Paleoproterozoic Magmatic Provenance of Detrital

International Geology Review, Vol. 46, 2004, p. 127–157. Copyright © 2004 by V. H. Winston & Son, Inc. All rights reserved.

Paleoproterozoic Magmatic Provenance of Detrital Zircons, Porongos Complex Quartzites, Southern Brazilian Shield LÉO AFRANEO HARTMANN,1 RUY PAULO PHILIPP, Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500; 91500-000 Porto Alegre, Rio Grande do Sul, Brazil

DUNYI LIU, YUSHENG WAN, YANGIN WANG, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, People’s Republic of China

JOÃO ORESTES S. SANTOS, Brazilian Geological Survey (CPRM), Rua Banco da Província, 105, Porto Alegre, Rio Grande do Sul, Brazil AND

MARCOS A. Z. VASCONCELLOS

Instituto de Física, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500; 91500-000 Porto Alegre, Rio Grande do Sul, Brazil

Abstract Provenance investigation using detrital zircon U-Pb SHRIMP (sensitive high-resolution ion microprobe) age dating for six quartzite samples (167 spot determinations on 166 grains) indicates that metasedimentary rocks of the Porongos Complex from the southern Brazilian shield were derived almost entirely from Paleoproterozoic sources. Intensive study of igneous and metamorphic rocks in this portion of southwestern Gondwana employing SHRIMP geochronology over the past five years provides important evidence for provenance investigation of zircon grains. Ages of magmatic sources of the zircon detritus in the deformed quartz sandstones show an eight-peak distribution (known equivalent rocks in parenthesis): 2470 Ma (Neto Rodrigues Gneiss), 2350 Ma (Santa Maria Chico Complex Granodiorite), 2200 Ma (Encantadas Complex Tonalite), 2140 Ma (Paso Severino Dacite, Miséria Mylonite), 2100 Ma (Sierra Azul Granite), 2080 Ma (Chacofy Tonalite, Villa Monica Monzogranite), and 2040 and 2020 Ma (Itapema Monzogranite). Ages between 2260 and 2000 Ma correspond to the Trans-Amazonian Cycle, and were already known in the basement of the region, but the 2470 Ma and 2350 Ma ages are a new contribution regarding the tectonic evolution of the southern Brazilian shield. Tectonic scenarios for the Porongos basin fill are restricted to two different environments: (1) cratonic cover, if the fill is near 1500 Ma; or (2) passive margin if the age of fill is near 1000 Ma (this explanation requires additional studies). We have thus elucidated a major problem in the provenance of detrital zircon in the southern Brazilian shield, because we now know that the protolith sediments of the Porongos Complex were deposited under stable tectonic conditions and derived from Paleoproterozoic sources. This occurred after cratonization of the crust and formation of supercontinent Columbia following the end of the Trans-Amazonian Cycle. Nearly all zircon analyses have Th/U ratios higher than 0.2. Thus, only magmatic crystals survived the sedimentary processes. We show that detailed study of detrital zircon is a powerful tool for the understanding of the provenance of sandstones.

Introduction T HICK QUARTZ SANDSTONE UNITS are markers of mature sedimentation, reflecting stable geological conditions. Tectonic quiescence required for formation of nearly pure quartz detrital units and shales is characteristic of cratons and passive margins, and 1Corresponding

author; email: [email protected]

0020-6814/04/715/127-31 $25.00

the presence of these sedimentary rocks delimits rather tightly the tectonic setting prevailing in a continent at a specific time, and is highly relevant for the reconstruction of Precambrian supercontinents such as Paleoproterozoic–Mesoproterozoic Columbia and Neoproterozoic Gondwana. The southern Brazilian shield (Fig. 1) is key to the reconstruction of supercontinents, because it is

127

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HARTMANN ET AL.

FIG. 1. Geological map of the southern Brazilian shield (Hartmann et al., 2000a). Its location in South America is indicated in the inset. The location of Figure 2 also is shown.

PROVENANCE OF DETRITAL ZIRCONS

located on the southwestern portion of the proposed geometry of both supercontinent Columbia (Rogers and Santosh, 2002; supercontinent Atlantica of Rogers, 1996; Hartmann, 2002) and Gondwana (Unrug, 1997 ). In add ition, the zircon U-Pb SHRIMP geochronology of this portion of continental crust has been extensively studied in the past five years (e.g., Leite et al., 1998; Hartmann et al., 2000a, 2000b), which makes this region ideal for provenance studies of sedimentary detritus. Nevertheless, despite considerable advances in the understanding of the regional tectonic evolution (Hartmann et al., 2000a), major problems remain unresolved. For instance, the large quartzite units in the Porongos Complex—deformed quartz sandstones and associated metapelites (Figs. 2, 3, 4, and 5)—have been commonly interpreted as of Late Neoproterozoic age (Fragoso-César et al., 1982, 1986; Fragoso-César, 1991). However, they could be equivalent to the Las Tetas Complex quartzites located 400 km to the south in Uruguay (Hartmann et al., 2001). Thus, they could be much older, possibly 3.3–3.1 Ga (Orosirian). Correlation with the Brusque Complex situated 700 km to the northeast has also been attempted (Hasui et al., 1975; Jost and Bitencourt, 1980; Almeida et al., 2000; Shukowski et al., 1991; Hallinan and Mantovani, 1993), and these quartzites are now known to contain only Paleoproterozoic detrital zircon grains (Hartmann et al., 2003a ). Although some of these Porongos quartzite units could correspond to compositionally modified granitic mylonites (Fragoso-César, 1991; Fernandes et al., 1992a, 1992b), we have demonstrated that, according to zircon morphology, all the studied quartzites have a sedimentary origin. An allochthonous emplacement of the Porongos Complex—a suspect terrane—was proposed by Basei and Hawkesworth (1994). This hypothesis is tested in the present study based on the knowledge that detrital zircon ages from allochthonous terranes differ from the associated shield. Because of the importance of the Porongos Complex quartzite units for the reconstruction of paleotectonic environments in the Precambrian supercontinents, we here report an extensive investigation of the ages of detrital zircon grains from this unit. Zircon is a common detrital mineral in quartz sandstones because it resists weathering and transportation in sedimentary environments. Zircon is the most important mineral in geochronology, due to its high U and low common Pb contents, together with its occurrence in many rock types and high resis-

129

tance to recrystallization during crustal processes (e.g., Heaman and Parrish, 1991; Nutman, 2001). The youngest detrital zircon grain in sandstone establishes the oldest possible age for the basin fill. We made use of the 25 µm spot analytical capability of the sensitive high-resolution ion microprobe (Beijing SHRIMP II) to obtain 167 207 Pb/ 206 Pb ages on 166 detrital zircon grains from six quartzite samples collected along the NE-aligned Porongos Complex. Our aim is to determine the maximum time limit for the attainment of stable tectonic conditions for deposition of thick quartz sandstones and shales during the Precambrian evolution of Proterozoic supercontinents Columbia or Gondwana, or possibly of an Archean supercontinent. The age stratigraphy of geological events prior to sandstone deposition is here compared with the known ages of previously SHRIMP-dated rocks in order to establish potential zircon sources. A possible bias in the preferential preservation of magmatic zircon crystals in the quartz sandstone is also tested in this investigation.

Regional Geology and Previous Geochronologic Study The southern Brazilian shield offers an outstanding opportunity for the investigation of the successive deformation of the crust from the Paleoarchean to the Neoproterozoic because of the superposition of four orogenic cycles, each composed of a succession of several orogenies (Hartmann et al., 2000a, 2001). Major stratigraphic entitites are briefly described in Table 1. Table 2 lists the major orogenic episodes of the southern Brazilian craton. As summarized by Hartmann (2002), the regional primeval crust was formed in the Paleoarchean to Mesoarchean (3.41 Ga) during the Uruguayan Cycle (3.5–3.0 Ga), with zircon dating only in the La China and Las Tetas complexes of Uruguay (Hartmann et al., 2001). The La China Complex is a deformed granite-greenstone belt, but the Las Tetas Complex has extensive, thick quartzite and metapelite units. All 11 detrital zircon grains that were dated in this complex are older than 3.1 Ga. This was taken into consideration in the formulation of working hypotheses for the provenance of the Porongos Complex quartzites, because the Las Tetas Complex is located some 400 km to the south. Thus, both quartzite-metapelite units could be coeval. The protoliths of the granulitic complexes in southern Brazil and Uruguay were formed over a

Granodiorite, monzogranite, syenogranite

Conglomerate, sandstone, thin shale, trachyandesite and rhyolite

Seival Association

Camaquã Basin

Metapelite, quartzite, marble, basalt

Brusque Complex

Some hydrothermal alteration

Amphibolite facies Greenschist to amphibolite facies

Amphibolite facies

Magmatic, mylonitic foliation

Complex, S2 subhorizontal

Complex, S1 subhorizontal

Complex

Complex

2.02

2.26–2.02

2.0–0.9

2.25–2.03

0.62–0.47

0.61–0.55

0.75–0.70 0.80–0.56

0.88–0.75–0.70

2.1–0.56

2.48–0.56

2.55–2.03

Hydrothermal alteration 0.78–0.63–0.59

Amphibolite and greenschist facies

Amphibolite facies

Amphibolite and greenschist facies

Amphibolite facies

Thrust-deformed in the basal units Some hydrothermal lteration

Little deformed

Steep dipping S2

Complex

Amphibolite facies

Amphibolite facies

Granulite facies

2.6–2.07

5

7

7

5

5

4

6

5

5

4

4

3

2

1

1

Ref.1

= reference: 1 = Hartmann et al., 2001; 2 = Santos et al., 2002, 3 = Hartmann et al., 1999a; 4 = Hartmann et al., 1998; 5 = Hartmann et al., 2000a; 6 = Chemale et al., 1995a; 7 = Hartmann et al., 2002.

1Ref.

Granodiorite, monzogranite, syenogranite, mylonite

Tonalite, monzogranite, metapelite

Camboriú Complex

Quartzite, metapelite, basalt, rhyolite

Albite-muscovite schist, talc schist

Passo Feio Norte Formation

Porongos Complex

Andesite, rhyolite, basalt, quartzite, metapelite Complex

Vacacaí Complex

Tonalite, trondhjemite, granodiorite, syenogranite, amphibolite

Tonalite, trondhjemite, granodiorite, harzburgite, basalt

Cambaí Complex

Streep dipping S2

Passo Feio Sul Formation

Mylonitic foliation

Tonalitic to granodioritic

Basalt, metapelite, marble

Neto Rodrigues Gneisses

Tonalite, trondhjemite, granodiorite, mafic rock, Subvertical EW to NE pyroxenite, metapelite

Santa Maria Chico Granulitic Complex

Valentines Granulitic Complex Mafic, ultramafic, granitic and metasedimentary Complex superposition of foliations Granulite facies rocks

Encantadas Complex

3.41–3.15–2.7

Age, Ga

Strongly deformed, S2 subhorizontal Low amphibolite facies 3.3–3.1–2.6

Quartzite, metapelite, conglomerate

Metamorphism

Las Tetas Complex

Structure

Tonalite, trondhjemite, granodiorite, mafic and Strongly deformed, commonly sub- Amphibolite facies ultramafic rocks vertical foliation

Lithology

La China Complex

Stratigraphic unit

Dom Feliciano Pelotas Batholith and FloriBelt anópolis Batholith

Brusque Belt

Porongos Belt

São Gabriel Block

Nico Pérez Terrane

Geotectonic unit

TABLE 1. Description of Major Geological Units from the Southern Brazilian Shield

130 HARTMANN ET AL.


131

TABLE 2. Description of Paleoproterozoic Orogenies in the Southern Brazilian Shield Orogenic cycle

Time span, Ma

Neto Rodrigues

2480

Neto Rodrigues gneiss

1

2330

Santa Maria Chico Granulitic Complex—granodiorite protolith

4

Ivaró Trans-Amazonian

Unit, known rocks

Ref.1

Orogeny

2260–2020

3

Encantadas

2260

Encantadas Complex tonalites—tonalite, granodiorite, monzogranite

2

Florida

2150

San José Belt, Uruguay

5

Sierra Azul

2100

Tandilia Belt, Argentina

4

Balcarce

2070

Tandilia Belt, Argentina

4

Camboriú

2020

Camboriú Complex—gneiss, migmatite, monzogranite

6

1Ref.

= reference: 1 = Remus et al., 2000; 2 =Hartmann et al., 2000a; 3 = Santos et al., 2002; 4 = Hartmann et al., 2002; 5 = Hartmann et al., 2001; 6 = Silva et al., 2000.

500 m.y. period at about 2.7 Ga, during the Jequié Cycle (3.0–2.5 Ga), or at about 2.25 Ga according to zircon core dating (Santos et al., 2003a). The age of detrital zircon grains in a conglomerate obtained from the amphibolite-facies Las Tetas Complex is about 2764 Ma (Hartmann et al., 2001). Minerals or rocks of the same age had not been previously observed in the Porongos Complex region, but are searched for in this SHRIMP zircon study. The major crust-building event in the southern Brazilian shield was the Trans-Amazonian Cycle (Hartmann and Delgado, 2001), which occurred between 2260 and 2000 Ma (Santos et al., 2003a). The great significance of this orogenic cycle for the southern Brazilian shield is shown by the widespread occurrence of Paleoproterozoic rocks in the Tandilia Belt of central Argentina (Hartmann et al., 2002), the Piedra Alta Terrane and other regions of Uruguay (Hartmann et al., 2001; Santos et al., 2003a), and in several terranes in southern Brazil (Hartmann et al., 2000a, 2002; Santos et al., 2003a). The Encantadas Complex (Porcher and Fernandes, 1990; Remus et al., 1990; Tommasi et al., 1992, 1994), located in the core of the Porongos Belt (Figs. 2 and 3), is a tonalite-trondhjemite-granodiorite (TTG) unit formed during the Trans-Amazonian Cycle (Hartmann et al., 1999b), which contains 100–500 m long lenses of ultramafic amphibolite dated at 2257 ± 12 Ma (Hartmann et al., 2003b). Several major Trans-Amazonian Cycle events may also be represented in the Porongos Complex quartzites, because they occur in the immediate

vicinity or enclosed in the Encantadas Complex TTG association. The southern Brazilian shield was first cratonized at about 2.0 Ga (the second cratonization occurred at 550 Ma). This is confirmed by the presence of undeformed diabase dikes intruded in the crust at 1790 ± 5 Ma (Halls et al., 2001) and by the absence of zircon ages between 2.0 and 0.9 Ga (see the extensive SHRIMP geochronologic studies summarized by Hartmann et al., 2000a). In the late Neoproterozoic, this supercontinent Columbia was deformed during the accretion of juvenile island arcs to active continental margins, including southern Brazil (e.g., Babinski et al., 1996), and crustal reworking along the entire eastern portion of South America (e.g., Brito Neves and Cordani, 1991), in association with mafic magmatism. The Brasiliano Cycle in southern Brazil is composed of two major NE-SW–trending belts, the São Gabriel Belt on the west and the younger Dom Feliciano Belt on the east. The São Gabriel Belt (São Gabriel Block of Jost and Hartmann, 1984) was formed between 880 and 720 Ma. It encloses Paleoproterozoic and Archean remnants (Santa Maria Chico Complex). The Dom Feliciano Belt was built between 620 and 595 Ma. Both belts are cut by postorogenic plutons such as the Lavras, Caçapava, and Encruzilhada granites (595–550 Ma). The Porongos Comple x (Hartm ann et al., 2000c) is 170 km long and 15–30 km wide (Figs. 1–5). Its contact with the Dom Feliciano Belt to the east is along subvertical shear zones, whereas the

132

HARTMANN ET AL.

FIG. 2. Geological map of the Porongos Belt (Porcher and Fernandes, 1990; Hartmann et al., 2000c), southern Brazil. All samples analyzed by SHRIMP zircon U-Pb isotopes are located; samples 1–6 are from this investigation, sample 7 is from Hartmann et al. (2002), and samples 8–10 are from Hartmann et al. (2000c) and C. C. Porcher (pers. commun.). The locations of Figures 3, 4, and 5 are indicated.

possible continuity to the west is hidden below the Upper Neoproterozoic–Lower Paleozoic sedimentary rocks of the Camaquã Basin. Rocks of the

Phanerozoic Paraná Basin cover the belt on the northern and southern limits. The Dom Feliciano Belt is a Neoproterozoic orogen resulting from


133

FIG. 3. Detailed geological map of the Porongos and Encantadas complexes near the town of Santana da Boa Vista (Porcher and Fernandes, 1990; Hartmann et al., 2000c). The locations of sampling sites are indicated.

crustal reworking during the Brasiliano Cycle of orogenies (e.g., Mantovani et al., 1987; Frantz and Nardi, 1992; Babinski et al., 1997; Frantz et al., 2000; Philipp et al., 2003). The age of zircon from the Dom Feliciano Belt granitic rocks is predominantly 620–595 Ma, but older xenoliths (about 780 Ma) are present (Chemale et al., 1995a, 1995b; Silva et al., 1999). It has been commonly asserted (e.g., Porcher and Fernandes, 1990; FragosoCésar, 1991) that the Porongos Complex quartzites and metapelites are part of this Neoproterozoic belt, although Chemale et al. (1995a) restricted the Dom Feliciano Belt to the younger graniti cgneissic rocks situated to the east of the Porongos Complex. The Porongos Complex has also been considered to be part of the Tijucas Belt (Hasui et al., 1975; Jost and Bitencourt, 1980) through

correlation with the Brusque Complex situated 700 km to the northeast. The Tijucas schist belt is associated with TTG rocks in its southern and northern extensions. Extensive, subvertical transcurrent shear zones cross the shield in a northeasterly direction. These were formed during the final stages of the Brasiliano Cycle (Leite et al., 2000; Fernandes and Koester, 1999; Philipp et al., 2003) at about 620-595 Ma. Peraluminous granitic magmas are associated with the shear zones (Picada, 1971; Nardi and Frantz, 1995). The Late Neoproterozoic-Early Paleozoic Camaquã Basin was formed in the foreland of the Dom Feliciano Belt and displays thrust-faulting in its basal formations. The basin was later filled with molasse debris and trachyandesitic to rhyolitic volcanic rocks.

134

HARTMANN ET AL.

FIG . 4. Geological map of the Aberto dos Cerros region, Porongos and Encantadas complexes (Porcher and Fernandes, 1990). The location of the sampling site is indicated.

The main unit in the Porongos belt is the Porongos Complex, made up of deformed volcanic and sedimentary rocks interleaved with deformed granites, all recrystallized at greenschist- to low amphibolite–facies metamorphic conditions. The quartzite units form elevated hills above the pelitic schist of the lowlands (Fig. 6). The Encantadas Complex (Tessari and Picada, 1966; Jost and Bitencourt, 1980; Orlandi Filho and Pimentel, 1998; Ramgrab and Wildner, 1998) underlies the Porongos Complex and forms a granitic-gneissic core of tonalites, trondhjemites, and granodiorites, exposed at the type locality near Santana da Boa Vista, and also in the Aberto do Cerro region. Quartzites from both localities are included in this investigation. The main rock types of the Porongos Complex are chlorite-muscovite and garnet-biotite pelitic schist, quartzite (Fig. 6), andesite, rhyolite, minor basalt, and mylonitic granite (Jost and Bitencourt, 1980; Marques et al., 1998; Porcher and Fernandes, 1990). Other rocks occur in minor amounts, such as

marble, calc-silicate gneiss, graphite schist, metaconglomerate, and ultramafic rocks (Jost and Bitencourt, 1980; Remus et al., 1991; Marques et al., 1998). The ultramafic rocks are exposed in both the northern and southern parts of the belt as hornblende ultramafite, serpentinite, anthophyllite schist, and chlorite schist. The Capané chromites in the northern part of the belt are chemically similar to mantle chromites and therefore suggest the presence of an ophiolite (Marques et al., 1996). The quartzite bodies presently investigated are part of a 2000 m thick sequence of metapelite, quartzite, and marble, included in the Irapuazinho Formation (Jost and Bitencourt, 1980). Porongos Complex volcanic and sedimentary protoliths have been interpreted as a passive continental margin sequence deposited over a sialic basement constituted of the Encantadas Complex (Jost and Bitencourt, 1980). Backarc (Fragoso-César et al., 1982; Fernandes et al., 1992a, 1992b, 1995, 1998) and forearc (Issler, 1983) environments have also


135

FIG. 5. Geological map of the Capané antiform, northern part of the Porongos Complex (modified from Marques et al., 1998). Dated sample location is indicated.

FIG . 6. Field photographs (A–C) of the studied quartzites and (D) photomicrograph of quartzite, sample 5 (Figueira quartzite). Rounded zircon crystals shown in (D) are in in a strongly deformed quartz matrix, sample 5.

136

HARTMANN ET AL.

TABLE 3. Previous Geochronology of Porongos Complex Rock

Method

Age

Andesite

Rb-Sr whole-rock isochron

1542 ± 83 789 ± 39

Capané rhyolite

Rb-Sr whole-rock isochron

Meta-andesite

Zircon multigrain U-Pb

1350 ± 227

Rhyolite

Zircon U-Pb SHRIMP

783 ± 6 > 1500

been proposed. The main arc situated to the east of the Porongos Belt corresponds to the Pelotas Batholith. Peralkaline, deformed rhyolites and granites (Capané Gneiss) occur in the northern and southern parts of the Porongos Complex (Marques et al., 1996). The approximate age of the Encantadas Complex granitic rocks is known from Rb-Sr isochron determinations as ~2.1 Ga (Cordani et al., 1974; Soliani Jr., 1986). Zircon U-Pb SHRIMP dating has placed the magmatic event at 2256 Ma and the main deformation at 2052 Ma (Hartmann et al., 2000a, 2000c, 2002). Before the present investigation, however, the maximum depositional age of Porongos Complex quartzites and metapelites was open to speculation (Table 3), because of lack of robust geochronological data and complex field relationships. Multigrain U-Pb dating of zircon from a meta-andesite yielded an age of 1350 ± 227 Ma, and was interpreted by Wildner et al. (1996) as the magmatic age of the rock. Rb-Sr isochron ages of 1542 ± 83 Ma and 789 ± 39 Ma in andesitic rocks (Table 3) were considered to represent volcanism occurring near 790 Ma (Soliani Jr., 1986). Hartmann et al. (2000c) placed the magmatic zircon U-Pb SHRIMP age of a Porongos Complex rhyolite at 783 ± 6 Ma (>1500 Ma inheritance in zircon cores). The Rb-Sr whole-rock isochron age of a deformed, peralkaline Capané Rhyolite from the northern portion of the belt suggested that the magmatic event occurred at ~531 Ma for the magmatic event (J.-M. Lafon, pers. commun.). Knowledge of the ages of detrital zircon grains from the deformed quartz sandstones (now quartzites) from the Porongos Complex is essential to unravel the tectonic history of this multiply deformed Precambrian terrain and its relationships to supercontinent agglomeration and dispersal.

Geochronology Field mapping of the region was conducted by the authors over the last 20 years with emphasis on the

531

Reference Soliani, 1986 J.-M. Lafon, pers. commun. Wildner et al., 1996 Hartmann et al. , 2000a

supervision of senior undergraduate geology students from the Universidade do Vale do Rio dos Sinos (UNISINOS) and Universidade Federal do Rio Grande do Sul (UFRGS) between 1985 and 1995. Six key samples were selected from major Porongos Complex quartzite bodies for this investigation. These samples are numbered: 1, Alto Bonito quartzite; 2, Aberto dos Cerros quartzite; 3, Godinho quartzite; 4, Coxilha do Raio quartzite; 5, Figueiras quartzite; and 6, Jaíba quartzite. Field descriptions, locations, and petrographic descriptions are given in Appendix 1. Cathodoluminescence images of zircon show euhedral internal zoning and rounded external shapes in all six samples. This is exemplified by zircon from sample 5 (Fig. 7). Several zircon crystals display internal sector zoning (e.g., Figs. 7A, 7D, 7K, and 7R), in addition to fine euhedral zoning. Rounded internal patches in Figs. 7l and 7m could be due either to late magmatic or metamorphic recrystallization. Zircon separation was carried out at UFRGS, using standard procedures of crushing 2–3 kg of rock, milling and sieving, followed by heavy liquid concentration and Frantz isodynamic magnetic separation. Roughly 100–200 zircon grains were handpicked for each sample. Selected zircon grains were mounted on an epoxy disc together with chips of CZ3 (564 Ma) and TEM (417 Ma) zircon standards, ground, and polished until nearly half of the grains was removed. The mount was then microphotographed in transmitted and reflected light. Only areas free of fractures and mineral inclusions were selected for spot dating. For visualization of internal morphology (to support age interpretation), imaging was performed at UFRGS after SHRIMP dating using a scanning electron microscope (i.e., backscattered electrons) and an optical cathodoluminescence device attached to an electron microprobe. After optical photography in


137

FIG . 7. Cathodoluminescence images of rounded, spherical to eliptical zircon crystals from the dated quartzite sample 5 (Figueira quartzite, Porongos Complex). The figure shows minor fractures, strong euhedral zoning typical of magmatic zircon, and sector zoning in some crystals. The analyzed spots are shown as black and white circles; ages are in Ma.

Beijing, the mount was cleaned and gold-coated for uniform electrical conductivity during the SHRIMP analyses. The isotopic composition of the zircon grains was determined using the Beijing SHRIMP II equ ipment (De Laeter and Kennedy, 1998) installed at the Chinese Academy of Geological Sciences, using methods described by Compston et al. (1992) and Smith et al. (1998). Circular areas of 20–30 µm were analyzed from morphologically distinct zones chosen within zircon grains. Replicated analyses of the TEM standard were also performed

in the same epoxy mount. Corrections for common Pb were made using the measured 204 Pb and the Pb isotopic composition of Stacey and Kramers (1975). For each spot analysis, the initial 60–90 seconds were used to rasterize and remove the gold, avoiding the introduction of common Pb from the coating. Each analysis is the result of a combination of four to five SHRIMP scans. In sample 5, several analyses are the result of only two scans, but some analyses resulted from five scans. Results with more than 1% common Pb correction were not used to calculate the ages, such as those on grains

138

HARTMANN ET AL.

TABLE 4. SHRIMP U-Pb Isotopic Analyses (n = 167) of Detrital Zircon Grains (n = 166) of Six Quartzite Samples from the Porongos Complex1 Isotopic Ratios3, 4

Ages

Pb

4f2062

206Pb

207Pb

207Pb

ppm

(%)

238 U

235U

206Pb

0.09

19

0.272

0.3739±1.60

6.6531±2.74

0.1291±2.22 0.1074±2.88

2048±28

2085 ± 39

98

32

0.88

12

0.270

0.3773±1.50

6.9060±2.05

0.1327±1.40 0.1099±2.46

2064±26

2135 ± 24

97

26

0.42

20

0.054

0.3766±0.65

6.7196±1.06

0.1294±0.83 0.1056±1.51

2060±12

2090 ± 15

99

113 0.51

73

0.046

0.3211±0.75

5.7676±1.22

0.1303±0.97 0.1078±1.41

1795±12

2102 ± 17

85

0.57

47

0.080

0.3733±1.39

6.6668±2.63

0.1295±2.24 0.1006±5.47

2045±24

2092 ± 39

98

19

0.40

16

0.289

0.3877±1.40

6.8868±2.75

0.1288±2.36 0.1070±4.19

2112±25

2082 ± 42 101

43

25

0.60

14

0.287

0.3948±1.86

7.2004±2.56

0.1323±1.76 0.1148±2.78

2145±34

2128 ± 31 101

36

22

0.63

12

0.098

0.3845±1.07

6.8821±1.56

0.1298±1.14 0.1088±2.46

2097±19

2096 ± 20 100

1-8.1

106

35

0.34

35

0.095

0.3725±0.81

6.6742±1.24

0.1299±0.94 0.1060±1.56

2041±14

2097 ± 17

1-9.1

142

66

0.48

45

0.051

0.3886±1.28

6.9369±1.99

0.1295±1.52 0.1148±2.35

2116±23

2091 ± 27 101

1-10.1

60

28

0.47

20

0.000

0.3788±1.18

6.6283±2.13

0.1269±1.77 0.1093±2.32

2071±21

2056 ± 31 101

1-11.1

61

50

0.86

20

0.332

0.3876±1.04

6.6844±1.58

0.1251±1.19 0.1137±1.54

2111±19

2030 ± 21 104

1-12.1

120

121 1.04

40

0.204

0.3993±1.01

7.1315±1.80

0.1295±1.49 0.1086±3.10

2166±19

2091 ± 26 104

1-15.1

81

38

0.48

22

0.247

0.3936±1.23

6.7981±2.03

0.1253±1.61 0.1098±2.91

2139±22

2033 ± 29 105

1-16.1

54

24

0.46

18

0.085

0.3857±0.94

6.9314±1.30

0.1303±0.90 0.1097±1.47

2103±17

2102 ± 16 100

1-17.1

160

85

0.55

53

0.025

0.3796±0.86

6.7493±1.51

0.1289±1.23 0.1066±2.07

2075±15

2084 ± 22 100

1-18.1

115

65

0.58

38

0.192

0.4661±1.10 10.0821±1.63

0.1569±1.19 0.1286±2.17

2467±23

2422 ± 20 102

1-19.1

68

39

0.59

27

0.140

0.4511±1.18

9.9698±1.77

0.1603±1.32 0.1213±2.01

2400±24

2459 ± 22

1-20.1

77

71

0.95

30

0.254

0.4193±1.36

7.2576±2.51

0.1255±2.11 0.1202±3.24

2257±26

2036 ± 37 111

1-21.1

48

31

0.66

17

0.427

0.3989±1.47

7.0092±2.46

0.1274±1.98 0.1122±2.40

2164±27

2063 ± 35 105

1-22.1

64

65

1.06

22

0.194

0.4103±0.95

7.1692±1.53

0.1267±1.21 0.1185±1.99

2216±18

2053 ± 21 108

2-1.1

300

90

0.31

99

0.069

0.3834±0.57

6.8099±0.93

0.1288±0.74 0.1086±1.73

2092±10

2082±13

101

2-3.1

63

56

0.93

22

0.197

0.4002±1.30

6.9920±2.23

0.1267±1.82 0.1147±2.29

2170±24

2053±32

106

2-4.1

84

118 1.45

29

0.117

0.4039±1.05

7.4421±1.92

0.1336±1.63 0.1185±1.70

2187±19

2146±28

102

2-5.1

32

31

1.02

11

0.229

0.4046±1.76

7.0253±3.05

0.1259±2.51 0.1148±3.05

2190±33

2042±44

107

2-6.1

135

82

0.63

45

0.095

0.3864±0.83

6.8923±1.44

0.1293±1.18 0.1094±1.88

2106±15

2089±21

101

2-7.1

64

28

0.45

21

0.124

0.3910±1.18

6.9204±1.87

0.1283±1.45 0.1109±2.30

2127±21

2076±26

102

2-8.1

60

95

1.64

24

0.065

0.4681±1.24 10.3861±1.75

0.1609±1.24 0.1327±1.65

2475±25

2466±21

100

2-9.1

142

39

0.29

47

0.100

0.3846±0.81

6.7946±1.33

0.1281±1.06 0.1101±2.48

2098±15

2072±19

101

2-10.1

42

32

0.80

18

0.028

0.4894±1.73 10.9525±2.28

0.1623±1.48 0.1351±2.49

2568±37

2480±25

104

2-11.1

285

110 0.40

91

0.065

0.3700±0.61

6.5389±0.93

0.1282±0.71 0.1037±1.26

2029±11

2073±12

98

2-12.1

176

87

0.51

57

0.070

0.3795±0.77

6.7282±1.23

0.1286±0.96 0.1071±1.59

2074±14

2079±17

100

2-13.1

56

24

0.44

19

0.296

0.3970±1.64

6.9124±2.49

0.1262±1.87 0.1080±3.74

2155±30

2047±33

105

2-14.1

38

25

0.69

14

0.206

0.4173±1.55

7.4356±2.43

0.1292±1.88 0.1194±2.61

2248±29

2088±33

108

2-15.1

147

103 0.72

49

0.008

0.3888±0.97

6.8170±1.36

0.1272±0.95 0.1090±1.43

2117±17

2059±17

103

2-16.1

80

109 1.42

26

0.097

0.3871±1.08

6.9132±1.72

0.1295±1.34 0.1107±1.56

2109±19

2092±24

101

2-17.1

203

17

0.09

71

0.040

0.4086±0.69

7.8098±1.03

0.1386±0.76 0.1109±2.86

2209±13

2210±13

100

2-18.1

127

45

0.36

42

0.158

0.3853±0.87

6.8333±1.49

0.1286±1.22 0.1063±2.81

2101±16

2079±21

101

2-19.1

271

99

0.38

88

0.159

0.3768±0.64

6.4430±1.05

0.1240±0.84 0.0443±3.50

2061±11

2015±15

102

2-20.1

162

157 1.00

63

0.024

0.4498±0.78

9.5912±1.21

0.1546±0.93 0.1221±1.19

2394±16

2398±16

100

U

Th

Spot

ppm ppm

1-0.0

238

21

1-1.1

38

1-2.1

63

1-3.1

227

1-4.1

169

93

1-5.1

49

1-6.1 1-7.1

Th/U

208 Pb 232 Th

206Pb 238U

207Pb 206Pb

Conc.5 (%)

Sample 1, Alto Bonito quartzite (n = 21)

97

98

Sample 2, Aberto dos Cerros quartzite (n = 34)

Table continues

139


TABLE 4. Continued Sample 2, Aberto dos Cerros quartzite (n = 34) (continued) 2-21.1

37

21

0.57

15

0.051

0.4806±1.82 11.0696±2.39

0.1671±1.54 0.1414±2.74

2530±38

2528±26

2-22.1

148

55

0.39

49

0.118

0.3815±0.81

6.8599±1.31

0.1304±1.04 0.1100±2.03

2083± 14

2103±18

100 99

2-23.1

134

91

0.71

45

0.011

0.3921±0.85

6.9713±1.31

0.1290±1.00 0.1124±1.39

2132± 16

2084±18

102

2-24.1

99

111 1.16

32

0.037

0.3806±0.99

6.8464±1.63

0.1305±1.31 0.1106±1.56

2079± 17

2104±23

99

2-25.1

38

32

0.88

12

0.270

0.3741±1.61

6.6601±2.73

0.1291±2.22 0.1075±2.88

2049± 28

2086±39

98

2-26.1

136

55

0.41

48

0.056

0.4073±1.09

7.6896±1.50

0.1369±1.04 0.1117±2.06

2203± 20

2189±18

101

2-28.1

169

11

0.06

55

0.006

0.3780±0.95

6.5829±1.36

0.1263±0.97 0.1146±3.31

2067± 17

2047±17

101

2-29.1

87

42

0.50

46

0.147

0.6137±1.04 19.9566±1.58

0.2358±1.19 0.1584±2.16

3085± 26

3092±19

100

2-30.1

75

60

0.83

29

0.169

0.4532±1.16 10.1908±1.73

0.1630±1.29 0.1265±2.02

2410± 23

2488±22

97

2-31.1

40

15

0.40

13

0.587

0.3708±1.56

6.6139±3.02

0.1292±2.62 0.0970±6.73

2033± 27

2089±46

97

2-32.1

48

38

0.82

16

0.445

0.3773±1.43

6.4856±2.54

0.1246±2.11 0.1046±2.86

2064± 25

2024±37

102

2-33.1

298

255 0.88

96

0.014

0.3756±0.59

6.6770±0.91

0.1289±0.69 0.0955±0.94

2056± 10

2084±12

99

2-34.1

135

42

0.32

44

0.076

0.3807±0.87

6.7741±1.38

0.1290±1.07 0.1073±2.18

2080± 16

2085±19

100

2-35.1

113

58

0.52

39

0.163

0.4030±1.01

7.4128±1.62

0.1334±1.28 0.1107±2.18

2183± 19

2143±22

102

2-36.1

108

34

0.32

37

0.036

0.4044±0.95

7.3964±1.56

0.1327±1.24 0.1128±2.94

2189± 18

2133±22

103

3-1

50

33

0.67

18

0.007

0.4261±0.96

8.4489±1.82

0.1438±1.54 0.1191±1.51

2288±19

2274±27

101

3-2

109

67

0.64

44

0.073

0.4646±0.65 10.4478±0.95

0.1631±0.70 0.1269±1.35

2460±13

2488±12

99

3-3

226

102 0.47

69

0.001

0.3536±0.51

6.3943±0.72

0.1311±0.52 0.0957±0.87

1952±9

2113±9

92

3-4

143

Sample 3, Godinho quartzite (n = 31)

51

0.37

47

0.138

0.3860±0.89

6.7669±1.35

0.1272±1.02 0.1092±2.23

2104±16

2059±18

102

22

0.27

28

0.026

0.3954±1.14

7.1831±1.69

0.1318±1.24 0.1125±3.55

2148±21

2122±22

101

231 0.78 103

0.032

0.3889±0.49

6.9680±0.80

0.1300±0.63 0.1129±1.00

2118±9

2097±11

101

3-5

83

3-6

308

3-7

184

3-8

127

3-9

23

20

3-10

103

3-11 3-12 3-13.1 3-13.2

73

0.41

63

0.111

0.3997±0.63

7.1546±1.01

0.1298±0.79 0.1174±1.46

2168±12

2096±14

103

188 1.53

41

0.023

0.3758±0.76

6.4992±1.22

0.1254±0.95 0.1093±1.08

2057±13

2035±17

101

0.88

8

0.083

0.3955±1.75

7.6188±2.69

0.1397±2.05 0.1149±2.83

2148±32

2224±35

97

40

0.40

34

0.062

0.3889±0.83

7.0823±1.37

0.1321±1.09 0.1122±2.13

2118±15

2126±19

100

68

51

0.78

22

0.015

0.3832±1.01

6.8719±1.59

0.1301±1.24 0.1127±1.65

2091±18

2099±22

100

114

30

0.28

38

0.186

0.3873±0.78

7.0492±1.31

0.1320±1.05 0.1184±2.78

2110±14

2125±18

99

40

51

1.34

16

0.128

0.4707±1.31 10.5491±2.69

0.1626±2.35 0.1350±1.95

2486±27

2482±40

100 99

38

49

1.34

14

0.477

0.4286±1.02

8.7757±1.89

0.1485±1.59 0.1195±1.77

2299±20

2329±27

3-14

185

67

0.37

59

0.032

0.3697±0.65

6.4893±1.05

0.1273±0.83 0.1079±1.52

2028±11

2061±15

98

3-15

83

43

0.54

29

0.268

0.4100±1.17

7.5864±1.91

0.1342±1.51 0.1154±2.99

2215±22

2154±26

103

3-16

103

64

0.64

33

0.011

0.3755±0.84

6.6782±1.38

0.1290±1.09 0.1066±1.62

2055±15

2084±19

99

3-17

172

48

0.29

50

0.015

0.3396±0.81

5.7890±1.18

0.1236±0.86 0.1092±1.95

1885±13

2009±15

94

3-18

110

76

0.72

38

0.174

0.4087±0.81

7.8274±1.31

0.1389±1.03 0.1173±1.58

2209±15

2214±18

100

3-19

269

188 0.72

84

0.017

0.3643±0.54

6.3610±0.88

0.1266±0.70 0.1067±0.96

2003±9

2052±12

98

3-20

162

54

0.035

0.3851±0.69

6.8731±1.07

0.1295±0.82 0.1080±1.40

2100±12

2091±14

100

3-21

351

111 0.33 108

0.028

0.3599±0.47

6.1928±0.78

0.1248±0.62 0.1027±1.33

1982±8

2026±11

98

3-22

158

90

0.59

53

0.163

0.3891±0.72

7.3889±1.41

0.1377±1.22 0.1204±2.17

2119±13

2199±21

96

3-23

45

21

0.48

15

0.280

0.3781±1.07

6.5727±1.87

0.1261±1.54 0.1059±3.03

2068±19

2044±27

101

3-24

20

14

0.72

7

0.613

0.4104±2.19

7.5705±6.58

0.1338±6.20 0.1057±11.3

2217±41

2148±99

103

3-25

211

71

0.35

66

0.019

0.3652±0.62

6.1595±1.06

0.1223±0.86 0.1051±1.82

2007±11

1990±15

101

3-26

93

30

0.33

30

0.147

0.3802±0.93

6.8706±1.50

0.1311±1.18 0.1040±2.59

2077±16

2112±21

98

3-27

215

91

0.44

77

0.001

0.4158±0.60

7.8089±0.90

0.1362±0.67 0.1168±1.12

2241±11

2180±12

103

3-28

30

24

0.84

10

0.054

0.3830±1.60

6.8898±2.59

0.1305±2.03 0.1125±2.71

2090±29

2104±36

99

3-29

26

28

1.12

9

0.309

0.4247±1.74

7.8393±2.95

0.1339±2.39 0.1181±2.94

2282±33

2149±42

106

3-30

95

72

0.78

32

0.024

0.3915±0.90

7.4450±1.38

0.1379±1.05 0.1119±1.54

2130±16

2201±18

97

58

0.37

Table continues

140

HARTMANN ET AL.

TABLE 4. Continued Sample 4, Coxilha do Raio quartzite (n = 31) 4-1.1

75

30

0.41

25

0.572

0.3928±0.92

6.7289±1.67

0.1242±1.40 0.1029±3.59

2136±17

2018±25

106

4-2.1

60

28

0.49

21

0.679

0.4033±1.19

7.3860±2.55

0.1328±2.25 0.1090±5.17

2184±22

2136±39

102

4-3.1

177

84

0.49

61

0.173

0.4015±0.72

7.1007±1.29

0.1283±1.07 0.1140±2.10

2176±13

2074±19

104

4-4.1

44

45

1.06

14

1.219

0.3801±1.84

6.2660±4.74

0.1196±4.37 0.1075±4.76

2077±33

1950±78

107

4-5.1

139

68

0.51

45

0.145

0.3787±0.79

6.7152±1.30

0.1286±1.03 0.1094±1.76

2070±14

2079±18

100

4-6.1

69

41

0.61

22

0.556

0.3775±1.17

6.6048±2.53

0.1269±2.25 0.1034±4.05

2064±21

2055±40

100

4-7.1

46

19

0.43

16

0.714

0.3945±1.43

6.9131±3.61

0.1271±3.32 0.1017±9.04

2144±26

2058±59

104

4-8.1

227

130

0.59

76

0.200

0.3876±0.66

6.6623±1.11

0.1247±0.89 0.1086±1.44

2112±12

2024±16

104

4-9.1

64

30

0.49

22

0.576

0.3921±1.20

6.7010±2.55

0.1239±2.25 0.1053±4.82

2133±22

2014±40

106

4-10.1

76

27

0.37

26

0.061

0.3950±1.28

7.0180±1.81

0.1289±1.27 0.1147±2.45

2146±23

2082±22

103

4-11.1

90

197

2.26

28

0.287

0.3641±0.99

6.2436±1.73

0.1244±1.42 0.1032±1.35

2002±17

2020±25

99

4-12.1

80

104

1.34

27

0.153

0.3993±1.10

7.0800±2.88

0.1286±2.66 0.1157±2.52

2166±20

2079±47

104

4-13.1 253

234

0.96

79

0.103

0.3638±0.64

6.3890±1.01

0.1274±0.78 0.0999±1.04

2000±11

2062±14

97

88

93

1.09

29

0.284

0.3885±1.01

6.8321±1.93

0.1276±1.64 0.1102±1.92

2116±18

2065±29

102

4-15.1 255

181

0.73

87

0.089

0.3979±0.64

7.3878±0.99

0.1347±0.76 0.1112±1.13

2159±12

2160±13

100

4-16.1 244

164

0.70

81

0.010

0.3891±0.65

7.0101±1.03

0.1307±0.80 0.1132±1.17

2119±12

2107±14

101

27

15

0.57

10

0.577

0.4102±1.82

7.7086±3.82

0.1363±3.37 0.1110±6.94

2216±34

2181±59

102

4-18.1 125

63

0.52

40

0.116

0.3697±0.86

6.4134±1.38

0.1258±1.08 0.1041±1.73

2028±15

2040±19

99

4-19.1 165

73

0.45

51

0.216

0.3603±0.76

6.1751±1.25

0.1243±0.99 0.0999±1.80

1984±13

2019±18

98

4-20.1 170

123

0.75

44

0.591

0.3024±0.96

5.4956±1.75

0.1318±1.46 0.0999±2.04

1703±14

2122±26

80

4-21.1 108

76

0.72

35

0.188

0.3719±0.95

6.4396±1.71

0.1256±1.43 0.1020±2.16

2038±17

2037±25

100

21

27

1.31

7

1.017

0.3795±2.02

6.4438±3.44

0.1232±2.79 0.1017±3.10

2074±36

2002±50

104

4-23.1 214

104

0.51

74

0.208

0.4004±0.64

7.5068±1.09

0.1360±0.88 0.1159±1.68

2171±12

2177±15

100

4-14.1

4-17.1

4-22.1 4-24.1

99

49

0.52

33

0.257

0.3838±0.95

6.6289±1.79

0.1253±1.52 0.1050±2.92

2094±17

2033±27

103

4-25.1

66

34

0.53

22

0.087

0.3804±1.12

6.7354±1.88

0.1284±1.51 0.1120±2.49

2078±20

2077±27

100

4-26.1

74

82

1.14

25

0.072

0.3879±1.32

6.7074±1.87

0.1254±1.33 0.1103±1.79

2113±24

2035±24

104

4-27.1 162

84

0.53

56

0.082

0.4014±0.89

7.1599±1.37

0.1294±1.04 0.1155±1.90

2176±16

2089±18

104

4-28.1 134

108

0.83

48

0.177

0.4165±0.82

7.7492±1.36

0.1349±1.09 0.1179±1.54

2245±16

2163±19

104

4-29.1 120

67

0.58

45

0.030

0.4340±0.84

8.1602±1.29

0.1364±0.98 0.1217±1.59

2324±16

2181±17

107

4-30.1

62

28

0.47

27

0.598

0.5075±1.24 11.1544±2.07

0.1594±1.65 0.1322±4.40

2646±27

2449±28

108

4-31.1

76

33

0.45

26

0.061

0.3999±1.05

0.1260±1.33 0.1174±2.35

2168 19

2043 24

106

5-27

180

199

1.14

69

0.088

0.4450±0.53

9.7689±0.74

0.1592±0.52 0.1233±0.76

2373±10

2447±9

97

5-28

179

134

0.78

57

0.119

0.3704±0.53

6.8352±0.85

0.1339±0.67 0.0955±1.05

2031±9

2149±12

95

5-29

108

73

0.69

43

0.064

0.4578±0.66 10.1107±0.93

0.1602±0.65 0.1240±1.11

2430±13

2457±11

99

5-30

93

72

0.80

31

0.079

0.3893±0.76

7.3539±1.25

0.1370±1.00 0.1099±1.30

2119±14

2190±17

97

5-3x

246

168

0.71

68

0.279

0.3222±0.47

5.4738±0.85

0.1232±0.71 0.0722±1.24

1800±7

2004±13

90

5-37

51

34

0.69

20

0.232

0.4550±1.08 10.2211±1.59

0.1629±1.17 0.1309±2.23

2418±22

2486±20

97

5-38

204

308

1.56

78

0.070

0.4455±0.49

0.1619±0.60 0.1246±0.66

2375±10

2475±10

96

6-1.1

265

81

0.31

89

0.058

0.3896±0.65

6.9813±0.91

0.1300±0.63 0.1142±1.42

2121±12

2097±11

101

6-2.1

117

56

0.50

42

0.067

0.4212±0.93

8.0169±1.31

0.1380±0.92 0.1219±1.75

2266±18

2203±16

103

6-3.1

154

56

0.38

51

0.051

0.3864±0.66

6.7920±1.03

0.1275±0.78 0.1105±1.31

2106±12

2063±14

102

6-4.1

261

236

0.93

83

0.004

0.3686±0.52

6.2846±0.81

0.1237±0.63 0.1085±0.80

2023±9

2010±11

101

6-5.1

112

25

0.23

38

0.164

0.3918±0.83

7.2890±1.38

0.1349±1.10 0.1252±3.26

2131±15

2163±19

99

6-6.1

44

25

0.58

15

0.106

0.4023±1.31

7.1962±2.10

0.1297±1.65 0.1195±2.51

2180±24

2094±29

104

6-7.1

64

42

0.67

22

0.036

0.3928±1.00

6.8819±1.60

0.1271±1.25 0.1116±1.81

2136±18

2058±22

104

6.9481±1.70

Sample 5, Figueiras quartzite (n = 7)

9.9422±0.78

Sample 6, Jaíba quartzite (n = 43)

Table continues

141


TABLE 4. Continued Sample 6, Jaíba quartzite (n = 43) (continued) 6-8.1

169

79

0.48

55

0.032

0.3788±0.65

6.6885±1.04

0.1281±0.81 0.1091±1.37

2071± 12

2072±14

100

6-9.1

54

38

0.72

18

0.292

0.3868±1.35

6.8451±2.11

0.1284±1.61 0.1089±2.51

2108±24

2076±28

102

6-10.1

40

21

0.53

13

0.017

0.3829±1.28

6.7724±2.03

0.1283±1.58 0.1142±2.42

2090±23

2075±28

101

6-11.1 135

71

0.55

46

0.022

0.3949±0.86

6.9487±1.20

0.1276±0.83 0.1139±1.36

2145±16

2065±15

104

6-12.1 186

78

0.43

59

0.014

0.3670±0.63

6.4829±0.97

0.1281±0.74 0.1027±1.25

2015±11

2072±13

97

6-13.1 108

110

1.05

36

0.033

0.3831±0.79

6.7570±1.22

0.1279±0.93 0.1103±1.18

2091±14

2070±16

101

6-14.1

57

16

0.29

20

0.027

0.4156±1.32

7.8421±1.80

0.1368±1.23 0.1283±2.56

2241±25

2188±21

102

6-15.1 220

118

0.55

75

0.037

0.3981±0.57

7.4200±0.87

0.1352±0.66 0.1152±1.07

2160±10

2167±12

100

6-16.1

81

105

1.34

27

0.118

0.3857±0.91

6.8623±1.46

0.1290±1.14 0.1101±1.34

2103±16

2085±20

101

6-17.1

46

53

1.19

16

0.077

0.3937±1.21

6.9179±1.93

0.1274±1.51 0.1127±1.83

2140±22

2063±27

104

6-18.1

45

25

0.57

14

0.136

0.3703±1.26

6.6939±2.02

0.1311±1.58 0.1059±2.53

2031± 22

2113±28

96

6-19.1 116

51

0.45

38

0.046

0.3821±0.97

6.8751±1.34

0.1305±0.93 0.1101±1.58

2086±17

2104±16

99

6-20.1

54

23

0.44

18

0.122

0.3831±1.14

6.7914±1.96

0.1286±1.59 0.1081±3.18

2091±20

2078±28

101

6-21.1

85

70

0.85

28

0.092

0.3856±0.94

6.7888±1.43

0.1277±1.08 0.1076±1.45

2103±17

2066±19

102

6-22.1 171

100

0.60

56

0.045

0.3791±0.80

6.7698±1.12

0.1295±0.79 0.1078±1.26

2072±14

2091±14

99

6-23.1

79

35

0.45

27

0.189

0.3886±0.94

7.2760±1.55

0.1358±1.24 0.1090±2.46

2117± 17

2174±22

97

6-24.1

77

35

0.47

27

0.010

0.4062±1.18

7.5096±1.60

0.1341±1.08 0.1198±1.89

2197±22

2152±19

102

6-25.1

72

48

0.69

23

0.363

0.3647±1.09

6.3339±1.88

0.1260±1.53 0.1031±2.26

2004± 19

2042±27

98

6-26.1 114

77

0.70

38

0.096

0.3930±0.77

7.1510±1.46

0.1320±1.24 0.1120±1.37

2137±14

2124±22

101

6-27.1

70

31

0.45

24

0.124

0.3908±0.97

7.1497±1.60

0.1327±1.28 0.1144±2.37

2127±18

2134±22

100

6-28.1

93

48

0.53

30

0.018

0.3704±0.91

6.5756±1.41

0.1288±1.08 0.1055±1.65

2031± 16

2081±19

98

6-29.1

90

60

0.69

31

0.106

0.4007±0.96

7.2038±1.50

0.1304±1.15 0.1173±1.59

2172±18

2103±20

103

6-30.1 120

66

0.57

39

0.012

0.3784±0.93

6.7952±1.29

0.1302±0.90 0.1087±1.46

2069±16

2101±16

98

6-31.1 178

85

0.49

60

0.043

0.3899±0.62

6.8915±0.99

0.1282±0.76 0.1111±1.26

2123±11

2073±13

102

6-32.1 155

46

0.31

49

0.011

0.3667±0.69

6.2109±1.10

0.1228±0.86 0.1061±1.67

2014±12

1998±15

101

6-33.1

85

38

0.46

28

0.134

0.3792±0.92

6.8354±1.53

0.1307±1.23 0.1080±2.35

2072± 16

2108±22

98

6-34.1

96

52

0.56

31

0.007

0.3786±0.84

6.6330±1.30

0.1271±0.99 0.1106±1.45

2070±15

2058±17

101

6-35.1 434

35

0.08 167

0.034

0.4483±0.51

9.2540±0.66

0.1497±0.42 0.1154±1.94

2388±10

2343±7

102

6-36.1 141

93

0.68

54

0.055

0.4444±0.86

9.7847±1.10

0.1597±0.69 0.1235±1.21

2370±17

2452±12

97

6-37.1 169

47

0.29

53

0.058

0.3627±0.65

6.3717±1.03

0.1274±0.80 0.0991±1.85

1995±11

2063±14

97

6-38.1

44

31

0.73

14

0.115

0.3700±1.53

6.6626±2.23

0.1306±1.62 0.1076±2.51

2030± 27

2106±28

96

6-39.1 183

116

0.66

61

0.034

0.3852±0.62

6.8106±0.98

0.1282±0.76 0.1111±1.10

2101±11

2074±13

101

6-40.1 198

71

0.37

103

6-41.1 642

254

6-42.1 101 6-43.1

72

67

0.063

0.3956±0.80

7.0837±1.16

0.1299±0.84 0.1128±1.82

2149±15

2096±15

0.41 205

0.004

0.3709±0.37

6.6865±0.55

0.1308±0.40 0.1035±0.69

2034±6

2108±7

96

59

0.60

34

0.000

0.3866±0.85

6.9335±1.30

0.1301±0.99 0.1117±1.45

2107±15

2099±17

100

39

0.56

25

0.024

0.4022±1.01

7.3721±1.56

0.1329±1.19 0.1206±1.76

2179±19

2137±21

102

1 Data

with more than 1% common lead correction and 5% or more discordant are not used in age calculations (rows in italics). Sample 5 only includes analyses with five scans; those with two scans not shown. 2 Common 206Pb/total 206Pb based on measured 204 Pb. 3 All Pb in ratios are radiogenic component. Errors in %. 4 Uncertainties are 1 . 5 Concordance: 100t (206Pb/238U)/t(207Pb/206Pb).

4-4.1 and 4-22.1 (Table 4). The uncertainty in all pooled ages is 95% except when indicated (68.5% confidence level), whereas errors in Table 4 are related to 1 sigma (%). All ages are expressed as weighed mean 207 Pb/206 Pb ages. SQUID software

was used for data reduction (Ludwig, 2001) and plots were prepared with Isoplot/Ex (Ludwig , 1999). Only SHRIMP II U-Pb zircon ages are mentioned throughout this paper, unless otherwise stated.

142

HARTMANN ET AL.

FIG. 8. A and B. Isotopic analyses of zircon crystals from sample 1 (Alto Bonito quartzite, Porongos Complex), displayed in a concordia diagram. C. Frequency histogram and probability curve of 206 Pb/207 Pb ages of sample 1.

The U content of zircon from sample 1, Alto Bonito quartzite, varies between 43 and 227 ppm. All Th/U ratios indicate magmatic origin (0.42–

1.06). SHRIMP dating (Fig. 8 and Table 4) of 21 spots on 21 zircon grains from sample 1 yields an array of data from 2459 ± 22 to 2030 ± 21 Ma, with concentration at 2087 ± 9 Ma (n = 18; MSWD = 1.1; 1 ). The main age peak on a Gaussian cumulative curve (Fig. 8C) is 2096 Ma. Secondary peaks are observed at 2030, 2055, 2134, and 2446 Ma. The U content of zircon from sample 2, Aberto dos Cerros quartzite, varies from 32 to 298 ppm, with most Th/U ratios indicating a magmatic origin (0.36–1.45). Only two grains (2-17 and 2-28; Table 4) have low Th/U ratios (0.06 and 0.09) and may therefore be of metamorphic origin. SHRIMP dating (Fig. 9 and Table 4) of 34 spots on 34 zircon grains from sample 2 yields ages between 3092 ±19 (grain 2-29) and 2015 ± 15 Ma (grain 2-19), with a concentration at 2078 ± 5 Ma (n = 16; MSWD = 0.98). The main age peak on a Gaussian cumulative curve (Fig. 9C) corresponds to 2082 Ma (Fig. 9C). Four additional peaks are present at 2481, 2210, 2141, and 2019 Ma (Fig. 9C). The U content of zircon from sample 3, Godinho quartzite, varies between 20 and 269 ppm. All Th/U ratios indi cate magmatic origin (0.27– 1.5 3). SHRIMP dating (Fig. 10 and Table 4) of 31 spots on 30 zircon grains from sample 3 yields an array of data from 2488 ± 12 Ma (grain 3-2) to 1990 ± 15 Ma (grain 3-25), with a concentration at 2079 ± 14 Ma (n = 16; MSWD = 1.3). In samples 1 and 2, the concordia plots have lower intercepts towards zero, but in sample 3 the lower intercept is at 1101 ± 230 Ma. Eight age peaks are identified on a Gaussian cumulative curve at 1998, 2029, 2056, 2096 (main peak), 2115, 2181, 2207, and 2488 Ma (Fig. 10C). The U content of zircon grains from sample 4, Coxilha do Raio quartzite, varies between 21 and 255 ppm. All Th/U ratios indicate a magmatic origin (0.41–2.26). SHRIMP dating (Fig. 11 and Table 4) of 31 spots on 31 zircon grains from sample 4 yields an array of data from 2449 ± 28 Ma (grain 4-30) to 1950 ± 78 Ma (grain 4-4), but with a concentration at 2056 ± 14 Ma (n = 21; MSWD = 1.9; 1 ). Four age peaks on a Gaussian cumulative curve are identified at 2029, 2080 (main peak), 2166, and 2453 Ma (Fig. 11C). The U content of zircon from sample 5, Figueiras quartzite, varies between 51 and 246 ppm. All Th/U ratios indicate magmatic origin (0.69– 1.56). SHRIMP dating (Fig. 12A and Table 4) of 7 spots on 7 zircon grains (analyses with two scans not shown) from sample 5 yields an array of data from 2486 ± 20 Ma (grain 5-37) to 2004 ± 13 Ma (grain 5-3x).


FIG. 9. A and B. Isotopic analyses of zircon crystals from sample 2 (Aberto dos Cerros quartzite, Porongos Complex), displayed on a concordia diagram. C. Frequency histogram and probability curve of 206 Pb/207 Pb ages of sample 2.

The ages scatter between 2004 and 2486 Ma. Four ages are older than 2400 Ma and are grouped at 2451 ± 12 Ma (n = 2) and 2477 ± 18 Ma (n = 2). The results obtained from five scans each were dupli-

143

FIG . 10. A and B. Isotopic analyses of zircon crystals from sample 3 (Godinho quartzite, Porongos Complex), displayed in a concordia diagram. C. Frequency histogram and probability curve of 206 Pb/207 Pb ages of sample 3.

cated at the spots previously analyzed with two scans. The age peaks identified on a frequency histogram of two scan analyses are shown in Figure 12B.

144

HARTMANN ET AL.

FIG . 12. A. Isotopic analyses of zircon crystals from sample 5 (Figueiras quartzite, Porongos Complex), displayed in a concordia diagram. Only analyses with five scans are shown. B. Frequency histogram of 206 Pb/207 Pb ages of sample 5; only analyses with two scans are shown, because the seven analyses with five scans were replicated on the same spots.

43 zircon grains from sample 6 yields an array of data from 2452 ± 12 Ma (grain 6-36) to 1998 ± 15 Ma (grain 6-32). Six age peaks are identified on a Gaussian cumulative curve at 2010, 2070 (main peak), 2099, 2168, 2343, and 2453 Ma (Fig. 13C).

Interpretation FIG. 11. A and B. Isotopic analyses of zircon crystals from sample 4 (Coxilha do Raio quartzite, Porongos Complex), displayed in a concordia diagram. C. Frequency histogram and probability curve of 206 Pb/207 Pb ages of sample 4.

The U content of zircon from sample 6, Jaíba quartzite, varies between 16 and 642 ppm. All Th/U ratios indic ate a magmatic origin (0.23–1.34). SHRIMP dating (Fig. 13 and Table 4) of 43 spots on

The quartzites (>90% detrital quartz in most samples) and feldspar-bearing quartzites (>80% quartz), consisting mainly of rounded, detrital quartz grains in all six samples (with a small volume of rounded feldspar grains), which demonstrates that the quartzites are deformed quartz sandstones and not altered granitic mylonites. Thus, a presumed origin from altered granitic mylonites is not supported.


The Porongos Complex was not deposited as a Neoproterozoic backarc or forearc association, because it does not contain zircon grains from the postulated arc, the Pelotas Batholith (780–590 Ma), or from any other Neoproterozic unit in the region such as the juvenile São Gabriel Belt (900–700 Ma). Backarc and forearc basins commonly have voluminous arc-related detritus (Busby and Ingersoll, 1995), which could be proven in the complex. In addition, all dated zircon grains in the Porongos Complex are much older (>1998 Ma) than the oldest known Brasiliano Cycle zircon (~900 Ma; Hartmann et al., 2000a). A Neoproterozoic arc-related geotectonic setting and terminology should not be considered any longer for the Porongos Complex. Some remarkable features emerge from the provenance interpretation of the isotopic data set. The 167 207 Pb/206 Pb ages obtained on the 166 detrital zircon grains from the six Porongos Complex quartzite samples show provenance nearly restricted to a 2488–1998 Ma Paleoproterozoic province (only two ages are Archean: 2528 ± 28 Ma and 3092 ± 19 Ma). The scarcity of Archean ages in the dated zircon grains indicates that the La China and Las Tetas complexes from Uruguay in the south, and Archean rocks from the Santa Catarina Granulitic Complex in the north, or equivalent units, were not sampled during the basin filling. Zircon detritus originated predominantly from sources similar to the nearby Encantadas Complex, i.e. from a Trans-Amazonian Cycle terrain, and from some older Paleoproterozoic sources (dominantly Siderian) such as the Neto Rodrigues Gneiss (Hartmann et al., 1998) and the Santa Maria Chico Granulitic Complex (Hartmann et al., 1999a). Archean continents in the southern Brazilian shield and West Africa were presumably far from the primeval Porongos Complex basin, because nearly only Paleoproterozoic zircons were shed into the basin. A Paleoproterozoic belt of comparable ages and sources (Birimian–Eburnean orogeny) occurs ~3,000 km to the ENE (in supercontinent Gondwana recons truction) in the Kedougou-Kéniéba Inlier, Senegal, West Africa (Hirdes and Davis, 2002), and seems to be comparable to the region studied. The marked cyclic distribution of detrital zircon ages in the Porongos Complex quartzites is very informative, because zircon ages from sedimentary rocks are often used for the investigation of orogenic pulses in provenance terrains (Machado et al., 1996; Rainbird et al., 2001; Nelson, 2001; Eriksson et al., 2001; Scott et al., 2002; Lahtinen et al., 2002; San-

145

FIG . 13. A and B. Isotopic analyses of zircon crystals from sample 6 (Jaíba quartzite, Porongos Complex), displayed in a concordia diagram. C. Frequency histogram and probability curve of 206 Pb/207Pb ages of sample 6.

tos et al., 2002, 2003b). Because intraplate magmatic processes commonly generate mafic rocks and small volumes of felsic or alkaline rocks, only a

146

HARTMANN ET AL.

FIG. 14. Frequency histogram (A) and cumulative frequency diagram (B) of 164 206 Pb/207 Pb analyses of zircon crystals from six samples, Porongos Complex.

small volume of zircon grains is newly formed in cratons. The zircon age peaks in quartz sandstones are therefore correlated with orogenic processes in the crust, because large volumes of felsic rocks are formed during orogenesis (Nutman, 2001). Only destructive plate-margin processes (accretionary subduction zone or continental collision) can cause crystallization or recrystallization of large volumes of zircon. We thus interpret the age peaks obtained in this investigation as orogenic magmatic events that occurred during the Paleoproterozoic in the southern Brazilian shield, as registered in the Porongos Complex quartzite zircon grains. An evaluation is also made of the relative contribution of magmatic versus metamorphic zircon to the quartz sands in the basin fill. The work of Hartmann et al. (2000a, 2002) has previously suggested the presence of two orogenies for the Trans-Amazonian Cycle in the southern Brazilian shield. However, Santos et al. (2003a) described four main orogenies between 2.26 and 2.01 Ga. The present data support the interpretation of four orogenies during this cycle, because the age peaks coincide with the timing attributed to the four events by Santos et al. (2003a). In the present study, marked peaks are observed at 2.27–2.20 Ga (5% of analyzed grains), 2.18–2.13 Ga (10%), 2.08–2.04 Ga (58%), and 2.02–2.00 Ga (8%). In addition, a previously unrecorded, strong age peak is observed at 2.10 Ga (2.11–2.09 Ga, 12% of analyzed zircon grains). Off-peak ages add up to 7%. A nearly continuous zircon production (interpreted here as crust production) is observed between 2.22 Ga and

2.00 Ga (Fig. 14), but the age peaks may actually correspond to strong, relatively sho rt-lived magmatic events, possibly discrete orogenies of the Trans-Amazonian Cycle (2.27–2.00 Ga). A novel observation is the presence of two age peaks in the Siderian period of the Paleoproterozoic, one at 2.48–2.44 Ga (7% of 168 analyzed grains) and another at 2.34–2.33 Ga (2%). The 2.47 Ga age was previously obtained by U-Pb SHRIMP analyses on magmatic zircons from the Neto Rodrigues gneisses located 80 km to the west of the Porongos Belt (Hartmann et al., 1998). Some granulite facies zircons have also yielded similar ages (Hartmann et al., 1999a). A few detrital zircon grains in the Passo Feio Norte deformed basin, located 30 km north of the Neto Rodrigues Gneiss, were also traced to the Siderian period at 2.47 Ga (Remus et al., 2000). Therefore, the present isotopic determinations indicate pre-Trans-Amazonian orogenic activity in the southern Brazilian shield, possibly representing two Siderian orogenies (peaks at 2.48–2.44 Ga and 2.34–2.33 Ga) because these zircon ages have been found in four different igneous and sedimentary rocks collected over a large portion (110 × 95 km) of the southern Brazilian shield. The observation of a zircon age peak near 2.34 Ga in the southern Brazilian shield is a major contribution to the elucidation of the tectonic evolution of the region. The previous record was restricted to a Th*-Pb electron microprobe age on monazite obtained by Tickyji et al. (2003) on a felsic garnet gneiss from the Santa Maria Chico Granulitic Complex (2.35 Ga). A geological event was dated in


zircon by conventional U-Pb at 2350 ± 30 Ma in the Santa Catarina Granulitic Complex, situated 700 km to the north of the Porongos Complex (Basei et al., 2000). The isotopic data are consequently interpreted here as the result of orogeny in the southern Brazilian shield near 2.34 Ga, caused by the accretion of continental masses before the Trans-Amazonian Cycle. The orogenies at 2.47 Ga and at 2.34 Ga are not part of the Trans-Amazonian Cycle (2.26– 2.00 Ga), because they are much older than the beginning of the Encantadas Orogeny at 2260 Ma. We propose the name Neto Rodrigues Orogeny for the event identified near 2.47 Ga (2.48–2.44), related to the type locality at Neto Rodrigues waterfall situated to the south of the town of Caçapava do Sul. The name Ivaró Orogeny is proposed for the event identified at 2.34 Ga (2.34–2.33 Ga), according to the type locality in the Santa Maria Chico granulites along the Ivaró Creek, situated west of the village of Ibaré. Therefore, in our interpretation, the Trans-Amazonian Cycle includes an orogeny at 2.11–2.09 Ga, in addition to the four orogenies previously dated by Santos et al. (2003a). The age peak near 2.10 Ga (Balcarce Granite, Brusque Complex) is well marked. Rocks of this age occur in the Sierra Azul Orogeny, presently proposed to identify the newly observed orogeny, as seen in the Sierra Azul Granite, Tandilia Belt of Argentina (Hartmann et al., 2002). The strongest age peak obtained by SHRIMP geochronology of detrital zircon grains from a Brusque Complex quartzite is also near 2.10 Ga (2105 ± 4 Ma), suggesting that the two belts (Brusque and Porongos) are coeval. Rocks of this age (2105 ± 4 Ma) have not been found in the Camboriú Complex, which is the basement of the Brusque Complex. Supe rcontinent Columbia was presumably formed by the collisions of island arcs, oceanic plateaus, microcontinents, and larger continental masses during the entire Trans-Amazonian Cycle between 2.26 and 2.01 Ga. Th/U ratios between 0.2 and 1.0 (Fig. 6) occur in zircon grains related to all the Paleoproterozoic ages identified (2.48–2.00 Ga), which means that the full range of granitic rocks from syenogranite to tonalite was formed almost continuously during the Trans-Amazonian Cycle. Some zircon Th/U ratios higher than 0.8 probably correspond to basaltic rocks (Nutman, 2001) that are known in the terrane, and possibly to A1-type granites, still unrecognized. At 2.0 Ga and somewhat later, major tectonic processes ceased and the crust cratonized, leading to favorable conditions for the

147

deposition of quartz sandstone and shale units such as those of the Porongos Complex. The age results from the six samples compare fairly well with the zircon SHRIMP U-Pb age distribution from a Brusque Complex quartzite (Hartmann et al., 2003a), sampled 700 km to the north of the Porongos Complex, both in the Tijucas Belt. The basement of the Brusque Complex schists is also made of a tonalitic-granodioritic gneiss association (Camboriú Complex), and is comparable to the basement of the Encantadas Complex in the core of the Porongos Belt. The large intervening area between the two exposed supracrustal geological units is covered by sedimentary and volcanic rocks of the Paraná Basin. This has prevented meaningful correlations. As summarized in Table 5, 12 age peaks are identified in the present zircon investigation of the Porongos Complex quartzites. All 12 age peaks have been recorded in previous zircon U-Pb SHRIMP investigations of igneous, metamorphi c, and metasedimentary rocks from the southern Brazilian shield (Table 6). These and coeval rocks are the source of the zircon grains now found in Porongos Complex quartzites. Timing of deformation is poorly established in the Porongos and Brusque complexes. This precludes the correct placement of the two units in the evolution of an orogenic belt. But the age of deformation is probably Neoproterozoic (about 0.78 Ga), because no major geological events were identified between 2.0 and 0.78 Ga in the extensive SHRIMP studies previously undertaken on rocks from the southern Brazilian shield (e.g., Hartmann et al., 2000a). The available geochronological evidence suggests deformation early in the Brasiliano Cycle (0.78 Ga). In this case, the Porongos and Brusque complexes are part of the Tijucas Belt, as originally proposed by Hasui et al. (1985) and Jost and Bitencourt (1980). The Tijucas Belt includes therefore all units deformed early (800–700 Ma) in the evolution of the Brasiliano Cycle. Because of the strong Neoproterozoic tectonic overprint, gneissic remnants of Paleoproterozoic age have been first recognized recently with SHRIMP geochronology (e.g., Hartmann et al., 2000a), after preliminary identification of remnants with Rb-Sr geochronology (e.g., Cordani et al., 1974). However, granites and gneisses dated at 2.26–2.00 Ga have been identified in large segments and as small remnants in the southern Brazilian shield.

(31) 28 1990 ± 15

(31) 28 2018 ± 25

3

4

6

3093

2452 ± 12 (1)

2453

–

2453

2476

–

2451 ± 12 (2)

2453

–

2477 ± 9

2449 ± 28 (1)

–

2488

–

–

–

–

2343

2343 ± 7 (1)

–

–

2333

2329 ± 27 (1)

2399

2398 ± 16 (1)

–

2446

2477 ± 8 (2)

2481

3092 ± 19 2477 ± (3) (1)

–

IV

2438 ± 11 (2)

III

I

II

Siderian

Mesoarchean

Pre- Trans-Amazonian ages

–

–

–

–

2275

2274 ± 27 (1)

–

–

–

–

V

2200

2203 ± 16 (1)

2192

2190 ± 17 (1)

–

–

2207

2207 ± 10 (4)

2210

2210 ± 13 (1)

–

–

VI

2168

2161 ± 20 (5)

–

–

2166

2164 ± 10 (6)

–

–

–

–

–

–

VII

–

–

2152

2149 ± 12 (1)

–

–

–

–

2141

2140 ± 14 (3)

2134

2130 ± 23 (2)

VIII

Rhyacian

–

–

2080

2079 ± 14 (16)

2082

2078 ± 5 (16)

2096

2087 ± 9 (18)

X

2099

2070

2101 ± 5 2069 ± 4 (18) (10)

–

–

–

–

2115

2109 ± 10 (10)

–

–

–

–

IX

Trans-Amazonian ages

–

–

–

–

2059 ± 14 (16)

2056

2056 ± 9 (4)

2049

2044 ± 13 (3)

2055

2043 ± 11 (6)

XI

2010 (2)

2015 ± 6 (2)

2006

2004 ± 13 (1)

2029

2020 ± 11 (5)

2029

2023 ± 15 (3)

2019

2016 ± 14 (2)

2030

–

XII

Orosirian

= sample number (1 to 6); first row contains ages calculated on concordia plots; ages of second row represent Gaussian curve peaks (figs x-y); number between brackets after the age indicates the number of analyses used on each age calculation; (n) = total number of analyses of each sample; n = total number of analyses used on age calculations of each sample on concordia plot; I-XII = ages of the main detected magmatic events.

1#

(43) 43 1998 ± 15

2004 ± 13

(34) 32 2015 ± 15

2

(7) 6

(22) 19 2030 ± 21

1

5

(n) n Youngest grains

#

TABLE 5. Main U-Pb SHRIMP Ages (n = 167) of Zircon Grains (166) from Six Quartzite Samples from the Porongos Complex1

148 HARTMANN ET AL.

2066 ± 9 Villa Monica

La Plata Craton Tandilia Belt, Argentina4

2073 ± 7 Chacofy

2077 ± 6 Isla Mala5

2109 ± 15 Azul

2113 ± 7 Isla Mala5

2108 ± 5 Caçapava9

2098 ± 6 Brusque0 2106 ± 7 Águas Mornas7

IX

2146 ± 5 Paso Severino3 2140 ± 6 Rivera3

2132 ± 23 Encantadas8 2149 ± 16 (2) Passo Feio9 2134 ± 15 Caçapava9 2131 ± 18 Arroio Ratos1

2148 ± 5 Brusque0

VIII

VII

VI

2168 ± 7 Ruta 30 2170 ± 14 Villa Monica 2176 ± 6 Punta Tota

2168 ± 8 Rivera3

2177 ± 23 S.M. Chico6

2194 ± 6 Balcarce 2197 ± 5 Cerro Triunfo

2206 ± 31 Caçapava6

2167 ± 20 2191 ± 12 Camboriú7 Águas Mornas7 2168 ± 18 2205 ± 9 S.C. Granulite2 Itapema0 2175 ± 13 Águas Mornas7 2175 ± 7 Itapema0

2219 ± 7 Brusque0 2201 ± 7 Pres. Nereu7

Possible sources

2179 ± 6 Brusque0 2162 ± 15 Miséria0

Rhyacian

2234 ± 15 Calvario

2224 ± 4 Rivera3

2256 ± 8 Encantadas8 2241 ± 29 Passo Feio9 2279 ± 14 Caçapava9

V

2377 ± 9 Punta Tota

2389 ± 25 Passo Feio9 2344 ± 16 Caçapava6

2336 ± 24 Pres. Nereu7

IV

II

Mesoarchean I

2449 ± 16 2477 ± 8 (3) 3084 ± 10 (2) Caçapava6 Passo Feio9 Passo Feio9 2449 ± 18 Passo Feio9 2448 ± 7 Neto Rodrigues10

Siderian III

(superscripts): 0 = Hartmann et al., 2003a,2003b; 1 = Leite et al., 2000; 2 = Hartmann et al., 2000a; 3 = Santos et al., 2002; 4 = Hartmann et al., 2002; 5 = Leite et al., 2000; 6 = Leite et al., 1998; 7 = Silva et al., 2000; 8 = C. C. Porcher , pers. commun.; 9 = Remus et al., 2000. An asterisk after a reference number indicates an age that has been recalculated in the present study.

1References

2056 ± 6 Soca3 2065 ± 9 Isla Mala5 2058 ± 3 Valentines3

La Plata Craton Piedra Alta belt, Uruguay3, 5

2023 ± 23 2052 ± 5 2078 ± 13 S.M. Chico6 Encantadas8 Arroio Ratos1 2013 ± 12 2058 ± 32 2069 ± S.M. Chico6 Caçapava6 Passo Feio9 2021 ± 21 2053 ± 6 Passo Feio9 Caçapava9

La Plata Craton Southern Brazil Shield Rio Grande do Sul 1, 6, 8, 9, 10

2070 ± 16 Brusque0

X

2023 ± 7 2043 ± 11 Brusque0 Pres. Nereu7 2012 ± 22 2048 ± 15 Pres. Camboriú7* 7* Nereu 2021 ± 7 2044 ± 8 Camboriú7* Camboriú7 2021 ± 14 Itapema0

XI

La Plata Craton Southern Brazil Shield Santa Catarina0, 2, 7

Main magmatic events

Orosirian XII

TABLE 6. Previously Dated Rocks from the Southern Brazilian Shield (zircon U-Pb SHRIMP): Possible Provenance of Dated Zircon Grains from the Porongos Complex1


149

150

HARTMANN ET AL.

FIG. 15. Th/U ratios of analyzed zircon crystals versus ages (n = 164 analyses).

The Porongos Complex may be a deformed Mesoproterozoic basin as suggested by Basei et al. (2000), but the correct placement of the Porongos Complex in an orogenic belt must await further geochronologic studies to determine accurately the age of deformation. The present study indicates that the detritus was shed from the underlying Encantadas Complex and other rocks in the shield, therefore not supporting the allochthonous terrane hypothesis proposed by Basei and Hawkesworth (1994). The youngest zircon grain we identified was dated at 1998 ± 15 Ma. The Porongos basin fill is consequently younger than 1998 ± 15 Ma, but a more precise age of the fill cannot be established based on the present data. The oldest Neoproterozoic granitic zircon is ~780 Ma (Silva et al., 1999) in the contiguous Dom Feliciano Belt, but ~880 Ma zircon is known from the Passinho Diorite located 80 km to the west (Leite et al., 1998). This sets limits on the age of Porongos basin fill between 1998 ± 15 Ma and 880 Ma. Further investigations are required to tighten the age parameters for the Porongos basin fill. The possible tectonic scenarios for the Porongos Complex basin generation and fill are restricted to: (1) a cratonic cover of supercontinent Columbia— sedimentation during Late Paleoproterozoic (Statherian) or Mesoproterozoic; and (2) a passive margin of supercontinent Columbia prior to Brasiliano Cycle tectonics—deposition near 1000 Ma (the Brasiliano Cycle deformation started at 900 Ma). The basin does not show the usual characteristics of foreland basins, because of the absence of coarse clastic sediments, so it cannot be related to the Trans-Amazon Cycle. In short, the cratonic provenance of the sand is well-established based on the large amount of

quartz (e.g., Potter, 1994), which implies that the basin was not filled in an orogenic environment. Additional robust age determinations, e.g., U-Pb SHRIMP of xenotime overgrowths on zircon crystals or baddeleyite ages from the associated volcanic rocks, may lead to the delimitation of these hypotheses into a better constrained geotectonic scenario. Because of the difficulties related to the determination of specific zircon provenance (Nelson, 2001), we rely on known, dated exposures from the southern Brazilian shield (Table 6) and on the chemical composition of the analyzed grains to delimit source rocks. A tonalite from the Encantadas Complex was dated by U-Pb SHRIMP at 2256 ± 8 Ma by C. C. Porcher (unpubl.) and is presently interpreted as the source of the detrital zircon grains of that age. The age of metamorphic rims on zircon from the same tonalite sample is 2052 ± 5 Ma (Th/U 0.2). Age peaks near 2.15 Ga and 2.07 Ga are known from: (a) the Piedra Alta Terrane, Uruguay (Hartmann et al., 2001) 800 km to the south; (b) from the Tandilia Belt, Argentina (Hartmann et al., 2002), 2000 km to the south; (c) from the Arroio dos Ratos Complex 50 km to the north; (d) and from the Camboriú Complex, Santa Catarina, 700 km to the north. These Paleoproterozoic units are possibly part of a formerly continuous belt of Trans-Amazonian Cycle rocks exposed in the southern Brazilian shield—an unnamed orogen that has partly eroded and shed into the Porongos basin. The chemistry of the zircon grains we analyzed is indicative of an even more varied provenance. The Th/U ratios (Fig. 15) vary between 0.27 and 2.26, except for four analyses, and reflect a dominant provenance from syenogranite, monzogranite, and granodiorite (commonly Th/U = 0.2–0.6), and from tonalite, trondhjemite, and diorite (commonly Th/U = 0.5–1.0). However, Th/U ratios >0.8 more likely reflect a basaltic source (Nutman, 2001) or an A1type granitic source (Santos et al., 2003c). Such granite types are not known from the southern Brazilian shield. The Belizário ultramafic amphibolite from the adjoining Encantadas Complex contains zircon crystals dated at 2257 ± 12 Ma (Hartmann et al., 2003b), with Th/U = 0.10–1.75, and could be one of the sources of the high Th/U zircon grains, but more varied mafic sources are likely. Zircon crystals and portions of crystals that have been recrystallized or newly crystallized during metamorphism tend to have low Th/U ratios

151


(