ 449 40420 ^ 454 38470 ^ 443 38560 ^ 443 38870 ^ 444 38570 ^ 444 39280 ^ 449 40250 ^ 451 40810 ^ 455 40630 ^ 453 40930 ^ 457 40810 ^ 455 41230 ^ 457 40960 ^ 454 41740 ^ 461 38300 ^ 442 38493 ^ 422 37214 ^ 415 37411 ^ 418 38919 ^ 422 39981 ^ 429 39928 ^ 428 40254 ^ 430 36549 ^ 414 36485 ^ 414 38993 ^ 423 37568 ^ 419 36953 ^ 415 40066 ^ 428 40838 ^ 433

7359 ^ 327 7834 ^ 343 7742 ^ 345 7843 ^ 345 7743 ^ 344 8356 ^ 340 8259 ^ 358 8130 ^ 356 6903 ^ 341 8131 ^ 353 6648 ^ 289 8439 ^ 360 7199 ^ 303 8651 ^ 334 8499 ^ 359 8509 ^ 363 8389 ^ 359 8232 ^ 354 7533 ^ 319 8409 ^ 335 7468 ^ 330 7296 ^ 339 7633 ^ 346 6786 ^ 337 7109 ^ 330 7077 ^ 311 6950 ^ 305 7929 ^ 314 7150 ^ 307 6805 ^ 295 7085 ^ 298 7034 ^ 298 7103 ^ 303 6036 ^ 287 8498 ^ 318 8662 ^ 328 8208 ^ 329 7913 ^ 313 7384 ^ 302 7035 ^ 293 6769 ^ 292 7781 ^ 320 7977 ^ 324 8208 ^ 309 7682 ^ 317 7541 ^ 317 7459 ^ 297 7017 ^ 293

2164 ^ 73 2278 ^ 73 2291 ^ 74 2289 ^ 73 2273 ^ 73 2247 ^ 68 2344 ^ 71 2347 ^ 73 2361 ^ 86 2345 ^ 73 1910 ^ 71 2360 ^ 71 1826 ^ 63 2310 ^ 69 2405 ^ 71 2403 ^ 72 2409 ^ 73 2299 ^ 71 2096 ^ 70 2231 ^ 68 2229 ^ 75 2274 ^ 79 2295 ^ 76 2282 ^ 85 2209 ^ 79 2025 ^ 72 1953 ^ 71 2152 ^ 70 1968 ^ 70 1917 ^ 71 1921 ^ 69 1958 ^ 70 1972 ^ 71 1879 ^ 77 2289 ^ 68 2393 ^ 69 2301 ^ 71 2192 ^ 70 2036 ^ 69 1948 ^ 69 1898 ^ 69 2345 ^ 75 2381 ^ 75 2265 ^ 69 2347 ^ 77 2270 ^ 75 2093 ^ 70 1950 ^ 69

2-F1 2-F2 2-F3 2-F4 2-F5 2-F6 2-F7 2-F8 2-F9 2-F10 2-F11 2-F12 2-F13 2-F14 2-F15 2-F16 2-F17 2-F18 2-F19 2-F20 2-F21 2-F22 2-F23 2-F24 2-F25 2-F26 2-F27 2-F28 2-F29 2-F30 2-F31 2-F32 2-F33 2-F34 2-F35 2-F36 2-F37 2-F38 2-F39 2-F40 2-F41

2198 ^ 785 1094 ^ 1431 1242 ^ 1279 1676 ^ 1017 2722 ^ 629 2945 ^ 397 2613 ^ 500 1503 ^ 992 3067 ^ 379 5932 ^ 248 6664 ^ 295 5185 ^ 254 5901 ^ 239 5882 ^ 238 6190 ^ 324 6329 ^ 229 5250 ^ 265 4424 ^ 300 6068 ^ 237 6329 ^ 297 25786 ^ 194 10192 ^ 196 4067 ^ 297 4466 ^ 278 6001 ^ 238 5691 ^ 300 12749 ^ 184 8997 ^ 202 2122 ^ 773 5513 ^ 247 7177 ^ 233 6058 ^ 270 12010 ^ 183 3087 ^ 376 2424 ^ 695 5211 ^ 258 6098 ^ 252 12015 ^ 225 7661 ^ 135 2786 ^ 428 2299 ^ 596

73450 ^ 605 83820 ^ 655 84240 ^ 655 81480 ^ 645 70690 ^ 594 43240 ^ 457 51280 ^ 498 74440 ^ 612 42920 ^ 458 38040 ^ 433 45350 ^ 471 37450 ^ 429 38210 ^ 438 38060 ^ 434 46240 ^ 475 38750 ^ 437 38480 ^ 438 37740 ^ 433 38830 ^ 441 44390 ^ 464 41190 ^ 458 40080 ^ 442 40980 ^ 447 35300 ^ 419 38270 ^ 436 40180 ^ 443 40490 ^ 447 37200 ^ 430 91960 ^ 695 37950 ^ 433 40870 ^ 449 38910 ^ 431 38500 ^ 438 42830 ^ 456 74290 ^ 610 38290 ^ 435 40680 ^ 447 43270 ^ 460 18980 ^ 327 43120 ^ 458 56040 ^ 521

7457 ^ 289 8119 ^ 295 7959 ^ 292 7818 ^ 293 7147 ^ 288 4838 ^ 264 5498 ^ 292 6979 ^ 300 4768 ^ 286 5625 ^ 291 7357 ^ 293 5096 ^ 289 5365 ^ 294 5345 ^ 290 7517 ^ 292 5525 ^ 292 5336 ^ 295 5236 ^ 303 5435 ^ 294 7107 ^ 292 14371 ^ 322 7104 ^ 284 4807 ^ 292 4876 ^ 295 5465 ^ 295 6607 ^ 274 8094 ^ 301 6563 ^ 297 7258 ^ 304 5245 ^ 288 6234 ^ 297 6338 ^ 276 7583 ^ 297 4768 ^ 289 7148 ^ 297 5246 ^ 297 5834 ^ 302 9396 ^ 306 4027 ^ 179 4948 ^ 300 5838 ^ 297

1960 ^ 72 1981 ^ 68 1924 ^ 67 1915 ^ 68 1901 ^ 72 1917 ^ 97 1934 ^ 96 1876 ^ 76 1887 ^ 104 1997 ^ 92 2207 ^ 79 1926 ^ 98 1914 ^ 95 1914 ^ 93 2275 ^ 80 1905 ^ 90 1968 ^ 98 2056 ^ 108 1902 ^ 92 2202 ^ 81 2182 ^ 41 1949 ^ 68 1847 ^ 102 2005 ^ 109 1934 ^ 93 2258 ^ 84 1966 ^ 63 1982 ^ 78 1580 ^ 62 1925 ^ 95 1972 ^ 83 2174 ^ 85 1950 ^ 66 1888 ^ 105 1848 ^ 72 1948 ^ 99 1969 ^ 91 2229 ^ 63 1833 ^ 71 1981 ^ 111 1940 ^ 92

(1 –2 mm), strongly exsolved porphyroblasts of plagioclase, pyroxene, and garnet. Denoting M2, the rest of the rock was recrystallized to a granoblastic aggregate by a strong, syn-to posttectonic event. The M2 assemblage of orthopyroxene þ clinopyroxene þ plagioclase þ garnet þ hornblende þ biotite þ ilmenite þ magnetite þ quartz (þ scapolite) experienced metamorphic conditions of 800 – 850 8C and 9.4 kbar (Hartmann, 1991). A subsequent, strong thermal metamorphism (M3) in the hornblende hornfels facies,

caused by intrusive granites, recrystallized the orthopyroxene to talc or hornblende/actinolite/cummingtonite in randomly oriented aggregates. Finally, epidote and chlorite generation occurred along fractures related to an M4 event (Hartmann, 1991). Previous geochronological data include a whole-rock Pb – Pb isochron that dates a collisional event in the TransAmazonian cycle (Soliani, 1986) and a Sm –Nd mineral isochron of 2.1 Ga for the M2 event (Hartmann, 1987). Zircon U – Pb SHRIMP isotopic data indicate an Archean age (ca. 2.55 Ga) for the trondhjemite protolith, a Paleoproterozoic age (ca. 2.02 Ga) for the M2 event, and a Neoproterozoic age (ca. 900 Ma) for the amphibolite facies retrogression (Hartmann et al., 1999). However, cores of

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703

Fig. 3. Results from the Santa Maria Chico granulitic complex. (a) Pb/Thp plot for monazites located in the matrix and garnet rim, one of which appears in Fig. 2; analytical uncertainties are plotted as error bars. (b) Histogram of all individual ages ðn ¼ 129Þ: (c,d) Age distribution observed in grains D (c) and F (d).

zircon crystals from an amphibolite facies granodiorite from the granulitic complex yield a protolith age of 2.35 Ga (U –Pb SHRIMP; Hartmann, unpublished data), and rims show similar ages as the granulite facies trondhjemite (2.02 Ga). Varied protolith ages probably should be expected in the complex terrain. Two samples of a felsic garnet-bearing granulite (308500 S –548180 W) were selected for U – Th –Pb dating. They are probably meta-graywackes and composed of 5 –10 mm porphyroblasts of garnet with albite, biotite, and quartz, as well as minor K-feldspar, rutile, magnetite, zircon, and monazite. This mineral assemblage corresponds to the M2 metamorphic event; no textural remnants of the M1 event were found in the assemblage of major minerals. The Cambaizinho Formation, located in the western portion of the Sa˜o Gabriel block (Fig. 1) as part of the Palma Group (Hartmann et al., 2000), consists of a supracrustal association of metasedimentary rocks with intercalations of mafic-ultramafic rocks and is intruded by syn-to posttectonic granitoids dated at ca. 700 Ma by Babinski et al. (1997) and Remus et al. (2001). The main deformational event (D2) under amphibolite facies conditions (M2) developed a pervasive, NE-trending, subvertical schistosity (Remus, 1990), which folded an earlier, flat-lying S1 schistosity (Chemale et al., 1995).

The metasedimentary rocks of the Cambaizinho Formation are mainly composed of biotite-bearing gneisses, quartz – feldspar schists, and mica schists, with minor amphibolite and quartzite. The diagnostic assemblages in metapelites are biotite þ muscovite þ staurolite þ garnet þ plagioclase þ quartz and biotite þ muscovite þ staurolite þ quartz þ grafite. Mineral assemblages in calcsilicate rocks are hornblende þ andesine þ sphene þ opaques and hornblende þ andesine þ diopside þ garnet þ epidote (Remus, 1990). A staurolite-bearing metapelite sampled in Sanga do Jobim (30818 0 S – 548010 W) has a mineral assemblage characteristic of the low amphibolite facies. The Passo Feio complex is usually included in the Vacacaı´ complex of the Sa˜o Gabriel block (Fig. 1) (Chemale et al., 1995). It is composed of a sequence of metasedimentary and metavolcanic rocks that surround the Cac¸apava granite (ca. 560 Ma; Bitencourt, 1983; Remus et al., 2000) and consists mainly of phyllite, pelitic schist, amphibolite, amphibole-bearing schist, and metavolcanoclastic rock, with minor metaarkose, quartzite, metagreywake, marble, Mg-rich schist, and metarrhyolite. The complex also records an early thrust-related deformational event associated with a greenschist to amphibolite facies metamorphic event (M1), which is overprinted by a retrometamorphic greenschist

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Fig. 4. Back-scattered electron images. (a,b) Sample 3 of Cambaizinho Formation. (c,d) Sample 5 of Passo Feio complex. (e,f) Sample 9 of Va´rzea do Capivarita complex.

facies event (M2) related to strike-slip tectonics (Bitencourt, 1983). Five samples of staurolite –biotite schist from the Passo Feio region (308370 S– 538240 W) at the southeastern border of the Cac¸apava granite also were selected. The schist is mainly composed of biotite, quartz, garnet, and staurolite, with minor zircon, monazite, and opaque minerals. The Va´rzea do Capivarita complex occurs as roof pendants in granitoids of the Encruzilhada do Sul suite (Fernandes et al., 1992; Vasquez, 1997) and the Cordilheira intrusive suite (Fragoso Cesar et al., 1986; Nardi and Frantz, 1995) of the Dom Feliciano belt (Fig. 1). It is composed of medium-to high-grade metamorphic rocks, mainly pelitic

gneisses, quartz-feldspar gneisses, quartzites, marbles, and calc-silicate gneisses. The complex records a first deformational event that generates a main foliation (N808– 858E, 108–308NW or SE) associated with a metamorphic event of upper amphibolite facies and low to intermediate pressure. A second deformational event consists of NE-trending, subvertical shear zones (Fernandes et al., 1992). The assemblages in pelitic gneisses are andaluzite þ sillimanite þ biotite þ plagioclase þ K-feldspar þ quartz and cordierite þ sillimanite þ garnet þ biotite þ Kfeldspar þ quartz þ spinel. A biotite-bearing pelitic gneiss from a quarry in Encruzilhada do Sul (308190 S– 528190 W) was chosen to represent the Va´rzea do Capivarita complex.

H. Tickyj et al. / Journal of South American Earth Sciences 16 (2004) 699–713 Table 5 Analytical results from Cambaizinho complex

3-1 3-2 3-3 3-4 3-5 3-61 3-62 3-7 3-8 3-9 3-10 3-11 3-121 3-122 3-123 3-13 3-141 3-142 3-15 3-16 3-171 3-172 3-173 3-18 3-19 3-20 3-21 3-22 3-23 3-24 3-25 3-261 3-262 3-27 3-281 3-282 3-291 3-292 3-30 3-31 3-32 3-331 3-332 3-333

705

Table 6 Analytical results from Passo Feio complex

U (ppm)

Th (ppm)

Pb (ppm)

Age (Ma)

2449 ^ 237 2423 ^ 235 2676 ^ 238 2402 ^ 235 2289 ^ 239 2202 ^ 240 2349 ^ 236 2013 ^ 235 3784 ^ 248 2322 ^ 238 1969 ^ 233 2137 ^ 238 2228 ^ 237 3100 ^ 243 2222 ^ 239 2986 ^ 244 2687 ^ 242 2992 ^ 240 2527 ^ 242 3103 ^ 244 1947 ^ 237 1813 ^ 235 1984 ^ 238 1733 ^ 236 2308 ^ 239 2197 ^ 239 2066 ^ 239 3440 ^ 224 5999 ^ 260 3646 ^ 247 1915 ^ 237 1605 ^ 234 2132 ^ 241 2371 ^ 237 2778 ^ 246 1939 ^ 236 2367 ^ 237 1797 ^ 227 2100 ^ 241 2192 ^ 240 1861 ^ 233 2505 ^ 238 2501 ^ 240 1911 ^ 236

15990 ^ 320 25214 ^ 362 32198 ^ 394 27715 ^ 374 22961 ^ 355 13841 ^ 312 14389 ^ 310 16668 ^ 326 29201 ^ 383 23980 ^ 357 27631 ^ 375 15441 ^ 320 18595 ^ 336 18108 ^ 334 16308 ^ 323 19779 ^ 335 24828 ^ 360 19227 ^ 334 22796 ^ 351 20721 ^ 339 19652 ^ 338 17737 ^ 327 17690 ^ 328 17129 ^ 325 24882 ^ 361 16086 ^ 320 19929 ^ 338 13692 ^ 274 33101 ^ 397 23136 ^ 352 23725 ^ 356 16330 ^ 322 18748 ^ 336 18649 ^ 329 21040 ^ 346 17814 ^ 326 20208 ^ 339 33193 ^ 396 18533 ^ 34 16897 ^ 323 31765 ^ 392 25213 ^ 360 25644 ^ 365 18772 ^ 331

715 ^ 241 908 ^ 252 1072 ^ 260 874 ^ 249 706 ^ 248 329 ^ 249 681 ^ 247 656 ^ 248 1290 ^ 257 895 ^ 256 924 ^ 259 471 ^ 248 513 ^ 252 673 ^ 255 570 ^ 256 437 ^ 238 788 ^ 253 729 ^ 255 651 ^ 253 823 ^ 259 692 ^ 252 475 ^ 248 574 ^ 250 467 ^ 249 841 ^ 255 451 ^ 247 653 ^ 250 647 ^ 206 1409 ^ 266 893 ^ 255 788 ^ 254 518 ^ 245 738 ^ 253 511 ^ 246 674 ^ 248 556 ^ 242 639 ^ 247 872 ^ 255 601 ^ 246 778 ^ 255 867 ^ 254 833 ^ 254 811 ^ 256 489 ^ 251

659 ^ 190 608 ^ 143 581 ^ 118 546 ^ 134 516 ^ 157 351 ^ 227 681 ^ 208 625 ^ 200 685 ^ 115 628 ^ 150 602 ^ 141 469 ^ 210 443 ^ 184 530 ^ 167 537 ^ 200 332 ^ 160 521 ^ 143 558 ^ 164 468 ^ 158 591 ^ 152 590 ^ 182 448 ^ 199 528 ^ 196 458 ^ 207 576 ^ 147 433 ^ 205 544 ^ 178 575 ^ 159 592 ^ 92 566 ^ 134 584 ^ 158 534 ^ 218 635 ^ 184 433 ^ 179 499 ^ 160 512 ^ 195 509 ^ 169 497 ^ 124 527 ^ 186 713 ^ 193 510 ^ 128 554 ^ 142 533 ^ 142 437 ^ 191

The Treˆs Figueiras granite (328130 S – 538020 W) occurs in the southern portion of the Dom Feliciano belt and, on the basis of its mineralogy, structure, and geochemistry, can be correlated with the Cordilheira granitic suite (Philipp, 1998; Philipp et al., 2003). It is a NE-elongated, syntectonic body emplaced into the Arroio Grande shear zone with a mainly syenogranitic composition with minor monzogranite. The syenogranites are made up of quartz, K-feldspar, and muscovite as major minerals, with plagioclase, biotite, garnet, and tourmaline as accessory minerals. The Treˆs Figueiras granite also has a tectonic foliation parallel to the shear zone and shows a protomylonitic texture in most of the body, with a mylonitic texture in a minor portion.

4-1 4-21 5-11 5-12 5-14 5-15 5-31 5-32 5-33 5-34 5-35 5-41 5-43 5-45 5-62 5-63 5-64 5-65 5-73 5-81 5-82 5-91 5-92 5-93 5-94 5-95 5-101 6-21 6-22 6-31 6-41 6-42 6-71 6-81 7-21 7-51 7-61 7-71 7-91 7-101 8-71 8-81

U (ppm)

Th (ppm)

Pb (ppm)

Age (Ma)

3185 ^ 417 2911 ^ 411 2293 ^ 410 2583 ^ 409 2098 ^ 408 3248 ^ 419 4478 ^ 434 6313 ^ 451 3840 ^ 431 4515 ^ 435 4464 ^ 437 2572 ^ 411 2566 ^ 405 2403 ^ 411 2412 ^ 408 2653 ^ 411 4058 ^ 432 4059 ^ 400 2212 ^ 399 2590 ^ 409 2928 ^ 409 2259 ^ 399 3395 ^ 420 2176 ^ 401 3569 ^ 420 2271 ^ 398 2383 ^ 406 2222 ^ 401 2003 ^ 400 1524 ^ 381 1914 ^ 391 5205 ^ 441 2215 ^ 401 2186 ^ 405 3442 ^ 436 1831 ^ 369 4466 ^ 443 1869 ^ 377 1964 ^ 401 3928 ^ 436 786 ^ 354 537 ^ 325

14424 ^ 524 14470 ^ 524 33500 ^ 665 33490 ^ 663 27338 ^ 624 40785 ^ 710 48073 ^ 758 37514 ^ 694 52702 ^ 788 39504 ^ 703 49422 ^ 764 38938 ^ 695 41840 ^ 715 31452 ^ 652 32552 ^ 682 36338 ^ 680 51188 ^ 771 50678 ^ 769 24474 ^ 596 32453 ^ 654 35287 ^ 665 28641 ^ 626 40871 ^ 712 28597 ^ 627 42089 ^ 720 31830 ^ 649 35140 ^ 677 49489 ^ 769 47589 ^ 755 47819 ^ 761 53304 ^ 790 36696 ^ 694 65867 ^ 874 34603 ^ 672 43092 ^ 726 141995 ^ 1272 27169 ^ 616 120665 ^ 1166 44710 ^ 735 15514 ^ 531 30067 ^ 641 30435 ^ 642

440 ^ 406 513 ^ 400 654 ^ 407 784 ^ 379 672 ^ 377 850 ^ 405 846 ^ 389 946 ^ 386 1412 ^ 388 999 ^ 381 933 ^ 389 710 ^ 408 808 ^ 383 558 ^ 404 601 ^ 403 883 ^ 401 1017 ^ 410 1430 ^ 413 602 ^ 394 737 ^ 403 662 ^ 378 557 ^ 401 1071 ^ 412 709 ^ 394 819 ^ 406 549 ^ 399 993 ^ 377 1093 ^ 430 996 ^ 436 1087 ^ 430 1101 ^ 435 843 ^ 425 1632 ^ 442 622 ^ 422 1022 ^ 425 3160 ^ 480 627 ^ 399 2855 ^ 467 894 ^ 415 539 ^ 390 593 ^ 414 568 ^ 405

397 ^ 331 477 ^ 341 357 ^ 208 418 ^ 201 439 ^ 245 370 ^ 165 303 ^ 139 365 ^ 147 482 ^ 131 412 ^ 156 327 ^ 136 336 ^ 181 360 ^ 170 318 ^ 215 333 ^ 208 438 ^ 185 353 ^ 133 498 ^ 134 424 ^ 259 403 ^ 204 331 ^ 188 346 ^ 232 460 ^ 165 443 ^ 229 341 ^ 158 314 ^ 212 515 ^ 194 430 ^ 152 411 ^ 162 459 ^ 163 413 ^ 147 352 ^ 160 497 ^ 121 334 ^ 204 420 ^ 154 476 ^ 68 337 ^ 199 501 ^ 77 391 ^ 168 425 ^ 282 406 ^ 256 394 ^ 260

The protomylonitic texture is defined by elongate to equidimensional porphyroclasts of muscovite and K-feldspar (0.8 – 3.0 mm) wrapped in an equigranular matrix of feldspars, muscovite, quartz, garnet, and tourmaline. The Camboriu´ complex (Fig. 1), part of the northern portion of the Dom Feliciano belt (Chemale et al., 1995), is composed of an association of fine-banded ortho-and paraderived rocks that exhibit high-grade metamorphism, including an event of partial melting. The migmatitic processes are characterized by several thin granitic injections. The Itapema (ca. 2.0 Ga, zircon U – Pb SHRIMP; Hartmann et al., 2003) and Serra dos Macacos granites intrude it. Chemale et al. (1995) describe the Camboriu´ complex as an association of gneisses, migmatites,

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Table 7 Analytical results from Va´rzea do Capivarita complex

9-41 9-42 9-43 9-44 9-51 9-52 9-53 9-54 9-55 9-56 9-61 9-62 9-63 9-64 9-65 9-71 9-72 9-73 9-74 9-91 9-92 9-93 9-94 9-95 9-96 9-121 9-122 9-123 9-124 9-125 9-141 9-142 9-143 9-144 9-145 9-146 9-147 9-148

U (ppm)

Th (ppm)

Pb (ppm)

Age (Ma)

4256 ^ 247 2712 ^ 238 2137 ^ 232 4515 ^ 247 3726 ^ 245 3553 ^ 242 4730 ^ 249 3838 ^ 243 3351 ^ 241 3140 ^ 239 3003 ^ 239 3556 ^ 240 4018 ^ 246 4232 ^ 249 3686 ^ 244 3211 ^ 241 3227 ^ 241 2423 ^ 235 3808 ^ 246 4158 ^ 248 2207 ^ 231 3730 ^ 244 4556 ^ 250 3879 ^ 246 3342 ^ 243 3537 ^ 245 2420 ^ 234 2356 ^ 233 2031 ^ 230 2358 ^ 231 5589 ^ 255 3443 ^ 242 7517 ^ 267 2284 ^ 234 2949 ^ 237 4269 ^ 248 4438 ^ 257 1502 ^ 214

33821 ^ 410 26824 ^ 376 31318 ^ 400 31883 ^ 400 31494 ^ 400 33612 ^ 408 33245 ^ 408 31941 ^ 401 30193 ^ 395 31635 ^ 400 35730 ^ 420 35438 ^ 418 30817 ^ 397 50832 ^ 497 34852 ^ 416 36311 ^ 425 35207 ^ 419 37067 ^ 427 33683 ^ 413 33009 ^ 408 32292 ^ 404 33693 ^ 412 35970 ^ 423 34372 ^ 414 27257 ^ 382 27602 ^ 382 34238 ^ 414 34429 ^ 414 35896 ^ 422 41907 ^ 452 31370 ^ 399 34938 ^ 417 32648 ^ 406 31335 ^ 400 41870 ^ 452 32768 ^ 408 79920 ^ 638 57680 ^ 530

1076 ^ 235 519 ^ 233 774 ^ 252 830 ^ 234 755 ^ 219 916 ^ 235 856 ^ 240 1043 ^ 234 731 ^ 244 965 ^ 237 951 ^ 236 942 ^ 234 922 ^ 260 1569 ^ 228 974 ^ 236 859 ^ 239 893 ^ 238 756 ^ 238 872 ^ 238 819 ^ 222 836 ^ 275 858 ^ 239 1075 ^ 246 827 ^ 235 607 ^ 218 688 ^ 236 977 ^ 247 953 ^ 253 759 ^ 236 859 ^ 240 1044 ^ 263 840 ^ 237 1249 ^ 224 713 ^ 255 994 ^ 225 943 ^ 263 2081 ^ 260 1055 ^ 249

502 ^ 100 326 ^ 135 451 ^ 126 398 ^ 104 387 ^ 111 452 ^ 107 393 ^ 100 522 ^ 107 398 ^ 118 513 ^ 114 466 ^ 106 447 ^ 102 468 ^ 110 539 ^ 77 463 ^ 103 410 ^ 106 436 ^ 107 376 ^ 110 422 ^ 106 393 ^ 105 472 ^ 122 418 ^ 107 471 ^ 96 393 ^ 103 356 ^ 127 393 ^ 124 516 ^ 114 504 ^ 115 399 ^ 114 387 ^ 100 469 ^ 97 407 ^ 106 487 ^ 85 411 ^ 125 431 ^ 96 451 ^ 104 491 ^ 57 377 ^ 82

and high-K calc-alkaline granites that experienced its main deformation during the Paleoproterozoic and younger transcurrent tectonics (Hartmann et al., 2000). According to Philipp (2001), the complex is composed of granodioritic to tonalitic gneisses, with tabular intercalations of amphibolites, calc-silicate gneisses, pelitic gneisses, and granitic rocks, and its deformation began with low-angle ductile tectonics in high-grade metamorphic conditions, contemporaneous with the generation of migmatitic bodies. The complex later was affected by transcurrent tectonics during the refolding of previous structures or the development of mylonitic rocks in high-strain shear zones. Three samples of pelitic gneiss (278020 S – 488370 W) from the Camboriu´ complex were selected for this study. The gneiss has a paleosome of garnet þ sillimanite þ biotite and a finebanded structure that consists of regular alternations of biotite þ sillimanite þ garnet with feldspars and quartz. The pelitic gneiss occurs as small bodies with gradational

boundaries in rocks of monzogranitic to tonalitic composition. The Brusque complex is a rift sequence that contains mostly clastic metasedimentary rocks, with minor intercalations of mafic and felsic metavolcanic rocks. The metasediments are characterized by a thick sequence of immature clastic rocks, such as turbidites, K-graywackes, and tholeiitic volcaniclastic sediments, as well as basic to ultrabasic subaqueous flows of alkaline to subalkaline composition. This unit shows early thrust tectonics, which created the regional flat-lying surface, superimposed at varying degrees by the N458E-trending strike-slip shear zones. The complex is intruded by the Neoproterozoic Compra Tudo, Valsungana, and Serra dos Macacos granites. It also has several 1 –10 m syntectonic, granitic sheets (UFRGS, 2000; Philipp et al., 2001). One such sheet, sampled near the Plaza Itapema Hotel (278040 S– 488360 W), is a 20-cm thick, foliated, fine-grained leucogranite composed of porphyroclasts of quartz, K-feldspar, and muscovite in a matrix of quartz, feldspars, muscovite, and biotite. The sheet was injected during the final stages of the deformation that caused the flat-lying structure in the surrounding Brusque group schists. Therefore, its magmatic age must be close to that of the flat-lying structure in the schists.

3. Analytical procedures Monazite was identified using both optical and electron microscopes, the latter coupled with an energy dispersive system (EDS). The internal structure of each monazite grain was observed using a scanning electron microscope in the BSE mode at Centro de Microscopia Eletroˆnica, Universidade Federal do Rio Grande do Sul. The chemical analyses of the monazite crystals were performed with a Cameca SX 50 electron microprobe, equipped with an EDS and four wavelength dispersive spectrometers, installed at Centro de Estudos em Petrologia e Geoquı´mica, Instituto de Geocieˆncias, Universidade Federal do Rio Grande do Sul. Quantitative analyses of monazite on polished thin sections were carried out in two steps. First, elements identified in the monazite crystal were analyzed using the following instrumental conditions: accelerating voltage ¼ 15 keV, beam current ¼ 50 nA, beam size ¼ 1 mm, and acquisition times projected to quantify major and minor elements (25 –50 s at peak position). Spectral lines and elements were selected as follows: Ka for Si, Al, P, Ca, and Fe; La for Y, Ce, La, Ho, and Tm; Lb for Sm, Nd, Gd, Pr, and Dy; Ma for Th and Pb; and Mb for U. These results were used to evaluate the lateral homogeneity of major and minor elements. Second, the average value for each element was used as input information for the Cameca microprobe to quantify trace elements. The experimental conditions for

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Fig. 5. Back-scattered electron images. (a,b) Sample 10 of Treˆs Figueiras granite. (c,d) Sample 12 of Camboriu´ complex. (e,f) Sample 15 of Plaza Itapema granite.

the measurement of Th, U, Pb, and Y contents were accelerating voltage 15 keV, beam current 100 nA, and beam size 1 mm. The counting times varied: 300 s in samples 1– 3 and 9– 14 and 100 s in samples 4 –8 and 15 at peak position. The shorter counting time applies to measurements that require a greater number of points, such as line profiles and surface maps, though higher errors are expected, and the geological results indicated by these grains therefore are interpreted carefully. Matrix correction used a PAP model in all cases. Spectral interferences of Y on Pb Ma were corrected; a U correction was also made when high Th contents were measured (Scherrer et al., 2000).

The confidence interval (95%) for concentrations and detection limits was calculated following the procedure of Ancey et al. (1977). Standards included synthetic thorite (ThSiO4) for Th; synthetic oxide (UO2) for U; natural galena (PbS) for Pb; anorthite glass for Si, Al, and Ca; synthetic phosphate Ca5(PO4)3(F,OH) for P; natural olivine for Fe; and synthetic glasses—REE1 for Gd and Tm; REE2 for Sm and Nd; REE3 for Y, Ce, La, and Pr; and REE4 for Ho and Dy. To calculate ages, we follow the procedure proposed by Suzuki and Adachi (1991) and Montel et al. (1996). Nonradiogenic Pb is considered negligible; an age is calculated for each analyzed spot with the following

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Eq. (2) can be rewritten as follows:

Table 8 Analytical results from Treˆs Figueiras granite

10-71 10-72 10-73 10-74 10-75 10-31 10-32 10-33 10-41 10-42 10-51 10-52 10-53 11-51 11-52 11-53 11-54 11-55 11-11 11-12 11-71 11-72 11-73 11-74 11-75 11-61 11-62 11-63 11-64 11-65

t ¼ ð1=l232 Þlnð1 þ aðA232 =A208 ÞÞ;

U (ppm)

Th (ppm)

Pb (ppm)

Age (Ma)

4877 ^ 257 1607 ^ 224 1489 ^ 224 175 ^ 125 2933 ^ 241 7239 ^ 272 2651 ^ 240 913 ^ 206 4192 ^ 252 721 ^ 190 1950 ^ 231 1321 ^ 217 2390 ^ 235 3751 ^ 252 4442 ^ 260 4445 ^ 258 2068 ^ 231 3281 ^ 247 2613 ^ 240 3076 ^ 248 1927 ^ 226 1285 ^ 209 777 ^ 189 1960 ^ 223 1903 ^ 223 1586 ^ 218 1463 ^ 214 1598 ^ 216 2310 ^ 231 1808 ^ 226

39471 ^ 429 36938 ^ 416 38847 ^ 425 31453 ^ 390 51508 ^ 476 53648 ^ 489 43848 ^ 445 31831 ^ 391 40165 ^ 431 41856 ^ 432 39855 ^ 429 37245 ^ 413 49022 ^ 466 76159 ^ 580 75573 ^ 579 79273 ^ 593 75136 ^ 578 89018 ^ 628 83390 ^ 608 68644 ^ 554 78014 ^ 588 56038 ^ 494 52861 ^ 481 66784 ^ 535 60870 ^ 512 61457 ^ 514 58564 ^ 504 73770 ^ 568 54885 ^ 488 61510 ^ 521

1025 ^ 266 765 ^ 250 876 ^ 255 529 ^ 240 1302 ^ 273 1452 ^ 277 1040 ^ 256 566 ^ 239 1105 ^ 267 756 ^ 243 729 ^ 254 737 ^ 246 1104 ^ 256 1816 ^ 293 1921 ^ 296 2212 ^ 312 1764 ^ 291 2280 ^ 301 2104 ^ 295 1819 ^ 300 1659 ^ 280 944 ^ 256 1200 ^ 249 1402 ^ 274 1474 ^ 265 1288 ^ 266 1312 ^ 260 1613 ^ 268 1240 ^ 273 1451 ^ 267

414 ^ 91 405 ^ 116 447 ^ 113 369 ^ 151 475 ^ 82 420 ^ 67 442 ^ 95 364 ^ 139 458 ^ 92 382 ^ 111 353 ^ 108 396 ^ 119 434 ^ 89 458 ^ 60 475 ^ 59 525 ^ 57 480 ^ 65 509 ^ 54 510 ^ 58 514 ^ 67 439 ^ 64 351 ^ 85 483 ^ 90 428 ^ 70 490 ^ 75 431 ^ 76 462 ^ 80 455 ^ 67 443 ^ 81 480 ^ 77

formula (Williams et al., 1999): Pb ¼ThðA208 =A232 Þ½el232t 2 1 þ UðA206 =A238 Þ0:9928  ½el238t 2 1 þ UðA207 =A235 Þ0:0072½el235t 2 1; ð1Þ where Pb, Th, and U are concentrations in ppm; A208, A232, A206, A207, and A205 are the atomic masses of 208Pb, 232Th, 206 U, 238U, 207Pb, and 235U, respectively; and l is a decay constant for each isotope (l232 ¼ 4:9475 £ 10211 =yr; l238 ¼ 1:55125 £ 10210 =yr; l235 ¼ 9:8485 £ 10210 =yrÞ: After the age is determined for each analytical point, the U concentration is converted into equivalent Th content (i.e. the amount of Th necessary to produce the same amount of Pb). This value is added to the measured values of Th, which results in an apparent Thp amount, and the data are plotted in a Pb versus Thp diagram. When the analyzed spots are coeval and remain in a closed system, they plot along a straight line. We use the Isoplot/EX program (Version 2.49, Ludwig, 2001) to determine the slope of a best-fit regression line and the associated error. We calculate a first approximation for age ðtÞ using the general equation for radioactive decay: Pb=A208 ¼ Th=A232 ðelt 2 1Þ:

ð2Þ

ð3Þ

where a is the ratio between Pb and Th. The value of ðtÞ calculated using Eq. 3 provides a new set of Thp values, and a second approximation is calculated for the age. This procedure is repeated until ðtÞ remains unchanged. The associated error at the 95% level is calculated by error propagation, taking into account the analytical uncertainties of the Th, U, and Pb measurements. In the case of surface mapping (grains D and F, Santa Maria Chico granulitic complex), we use only Eq. (1) to calculate the ages.

4. Results 4.1. Santa Maria Chico granulitic complex We distinguish between two populations of monazite. The main population consists of rounded grains (50 – 120 mm) included in garnet. Those situated in the rims of garnet (Fig. 2) show marked oscillatory or normal zoning, whereas those located in the core are homogeneous or display weak zoning. For an example of garnet matrix and monazite location see Fig. 2. A few interstitial, irregular, rounded grains (, 40 mm) also were found in the matrix. A total of 129 analyses was made in six grains (Tables 1– 4). Three grains situated in the rim of garnet and one in the matrix were selected for a Thp – Pb isochron diagram, with 10 points analyzed for each grain. An age of 1899 ^ 43 Ma (MSWD ¼ 1.2) was obtained using an isochron diagram (Fig. 3a) in the monazite grains situated in garnet rims and matrix. When all results are plotted on a bar graph with frequency of occurrence for all ages measured, we observe a bimodal distribution. One maximum is centered near 2300 Ma and the other near 1950 Ma (Fig. 3b). Two grains from garnet cores were selected to evaluate the ages of the exposed internal portion of the grain, in view of the observed BSE zoning in these particular grains. Grain D has a small range of variation in its Th, U, and Pb concentrations (Table 3). Ages at the central part of the grain are 2.4– 2.2 Ga, whereas those at the rim appear near 2.0 Ga. The few ages in grain F indicate a range of 2.27– 2.17 Ga; the frequent rim ages point to the interval 2.00– 1.84 Ga. The contents of Th, U, and Pb vary widely (Table 4, Fig. 3c and d). 4.2. Cambaizinho formation Monazite is small (10 – 15 mm) and abundant, occurs in the matrix and as inclusions in feldspar and quartz, and displays a rounded, irregular shape without apparent chemical zonation (Fig. 5). The shapes of the grains and their locations in the matrix are shown in Fig. 4a and b.

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709

Fig. 6. Pb versus Thp plot for monazite from (a) Cambaizinho Formation, (b) Passo Feio complex, (c) Va´rzea do Capivarita complex, (d) Treˆs Figueiras granite, (e) Camboriu´ complex, and (f) Plaza Itapema granite. Analytical uncertainties are plotted as error bars.

According to more than 40 measurements in 33 grains, most grains are included in quartz (14) or situated in the matrix (11); five grains are included in plagioclase and three in K-feldspar (Tables 1 and 5). We obtain an isochron age of 643 ^ 129 Ma (MSWD ¼ 0.66) (Fig. 6a). 4.3. Passo Feio complex Monazite occurs as inclusions in biotite, staurolite, and garnet, as well as isolated in the matrix or in contact with apatite. Monazite crystals have variable sizes (20 – 100 mm) and mostly oval to rounded shapes (Fig. 4c and d). In Tables 1 and 6, we present the results of analyses performed on 26 grains. We obtain an age of 510 ^ 68 Ma (MSWD ¼ 0.48) for this unit (Fig. 6b).

4.4. Va´rzea do Capivarita complex Large (50 – 200 mm), oval-shaped monazite occurs as inclusions in garnet or discrete crystals in the matrix, with complex chemical zonation (Fig. 4e and f). The results of analyses of seven grains appear in Tables 1 and 7. An isochron fitting results in an age of 552 ^ 90 Ma (MSWD ¼ 0.77) (Fig. 6c). 4.5. Treˆs Figueiras granite Monazite of variable size (20 – 150 mm) occurs in the matrix and as inclusions in biotite, muscovite, and plagioclase. Both crystals and grains are subhedral to anhedral with oscillatory, magmatic zonation, and some

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Table 9 Analytical results from Camboriu´ complex

12-1A 12-1B 12-1C 12-1D 12-3A 12-3B 12-5A 12-5B 12-6A 12-6B 13-1A 13-1B 13-1C 13-2A 13-2B 13-2C 13-3A 13-4A 13-4B 13-4C 13-4D 13-6A 13-6B 13-6C 14-1A 14-1B 14-2A 14-2B 14-3A 14-4A 14-4B 14-5A 14-5B

Table 10 Analytical results from Plaza Itapema granite

U (ppm)

Th (ppm)

Pb (ppm)

Age (Ma)

1741 ^ 223 1581 ^ 219 1155 ^ 216 1251 ^ 221 2124 ^ 232 14361 ^ 301 5787 ^ 261 2445 ^ 237 893 ^ 201 2408 ^ 238 3953 ^ 252 4997 ^ 257 836 ^ 196 9942 ^ 278 4446 ^ 252 1297 ^ 211 3704 ^ 248 2137 ^ 232 3505 ^ 248 8628 ^ 276 8654 ^ 277 6699 ^ 269 6001 ^ 262 7272 ^ 269 6465 ^ 267 4546 ^ 255 3389 ^ 245 4258 ^ 255 2801 ^ 241 8092 ^ 275 7652 ^ 275 3034 ^ 243 3841 ^ 249

70000 ^ 546 65534 ^ 527 32591 ^ 390 26750 ^ 365 49377 ^ 364 45162 ^ 451 25340 ^ 363 34875 ^ 399 44035 ^ 440 54272 ^ 487 39948 ^ 427 44061 ^ 440 47896 ^ 456 33384 ^ 397 37504 ^ 413 50530 ^ 465 41959 ^ 432 55121 ^ 489 49160 ^ 464 43899 ^ 443 48348 ^ 460 43738 ^ 446 40833 ^ 429 55174 ^ 490 44403 ^ 445 47676 ^ 458 47679 ^ 457 38327 ^ 421 42599 ^ 436 47412 ^ 460 47890 ^ 461 42082 ^ 431 41653 ^ 430

1595 ^ 255 1333 ^ 254 709 ^ 237 607 ^ 233 1191 ^ 258 2163 ^ 281 1124 ^ 271 839 ^ 235 793 ^ 243 1379 ^ 288 1099 ^ 290 1230 ^ 257 897 ^ 242 1497 ^ 264 1097 ^ 246 1199 ^ 282 1086 ^ 262 1347 ^ 258 1403 ^ 257 1793 ^ 293 1676 ^ 303 1533 ^ 304 1406 ^ 292 1854 ^ 293 1306 ^ 288 1224 ^ 268 1227 ^ 248 915 ^ 254 1148 ^ 250 1717 ^ 295 1588 ^ 295 1086 ^ 243 1060 ^ 246

470 ^ 69 421 ^ 74 435 ^ 134 439 ^ 154 471 ^ 90 522 ^ 56 564 ^ 109 437 ^ 114 377 ^ 106 494 ^ 83 464 ^ 95 454 ^ 82 396 ^ 99 506 ^ 75 470 ^ 94 488 ^ 91 448 ^ 92 483 ^ 83 515 ^ 83 552 ^ 70 487 ^ 67 520 ^ 78 517 ^ 83 522 ^ 66 445 ^ 78 437 ^ 81 466 ^ 86 392 ^ 98 494 ^ 96 517 ^ 69 486 ^ 71 466 ^ 95 437 ^ 92

crystals show recrystallized rims due to a late shearing event (Fig. 5b –d). We present the analytical results from eight grains in Tables 1 and 8. The isochron method yields an age of 558 ^ 57 Ma (MSWD ¼ 0.57) (Fig. 6d).

15-A11 15-A12 15-A13 15-A14 15-A15 15-A21 15-A22 15-A23 15-A24 15-A25 15-A26 15-A27 15-A31 15-A32 15-A33 15-A34 15-A35 15-A41 15-A42 15-A43 15-B11 15-B13 15-B14 15-B15 15-B17 15-B18 15-B19 15-E11 15-E12 15-E15

U (ppm)

Th (ppm)

Pb (ppm)

Age (Ma)

1876 ^ 388 1964 ^ 391 1676 ^ 382 1427 ^ 385 1379 ^ 365 2808 ^ 407 3064 ^ 414 2418 ^ 401 1895 ^ 386 2294 ^ 400 1606 ^ 370 1918 ^ 377 1231 ^ 374 1684 ^ 386 2971 ^ 414 1399 ^ 359 1417 ^ 358 1471 ^ 361 787 ^ 284 1129 ^ 339 11007 ^ 531 – 3870 ^ 423 2498 ^ 405 2858 ^ 410 9753 ^ 518 10698 ^ 531 991 ^ 353 666 ^ 313 527 ^ 304

50833 ^ 916 53618 ^ 951 51939 ^ 927 45540 ^ 869 61354 ^ 1022 63320 ^ 1043 67580 ^ 1090 62866 ^ 1040 69881 ^ 1115 75671 ^ 1174 87345 ^ 1295 85925 ^ 1275 38580 ^ 797 47942 ^ 891 58694 ^ 1003 79383 ^ 1212 81369 ^ 1231 88428 ^ 1308 108963 ^ 1515 48850 ^ 876 127744 ^ 1707 62137 ^ 1030 16725 ^ 568 26664 ^ 671 29858 ^ 699 131327 ^ 1744 124692 ^ 1676 48298 ^ 893 53956 ^ 950 42294 ^ 831

1236 ^ 417 1387 ^ 422 1243 ^ 420 1240 ^ 426 1805 ^ 424 1739 ^ 428 1562 ^ 436 1773 ^ 432 1659 ^ 433 1746 ^ 440 2012 ^ 449 2358 ^ 445 846 ^ 411 1004 ^ 418 1612 ^ 422 1846 ^ 440 2249 ^ 443 2516 ^ 446 2469 ^ 461 1355 ^ 378 3946 ^ 532 1477 ^ 396 446 ^ 394 777 ^ 398 813 ^ 405 3769 ^ 519 3484 ^ 523 948 ^ 419 1008 ^ 424 673 ^ 413

483 ^ 151 514 ^ 144 483 ^ 151 549 ^ 173 608 ^ 131 534 ^ 120 449 ^ 116 557 ^ 125 486 ^ 117 468 ^ 109 484 ^ 100 568 ^ 99 443 ^ 200 420 ^ 162 524 ^ 127 490 ^ 108 581 ^ 106 599 ^ 98 492 ^ 85 573 ^ 148 536 ^ 62 521 ^ 139 341 ^ 279 497 ^ 235 463 ^ 211 514 ^ 62 486 ^ 64 411 ^ 167 401 ^ 156 342 ^ 193

545 ^ 55 Ma (MSWD ¼ 0.68) is obtained from all measurements (Fig. 6f). Table 11 shows the Th, U, and Pb concentration range for all units, as well as the values of age and errors obtained by using a nonforced isochron fit to zero content of Pb and Thp. Ages obtained from zircon grains of the same rocks using other methods are also included.

4.6. Camboriu´ complex—Balnea´rio Camboriu´ xenolith Monazite grains are variable in size (20 –200 mm), with rounded to oval shapes and complex chemical zonation. The crystals occur mainly as inclusions in biotite, with a few in the matrix (Fig. 5b and c), and some zoning is observed (Fig. 5d). Of the 14 analyzed grains (Tables 1 and 9), half were included in biotite. The isochron method yields an age of 565 ^ 77 Ma (MSWD ¼ 0.63) (Fig. 6e). 4.7. Plaza Itapema granite Monazite crystals (20 – 200 mm) occur mainly in the matrix in a close relationship with apatite, though a few are included in muscovite, plagioclase, and quartz. They have an oval shape and complex chemical zonation (Fig. 5e and f). Six grains (one in plagioclase) were analyzed, adding to more than 30 analyses (Tables 1 and 10). An isochron age of

5. Discussion and conclusions With this study, we evaluate the results of chemical dating of monazite with the EPMA technique in igneous and metamorphic rocks from the southern Brazilian Shield. The experimental conditions produce a detection limit at 95% level ofp180 ffiffiffiffiffiffippm for U, Th, and Pb using the relation DL , 3 Cbkg ; where Cbkg is the number of counts at the background of elements obtained in the sample. This detection limit is used in some cases in the literature as the absolute error, but we use instead the procedure of Ancey et al. (1977), though it produces comparatively higher absolute errors, because we also take into account errors in the measurements of standards. We obtain the ages depicted in Table 1 from isochron fits without including the value of the origin of the Pb versus

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Table 11 Concentration ranges for Th, U, and Pb Sample

Th wt% range

U wt% range

Pb wt% range

Isochron ages (Ma)

Santa Maria Chico complex Grain D Grain F Cambaizinho formation Passo Feio complex Va´rzea do Capivarita complex Treˆs Figueiras granite Camboriu´ Complex Plaza Itapema complex

2.00–10.71 3.64–4.21 1.90–9.19 1.37–3.31 1.44–6.59 2.68–5.08 3.18–8.90 2.68–7.00 1.67–13.13

0.03– 0.30 0.75– 1.15 0.11– 2.58 0.16– 0.60 0.05– 0.63 0.20– 0.75 0.07– 0.72 0.08– 1.44 0.05– 1.10

0.22– 0.95 0.60– 0.87 0.40– 1.44 0.03– 0.14 0.04– 0.31 0.05– 0.16 0.05– 0.23 0.06– 0.22 0.04– 0.38

1899 ^ 43

Thp diagram. When the fit is forced to include this value, errors in the slope can be reduced to ^ 16 Ma, as in the case of Santa Maria Chico complex. However, for the Passo Feio, Va´rzea do Capivarita, Camboriu´, and Plaza Itapema complexes and the Treˆs Figueiras granite, the same procedure produces different results depending on the error assumed. Lower age values are obtained from isochrons gathered by forcing the data through the origin and using an error for this point similar to that calculated for Thp and Pb. If the error for this virtual point is stipulated at 180 ppm, even lower ages appear. In this case, some very low, geologically unrealistic ages are observed. However, the observed intercepts with forced or nonforced fittings are always at the limit of absolute errors calculated with the procedure we adopt. The smallest difference observed between ages obtained with forced and nonforced fittings is for the Plaza Itapema granite, where Thp and Pb concentrations extend to higher values. For the other samples, the variability of these elements is lower, which concentrates the data to a restricted

643 ^ 129 MSWD ¼ 0.66 510 ^ 68 MSWD ¼ 0.48 552 ^ 90 MSWD ¼ 0.77 558 ^ 57 MSWD ¼ 0.57 565 ^ 77 MSWD-0.63 545 ^ 55 MSWD ¼ 0.68

Y intercept

Ages-other methods 2022 ^ 18

2137 ^ 180 2248 ^ 170 2247 ^ 200 2314 ^ 170 2245 ^ 210 2137 ^ 190

661 ^ 29 562 ^ 8

range. Extreme examples are the Va´rzea do Capivarita complex and Cambaizinho Formation, for which the inclusion of the value of the origin of the graph in the fitting procedure induces greater changes in the slope compared with the other samples. As a consequence, the precision of our data was reduced by the low measured Pb concentration. Because the precision of the ages obtained by the Th – U-total Pb method depends mainly on these factors, the results for these samples must be considered an approximation. Because this technique yields either the age of magmatic crystallization or metamorphic (re)crystallization of monazite, the results are most significant regarding the geotectonic evolution of the shield. All the ages obtained in this study belong to the Trans-Amazonian and Brasiliano orogenic cycles (Fig. 7), in agreement with Hartmann et al’s. (2000) zircon SHRIMP U – Pb isotopic results. This finding is of major significance, because the cyclicity of the orogenic evolution of a shield can be unraveled to a large extent by chemical dating of monazite. Because monazite recrystallizes during metamorphism, it is likely that older

Fig. 7. Bimodal distribution of all spot ages obtained from southern Brazilian monazite crystals. Ages are mostly concentrated in the time range of the TransAmazonian and Brasiliano cycles. A previously unknown age is registered near 2.35 Ga.

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events may be concealed, but the register of age zoning of monazite may be detected in some cases, as in the granulite sample. In the Santa Maria Chico granulitic complex, the ages of two different geological events were obtained (Fig. 3). The first event at 2.35 Ga may be either metamorphic or magmatic and represents the first mention of the age of the M1 event in this complex. More work is required to confirm this age, but the SHRIMP results for zircon cores substantiate the EPMA results for monazite. For M2, an age of 1.9 Ga was obtained, in agreement with the age of zircon from the same region dated by U –Pb SHRIMP (Hartmann et al., 1999). The 643 ^ 129 Ma age obtained for the Cambaizinho Formation monazite is attributed to the second metamorphic event recorded in this unit. Taking into account the error of the age, our result is similar to previous data that estimates ages of 661 ^ 29 and 697 ^ 3 Ma (Remus, 1990; Remus et al., 2001). For the Passo Feio complex, we report an age of 510 ^ 68 Ma. We believe this result corresponds to the retrometamorphic M2 event, related to strike-slip tectonics, because most of the analyzed monazites are situated in the matrix of the studied rocks. This age is in agreement with an estimated age of 562 ^ 8 Ma (Remus et al., 2000) for the syntectonic injection of the Cac¸apava granite, presumably the event responsible for the formation of the dated monazite. We also report the first age determination (552 ^ 90 Ma) of metamorphism in the Va´rzea do Capivarita complex that corresponds to the main metamorphic event recorded in this unit. In the Treˆs Figueiras granite, the dated monazite shows partial recrystallization due to strike-slip deformation. The 558 ^ 57 Ma age thus must represent a mixture of the magmatic age of the body and the recrystallization effect of a shearing event. An age of 565 ^ 77 Ma was calculated for the main metamorphic event affecting the Camboriu´ complex. The dated samples belong to xenoliths in an undated granite, correlated with the Itapema granite (ca. 2.0 Ga). Additional data are required to elucidate this dating. Finally, the dated monazites of the Plaza Itapema granite yield an age of 545 ^ 55 Ma, which constrains the metamorphic evolution of the Brusque complex to the Brasiliano orogenic cycle. Both the dated granitic sample and the schist from the Brusque complex were affected by the same thrusting event. In summary, the procedure for dating monazite by EPMA has been established and continues to evolve in the UFRGS electron microprobe laboratory, but these first results demonstrate the technique’s potential to identify orogenic cycles and explore little-known regions at minimal cost. The oldest crystals yield high-quality results, but the quality of the youngest ages is limited by large errors. In some cases, the Th/U ratios have low dispersion, which leads to conflicting results from weighed average

calculations. In these cases, the CHIME isochron method may not be the best for age calculation. Nevertheless, the technique has some advantages. One major advantage is the 1 – 3 mm spot dating of grains and the observation of their relationship with the texture of the rock, because different ages of monazite grains in the core and rim of garnets can be correlated with different geological events. For example, an older event at ca. 2.35 Ga is identified in monazite from granulitic rocks, most of which yield ages near 1.9 Ga. The ages obtained in monazite in this study are comparable to previous ages in zircon for the same regions using zircon U –Pb SHRIMP geochronology, which shows the reliability of the chemical dating technique. Ages near 2.02 Ga were obtained previously for the Santa Maria Chico granulitic complex by SHRIMP dating of zircon and are confirmed here. The most significant orogenic cycle in the region is the Trans-Amazonian cycle (Santos et al., 2003). The most important conclusion of this investigation is the intense tectonic activity of the Neoproterozoic Brasiliano cycle (750 – 550 Ma), including the formation of flat-lying foliation during continental collision, because the monazite ages of schists and deformed granites are all within this time span.

Acknowledgements Fundac¸a˜o de Amparo a` Pesquisa do Estado do Rio Grande do Sul (FAPERGS) financially supported this project through grants 98/60186.6 and 01/02677. The project of excellence in metallogeny and crustal evolution of southern South America (CNPq/PRONEX-UFRGS) provided maintenance of the electron microprobe laboratory. J.M. Montel and, particularly, S.R.F. Vlach made careful reviews and contributed significantly to the improvement of the paper.

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