Gondwana Research. Time frame of Ma juvenile accretion during the São Gabriel orogeny, southern Brazilian Shield

Transcription

1 Gondwana Research 19 (2011) Contents lists available at ScienceDirect Gondwana Research journal homepage: Time frame of Ma juvenile accretion during the São Gabriel orogeny, southern Brazilian Shield L.A. Hartmann a,, R.P. Philipp a, J.O.S. Santos b, N.J. McNaughton c a Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500; Porto Alegre, Rio Grande do Sul, Brazil b RedStone Resources, 110 East Parade, East Perth 6004 WA, Australia c John de Laeter Centre of Mass Spectrometry, Applied Physics, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia article info abstract Article history: Received 13 July 2009 Received in revised form 30 April 2010 Accepted 4 May 2010 Available online 20 May 2010 Keywords: Cambaí Complex Geochronology Neoproterozoic Juvenile Brazilian Shield São Gabriel orogeny The time frame of the three main geological events in the Neoproterozoic Cambaí Complex, juvenile São Gabriel belt in the southern Brazilian Shield is established by integrating field mapping, back-scattered electron imaging and sensitive high-resolution ion microprobe (SHRIMP II) U Pb dating of 96 zircon crystals from nine granitic and metasedimentary rock samples. The three events are: (1) voluminous flat-lying paragneisses (Cambaizinho Complex) and orthogneisses (Vila Nova gneisses) between 735 and 718 Ma, (2) tonalite trondhjemite association (Lagoa da Meia-Lua Suite) between 710 and 690 Ma, and (3) late granodiorite intrusions (Sanga do Jobim Suite) at 680 Ma. An additional older volcanic event (Campestre Formation) was dated at 753 Ma. These results are most significant for the reconstruction of West Gondwana International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction Understanding the time frame of Precambrian evolution is essential for the reconstruction of accretionary belts and evolution of supercontinents. Detailed field mapping clarifies the relative ages of geological units, such as the sequential intrusion of granitic rocks and shear zones, but robust geochronology is required for the reliable determination of the absolute ages of rock crystallization. Orogenic cycles commonly last for several hundred million years, the duration of a full Wilson Cycle of oceanic crust generation and consumption (Condie, 1997; Condie et al., 2009); such a cycle evolves from accretionary orogenies in the beginning to a great final collision of continental blocks. The Brasiliano Cycle ( Ma) in the southern Brazilian Shield includes the accretionary São Gabriel orogeny ( Ma) and the collisional Dom Feliciano orogeny ( Ma) (Hartmann et al., 2000; Heilbron and Machado, 2003; Heilbron et al., 2004) and extends into southeastern Brazil (Silva et al., 2005; Borba et al., 2006; Schmitt et al., 2008; Borba et al., 2008). We presently focus on the main accretionary events because of their importance for understanding the crustal evolution of this southwestern portion of Gondwana Supercontinent. Neoproterozoic juvenile terranes are extensive (N1000 km wide) in the Arabian Nubian Shield, but less extensive in South America. A large (about 500 km long) belt occurs in central Brazil, the Goíás Arc Corresponding author. address: leo.hartmann@ufrgs.br (L.A. Hartmann). (Pimentel and Fuck, 1992; Laux et al., 2005; Matteini et al., 2010) and a smaller (about 100 km long) belt in southernmost Brazil, the São Gabriel belt (Babinski et al., 1996) where the existence of Meso/ Neoproterozoic juvenile oceanic crust and island arc rocks formed during the Brasiliano orogenic events was demonstrated by Saalmann et al. (2005b). All three terranes were formed between 800 and 550 Ma, but accretion of granitic rocks to the crust occurred at different ages. The main peak in NE Africa was Ma and in central Brazil spread between 900 and 700 Ma. In the São Gabriel belt, previous investigations (e.g., Hartmann et al., 2000; Saalmann et al., 2005a, b, c, 2006a, b 2007) indicate an age peak near Ma, the youngest intrusion occurring at about 704 Ma. The knowledge of the ages of magmatic and metamorphic events is essential for the reconstruction of the evolution of West Gondwana, as seen in previous investigations (e.g., Vaughan and Pankhurst, 2008). Because tonalite trondhjemite granite (TTG) associations are most significant for the evolution of juvenile continental crust (Philipp et al., 2008; Senshu et al., 2009) and because detailed geochronological investigations can define the timing of generation of different portions of the juvenile crust, we concentrated field and laboratory (sensitive high-resolution ion microprobe SHRIMP II) investigations on the Cambaí Complex, southern Brazilian Shield (Fig. 1). We selected the Sanga do Jobim, Vila Nova and Palma regions (Fig. 1), because field relationships indicate that the rocks are particularly suitable for understanding the timing of events in this segment of Western Gondwana. Field mapping and petrography indicate that the juvenile terrane is constituted by three main units, namely (1) amphibolite X/$ see front matter 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi: /j.gr

2 L.A. Hartmann et al. / Gondwana Research 19 (2011) Fig. 1. Regional geological map of western Rio Grande do Sul shield. Inset shows location of studied area; areas in black are cratons (older than 1.0 Ga), gray area is the Brasiliano cycle mobile belt and in white is the Andean orogen. facies, voluminous, flat-lying gneisses of mostly sedimentary (Cambaizinho Complex ) and plutonic (Vila Nova gneisses, Cambaí Complex) derivation, (2) less deformed, diorite tonalite trondhjemite rocks (Lagoa da Meia-Lua Suite, Cambaí Complex), and (3) intrusive, little deformed, granodiorite and tonalite plutons (Sanga do Jobim Suite, Cambaí Complex). The intrusion of the granitic rocks was intimately associated with the development of the NE-trend, steep dipping shear zones. Greenschist and amphibolite facies volcanic belts also occur. Nine key rock samples were dated by SHRIMP using 96 zircon crystals; these were supported by back-scattered electron images of all the crystals. From these observations we delimit precisely the timing of accretionary events related to the consolidation of Supercontinent Gondwana in its southwestern margin. 2. Geology and samples The São Gabriel belt is located in the southern portion of the Brazilian Shield (Fig. 1), which includes all Precambrian rocks exposed at the surface in South America. The shield contains juvenile granitic and volcanic rocks of the Neoproterozoic Brasiliano Cycle in the region (Babinski et al., 1996). The concept of Brazilian Shield is distinct from the usage of South American Platform, because the shield does not include the cover intracratonic basins. The geology of the belt (Fig. 1) has been investigated for several decades (e.g., Jost and Hartmann, 1984; Chemale, 2000; Hartmann et al., 2007), including sensitive high resolution ion microprobe (SHRIMP) age determinations of zircons from many rocks (e.g., Hartmann et al., 2000). Several mapping projects were undertaken by senior undergraduate students of Universidade Federal do Rio Grande do Sul; the reports remain unpublished but can be accessed in the libraries (Bitencourt et al., 1996, 1997, 2001, 2002, 2004). Structural relationships and a plate tectonic model for the evolution of the São Gabriel belt, in the context of the Brasiliano Cycle, are presented by Saalmann (2004) and Saalmann et al. (2005a, b, c, 2006a, b). These relationships constitute the base of the present field and geochronological investigation. Mantle and crustal evolution in the São Gabriel belt and surrounding geotectonic units are interpreted by Gastal et al. (2005a, b) using Nd and Sr isotopes as a proxy for the processes. The São Gabriel belt has a complex arrangement of geological units, as seen on the geological map (Figs. 1 and 2) and stratigraphic column (Figs. 3 and 4); see also Appendix A. Thrusting to the SE further complicates the geological structure of the belt (Figs. 3 and 5). A large step forward in the understanding of the geological evolution of the belt was made by Saalmann (2004) and Saalmann et al. (2005a, b, c) and is used as the reference for this investigation. Particularly significant is the division of the Palma Group into a lower part (Cerro do Ouro Formation) and an upper part (Campestre Formation); the lower part has evidence of deformations D1 and D2, not observed in the upper part, which only displays D3 and D4 deformations (Figs. 3 and 5). The rocks of the Cambaizinho Complex also have evidence of D1 and D2 deformations. Also most significant is the observation that the granitic rocks of the Cambaí Complex (Lagoa da Meia-Lua Suite) were intruded coeval with the upper Palma Group (only the D3 and D4 structures of the Palma Group are present in the Lagoa da Meia-Lua Suite. The Vila Nova gneisses (Cambaí Complex) show structures formed during D1 and D2. The granitic rocks were deformed in subvertical shear zones, whereas the same deformational event caused flattening in the country-rock schists of the Palma Group Cambaizinho Complex The Cambaizinho Complex, composed mostly of paragneisses, is coeval with the lower Palma Group and has a strong flat-lying foliation developed under amphibolite facies conditions. Amphibolite-facies, flat-lying orthogneisses (Vila Nova gneisses) are also coeval, but are included in the Cambaí Complex, because this stratigraphic unit includes all granitic rocks in the São Gabriel belt. The Cambaizinho Complex includes mostly paragneisses and has the oldest rock unit in the São Gabriel belt. These paragneisses (sample RL1) occur in many outcrops as xenoliths in the orthogneisses, indicating that they are the oldest unit from field relationships. The paragneisses have mostly quartzo-feldspathic composition, interpreted as meta-arkoses, and have a significant volume of intercalated lenses of pelitic gneisses (garnet biotite, staurolite garnet and plagioclase quartz muscovite biotite assemblages), quartzites, calcitic and dolomitic marbles, and calc silicate gneisses. These lenses of paragneisses (Cambaizinho Complex) are segmented and form roof pendants (tens to hundreds of meters long) in the dominant dioritic tonalitic trondhjemitic orthogneisses (Vila Nova gneisses). In the westernmost portion of the São Gabriel belt, the paragneisses form a continuous body of 80 km length from NW Palma region, passing through Passo do Ivo and reaching the Cambaizinho creek, the type-

3 86 L.A. Hartmann et al. / Gondwana Research 19 (2011) Fig. 2. Geological map of Vila Nova region (many authors; most recently Bitencourt et al., 2001). Location of eight studied samples indicated; only one sample is located outside the map area. locality of the Cambaizinho Complex. Some slivers of other rocks are also intercalated in the paragneisses; these are mostly mafic ultramafic rocks such as magnesian schists, serpentinites, and minor deformed peridotite, gabbro, norite, troctolite and anorthosite. All these rocks were deformed and recrystallized during orogenic activity. Fig. 3. Stratigraphic description of studied area (Saalmann et al., 2005a).

4 L.A. Hartmann et al. / Gondwana Research 19 (2011) Fig. 4. Summary of stratigraphic names and ages (Saalmann et al., 2005a). The main exposure of quartz feldspar paragneisses occurs in the Rufino Farias region and still has remnants of sedimentary layering and the S1 metamorphic banding followed by three main folding episodes. The sedimentary layering is identified in continuous and compositionally distinct layers each typically with regular alternation of grain size; this is observed particularly in the quartz-feldspar gneisses, quartzites and marbles. The two oldest deformational events generated flat-lying structures, the S 1 metamorphic banding and its transposition into S 2.F 2 folds are isoclinal and are preserved as recumbent, rootless folds. This was followed by F 3 subvertical transcurrent faults, leading to the refolding of the previously formed structures and generating normal, dipping anticlines and synclines. This third deformational phase formed the regional structure marked by NE-trending foliation, dipping to NW and SE. A last F 4 phase is seen in refolding of F 3 fold axes, with formation of axial fracture cleavage oriented NW-SE. The paragneisses from the Cambaizinho Complex and the orthogneisses from the Cambaí Complex (Vila Nova gneisses) were metamorphosed during two events in the middle to the upper amphibolite facies (M 1 and M 2 ). These conditions were attained during development of foliations S 1 and S 2. A third M 3 is related to steep dipping shear zones and occurred in variable conditions from greenschist to low amphibolite facies Orthogneisses (Vila Nova gneisses, Cambaí Complex) This is the dominant unit in the Cambaí Complex. Steep shear zones also delimit the occurrence of orthogneisses (samples SL2, SL19), which are exposed in an elongated, extensive NE SW unit from Sanga do Jobim to Sanga do Velocindo about 20 km. These two creeks are located respectively to the west and to the east of the town of Vila Nova do Sul. The best exposures of orthogneisses are situated to the east of Rufino Farias village and in the beds of the following creeks: Sanga do Jobim, Cambaí, Laranjeiras, and Sanga do Velocindo, all in the Vila Nova do Sul region. The unit extends 50 km to the SW into the eastern part of Palma region. Fig. 5. NW SW cross-section of studied area (Saalmann et al., 2005a).

5 88 L.A. Hartmann et al. / Gondwana Research 19 (2011) The orthogneisses (Vila Nova gneisses, Cambaí Complex) are delimited to the west by magnesian schists and serpentinites. These are cut by younger bodies of diorite tonalite trondhjemite granodiorite (Lagoa da Meia-Lua Suite) and granodiorite (Sanga do Jobim Suite), both belonging to the Cambaí Complex. The gneisses form lens-shaped and tabular bodies; tonalites are dominant, but diorites and trondhjemites are also present. The trondhjemitic gneisses form thin (1 50 mm) bands in the tonalites, whereas the diorites are thicker ( cm). Banding is regular, continuous and ranges in thickness from millimeters to centimeters. It may show intense mineral orientation and biotite hornblende schlieren. Some primary magmatic, plutonic textures are preserved, such as blastoequigranular hipidiomorphic texture and blasto-poikilitic texture, particularly by prismatic plagioclase. The four deformational events found in the paragneisses are also present in the orthogneisses, namely S 1 metamorphic banding succeeded by three main folding episodes. Granoblastic plagioclase and quartz predominate in bands, which are intercalated with nematoblastic to lepidoblastic biotite and hornblende bands. The main mineral assemblage is plagioclase quartz hornblende biotite epidote, indicative of middle amphibolite facies metamorphic conditions. In the least deformed portions, the gneissic structure changes into a well-marked foliation of biotite and hornblende Lagoa da Meia-Lua Suite (Cambaí Complex) This suite is part of the Cambaí Complex and consists of tonalite, diorite, granodiorite and trondhjemite. The granitic rocks (dated samples RL6, RL12, RL15 and SL6) were intensely deformed in ductile conditions and range from tabular bodies with either subhorizontal or subvertical foliation to more massive structure. In general, the granitic bodies are elongated NE SW parallel with the metamorphic foliation of the paragneisses and orthogneisses. The foliation is marked by the orientation of biotite and hornblende, but mylonitic textures developed in high strain portions; these portions display consistent mineral stretching and partial recrystallization of plagioclase, K-feldspar and quartz. There is also formation of chlorite, epidote and white mica. In a few outcrops, the rocks are banded and have intercalation of decimeter to meter-thick bands of diorites and tonalites. Granitic bodies of the Lagoa da Meia-Lua Suite are mostly concordant with S 3 foliation of the orthogneisses, although some are locally discordant Sanga do Jobim Suite (Cambaí Complex) This suite includes the youngest granitic bodies of the Cambaí Complex judging from field relations and is composed (dated samples SL4 and RL4) of tabular, concordant granitic bodies and intrusive, elliptical bodies with variable composition from diorite and tonalite to granodiorite. The suite includes the Capivara diorite (Garavaglia et al., 2002), the Sanga do Jobim granodiorite and the Cerca de Pedra tonalite. Texture is equigranular, medium to coarse grained and principal minerals are plagioclase and quartz, with some biotite and hornblende, in addition to minor K-feldspar, epidote, titanite, zircon and apatite. Its structure is massive in most of the granitic body but there is some mineral orientation along the contacts with the other granites; protomylonitic to mylonitic textures occur in a few exposures. The nine dated samples from the Cambaí Complex and Cambaizinho Complex (Figs. 6 and 7; description in Appendix A) were selected from key exposures in the Vila Nova region (Fig. 1). One dated sample (RL1) is from the Cambaizinho Complex and two orthogneiss samples (SL19 and SL2) are from the Cambaí Complex. Judging from field relationships, all three samples are representative of the oldest events, because they have flat-lying foliation, gneissic structure, middle to upper amphibolite facies metamorphism, and granoblastic texture. Four dated samples (RL6, RL12, RL15, and SL6) belong to the main granitic unit (Lagoa da Meia-Lua Suite) that forms the juvenile crust. The two additional samples (SL4 and RL4) are from the youngest granitic intrusions (Sanga do Jobim Suite) based on field relationships magmatic structures and textures, little shear-zone deformation. This sampling, integrated with previous studies, forms the basis for a reliable time frame for the evolution of the São Gabriel orogeny (Cambazinho and Cambaí Complexes), the granitic and mediumgrade metasedimentary portion of crust formed and deformed during the São Gabriel orogeny. The three main geological structures observed in the field can thus be dated (1) flat-lying, amphibolite facies para- and orthogneisses, (2) volumetrically dominant, shearzone related diorite tonalite trondhjemite, and (3) late-tectonic granodiorite plutons. 3. SHRIMP U Pb zircon geochronology The overall time frame of rock formation and deformation is strongly bimodal in the southern Brazilian Shield (Hartmann et al., 2000, zircon/shrimp; Tickyj et al., 2004, monazite/electron microprobe) with one age peak at Ma, the Trans-Amazonian Cycle, and another at Ma, the Brasiliano Cycle, with few datable Archean rocks. Both orogenic cycles have similar evolution, starting with juvenile accretion of granitic rocks ( Ma and Ma) and ending with collision of continental plates ( Ma and Ma). The intervening time period ( Ma) corresponds to the position of the southern Brazilian Shield inside Columbia Supercontinent (Hartmann, 2002; Rogers and Santosh, 2009; Santosh et al., 2009; Ramos et al., 2010). The discovery of the juvenile terrane in the São Gabriel belt was based on Nd isotopes and widespread occurrence of tonalites, trondhjemites and granodiorites, and included the TIMS dating of zircon crystals from a diorite at 704 Ma (Babinski et al., 1996). Machado et al. (1990) had already established the magmatic age (zircon TIMS) of a rhyolite from the terrane at 753 Ma. Similar ages (zircon SHRIMP) were obtained by Leite et al. (1998) in the Mantiqueiras section of the Cambaí Complex. This timing was simplified and extensively used as Ma for the delimitation of the São Gabriel orogeny (e.g., Hartmann et al., 2000; Saalmann et al., 2005a, b, c). These previous zircon geochronological investigations are presently re-evaluated and integrated with new zircon SHRIMP geochronology of nine rocks to establish the time frame of the São Gabriel orogeny of juvenile accretion in the São Gabriel belt. Ninety six zircon crystals were separated from nine rock samples by crushing and milling 5 10 kg of each rock followed by heavy liquid and magnetic methods at the laboratories of Universidade Federal do Rio Grande do Sul. The crystals were mounted on an epoxy disc, polished to half their thicknesses and carbon coated for backscattered electron imaging at the University of Western Australia. The mount was repolished and gold coated for SHRIMP II U Pb isotopic determinations at Curtin University of Technology, Western Australia (Smith et al., 1998). Data reduction used the SQUID software (Ludwig, 2001) and plots were prepared with Isoplot/Ex (Ludwig, 1999). Our geochronological investigation included the study of the internal structure of the zircon crystals by electronic imaging, backscattered electrons (BSE), prior to isotopic study (Figs. 8 10), as done in many similar studies (e.g., Ali et al., 2009; Chen et al., 2010). All zircon crystals display complex internal structure with two dominant structural domains. One domain is the magmatic portion (commonly homogeneous and dark grey in BSE images) and the other is the metamorphic (shear-zone related) portion that is commonly bright grey in BSE image. In the following discussion, the light grey portions are considered the product of alteration by regional metamorphism of the dark gray portions, which are taken as the original magmatic composition of the crystals. This is based on extensive observation of

6 L.A. Hartmann et al. / Gondwana Research 19 (2011) Fig. 6. Field photos of some of the studied units. (a) Gt bt gneiss, Cambaizinho Complex (sample RL1), (b) Orthogneiss, Vila Nova gneisses, Cambaí Complex (sample SL19), (c) Tonalite (sample SL6) and trondhjemite pegmatite (sample RL6) from the Lagoa da Meia-Lua Suite, Cambaí Complex, and (d) Cerca de Pedra granodiorite (sample SL4), Sanga do Jobim Suite, Cambaí Complex. internal structure, chemical and isotopic compositions of zircons from South America (e.g., Hartmann et al., 2000; Silva, 2006). Zircon crystals from orthogneisses (Fig. 8) have complex internal structures. Titanite crystals from sample SL2 are rounded to embayed, have anhedral shapes, and range in size from 100 to 200 μm. They also have large quartz inclusions and narrow metamict seams. The fractures in the crystals end as they reach lighter (in BSE) parts of the crystal (interpreted as younger, recrystallized zircon). Crystals from samples RL1 and SL19 are similar. The zircon crystals from the main magmatic stage (samples RL6, RL12, RL15, and SL6) are all prismatic, size between 100 and 300 μm, aspect ratio 3:1 to 5:1 (Fig. 9). The crystals are mostly euhedral, but rounding is omnipresent and ranges from pronounced to angular. Mineral inclusions are common, and may include apatite. All crystals show some fractures in the dark grey portions (magmatic), and these are sealed in many places by light grey zircon (metamorphic). The imaged zircon crystals from sample RL6, trondhjemite BR290, have an aspect ratio of 4:1, lengths near 200 μm(fig. 9a, b) and euhedral Fig. 7. Photomicrographs of some of the studied samples. (a) Sample RL1, gt bt gneiss, Cambaizinho Complex, (b) Sample SL19, orthogneiss, Vila Nova gneisses, Cambaí Complex, (c) Sample RL6, trondhjemite, Lagoa da Meia-Lua Suite, Cambaí Complex, and (d) Sample RL4, Sanga do Jobim Tonalite, Sanga do Jobim Suite, Cambaí Complex.

7 90 L.A. Hartmann et al. / Gondwana Research 19 (2011) Fig. 8. Back-scattered electron images of zircon crystals from sample SL2, orthogneiss, Vila Nova gneisses, Cambaí Complex. Black circles indicate local and size of SHRIMP analyses. Analyses number and age shown. external faces, although some rounding is observed caused by metamorphism. Inclusions are present, probably apatite. Darkest patches are metamict, which are present in several crystals; these are restricted to an original euhedral zone of the crystal. Radial and crosscutting fractures are present in many crystals. Although complex, their internal structure is divided into dark grey and light grey portions. The light grey portions occur anywhere in the crystals, in the core, mantle and rim. It is noteworthy that fractures originally established in the dark grey portion were sealed during recrystallization of zircon in the light grey portion, a process described as fracture sealing by Hartmann et al. (1997), processes described by Geisler et al. (2007) and Harley et al. (2007). Sample RL12, the Santa Zélia granite, has zircon crystals μm in length and an aspect ratio 3:1 with many euhedral faces (Fig. 9). Mineral inclusions are common, and some are probably apatite. Some very dark portions are observed, both irregular or following approximately the internal euhedral structure, and are interpreted as metamict portions. Some crosscutting and radial fractures are also observed. The light grey portions occur mostly in the cores of crystals, but are also observed in the mantle and rims. Fractures are sealed by recrystallization (light grey in BSE) of the original magmatic zircon (dark grey in BSE). RL15, the Buriti meta-tonalite, has zircons with an outstanding internal structure displaying dark grey cores surrounded by light grey rims (Fig. 9). The crystals are μm long, with an aspect ratio of 2:1. Although magmatic euhedral faces are present in some crystals, rounding is a common feature in many crystals. Crosscutting fractures are also present; several fractures are restricted to the dark grey (magmatic) core of the crystal and sealed by metamorphic recrystallization (light grey portion in BSE) of the magmatic rims. Some of the fractures well-marked in the dark grey portion have faint extensions into the light grey portions, a result of partial sealing of the fracture during metamorphism. In sample SL6, the tonalite BR290, the zircon crystals are μm long, aspect ratio 5:1, and have euhedral external faces with very little rounding (Fig. 9). Few crosscutting fractures are observed in the dark grey portions which are sealed in places by light grey zircon. The recrystallized (light grey in BSE) zircon forms large bands, irregular patches and narrow rims on the zircon. Zircons from the two samples (SL4 Cerca de Pedra granodiorite and RL4 Sanga do Jobim granodiorite) are from little-deformed, well-defined intrusive bodies, and are similar in the two rocks (Fig. 10). The crystals are μm long, have an aspect ratio of 4:1, are euhedral, and have irregular to planar metamict portions (very dark in BSE). Some fractures crosscut the crystals, well-marked in the dark grey in BSE (magmatic) portions and faint to nonexistant in the light grey (metamorphic) portions. The fainting of the fracture trace into the metamorphic portion is due to relative intensity of fracture sealing. Dark grey cores are preserved in a few crystals, but alteration was very intense so that the light grey in BSE portions predominate and occur everywhere in the crystal. The significance of the U Pb ages of the zircon crystals (Fig. 11, Table 1) is better understood when the ages are associated with the U, Th contents and Th/U ratios of the zircon (Fig. 12, Table 1). In sequence, the three oldest, most deformed rocks (SL19, SL2, and RL1) are described first, followed by the main magmatic stage (samples RL6, RL12, RL15, and SL6) and then the two samples from the waning stage of granitic magmatism in the arc (SL4 and RL4). In sample SL19, the Vila Nova gneiss (tonalite), nine analyses of nine crystals reveal low U contents near 100 ppm and low Th contents of 50 ppm resulting in Th/U ratios near 0.5 (variation from 0.28 to 0.69). In sample SL2, the Vila Nova gneiss (diorite), seven analyses in seven crystals reveal higher U near 400 ppm and Th near 450 ppm resulting in high Th/U ratio (near 1.2) with very small variation ( ). In sample RL1, garnet biotite gneiss, 17 analyses in 10 crystals display highly variable U ppm and Th ppm. The Th/U ratios are accordingly highly variable ( ). The samples from the main magmatic stage (RL6, RL12, RL15, and SL6) belong to the Lagoa da Meia-Lua Suite, and are representative of the magmatism that was responsible for the largest volume of granitic magma that built the juvenile magmatic arc. Sample RL6, trondhjemite (Lagoa da Meia-Lua Suite), studied by nine analyses in nine crystals has high U of about 500 ppm ( ppm) and variable Th (ranging from 20 to 974 ppm). The Th/U ratios are consequently variable ranging from 0.13 to Sample RL12, the Santa Zélia granite (Lagoa da Meia-Lua Suite), was studied in six crystals and has high U contents between 296 and 1363 ppm, mostly high Th contents which varies between 52

8 L.A. Hartmann et al. / Gondwana Research 19 (2011) Fig. 9. Back-scattered electron images of analyzed zircon crystals. (a, b) Sample RL6, trondhjemite BR290, Lagoa da Meia-Lua Suite, Cambaí Complex, (c, d) Sample RL12, Santa Zélia granite, Lagoa da Meia-Lua Suite, Cambaí Complex, (e, f) Sample RL15, Buriti meta-tonalite, Lagoa da Meia-Lua Suite, Cambaí Complex, and (g, h) Sample SL6, tonalite BR290, Lagoa da Meia-Lua Suite, Cambaí Complex. Black circles indicate SHRIMP analysis position; spot number and age (Ma) shown. and 757 ppm. The Th/U ratios are near 0.50 but vary between 0.20 and Sample RL15, the Buriti meta-tonalite (Lagoa da Meia-Lua Suite), has 16 analyses in 10 crystals and mostly high U contents between 202 and 1389 ppm although one analysis is only 41 ppm. Th contents are mostly low 6 17 ppm, but some higher values were also obtained ( ppm). This sample shows a special feature, because Th/U ratios are bimodal, many are near 0.02 (metamorphic) and a few are near 0.5 (magmatic). In the BSE images, the best-preserved core is in Fig. 9e, f, because it is dark grey and homogeneous. This core also has the highest Th/U ratio of this sample (Fig. 12). The age of analysis b.1-2 is 766±14 Ma and is taken as the magmatic age of the Buriti metatonalite. The other cores are not as homogeneous and display some alteration by light grey portions (Fig. 9). This alteration is interpreted as causing partial resetting of the original magmatic age and lowering of the Th/U ratio (Fig. 12). Sample SL6, the tonalite BR290, is intercalated with sample RL 6, the BR290 trondhjemite, has six analyses in five crystals and U contents near 400 ppm (variation between 155 and 671 ppm) and Th near 100 (variation between 32 and 361 ppm). The Th/U ratio varies between 0.17 and The two analyzed samples from the waning stages of granitic intrusion in the juvenile terrane (SL4 and RL4) are part of the Sanga do Jobim Suite, a late-tectonic granitic phase in the juvenile magmatic

9 92 L.A. Hartmann et al. / Gondwana Research 19 (2011) Fig. 10. Back-scattered electron images of analyzed zircon crystals, Sanga do Jobim Suite, Cambaí Complex. (a, b) Sample SL4, Cerca de Pedra granodiorite, (c, d) Sample RL4, the Sanga do Jobim granodiorite. Fig. 11. Concordia plots of all nine analyzed samples.

10 L.A. Hartmann et al. / Gondwana Research 19 (2011) Table 1 U Pb zircon SHRIMP isotopic data from Cambaí Complex (and related Cambaizinho Complex) samples. Spot U Th Th 4f206 Isotopic ratios Ages 207 Pb 208 Pb 206 Pb 207 Pb 208 Pb 207 Pb 206 Pb Disc. ppm ppm U (%) 206 Pb 206 Pb 238 U 235 U 232 Th 206 Pb 238 U % RL1, Garnet biotite gneiss (Cambaizinho Complex) h ± ± ± ± ± ± ±10 8 h ± ± ± ± ± ± ±10 4 h ± ± ± ± ± ± ±18 1 h ± ± ± ± ± ±7 0 h ± ± ± ± ± ±10 0 h ± ± ± ± ± ±10 11 h ± ± ± ± ± ±10 2 h ± ± ± ± ± ± ±20 9 h ± ± ± ± ± ± ±27 1 h ± ± ± ± ± ± ±7 1 h ± ± ± ± ± ± ±9 4 h ± ± ± ± ± ± ±9 11 h ± ± ± ± ± ± ±6 4 h ± ± ± ± ± ± ±21 4 h ± ± ± ± ± ± ±5 18 h ± ± ± ± ± ±4 0 h ± ± ± ± ± ± ±6 0 SL19, orthogneiss (Vila Nova gneisses, Cambaí Complex) e ± ± ± ± ± ± ±7 6 e ± ± ± ± ± ± ±8 4 e ± ± ± ± ± ± ±8 2 e ± ± ± ± ± ± ±7 5 e ± ± ± ± ± ± ±8 2 e ± ± ± ± ± ± ±4 7 e ± ± ± ± ± ± ±5 4 e ± ± ± ± ± ± ±8 15 e ± ± ± ± ± ± ±7 0 SL2, orthogneiss (Vila Nova gneisses, Cambaí Complex) e ± ± ± ± ± ± ±2 0.8 e ± ± ± ± ± ± ±1 0.6 e ± ± ± ± ± ± ±2 1.5 e ± ± ± ± ± ± ±2 4.4 e ± ± ± ± ± ± ±2 3.5 e ± ± ± ± ± ± ±1 0.4 e ± ± ± ± ± ± ±2 2.9 RL15, Buriti meta-tonalite (Lagoa da Meia-Lua Suite, Cambaí Complex) b ± ± ± ± ± ± ±6 6 b ± ± ± ± ± ± ±17 26 b ± ± ± ± ± ± ±6 0 b ± ± ± ± ± ± ±6 2 b ± ± ± ± ± ± ±6 1 b ± ± ± ± ± ± ±6 3 b ± ± ± ± ± ± ±6 0 b ± ± ± ± ± ± ±8 6 b ± ± ± ± ± ± ±6 0 b ± ± ± ± ± ± ±8 4 b ± ± ± ± ± ± ±6 1 b ± ± ± ± ± ± ±6 0 b ± ± ± ± ± ± ±9 5 b ± ± ± ± ± ± ±8 5 RL6, trondhjemite BR-290 (Lagoa da Meia-Lua Suite, Cambaí Complex) intercalated with SL-6 tonalite k ± ± ± ± ± ± ±5 4 k ± ± ± ± ± ± ±5 0 k ± ± ± ± ± ± ±5 5 k ± ± ± ± ± ± ±5 2 k ± ± ± ± ± ± ±7 2 k ± ± ± ± ± ± ±6 2 k ± ± ± ± ± ± ±5 5 k ± ± ± ± ± ± ±8 1 k ± ± ± ± ± ± ±5 0 RL12, Santa Zélia granite (Lagoa da Meia-Lua Suite, Cambaí Complex) c ± ± ± ± ± ± ±7 1 c ± ± ± ± ± ± ±6 1 c ± ± ± ± ± ± ±7 0 c ± ± ± ± ± ± ±7 3 c ± ± ± ± ± ± ±7 1 (continued on next page)

11 94 L.A. Hartmann et al. / Gondwana Research 19 (2011) Table 1 (continued) Spot U Th Th 4f206 Isotopic ratios Ages 207 Pb 208 Pb 206 Pb 207 Pb 208 Pb 207 Pb 206 Pb Disc. ppm ppm U (%) 206 Pb 206 Pb 238 U 235 U 232 Th 206 Pb 238 U % RL12, Santa Zélia granite (Lagoa da Meia-Lua Suite, Cambaí Complex) c ± ± ± ± ± ± ±9 1 c ± ± ± ± ± ± ±6 3 c ± ± ± ± ± ± ±6 2 c ± ± ± ± ± ± ±7 2 c ± ± ± ± ± ± ±8 3 c ± ± ± ± ± ± ±7 5 SL6, tonalite BR290 (Lagoa da Meia-Lua Suite, Cambaí Complex), intercalated with RL-6 trondhjemite g ± ± ± ± ± ± ±3 2.2 g ± ± ± ± ± ± ±3 1.4 g ± ± ± ± ± ± ±5 2.4 g ± ± ± ± ± ± ±5 7.2 g ± ± ± ± ± ± ± g ± ± ± ± ± ± ±3 2.0 RL4, Sanga do Jobim tonalite (Sanga do Jobim Suite, Cambaí Complex) g ± ± ± ± ± ± ±6 2 g ± ± ± ± ± ± ±6 2 g ± ± ± ± ± ± ±6 4 g ± ± ± ± ± ± ±6 2 g ± ± ± ± ± ± ±6 2 g ± ± ± ± ± ±5 757 ±6 1 g ± ± ± ± ± ± ±6 2 g ± ± ± ± ± ± ±6 2 g ± ± ± ± ± ± ±6 2 SL4, Cerca de Pedra granodiorite (Sanga do Jobim Suite, Cambaí Complex) g ± ± ± ± ± ± ± g ± ± ± ± ± ± ±5 1.0 g ± ± ± ± ± ± ±3 4.6 g ± ± ± ± ± ±9 698 ±2 0.7 g ± ± ± ± ± ± ± g ± ± ± ± ± ± ±4 5.9 g ± ± ± ± ± ± ±3 7.0 g ± ± ± ± ± ± ±2 0.3 g ± ± ± ± ± ± ±5 3.9 g ± ± ± ± ± ± ±4 1.4 g ± ± ± ± ± ± ±3 2.8 g ± ± ± ± ± ±6 747 ±5 6.6 g ± ± ± ± ± ± ±2 1.5 g ± ± ± ± ± ± ±4 0.5 Notes: Isotopic ratios errors in %. All Pb in ratios are radiogenic component. Most are corrected for 204 Pb and some for 208 Pb (metamorphic, Th-poor grains or rims). disc. = discordance, as {t[ 206 Pb/ 238 U]/t[ 207 Pb/ 206 Pb]}. f206=(common 206 Pb) /(total measured 206 Pb) based on measured 204 Pb. Uncertainties are 1σ. arc. In sample SL4, the Cerca de Pedra granodiorite, 14 analyses in 10 crystals show variable U contents two analyses are 46 ppm and 66 ppm, one analysis is 5011 ppm, and many vary between 136 and 1020 ppm. Th contents also vary considerably two analyses are 15 and 19 ppm, one is 621 ppm, and many are between 35 and 578 ppm. The Th/U ratio varies less extensively between 0.13 and Sample RL4, Sanga do Jobim granodiorite, nine analyses in nine crystals display high U contents ( ppm, one analysis at 1419 ppm and one at 8939 ppm. Th content is low in one analysis (14 ppm) and high in many analyses ( ppm), one exception at 1687 ppm. The U Pb isotopic spot ages obtained by SHRIMP from the 96 analyses in nine rock samples from the juvenile terrane are 207 Pb/ 206 Pb ages for the Archean and Paleoproterozoic and 206 Pb/ 238 U ages for the Neoproterozoic. Only four individual spot ages are older than Neoproterozoic: one is Archean 2729±12 Ma (spot h.6-1) and three are Paleoproterozoic e.g., 2162±9 Ma (spot h.2-1, sample RL1), all four old ages were obtained in zircons from sample RL1, the garnet biotite gneiss. Most zircons from this paragneiss are Neoproterozoic, as are the ages of the 92 other analyses. The spot ages from the nine samples are concentrated in the time interval Ma, but a few are near 830 Ma or 520 Ma; both oldest and youngest Neoproterozoic ages are from sample RL1. The geochronology of each rock sample is described in sequence. Zircons from sample SL19, Vila Nova orthogneiss, have an age spread between 728 and 707 Ma. The concordia age of the sample is 718±2 Ma (Fig. 11), interpreted as the metamorphic age of the rock. Th/U ratios of the zircons are high, near 0.5; metamorphic compositions commonly remain high in tonalite zircons. Sample SL2 is an orthogneiss (diorite) from the Vila Nova gneisses (Cambaí Complex), and has an intercept age of 735±7 Ma, interpreted as the magmatic age, and two younger ages near 725 Ma, probably resetting by metamorphism without evidence for lead-loss. Sample RL1, a metasedimentary rock from the Cambaizinho Complex, has an intercept age of 579±6 Ma, interpreted as the age of shear-zone metamorphism of this sample. One Archean crystal is present, two Paleoproterozoic crystals and several crystals with ages near 800 Ma. The intercept age of sample RL6, trondhjemite BR290, is 694± 5Ma(Fig. 11). As seen on the BSE images, the spots were analyzed either on the dark (magmatic) or light (metamorphic) grey portion of the crystal. Because the age difference of the analyses from the two portions is not significant, we consider that magmatism and shear-

12 L.A. Hartmann et al. / Gondwana Research 19 (2011) Fig. 12. Age versus Th/U diagrams of all nine studied samples; magm. = magmatic, met. = metamorphic. zone metamorphism occurred within a short time-period. This timeperiod is delimited by the error of the intercept age ( 10 m.y.). Sample SL6, tonalite BR290, yields a concordia age of 690±2 Ma which overlaps with sample RL6; both occur in alternating bands in the same outcrop and have similar ages. This is in agreement with field and petrographic evidence that indicates that this outcrop is part of a magmatic body, the banding corresponding to magmatic flow. Babinski et al. (1996) dated three zircon crystals (conventional) from the same outcrop, from the tonalite band, and obtained an age of 704 Ma. Re-evaluation of the data shows that the correct interpretation is to use the age of the most concordant analysis, which yields 695 Ma for the magmatic age. This age is in agreement with the SHRIMP data presently reported. Babinski et al. (1996) determined the age of zircon from a diorite band (704 Ma, TIMS U Pb geochronology) in this same outcrop from which samples RL6 and SL6 were collected. This age is presently reevaluated as 695 Ma±5 Ma, because three zircon crystals were analyzed by TIMS (683, 695 and 712 Ma) and this age corresponds to the most concordant (2%) analysis. The three ages cannot be grouped, and one of the two remaining analyses is not taken into consideration because the crystal is much smaller than the other two. The third analysis is more discordant (6%). The recalculated age (695 Ma±5 Ma; Babinski et al., 1996) is here interpreted as the magmatic age of the rock, and is nearly identical with the SHRIMP ages of samples RL6 (694±5 Ma) and SL6 (690±2 Ma) presently reported. The concordia age of sample RL12, Santa Zélia granite, is 704± 3 Ma (Fig. 11). Altered and unaltered portions of the crystals yield similar ages, so the emplacement and shear-zone deformation ages must be similar, within the error ( 6 m.y.). Sample RL15, the Buriti meta-tonalite, yields an intercept age of analyses that have Th/U ratios near 0.02 and are therefore of metamorphic compositions. This intercept age of 696±5 (n=12; MSWD=1.6) Ma is thus interpreted as a metamorphic age related to shear-zone deformation. Three analyses have higher Th/U ratios near 0.50 and also older ages of 750±6 Ma (Fig. 12), interpreted as the magmatic age of the tonalite. The crustal residence age of this sample is therefore at least 54 m.y., a large time interval in comparison with the other samples from the Cambaí Complex. Sample SL4, the Cerca de Pedra granodiorite, has a concordia age of 682±1 Ma, interpreted as the magmatic age of the sample; several older crystals are inherited. Magmatic crystallization and shear-zone alteration occurred within 2 m.y. The concordia age of sample RL4, the Sanga do Jobim tonalite, is 680±2 Ma and interpreted as the magmatic age of the rock. Altered and unaltered portions of the crystals yield the same age and occurred within the error (±4 m.y.) of the analyses. 4. Discussion and conclusions The dating of 96 zircon crystals from nine rocks established the time frame for the evolution of the São Gabriel orogeny, particularly the granitic Cambaí Complex and metasedimentary Cambaizinho Complex in the Vila Nova region of the southern Brazilian Shield (Fig. 13). Nd isotopic geochemistry indicates the beginning of crustal formation at 1300 Ma (Saalmann et al., 2005a, b, c). However, these Nd model ages may be due to the mixing of younger magmas and sediments with older crust. The presence of older crust was envisaged by Saalmann et al. (2005a, b, c), because the geochemistry of the magmatic rocks is comparable to rocks in continental margin arcs. For instance, volcanic and granitic rocks have low to medium K 2 O contents between 0.2 and 3.0 wt.% in the SiO 2 interval wt.% (Ruy P. Philipp, unpublished data). The presence of Archean and Paleoproterozoic zircons in a paragneiss (sample RL1) confirms that

13 96 L.A. Hartmann et al. / Gondwana Research 19 (2011) Fig. 13. Summary of major orogenic events during the São Gabriel orogeny. the São Gabriel orogeny was established on active continental margin (Andean type) represented by the la Plata Craton. An overview of magmatic and metamorphic ages obtained in zircons in the present study is given in Table 2. Deep crustal levels are exposed as amphibolite-facies paragneisses (Cambaizinho Complex) and orthogneisses (Vila Nova gneisses, Cambaí Complex); the intense deformation that generated this flat-lying foliation occurred at Ma (Remus et al., 1999). The paragneisses resemble a deformed passive margin, which was present on the margin of the continent before this 730 Ma deformation occurred but they may also represent paleo-turbidites (trench) deposits. The volcanic rocks of the Campestre Formation formed at about 753 Ma and were situated closer to the surface and only deformed at a later time (699 Ma, Remus et al., 1999). Orthogneisses dated at 735 Ma are included in the Cambaí Complex. The main crust-forming event in the region was the melting and intrusion of the orthogneisses protoliths (Vila Nova gneisses). This was followed by the intrusion of diorite, tonalite, trondhjemite and granodiorite of the Lagoa da Meia-Lua Suite (Cambaí Complex). This event lasted about 15 m.y. from 705 to 690 Ma. This interval is reasonable for the construction of a volcanic arc. The intense shearzone metamorphism that affected the suite occurred during the same time span, because the magmatic and metamorphic ages of zircons are the same within error. The syntectonic nature of these granitic rocks reported by Saalmann et al. (2005a,b,c) is now confirmed by zircon dating. Granitic intrusions late in the shear-zone deformational event are included in the Sanga do Jobim Suite and were formed at 680 Ma; minor deformation occurred within the error of the analyses. The São Gabriel orogeny thus started with andesitic volcanism (Campestre Formation) at 753 Ma, followed by intrusion of orthogneisses protoliths (Vila Nova gneisses, Cambaí Complex) and their deformation with the passive margin (Cambaizinho Complex) at Ma and a major phase of juvenile granitic intrusions (Lagoa da Meia-Lua Suite, Cambaí Complex). The granitic intrusions of the Sanga do Jobim Suite (Cambaí Complex) occurred at 680 Ma in the waning stages of the orogeny. The crust remained stable for 50 m.y., because the oldest events of the Dom Feliciano orogeny occurred at 630 Ma and affected the São Gabriel belt by minor intrusion of granitic rocks and formation of the Camaquã basin. The collision of several microcontinents and oceanic terranes during the assembly of West Gondwana (e.g., Alkmim et al., 2001; Saalmann et al., 2006a) generated voluminous volcanic and granitic rocks. The robust dating of geological events in West Gondwana is relevant for investigations in peripheral domains (e.g., Bueno et al., 2009; Silva Filho et al., 2010; Zeh and Gerdes, 2010). This makes the present investigation most significant because we have now established the time frame of volcanism, deformation and granitic rock injection during the juvenile São Gabriel orogeny. Volcanism occurred at 753 Ma, followed by the main collisional event at 719 Ma, a major post-collisional event of tonalite trondhjemite intrusion at Ma, and ending with the injection of granodiorites at 680 Ma. Some of the most significant results of this investigation can be summarized as follows. 1. The São Gabriel orogeny occurred between 753 and 680 Ma, as registered in the Campestre Formation, Cambaizinho Complex and Cambaí Complex of the Palma and Vila Nova regions, southern Brazilian Shield. 2. The largest volume of juvenile granitic rocks was generated and deformed early in the orogeny (Vila Nova gneisses) Ma. 3. A significant granitic event (Lagoa da Meia-Lua Suite) occurred between 705 and 690 Ma. 4. Early deformation occurred at Ma generating flat-lying, amphibolite facies paragneisses, the Cambaizinho Complex, and orthogneisses, the Vila Nova gneisses, of the Cambaí Complex. 5. The final stages of the orogeny had intrusions of granodiorites at 680 Ma, the Sanga do Jobim Suite. 6. Cores of zircon crystals (Buriti meta-tonalite) are dated at 776 Ma, but their significance requires additional investigations. 7. Andesitic volcanism occurred earlier (753 Ma) in the Campestre Formation and was little deformed. Table 2 Ages and classification of the nine samples dated by SHRIMP II in this investigation; sample dated by Babinski et al. (1996) by TIMS included. Sample number Rock description Stratigraphic unit Magmatic age, Ma Metamorphic age, Ma RL1 Garnet biotite gneiss Cambaizinho Complex , inherited 579±6 SL19 Orthogneiss Vila Nova gneisses, Cambaí Complex 718±2 718±2 SL2 Orthogneiss Vila Nova gneisses, Cambaí Complex 735±7 725 RL15 Meta-tonalite Buriti meta-tonalite, Lagoa da Meia-Lua Suite, Cambaí Complex ±7 RL6 Trondhjemite BR-290 intercalated Lagoa da Meia-Lua Suite, Cambaí Complex 694±5 694±5 with SL6 tonalite RL12 Granite Santa Zélia granite, Lagoa da Meia-Lua Suite, Cambaí Complex 704±3 704±3 SL6 Tonalite BR290 intercalated with Lagoa da Meia-Lua Suite, Cambaí Complex 690±2 690±2 RL6 trondhjemite SL4 Granodiorite Sanga do Jobim tonalite, Sanga do Jobim Suite, Cambaí Complex 682±1 682±1 RL4 Tonalite Sanga do Jobim tonalite, Sanga do Jobim Suite, Cambaí Complex 680±2 680±2 Babinski et al. (1996) Diorite BR290 intercalated with SL6 and RL6 Lagoa da Meia-Lua Suite, Cambaí Complex 695±5 695±5