Journal of the Virtual Explorer

Initial preparation of monazite crystals for isotopic analysis was done at Department of Petrology and Metallogeny State University of São Paulo (UNESP), Brazil. Single monazite crystals, spiked with a ²⁰⁵Pb-²³⁵U tracer solution, were dissolved in 7 ml Teflon Savillex beakers using a solution of concentrated ultra pure H2SO4 (5 uL), 6M HCl (40 uL) and 7M HNO3 (40 uL). Dissolution of the monazite was achieved by placing the beakers on a conventional hot plate (at 125-140oC) for 24 hours to ensure complete dissolution. Samples were then partially dried, as the H2SO4 is difficult to evaporate, and conditioned with 3.1M HCl prior to microcolumn chromatography, adapted from Krogh (1973). Isotopic ratios were measured at Geoscience Institute, National University of Brasília (UnB), Brazil, using a Finnigan MAT multi-collector mass spectrometer equipped with an ion counting system.

Eleven samples of prismatic monazite were analyzed for both Pb and U isotopic compositions on single Re filaments using silica gel and phosphoric acid. The analyses were corrected for average mass discrimination of 0.12 ± 0.05% per mass unit for multi-collector analyses (based on replicate analyses of common Pb standard SRM 981). Uranium fractionation was monitored by replicate analyses of SRM U-500. Uncertainties in U-Pb ratios due to uncertainties in fractionation and mass spectrometry were around ± 0.5%, as all signals measured were relatively strong. Radiogenic Pb isotopes were calculated by correcting for modern blank Pb and for original no radiogenic original Pb corresponding to Stacey and Kramers (1975) model Pb for the approximate age of the sample. Uncertainties in radiogenic Pb ratios in the studied samples are typically ± 0.1%. Decay constants and isotopic ratios used in the age calculations are those listed by Steiger and Jäger (1977). Total procedure blanks over the course of analyses ranged from 10 to 46 pg for lead and 0.5 to 2 pg for uranium. The U-Pb monazite data (see Table 3) were regressed using the ISOPLOT/EX program of Ludwig (1999). For both samples, forced Model 1 regressions were performed as the analytical points were either concordant or nearly concordant with little spread in the data. Uncertainties in concordia intercept ages are given at the 2s level (Fig. 5).

Transmitted- and reflected-light microscopy of polished thin sections were used to determine the mineralogy and textures of datable samples. Samples were then crushed and suitable minerals (well-formed biotite and muscovite grains) were concentrated and hand-picked under a binocular microscope. The mineralogical study was conducted at the Geochronology Laboratory of the State University of São Paulo (USP) and at the Department of Geology of the Federal University of Rio de Janeiro (UFRJ), Brazil. The ⁴⁰Ar/³⁹Ar geochronology (see Vasconcelos et al., 2002) was carried out at the Geochronological Research Center (CPGeo) at the University of São Paulo, Brazil. The laboratory facility comprises two major units: a home built fully automated noble gas stainless steel ultra-high vacuum gas extraction and purification system, coupled with a continuous laser, and the MAP-215-50 mass spectrometer. The argon routine is limited to grains smaller than 2.1 mm, since this is the maximum diameter of the wells in the sample aluminum disks used for irradiation. Five to ten grains from each sample were loaded into these disks along with Fish Canyon sanidine standards (28.02 ± 0.28 Ma; Renne et al., 1998). The disks were wrapped in aluminium-foil, sealed in silica glass tubes and irradiated for 30 hours at the IPEN/CNEN IEA-R1 nuclear reactor, São Paulo, Brazil.

The samples were analyzed after cooling by the laser incremental heating, following procedures detailed by Vasconcelos et al. (2002). Argon isotope ratios (⁴⁰Ar/³⁹Ar, ³⁸Ar/³⁹Ar, ³⁷Ar/³⁹Ar, ³⁶Ar/³⁹Ar, ⁴⁰Ar*/³⁹Ar), the percentage of radiogenic argon (⁴⁰Ar*), the age obtained for each incremental heating step (identified with a letter after the grain number), J factors, laser beam intensity, ⁴⁰Ar/³⁶Ar discrimination, correction factors, and full system blanks are shown in the Appendix 1. The calculation of ⁴⁰Ar/³⁶Ar discrimination (McDougall and Harrison, 1999), was obtained by the measurement of the ⁴⁰Ar and ³⁶Ar isotopes from the air pipette of the extraction line, in order to account correcting factors for the production of interfering isotopes (³⁶Ar, ³⁷Ar, ³⁸Ar, ³⁹Ar, and ⁴⁰Ar) from Ca and K salts and glasses during sample irradiation. Full system blanks correspond to the masses of ⁴⁰Ar, ³⁹Ar, ³⁸Ar, ³⁷Ar, and ³⁶Ar present in the extraction line and mass spectrometer. The ⁴⁰Ar/³⁹Ar incremental heating analyses on three single grains from each sample (triplicate analysis) have provided plateau, ideogram and integrated ages (see Table 4). The plateau ages represent continuous steps (more than 50%) of the total ³⁹Ar released from a sample and for which no difference in age can be detected between any two fractions at the 95% confidence level (Fleck et al., 1977). All uncertainties in plateau, integrated, and weighted mean ages are given at 2s level. When the plateau ages showed comparable individual results within error, the resulting age-probability ideogram is taken as a robust estimate of the samples’ age (weighted mean age). Figure 3B contains some examples of weighted mean age corresponding to ideograms. Contrastingly, when the plateau ages are significantly different they indicate a complex thermal history of the sample. In these cases the range between the single grain ages has been considered realistic (Table 4; H693C and HFT361), as additionally supported by the new U-Pb monazite ages and the regional geochronologic background.

The ⁴⁰Ar/³⁹Ar ages (Ideogram/Integrated) are plotted in Fig. 3B which presents the different domains of the Brasília and Ribeira belts, north and south of the Jacutinga shear zone. Figure 6 shows four selected ⁴⁰Ar/³⁹Ar analyses for each domain distinguished in the area, whereas Table 4 presents the age variation of the ideograms for the selected group of samples. These ideograms display the thermal history against time within the selected domains.

Figure 3. Geologic map of SE Brazil

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Geologic map of SE Brazil (modified after CPRM, 2002) showing: A) U-Pb monazite ages; B) ⁴⁰Ar/³⁹Ar biotite and muscovite ages (bold numbers). Symbols as in Figure 2. Compare with Fig. 2 for sample numbers.

The timing of late collisional stages of the southern Brasília belt have been derived from U-Pb analyses of monazite grains and ⁴⁰Ar/³⁹Ar analyses of muscovite and biotite of high-grade paragneisses, schists, migmatites and late-tectonic pegmatites from the Reworked Cratonic Border, Socorro-Guaxupé Nappe System and the Metasedimentary Succession Domains of the Brasília and Ribeira belts (Heilbron et al., 2004). Monazite age is used to date peak metamorphism (De Wit et al., 2001; Foster et al., 2004; Gibson et al., 2004; Guillaume-Seydoux et al., 2002) interpreted to be related to the southern Brasília belt evolution, as supported by petrography and field inferences. Ar-Ar is used to date thermal episodes (Noce et al., 2004).

All samples studied were taken from locations far from the recognized late sub-vertical shear zones (Jacutinga, Jundiuvira, Camanducaia, São Bento do Sapucaí, Campo do Meio and Três Corações shear zones; Fig. 2) to avoid local tectonic effects. These shear zones are related to the Socorro-Guaxupé Nappe System of the Brasília belt or to the younger escape tectonic at the Central Ribeira belt. Twenty nine samples were analyzed by the ⁴⁰Ar/³⁹Ar method whilst eleven of them by U-Pb method as well (sample locations and descriptions are in Figs 2, 3 and Table 2). U-Pb monazite analyses are summarized in Table 3, and concordia diagrams are shown in Fig. 5. The ⁴⁰Ar/³⁹Ar ages are listed in Table 4, whilst the complete analytical data are shown in Appendix 1. Four plots of the ⁴⁰Ar/³⁹Ar dates illustrate the history of uplift and cooling for the studied domains (Fig. 6A-D).

Figure 4. Field photographs

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Field photographs. A) Paragranulite of the Guaxupé Domain, sample H693; B to D) paragneisses of the Socorro Domain (samples HFT353, HFT349, H704 respectively); E) Schist interlayered with quartzites of the Metasedimentary Succession Domain, sample H687.

The oldest populations of monazite were found in the Guaxupé Domain. U-Pb age determinations of monazite grains from a single sample of migmatite (HFT 359, Figs 3A, 5A and Table 3) yield a discordant age of around 642±1 Ma. This age represents an early generation of monazite growth related to an early metamorphic event. This event is roughly contemporaneous to that in the Socorro Domain, associated with the pre-collisional granites of Hackspacher et al. (2003).

Figure 5a. U-Pb concordia diagrams

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U-Pb concordia diagrams of monazite data from paragneisses. migmatites and pegmatites in the southern part of the: Brasília belt, Guaxupé Domain:

1. HFT359

2. HFT361

3. H693A.

4. Socorro Domain: H616,

5. FCS-11B,

6. HFT353B,

Figure 5b. U-Pb concordia diagrams

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U-Pb concordia diagrams of monazite data from paragneisses. migmatites and pegmatites in the southern part of the: Brasília belt, Guaxupé Domain:

7. HFT349,

8. H704B.

9. Reworked Cratonic Border: HFT363B,

10. H681.

11. Ribeira belt: H513B.

Granulites, migmatites and paragneiss leucosomes of both the Guaxupé and Socorro Domains (Brasília belt) yield concordant ages between of 620±2 Ma, sample HFT 359, Guaxupé Domain, and 625±1 Ma, sample H616, Socorro Domain (Figs 3A, 5D and Table 3). Nearly concordant U-Pb monazite ages between 613 and 607 Ma were also found (samples H693A, H361, FCS 11B, HFT 353, H 704, H 681, H513B in Figure 3a, Figure 4, Figure 5a and Table 3). Geological and textural evidence, such as monazite inclusions in garnet, suggest that these old monazite grains grew during peak metamorphism (e.g., De Wit et al., 2001; Foster et al., 2004; Gibson et al., 2004; Guillaume-Seydoux et al., 2002). In the Brasília belt these early-formed monazite grains were found in high-grade rocks associated with low angle shear zones, syn-collisional granites, which may have re-homogenized the monazite ages of older migmatites and paragneisses.

One sample of pegmatite was dated. Pegmatites are mostly undeformed except for local boudinage and are interpreted to be late tectonic. They are intrusive into garnet-schists of the Andrelândia Group (Metasedimentary Succession Domain), in the southern edge of the Brasília belt. This monazite sample yielded an almost concordant U-Pb age of 598±2 Ma (Fig. 5I, sample HFT363B).

The study of monazite growth ages marking thermal peak is complemented by the study of Ar-Ar cooling ages. This thermochronometer was used here to identify the overprinting of the Ribeira belt deformation on the Brasília belt. In the northern portion of the Nappe System, north of the Jacutinga shear zone, in the Guaxupé Domain, ⁴⁰Ar/³⁹Ar data from biotite (Group 1 in Table 4, Figure 3B), yield a fairly tight cluster of cooling ages (plateau and ideogram), between 599±1 and 587±3 Ma (Fig. 6A, sample HFT465). Muscovite sample (H559: 588±2 Ma) yields similar ages to biotite. This indicates that the northern part of the Nappe System cooled to ~350 oC some 20-30 myr after peak metamorphism.

In the southern portion of the Nappe System, south of the Jacutinga shear zone, ⁴⁰Ar/³⁹Ar biotite data show a varied cooling history. The oldest ideogram biotite ages yielded a value of 597±1 Ma (sample HFT355), similar to the northern part of the Nappe System, and interpreted to represent a relict of that cooling phase. The next oldest sample obtained in this region is younger, ranging between 571±1 to 566±1 Ma (Group 2 in Table 4, Figs 3B, 6B sample HFT358), and include muscovite age between 566-562 Ma (Group 2 in Table 4, sample HFT356).

A third group of samples south of the Jacutinga shear zone is associated with transpressional structures of the Central Ribeira belt, such as the NE-SW trending sub vertical strike-slip Jacutinga and Três Corações shear zones, which separate the Guaxupé and Socorro Domains. They yield ⁴⁰Ar/³⁹Ar biotite ideogram ages from 527±1 to 521±1 Ma (Group 3 in Table 4, e.g. samples HFT353 H681 and LUME, Figs 3B, 6C). The ⁴⁰Ar/³⁹Ar systematics in this area has been perturbed, as indicated by age spectra of the sample H687B that vary from 557±2 to 527±1 Ma. The youngest group of ages south of the Jacutinga shear zone yield biotite/muscovite ideogram ages from 510±2 to 491±1Ma (group 4 in Table 4, Figs 3B and 6D sample FDA). Six ⁴⁰Ar/³⁹Ar cooling age determinations in the eastern sector, near the boundary to the Ribeira Belt, show an age distribution between 501±2 and 491±1 Ma (see for example FDA, ANDT in Table 4).

Thus, the age patterns in the Nappe System south of the Jacutinga shear zone (Groups 2, 3 and 4), show that group 3 samples from the Jacutinga and Três Corações shear zones are younger than elsewhere (group 2), while group 4 probably reflects a new tectonic event. Their geographical distribution, closer to the Ribeira belt boundary, suggests a possible late overprint. The cooling ages of groups 3 and 4 are interpreted to have been reset by motion on vertical shear zones south of the Jacutinga and Três Corações shear zones producing a range of younger ages.

Figure 6. Incremental heating analysis of single grains

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⁴⁰Ar/³⁹Ar incremental heating analysis of single grains with plateau ideogram and integrated graphics.

1. North of Jacutinga shear zone, sample HFT 465. View Figure 6a [fullsize] (above).

2. South of the Jacutinga shear zone, sample HFT 358. View Figure 6b [fullsize].

3. South of Jacutinga shear zone, sample HFT 353. View Figure 6c [fullsize].

4. South of Jacutinga shear zone, sample FDA. View Figure 6d [fullsize].

See Table 4 for details.