Geochronology

U-Pb analyses

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).

40Ar/39Ar analyses

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

Geologic map of SE Brazil

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.


Samples and radiometric results

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

Field photographs

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.


Table 2. Sample Description

Sample Lithology Provenance Coordinates Analysis
Guaxupé Domain
HFT359 Banded, biotite paragneiss road between Poços and Machado Minas Gerais State- MG 21º 42' 0.6"S, 46º 22' 56"W gneissic banding (122/09) composed of narrow bands of quartz, plagioclase, biotite , K-feldspar and prismatic monazite crystals
HFT361 Augen gneiss road between Machado and Pouso Alegre- MG 21º 42' 52"S, 45º 55' 35"W gneissic banding (80/10) and thrust sense to west composed by K-feldspar porphyroblasts in a quartz-feldspathic matrix, biotite and well formed monazite as accessory mineral
H693 Paragranulite (Fig. 4A) alternating with kinzigite and metabasite road Varginha- Carmo Cachoeira-MG, at the Santo Antônio quarry 21º 33' 25"S, 45º 22' 07"W gneissic banding (232/11) and stretching lineation (252/8) with transport sense to northeast. It is composed by K-feldspar, garnet, kyanite and hypersthene, biotite and prismatic monazite
H465 Schist alternating with quartzite and granulitic gneiss road between Paraguaçu and Varginha-MG 21º 22' 51"S, 45º 30' 20"W foliation (172/12) composed by quartz, biotite and K-feldspar
H559 Schist intercalated with quartzite and calc-silicate road 381 between Três Corações and Carmo da Cachoeira- MG 21º 33' 37"S, 45º 08' 16"W foliation (212/15). It is composed by quartz, biotite, K-feldspar and muscovite
H690 Schist , gneiss and amphibolite road Cambuquira- Três Corações-MG 21º 45' 21"S, 45º 14' 18"W foliation. It is composed by bands of quartz, K-feldspar, plagioclase and biotite . Amphibolite boudins are stretched along the main banding
Socorro Domain
H616 Pegmatite cutting a migmatitic paragneiss road between Itajubá and Pedralva-MG 22º 23'54''S, 45º 32'00''W gneissic banding (175/10) and stretching lineation (182/8) with transport sense to north, composed by K-feldspar, plagioclase, biotite and prismatic monazite ;
FCS11B Pegmatite cutting granitoid road between Jundiaí and Itu-SP 23º 17'53''S, 47º 02'60''W Granitoid with an equigranular texture composed by quartz, plagioclase, biotite and K-feldspar, and prismatic monazite crystals
HFT353B Paragneiss (Fig. 4B) with migmatitic features road between Cambuí and Pouso Alegre-MG 22º 31' 23"S, 46º 01' 25"W gneissic banding (105/08), composed by bands of K–feldspar, plagioclase, biotite and garnet crosscut by quartz-feldspathic pegmatite. The sample shows prismatic monazite prismatic biotite grains.
HFT349 Paragneiss (Fig. 4C) and schist road between São Bento do Sapucaí and Paraisópolis-MG 22º 44' 29"S, 45º 44' 21"W gneissic banding (117/11) and asymmetric isoclinal fold with axis plunging 5 degrees to NNE and foliation parallel to banding, composed by narrow bands of quartz, plagioclase, biotite, K-feldspar and garnet. The sample yielded some prismatic monazite and biotite grains
H751 Schist , paragneiss and migmatite road between Sapucaí- Mirim and São Bento do Sapucaí-MG 22º 38' 34"S, 45º 44' 58"W foliation (147/52) composed by quartz, K-feldspar, biotite and muscovite. The sample yielded prismatic biotite grains.
HFT704B Pegmatite (Fig. 4D) related to intrusive granite and orthogneiss road SP 65 between Bom Jesus dos Perdões and Nazaré Paulista-SP 23º 10'15''S, 46º 23' 24''W gneissic banding (137/43) with stretching lineation (135/40) and NW sense of thrusting. The orthogneiss is composed by plagioclase, K-feldspar, biotite and is intruded by granite with K-feldspar, plagioclase and biotite. The sample yielded well-formed monazite grains.
H355 Augen gneiss road Bom Repouso to Borda da Mata- MG 22º 22' 06"S, 46º 08' 30"W gneissic banding (123/13), composed by K-feldspar prophyroblast in a quartz-feldspathic matrix. The sample yielded well-formed biotite grains
H351 Paragneiss road between Paraisópolis and Consolação-MG 22º 32' 34"S, 45º 50' 38"W gneissic banding composed by small bands of quartz, plagioclase, biotite and K-feldspar
Reworked Cratonic Border
HFT363B Pegmatitic veins cutting paragneiss with migmatitic features Sindicato quarry at the road between Borda da Mata and Ourofino-MG 22º 16' 45"S, 46º 10' 55"W banding (137/05), composed by bands of quartz, plagioclase, biotite and K-feldspar and quartz-feldspathic pegmatite. Amphibolite boudins are stretched along the main banding. Biotite , muscovite and rare monazite crystals are well preserved
H681 Paragneiss with migmatitic portions road Pouso Alegre- Silvianópolis-MG 22º 05' 35"S, 45º 53' 32"W gneissic banding (225/20) and stretching lineation (230/70), composed by K-feldspar, plagioclase, biotite and garnet. The sample present well formed biotite and monazite crystals
HFT358 Granitoid (syn to post-tectonic) road between Borda da Mata and Ipuíuna-MG 22º 05' 59"S, 46º 10' 57"W equigranular texture composed by quartz, plagioclase, biotite and K-feldspar. The sample present well formed biotite crystals
HFT356 Schist intercalating with gneiss and migmatite road between Jacutinga and Ourofino-MG 21º 03' 03"S, 46º 08' 37"W foliation ( 172/74) composed by quartz, biotite, K-feldspar and muscovite
Metasedimentary Sucession Domain
H687 Pegmatite cutting metassediments (Fig. 4E) in amphibolite facies road Cambuquira-Lambari-MG 21º 53' 29"S, 45º 15' 46"W foliation (340/10) and stretching lineation (64/00), composed by K-feldspar, quartz, muscovite and biotite crystals; The sample present well formed biotite and muscovite crystals
LUME Schist , quartzite and metaconglomerate of the Andrelândia Group road between Luminárias and Lavras-MG 21º 33' 30"S, 44º 57' 15"W foliation ( 272/15) composed by quartz, muscovite and biotite. The sample yielded prismatic muscovite crystals
ITMTR Schist , quartzite and metaconglomerate of the Andrelândia Group road between Luminárias and Lavras-MG 21º 21' 00"S, 44º 52' 00"W foliation (179/13) composed by quartz, muscovite and biotite. The sample yielded prismatic muscovite crystals
MN85 Schist , quartzite and metaconglomerate of the Andrelândia Group road between Três Corações and São Bento do Abade-MG 21º 40' 20"S, 44º 57' 34"W foliation (269/10) composed by quartz, muscovite and biotite
MR72 Schist and quartzite of the Andrelândia Group road between São Bento do Abade and Luminárias-MG 21º 40' 20"S, 44º 57' 00"W foliation (280/12) composed by quartz, muscovite and biotite
CAR1 Schist , quartzite and metaconglomerate of the Andrelândia Group road between São João Del Rei and São Tiago-MG 21º 27' 00"S, 44º 34' 00"W foliation ( 315/35) composed by quartz, muscovite and biotite
DIT Schist and quartzite of the Andrelândia Group road between Lavras and São João Del Rei-MG 21º 33' 00"S, 44º 24' 45"W foliation (137/17) composed by quartz, muscovite and biotite
ANDT Schist , quartzite and metaconglomerate of the Andrelândia Group road between Madre de Deus de Minas e Andrelândia-MG 21º 45' 00"S, 44º 18' 00"W foliation (142/23) composed by quartz, muscovite and biotite
FDA Schist , quartzite and metaconglomerate of the Andrelândia Group road between Madre de Deus de Minas e Andrelândia-MG 21º 52'15''S, 44º 15'35''W foliation (133/15) composed by quartz, muscovite and biotite
Ribeira belt
H513B Pegmatite cutting syntectonic granite at Ibiúna-SP 23º 43' 53''S; 47º 24' 26''W equigranular texture composed by quartz, plagioclase, biotite and K-feldspar, and well formed monazite crystals
H109 Schist , paragneiss and migmatite road BR-116 between Itatiaia and Resende-RJ 21º 33' 37"S, 45º 08' 16" W foliation (177/45) composed by quartz, biotite , K-feldspar and muscovite

U-Pb results

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

U-Pb concordia diagrams

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

U-Pb concordia diagrams

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

  1. HFT349,

  2. H704B.

  3. Reworked Cratonic Border: HFT363B,

  4. H681.

  5. 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).

Table 3. U-Pb monazite data from the southern Brasília belt

Sample       Pb206 Pb207* ± Pb206* ± Correl. Pb207* ± Pb206* ± Pb207* ± Pb207* ±
Fraction Wt. U Pb Pb204 U235 U238 Coeff. Pb206* U238 U235 Pb206*
  (mg) ppm ppm (obs.)   %   % (rho)   % Age (Ma) Age (Ma) Age (Ma)
Guaxupé Domain HFT359                                  
M(0.4A)2 0.018 4643 2459 12496 0.8564 0.28 0.1017 0.27 0.974 0.06108 0.06 624 2 628 2 642 1
M(0.5A)2 0.018 13357 6811 18461 0.8423 0.32 0.1010 0.32 0.994 0.06047 0.03 620 2 620 2 620 1
HFT361                                  
M(0.5A)2 0.014 652 1113 1170 0.8276 0.90 0.0982 0.71 0.785 0.06114 0.55 604 5 612 4 644 12
M(0.5A)4 0.012 688 1449 1471 0.8207 1.19 0.0990 0.55 0.473 0.06010 1.05 609 7 608 3 607 23
H693A                                  
M(0.5A)3 0.030 1962 1722 7371 0.8271 0.54 0.0996 0.53 0.990 0.06021 0.08 612 3 612 3 611 2
M(0.5A)4 0.020 3551 2075 7539 0.8271 0.72 0.0997 0.68 0.938 0.06019 0.25 612 4 612 4 611 5
Socorro Domain H616                                  
M(0.5A) 0.009 2623 1207 20271 0.8502 0.19 0.1020 0.18 0.955 0.06044 0.06 626 1 625 1 620 1
M(0.55A) 0.012 2377 942 16748 0.8506 0.28 0.1020 0.27 0.972 0.06048 0.07 626 2 625 2 621 1
M(0.6A) 0.011 1739 1191 8341 0.8484 0.21 0.1020 0.21 0.960 0.06032 0.06 626 1 624 1 615 1
FCS11B                                  
M(0.3A)"1" 0.008 4482 1571 2554 0.8069 0.76 0.0972 0.75 0.993 0.06021 0.09 598 5 601 5 611 2
M(0.3A)"2" 0.006 7218 2580 2611 0.8098 0.47 0.0977 0.46 0.980 0.06011 0.09 601 3 602 3 608 2
M(0.3A)"3" 0.008 2552 1051 2947 0.8185 0.56 0.0984 0.53 0.951 0.06033 0.17 605 3 607 3 615 4
HFT353B                                  
M(0.4A)1 0.010 1574 1035 2149 0.8176 0.39 0.0984 0.33 0.868 0.06025 0.19 605 2 606 2 612 4
M(0.4A)2 0.024 1235 1184 3561 0.8087 0.37 0.0974 0.3 0.814 0.06020 0.21 599 2 602 2 611 5
M(0.5A)2 0.028 900 977 3400 0.8137 0.25 0.0980 0.2 0.828 0.06020 0.14 603 1 605 1 611 3
HFT349                                  
M(0.4A)2 0.028 2234 1271 3453 0.8036 0.64 0.0970 0.63 0.980 0.06006 0.13 597 4 599 4 606 3
M(0.5A)2 0.026 5061 1564 13390 0.8165 0.46 0.0983 0.45 0.993 0.06027 0.05 604 3 606 3 613 1
H704B                                  
M(0.6A) 0.008 2282 810 13662 0.8197 0.17 0.0989 0.17 0.988 0.06012 0.03 608 1 608 1 608 1
M(0.65A) 0.012 1808 640 22393 0.8215 0.21 0.0992 0.2 0.963 0.06007 0.06 610 1 609 1 606 1
M(0.70A) 0.015 1684 1002 16594 0.8439 0.19 0.1017 0.18 0.957 0.06016 0.06 625 1 621 1 609 1
Reworked Cratonic Border HFT363B                                  
M(0.4A)2 0.020 16685 3194 5545 0.8022 0.67 0.0971 0.66 0.992 0.05993 0.09 597 4 598 4 601 2
M(0.5A)2 0.026 9919 2109 8026 0.7983 1.28 0.0962 1.27 0.994 0.06017 0.15 592 8 596 8 610 3
H681                                  
M(0.3A) 0.026 1184 491 1657 0.8146 0.50 0.0982 0.49 0.985 0.06016 0.09 604 3 605 3 609 2
M(0.5A) 0.009 1284 452 2074 0.8131 0.79 0.0981 0.57 0.745 0.06010 0.23 603 5 604 5 607 11
Ribeira Belt H513B                                  
M(0.3A)"1" 0.010 989 1214 1645 0.8234 0.17 0.0995 0.16 0.953 0.06004 0.05 611 1 610 1 605 1
M(0.3A)"2" 0.010 1465 702 2737 0.8193 0.21 0.0990 0.21 0.994 0.06002 0.02 609 1 608 1 604 1
M(0.3A)"3" 0.004 1130 978 927 0.8185 0.36 0.0988 0.34 0.947 0.06010 0.12 607 2 607 2 607 3

40Ar/39Ar results

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

Incremental heating analysis of single grains

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.


Table 4. 40Ar-39Ar data (single grains; triplicate) from the southern Brasília belt. See Table 2 for sample details. Errors are given in 2s.

Sample Rock/Mineral Lab # Plateau 1 Plateau 2 Plateau 3 Ideogram Integrated
    USP Ages (Ma) Ages (Ma) Ages (Ma) Ages (Ma) Ages (Ma)
Group 1- North of Jacutinga shear zone: northern part of the Socorro-Guaxupé domains              
HFT359 paragneiss/biotite 1512 601±1 598±1 598±1 599±1  
HFT361 augen gneiss/biotite 1513 588±1 586±1 591±1   584±1
H693C paragranulite/biotite 1485 574±1 578±1 586±1   579±1
H465 schist/biotite 1489 586±1 589±1 585±1 587±1  
H559 schist/muscovite 1491 591±1 587±1 587±1 588±2  
Group 2- South of Jacutinga shear zone              
HFT355 augen gneiss/biotite 1507 597±1 599±1 597±1 597±1  
HFT349 paragneiss/biotite 1494 572±1 570±1 572±1 571±1  
HFT351 paragneiss/biotite 1495 567±2 575±1 543±1   560±1
HFT356 schist/muscovite 1509 567±1 566±1 567±1 566±1  
HFT358 granitoid/biotite 1510 568±1 568±1 555±1 568±1  
Group 3- South of Jacutinga shear zone (hybrid ages)              
HFT353B paragneiss/biotite 1506 537±1 535±1 540±1 537±1  
HFT363B pegmatite/biotite 1515 538±1 537±1 525±1 537±1  
HFT363B pegmatite/muscovite 1516 562±1 561±1 562±1 562±1  
CAR1 schist/muscovite 927 525±1 536±3 521±2 526±3  
H687B pegmatite/biotite 1501 557±2 527±1 541±1   533±1
H687B pegmatite/muscovite 1502 528±1 539±1 535±1   533±2
H690 schist/biotite 1503 534±1 546±1 544±1   541±1
LUM E schist/muscovite 922 531±2 526±1 511±2 527±4  
MN85 schist/muscovite 929 553±2 545±2 523±1   537±2
H681 paragneiss/biotite 1499 521±1 522±1 518±1 521±1  
Group 4- South of Jacutinga shear zone              
H109 schist/biotite 1488 492±1 490±1 492±1 491±1  
H751 schist/biotite 1492 516±1 514±1 524±1   518±1
MR72 schist/muscovite 928 bad run 512±1 509±2 510±2  
ANDT schist/muscovite 921 499±1 504±1 501±1 501±2  
DIT schist/biotite 919 497±1 497±1 493±1 494±3  
DIT schist/muscovite 918 499±2 494±2 500±1 498±2  
FDA schist/biotite 932 501±1 501±1 501±1 501±1  
FDA schist/muscovite 931 502±2 496±1 495±1 496±1  
ITM TR schist/muscovite 924 500±1 506±1 507±2 503±2