39Ar geochronological evidence for multiple magmatic events during the emplacement of Tapira alkaline-carbonatite complex, Minas Gerais, Brazil

Journal of South American Earth Sciences 97 (2020) 102416

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40

Ar/39Ar geochronological evidence for multiple magmatic events during the emplacement of Tapira alkaline-carbonatite complex, Minas Gerais, Brazil

T

Fabiano T. Conceiçãoa,∗, Paulo M. Vasconcelosb, Letícia H. Godoya, Guillermo R.B. Navarroa a b

UNESP - Universidade Estadual Paulista, Instituto de Geociências e Ciências Exatas, Rio Claro, Brazil University of Queensland, School of Earth Science, Brisbane, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: alkaline-carbonatite rocks 40 Ar/39Ar geochronology APIP emplacement Brazil

The Alto Parnaíba Igneous Province (APIP) is a voluminous magmatic province composed of various alkalinecarbonatite complexes emplaced in the Brasilia Mobile Belt during the Cretaceous. Relative timing of emplacement of silicate and carbonate magmas in most of these complexes remains mostly unresolved due to conflicting geochronological results. To determine the duration of magmatism and to test the possible existence of multiple magmatic events, we employ 40Ar/39Ar phlogopite single crystal dating to determine the history of magma emplacement at the Tapira alkaline-carbonatite complex, Minas Gerais, Brazil. The new single crystal data indicate at least two magmatic events during the emplacement of this complex, the first at > 96.2 ± 0.8 Ma and the second at 79.15 ± 0.6 Ma. The first igneous event was responsible for emplacement of the silicate plutonic series, while the second event corresponds to the emplacement of primarily carbonatitic magmas, generating metasomatic phlogopite alteration in bebedourites. The ages of intrusion and cooling of the alkaline-carbonatite complexes in the APIP must be investigated in other complexes to determine if intrusion intervals of ~17 Ma or more are common regionally. Protracted intrusive events, if related to magma generation by passage of South America over a stationary Trindade plume, requires complex ponding and lateral magma flow below a slow-moving continent.

1. Introduction The Alto Parnaíba Igneous Province (APIP) was emplaced into metasedimentary rocks of the Brasilia Mobile Belt, at the border between the São Francisco Craton and the Phanerozoic Paraná Sedimentary Basin. It comprises kamafugites, kimberlites, lamproites and alkaline-carbonatite complexes that cluster along the border between the states of Goiás (Catalão I and Catalão II) and Minas Gerais (Serra Negra, Salitre, Araxá and Tapira) (Fig. 1a), Brazil (Morbidelli et al., 1995; Gomes et al., 1990, 2018; Araujo et al., 2001; CominChiaramonti and Gomes, 2005). These complexes result from voluminous magmatism that have been attributed to the passage of South America over the Trindade mantle plume, which affected central and southern Brazil, Paraguay, Argentina and Uruguay from the Early Cretaceous to the Eocene (Gibson et al., 1995; Thompson et al., 1998; Carlson et al., 2007; Bulanova et al., 2010) and purportedly provided enough heat to trigger the melting of the lithospheric mantle underneath (Comin-Chiaramonti and Gomes, 2005).

∗

Among the various types of intrusions in the APIP, the alkalinecarbonatite complexes are particularly important because, in addition to their petrological, tectonic, and geodynamic significance, they also host significant mineralization. For example, the Araxá and Catalão I and II are major global sources of Nb, and significant sources of P; Tapira is a world-class P reserve (~1 Gton) and also a major P producer (DNPM, 2018). Importantly, these complexes are also main reserves of REE in Brazil and globally, particularly Catalão I and II, Tapira and Araxá, which together account for 16.9% of the total global REE reserves (DNPM, 2018), despite an absence of production. Finally, deep weathering profiles overlying the igneous intrusions contain worldclass Ti reserves, which could become a significant resource if metallurgical challenges associated with processing of anatase were to be overcome (Conceição and Bonotto, 2006). Here, we focus on the age of emplacement of the Tapira alkaline-carbonatite igneous rocks intruded into metasedimentary rocks of the Precambrian Canastra Group (Ulbrich and Gomes, 1981), the most important current source of phosphates in Brazil (~1.9 Mton annual production from an ~ 1 Gton

Corresponding author. UNESP, IGCE, Avenida 24-A, 1515, CEP: 13506-900, Rio Claro, São Paulo, Brazil. E-mail address: [email protected] (F.T. Conceição).

https://doi.org/10.1016/j.jsames.2019.102416 Received 12 July 2019; Received in revised form 11 November 2019; Accepted 12 November 2019 Available online 21 November 2019 0895-9811/ © 2019 Elsevier Ltd. All rights reserved.

Journal of South American Earth Sciences 97 (2020) 102416

F.T. Conceição, et al.

fission track ages of 81.7 ± 7.9 and 78.6 ± 9.0 Ma for the bebedourite and carbonatite, respectively (Eby and Mariano, 1992). The large uncertainty reported for both methods and the fact that each method measures cooling below different temperature thresholds make a direct comparison of the results difficult. To address this absence of high-resolution geochronological data and to investigate whether the Tapira complex does indeed record a large spread in magmatic ages, we apply high-resolution laser-heating 40Ar/39Ar geochronology on single phlogopite crystals from syenites and bebedourites and use these results to assess the possible duration of the complex magmatic assemblage. Improved understanding of magmatic processes in these systems may lead to better understanding of possible magmatic sources and geodynamic control, and help to unravel the magma generation and fractionation processes that led to such large-scale P, Nb and REE mineralization. 2. Materials and methods Drill-cores, made available by Fosfertil, were sampled for petrography and geochronology (Fig. 1b). Two types of bebedourites (FTB1 and FTB2) and syenites (FTS) were collected from those drill-cores at ~117, ~95 and ~135 m depth. Phlogopite-bearing samples from FTB1, FTB2 and FTS were crushed, and phlogopite grains were separated and washed, first in distilled water and then absolute ethanol, in an ultrasound bath for 60 min. Suitable 1–2 mm clean single crystals of phlogopite were mounted in Al-disks together with Fish Canyon sanidine neutron fluence monitor (age 28.201 ± 0.046 Ma; Kuiper et al., 2008), following the geometry shown by Vasconcelos et al. (2002). The irradiation disks were closed with aluminium covers, wrapped in Al-foil and vacuum heat sealed into quartz vials and irradiated for 14 h at the Oregon State University TRIGA reactor (OSTR, CLICIT facility) at the Radiation Center, Oregon State University, USA. After a cooling period of approximately 5.5 months, two grains of each sample were analyzed by laser 40Ar/39Ar heating at the University of Queensland Argon Geochronology in Earth Sciences Laboratory (UQ-AGES), Brisbane, Australia, following the procedures detailed by Vasconcelos et al. (2002). A40Ar/36Ar value of 298.56 ± 0.31 for atmospheric argon was used for discrimination calculations (Lee et al., 2006). Aliquots of Ksilicate and CaSiO2 fused glasses were also co-irradiated and analyzed to obtain the following irradiation correction factors: (36Ar/37Ar)Ca = (2.57 ± 0.25) x 10−4, (39Ar/37Ar)Ca = (6.91 ± 0.94) ×10−4, and (40Ar/39Ar)K = (8 ± 3) x 10−4. The J factors used in age calculations were 0.003655 ± 0.000008 and 0.03639 ± 0.000014.

Fig. 1. (a) Map of Brazil showing the alkaline-carbonatite rocks from Alto Parnaíba Igneous Province (APIP) (modified from Ulbrich and Gomes, 1981) and predicted tracks of the South America plate over the Trindade hotspot, in dashed line (Morgan, 1983). (b) A geological map for the Tapira alkaline-carbonatite complex (a 35 km2 elliptical structure), based on drill-core information, shows the localities of samples dated in this study (adapted from Brod et al., 2000, 2013).

3. Results The 40Ar/39Ar results are illustrated in Fig. 2 and the complete data are listed in Table 1. In this work, an age plateau is defined as three or more contiguous steps, comprising at least 50% of total 39Ar released, with repeatable apparent ages at the 95% confidence level (2σ) (Fleck et al., 1977). A plateau-like segment describes contiguous steps that define a flat segment of apparent ages but do not fulfil the plateau definition either because the amount of gas in the contiguous steps is less than 50% or because the contiguous steps defining the flat segment in the spectrum are not within 2σ uncertainty. All phlogopite grains from bebedourites samples FTB1 and FTB2 yield compatible plateaus represented by more than 80% of the total amount of 39Ar released. The incremental heating spectra are mostly flat or slightly descending, suggesting only a small contribution from an excess argon component. The spectra do not show any evidence of alteration or resetting, indicating that radiogenic argon has been quantitatively retained during the sample's history. Two phlogopite grains from sample FTB1 (Fig. 2a and b) yield statistically indistinguishable plateau ages of 79.5 ± 0.8 and 79.1 ± 0.7 Ma. Similarly, two phlogopite grains from the bebedourite sample FTB2 (FTB21 and FTB22; Fig. 2d and e) yield reproducible plateau ages (78.9 ± 0.6 and

reserve with an average ~12 wt% P2O5 content - Brazil, 2018). In the APIP, previous geochronological results suggest a possible multi-stage history of magma emplacement. For example, the K–Ar results of Sonoki and Garda (1988) for the Araxá (89.4 ± 10.1 Ma and 97.6 ± 6.1 Ma) and Salitre (82.5 ± 5.6 Ma and 86.3 ± 5.7 Ma) complexes, and more recent U–Pb results (83 ± 2, 82 ± 4 Ma and 90 ± 4) for the Catalão II complex (Guarino et al., 2013) may be interpreted to suggest a protracted history of magmatism. Despite field and petrological relationships showing complex superposition of magmatic events at the various alkaline-carbonatite complexes in the APIP, a possible difficulty in identifying multiple stages of magmatism is that the age and duration of magma emplacement are not well resolved by the geochronological methods employed. For example, the emplacement of the Tapira complex has a K–Ar age of 71.2 ± 5.1 Ma for the syenite (Sonoki and Garda, 1988) and apatite

2

Journal of South American Earth Sciences 97 (2020) 102416

F.T. Conceição, et al.

Fig. 2. (a,b) 40Ar/39Ar results. All four incremental heating spectra for samples FTB1 and (d,e) FTB2 define compatible plateaus with statistically indistinguishable plateau ages. The isochrones obtained for all steps from each sample (c, for sample FTB1 and f for FTB2) define isochron ages intisguinshable from the plateau ages for the sample samples. In contrast, the (g and h) incremental heating spectra for phlogopite grains from samples FTS1 and FTS2 yield compatible and reproducible spectra that do not define true plateaus but suggest a minimum age of > 96 Ma for each of the grains. (i) A probability density plot obtained by combining all steps for the two FTS grains illustrates the well defined high-probability peak identifying the minimum age of ~97-96 Ma for the sample.

4. Discussion

79.5 ± 0.7 Ma). For all four grains, plateau ages are indistinguishable from integrated ages. The best age for each sample is derived from the 36 Ar/39Ar vs 36Ar/40Ar correlation diagrams, where inverse isochron ages regressed from all steps from the two grains from each sample are 79.2 ± 0.6 Ma for FTB1 and 79.1 ± 0.6 Ma for FTB2 (Fig. 2c and f). These ages are compatible with each other and with the plateau ages for each individual sample. The isochrons yield initial 40Ar/36Ar ratios of 310 ± 12 (FTB1) and 314 ± 14 (FTB2), consistent with the present atmospheric value (298.56 ± 0.31; Lee et al., 2006), with the possible presence of very minor excess argon components. Phlogopite crystals from two syenite samples (FTS11 and FTS12; Fig. 2g and h) shows much more complex age spectra that fail to yield plateaus and plateau ages according to the definition of Fleck et al. (1977). But both spectra are remarkably similar, yield young results at low temperatures, ascend rapidly at intermediate temperatures, and reach plateau-like segments at high temperatures that suggest a minimum age of crystallization of 96.2 ± 0.8 Ma for both grains. The integrated ages obtained for the two grains are also statistically indistinguishable at 70.5 ± 0.7 and 70.5 ± 0.8 Ma, respectively. The ascending spectra reveal significant Ar loss from the phlogopite crystals, suggest a minimum crystallization age of 96.2 ± 0.8 Ma (Fig. 2i) for the syenite samples, and require a more recent argon loss (reheating?) event.

4.1. Magmatic events during the emplacement of Tapira alkalinecarbonatite complex The 40Ar/39Ar integrated ages for our syenite samples (70.5 ± 0.7 and 70.5 ± 0.8 Ma) are more precise but within error from the K–Ar age of 71.2 ± 5.1 Ma obtained by Sonoki and Garda (1988) for a syenite sample from Tapira complex. The ascending nature of the incremental heating spectra, however, reveals that the ages of ~70 Ma are not true ages for the syenites, but an artifact associated with partial resetting of the ~96 Ma ages during a much more recent thermal event. Interestingly, the apparent age of resetting (~36 Ma, Fig. 2i), based on the first step in both incremental heating spectra for the syenite samples (McDougall and Harrison, 1999), does not correspond to any known thermal event in the region. Thus, it is possible to conclude that the emplacement age of the Tapira complex is at least 96.2 ± 0.8 Ma or older, that it was affected by at least one subsequent magmatic event at 79.15 ± 0.6 Ma; and that a second partial resetting event at ~36 Ma affected the isotopic system in many of the phlogopite samples. The exact age of this latest resetting event remains to be determined. Similar alkaline-carbonatite complexes elsewhere (Kenya, Norway, Sweden, Russia, India Canada, USA, Angola and South Africa) also exhibit two main magmatic events, where an early silicate stage is followed by a later carbonate stage (Le Bas, 1977). In addition, two magmatic events for emplacement of the Jacutinga alkaline-carbonatite complex, located 3

Sample

B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11 B11

B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21 B21

S11 S11 S11 S11 S11 S11 S11 S11 S11 S11 S11

Run ID

5604-01A 5604-01B 5604-01C 5604-01D 5604-01E 5604-01F 5604-01G 5604-01H 5604-01I 5604-01J 5604-02A 5604-02B 5604-02C 5604-02D 5604-02E 5604-02F 5604-02G 5604-02H 5604-02I 5604-02J

5605-01A 5605-01B 5605-01C 5605-01D 5605-01E 5605-01F 5605-01G 5605-01H 5605-01I 5605-01J 5605-02A 5605-02B 5605-02C 5605-02D 5605-02E 5605-02F 5605-02G 5605-02H 5605-02I 5605-02J

5606-01A 5606-01B 5606-01C 5606-01D 5606-01E 5606-01F 5606-01G 5606-01H 5606-01I 5606-01J 5606-02A

Ar/39Ar

4

0.008035 0.006648 0.007280 0.008140 0.008230 0.008740 0.008510 0.008330 0.008520 0.006960 0.010451

0.021930 0.001302 0.000432 0.000340 0.000620 0.000270 0.000394 0.000319 0.000267 0.000790 0.013800 0.001908 0.000650 0.000480 0.000830 0.000910 0.000600 0.000434 0.000610 0.000261

0.009170 0.000912 0.000660 0.000500 0.000870 0.000450 0.000376 0.000531 0.004700 0.000400 0.028950 0.001161 0.000616 0.000460 0.000330 0.000610 0.000236 0.000233 0.000151 0.000350

36

0.000059 0.000094 0.000130 0.000170 0.000160 0.000280 0.000430 0.000680 0.000610 0.000540 0.000079

0.000270 0.000063 0.000093 0.000100 0.000120 0.000140 0.000065 0.000035 0.000051 0.000410 0.000240 0.000093 0.000130 0.000140 0.000170 0.000170 0.000100 0.000039 0.000063 0.000035

0.000160 0.000072 0.000140 0.000180 0.000200 0.000240 0.000097 0.000088 0.003000 0.001300 0.000320 0.000056 0.000074 0.000100 0.000110 0.000130 0.000091 0.000046 0.000053 0.000520

36 Ar/39Ar Error

Tables 1 40 Ar/39Ar numerical data. Errors in this table are 1σ. Ar/39Ar

0.031 0.065 0.025 0.017 0.031 0.106 0.030 0.097 0.111 −0.039 0.023

0.368 0.002 0.011 0.003 0.013 −0.021 0.001 −0.005 0.012 0.013 0.530 0.191 0.021 0.030 0.019 0.028 0.015 0.012 0.021 −0.0005

0.074 0.011 0.009 0.088 0.050 0.011 0.020 0.335 3.210 0.330 0.073 −0.0015 0.007 0.020 0.003 0.002 −0.0062 0.0031 0.0055 0.055

37

0.0033 0.0063 0.0100 0.0130 0.0150 0.0430 0.0520 0.0810 0.0640 0.0630 0.0043

0.0180 0.0064 0.0120 0.0150 0.0140 0.0180 0.0100 0.0061 0.0077 0.0620 0.0260 0.0110 0.0170 0.0200 0.0240 0.0240 0.0130 0.0053 0.0071 0.0042

0.0160 0.0140 0.0200 0.0380 0.0310 0.0330 0.0140 0.0130 0.4800 0.2000 0.0170 0.0048 0.0080 0.0100 0.0130 0.0150 0.0095 0.0048 0.0048 0.0630

37 Ar/39Ar Error

Ar/39Ar

0.00969 0.01093 0.01025 0.01086 0.01098 0.00953 0.01034 0.01184 0.00880 0.01105 0.00976

0.01302 0.00904 0.00936 0.00893 0.01051 0.01019 0.00882 0.00837 0.00937 0.01051 0.01137 0.00984 0.01025 0.00882 0.00958 0.00926 0.00861 0.00843 0.00915 0.00981

0.01029 0.00836 0.00901 0.00960 0.00988 0.00846 0.00919 0.00970 0.01510 0.00740 0.01398 0.01140 0.00977 0.01035 0.00988 0.00994 0.00931 0.01006 0.00902 0.01181

38

0.00038 0.00030 0.00029 0.00032 0.00029 0.00055 0.00057 0.00075 0.00082 0.00072 0.00042

0.00048 0.00027 0.00032 0.00035 0.00042 0.00039 0.00025 0.00035 0.00033 0.00067 0.00041 0.00023 0.00025 0.00034 0.00035 0.00035 0.00031 0.00030 0.00020 0.00021

0.00038 0.00025 0.00039 0.00043 0.00037 0.00048 0.00041 0.00023 0.00320 0.00170 0.00054 0.00028 0.00026 0.00026 0.00031 0.00034 0.00029 0.00039 0.00023 0.00077

38 Ar/39Ar Error

Ar/39Ar

8.023 15.380 16.961 17.385 17.670 17.805 18.070 17.970 18.080 17.220 8.790

19.146 12.608 12.390 12.416 12.448 12.431 12.410 12.358 12.448 12.598 17.935 13.126 12.710 12.607 12.733 12.672 12.511 12.432 12.570 12.506

15.422 12.614 12.563 12.641 12.613 12.520 12.470 12.577 12.640 12.750 21.293 12.547 12.407 12.414 12.368 12.472 12.381 12.389 12.448 12.320

40

0.024 0.043 0.062 0.062 0.065 0.077 0.110 0.150 0.130 0.110 0.023

0.073 0.041 0.045 0.046 0.043 0.043 0.041 0.035 0.049 0.085 0.071 0.050 0.049 0.050 0.047 0.051 0.055 0.035 0.043 0.037

0.053 0.047 0.052 0.055 0.055 0.057 0.043 0.050 0.330 0.180 0.081 0.037 0.046 0.053 0.044 0.049 0.048 0.039 0.048 0.100

40 Ar/39Ar Error

Ar*/39Ar

5.625 13.400 14.788 14.954 15.215 15.200 15.530 15.490 15.540 15.140 5.671

12.630 12.219 12.261 12.312 12.264 12.348 12.291 12.261 12.368 12.360 13.860 12.572 12.516 12.464 12.486 12.401 12.332 12.302 12.388 12.427

12.689 12.341 12.366 12.499 12.357 12.386 12.358 12.446 11.500 12.670 12.650 12.199 12.223 12.278 12.269 12.288 12.309 12.319 12.402 12.210

40

0.034 0.055 0.073 0.082 0.082 0.110 0.160 0.240 0.220 0.190 0.041

0.120 0.046 0.053 0.055 0.056 0.059 0.046 0.037 0.051 0.150 0.100 0.057 0.062 0.065 0.069 0.072 0.063 0.037 0.047 0.038

0.075 0.052 0.067 0.077 0.081 0.091 0.052 0.057 0.960 0.440 0.140 0.041 0.051 0.061 0.055 0.063 0.055 0.041 0.051 0.190

40 Ar*/39Ar Error

70.12 87.12 87.19 86.02 86.11 85.38 85.94 86.20 86.00 87.91 64.51

65.96 96.91 98.96 99.17 98.53 99.35 99.05 99.22 99.36 98.10 77.25 95.77 98.47 98.87 98.06 97.86 98.57 98.96 98.56 99.37

82.28 97.84 98.44 98.88 97.97 98.93 99.11 98.94 90.80 99.30 59.43 97.24 98.52 98.91 99.21 98.53 99.43 99.44 99.64 99.20

%40Ar*

0.25 0.21 0.25 0.36 0.33 0.50 0.73 1.20 1.00 0.98 0.29

0.45 0.23 0.29 0.33 0.36 0.37 0.24 0.16 0.22 1.00 0.43 0.27 0.38 0.38 0.43 0.44 0.33 0.15 0.22 0.14

0.36 0.23 0.38 0.46 0.52 0.61 0.31 0.30 7.20 3.20 0.46 0.18 0.24 0.34 0.35 0.39 0.28 0.19 0.19 1.30

%40Ar* Error

0.22 0.34 0.45 0.51 0.51 0.70 1.00 1.50 1.30 1.20 0.26

0.74 0.29 0.33 0.35 0.35 0.37 0.29 0.23 0.32 0.93 0.65 0.36 0.39 0.41 0.44 0.45 0.39 0.23 0.29 0.24

0.47 0.33 0.42 0.48 0.51 0.57 0.33 0.36 6.00 2.80 0.86 0.26 0.32 0.39 0.35 0.40 0.35 0.26 0.32 1.20

Age Error

(continued on next page)

36.56 85.89 94.56 95.60 97.22 97.15 99.20 98.90 99.20 96.70 36.85

81.07 78.48 78.75 79.07 78.77 79.30 78.94 78.75 79.42 79.39 88.77 80.70 80.35 80.02 80.16 79.63 79.20 79.01 79.55 79.79

81.44 79.26 79.41 80.25 79.35 79.53 79.36 79.91 73.90 81.30 81.22 78.36 78.51 78.86 78.80 78.92 79.05 79.11 79.64 78.50

Age (Ma)

F.T. Conceição, et al.

Journal of South American Earth Sciences 97 (2020) 102416

Journal of South American Earth Sciences 97 (2020) 102416

0.01025 0.01164 0.00928 0.01033 0.01242 0.01077 0.01069 0.01330 0.01030 0.0091 0.0150 0.0220 0.0230 0.0370 0.0420 0.0650 0.1300 0.0810

0.00041 0.00032 0.00045 0.00044 0.00052 0.00057 0.00069 0.00130 0.00090

15.884 17.933 18.121 18.225 18.656 18.640 18.600 18.760 19.040

0.063 0.060 0.068 0.078 0.098 0.100 0.120 0.180 0.140

13.278 14.846 14.996 15.090 15.080 15.100 15.560 14.840 15.290

0.074 0.081 0.095 0.100 0.140 0.140 0.190 0.380 0.250

83.59 82.79 82.76 82.82 80.85 81.01 83.63 79.10 80.30

0.28 0.32 0.41 0.42 0.62 0.60 0.84 1.90 1.20

85.13 94.93 95.86 96.47 96.40 96.50 99.30 94.90 97.70

0.46 0.50 0.59 0.63 0.89 0.89 1.20 2.30 1.50

at the northern flank of the Ponta Grossa Arc (Brazil), also were proposed by Beccaluva et al. (2017) and Chmyz et al. (2017). Brod et al. (2013) interpret the evolution of the Tapira complex as the result of magmatic fractionation leading to carbonate-silicate liquid immiscibility, which finally resulted in magma degassing and late-stage CO2 metasomatism of previously formed magmatic rocks. Gomide et al. (2016) interpret stable (C, O and S) isotope compositions and trends, combined with whole rock geochemistry, to suggest that the carbonatite melts in the APIP were produced by liquid immiscibility at low lithospheric pressure. On the other hand, Brigatti et al. (1996) interpret the crystal geochemistry of phlogopite from Tapira as evidence for two magmatic systems, i.e. one an alkaline-silicate system and the other a carbonatite system. Brod et al. (2001), based in micro-analyses of Tapira phogopites, propose a magmatic evolution for the silicate plutonic series (SPS - from wehrlites to bebedourite B1, bebedourite B2 and syenites) and carbonatites (from C1 to C5) along independent trends. The authors suggest that the interaction of carbonatitic liquid with SPS resulted in metasomatic phlogopites in some of the previously formed silicate magmas, such as the bebedourites. Our new geochronological data suggest that the bebedourites should be associated with the second igneous event at 79.15 ± 0.6 Ma, indicating that this rocks would be younger than the syenites, a fact inconsistent with the petrographic and isotopic data reported by Brod et al. (2001, 2013) and Gomide et al. (2016) that suggest that syenites are evolved endmembers of the bebedourites series. However, mineral chemistry shows that bebedourites contain two distinct generations of phlogopites: primary phlogopites that show increasing Fe2+ and decreasing Mg2+ contents with magmatic evolution; and metasomatic phlogopites generated by alteration during carbonatite intrusion (Brod et al., 2001). We therefore interpret that our 79.15 ± 0.6 Ma age does not represent an emplacement age for the bebedourites, but it records the timing of Mg-metasomatism during carbonatite intrusion. This interpretation is consistent with the fact that apatites found in bebedourites and carbonatites yield fission-track ages of 81.7 ± 7.9 Ma and 78.6 ± 9.0 Ma, respectively (Eby and Mariano, 1992), values within error of our new 40Ar/39Ar age for the bebedourites (79.15 ± 0.6 Ma). Thus, our new geochronological results reveal at least two distinct magmatic events during the emplacement of Tapira alkaline-carbonatite complex. The first igneous event, at least 96.2 ± 0.8 Ma or older, as discussed above, was responsible for emplacement of the SPS and the second igneous event, at 79.15 ± 0.6 Ma, is associated with the carbonatite intrusion (Fig. 3). 4.2. Extended age of APIP magmatism Previous K–Ar mica ages (Sonoki and Garda, 1988) and U–Pb ages (using SIMS and LA-ICP-MS) on perovskites (Guarino et al., 2013), when interpreted in light of our new 40Ar/39Ar results for the Tapira alkaline-carbonatite complex, indicate that alkaline-carbonatite magmatism in the APIP province appears to have occurred in multiplestages between ~ 96 and ~79 Ma (Fig. 4) and that episodes may be coeval among the various magmatic centres. If this is correct – i.e., that magmatism is indeed regionally episodic and coeval and that it spans more than 17 Ma – the generation of magmas by heat anomalies associated with passage over a plume head (Gibson et al., 1995; Thompson et al., 1998; Carlson et al., 2007; Bulanova et al., 2010) would require significant lateral migration of magmas along the base of the lithosphere beneath a slow moving plate (Sleep, 2006). And the association of silicic and carbonatitic magmatism across the province calls for very similar coeval melting processes from a regionally carbonate-metasomatized mantle. To completely assess the contemporaneity of the various stages of magmatism at a regional scale, high-resolution 40Ar/39Ar of the various magmatic centres is still required. Finally, a process that would generate variable degrees of partial melting, at a regional scale, to generate the initial silicic and later carbonatitic melts, repeatable across the entire region, still eludes

S11 S11 S11 S11 S11 S11 S11 S11 S11 5606-02B 5606-02C 5606-02D 5606-02E 5606-02F 5606-02G 5606-02H 5606-02I 5606-02J

0.008730 0.010340 0.010470 0.010500 0.011990 0.011880 0.010230 0.013200 0.012600

0.000130 0.000170 0.000210 0.000220 0.000370 0.000360 0.000510 0.001200 0.000730

0.027 0.032 0.031 0.070 0.085 0.133 0.159 0.390 0.174

38

Sample Run ID

Tables 1 (continued)

36

Ar/39Ar

36 Ar/39Ar Error

37

Ar/39Ar

37 Ar/39Ar Error

Ar/39Ar

38 Ar/39Ar Error

40

Ar/39Ar

40 Ar/39Ar Error

40

Ar*/39Ar

40 Ar*/39Ar Error

%40Ar*

%40Ar* Error

Age (Ma)

Age Error

F.T. Conceição, et al.

5

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Fig. 3. Photograph of (a) bebedourite B2 and (b) syenite, with veins of calcite cutting all minerals from these rocks, indicating a carbonate event after the emplacement of the Silicate Plutonic Series. Di = diopside, Phl = phlogopite, Cal = calcite, Ap = apatite, Mgt = magnetite and Mc = microcline.

magmatism. Declaration of competing interest There is not a Conflict of Interest. Acknowledgements We thank the Vale SA for logistic support and University of Queensland (UQ) Argon Geochronology in Earth Science (UQ-AGES) laboratory, Austrlia. This investigation was funded by FAPESP-Brazil (Process No. 2005/59203-1). Conceição thanks specially CNPq, Brazil (Process No. 200775/2008-1) for its Post-Doctoral scholarship. The authors specially thank Robson Santos Aglisnkas, Benjamin E. Cohen and David S. Thiede for their general help during its development. Specially, Dr. Reinhardt Fuck (Regional Editor) and two anonymous referees are thanked for their detailed and insightful review comments, which helped to improve the manuscript.

Fig. 4. Previous K–Ar (Sonoki and Garda, 1988) and U–Pb (Guarino et al., 2013) ages for alkaline-carbonatite rocks found in the large APIP are compared to the 40Ar/39Ar results from this study, showing multiple stages of magmatism between ~ 96 and 79 Ma. Multiple magmatic and alteration events could be better resolved if high-resolution laser-heating single crystal 40Ar/39Ar geochronology were to be applied to the various K-bearing minerals (K-feldspars, phlogopite, plagioclase, etc.) often present in the various alkaline-carbonatite complexes in the APIP.

References Araujo, A.L.N., Carlson, R.W., Gaspar, G.C., Bizzi, L.A., 2001. Petrology of kamafugites and kimberlites from the Alto paranaíba alkaline province, Minas Gerais, Brazil. Contrib. Mineral. Petrol. 142, 163–177. Beccaluva, L., Bianchini, G., Natali, C., Siena, F., 2017. The alkaline-carbonatite complex of Jacutinga (Brazil): magma genesis and mode of emplacement. Gondwana Res. 44, 157–177. Brigatti, M.F., Medici, L., Saccani, E., Vaccaro, C., 1996. Crystal chemistry and petrologic significance of Fe3+-rich phlogopite from the Tapira carbonatite complex, Brazil. Am. Mineral. 81, 913–927. Brod, J.A., Gibson, S.A., Thompson, R.N., Junqueira-Brod, T.C., Seer, H.J., Moraes, L.C., Boaventura, G.R., 2000. The kamafugite-carbonatite association in the Alto paranaíba igneous province (APIP), southeastern Brazil. Rev. Bras. Geociencias 30, 408–412. Brod, J.A., Gaspar, J.C., Araújo, S.A., Gibson, S.A., Thompson, R.N., Junqueira-Brod, T.C., 2001. Phlogopite and tetra-ferriphlogopite from Brazilian carbonatite complexes: petrogenetic constrains and implications for mineral-chemistry systematics. J. Asian Earth Sci. 19, 265–296. Brod, J.A., Junqueira-Brod, T.C., Gaspar, J.C., Petronovic, I.A., Valente, S.C., Corval, A., 2013. Decoupling of paired elements crossover REE patterns and mirrored spider diagrams: fringerpriting liquid immiscibility in the Tapira alcaline-carbonatite complex, SE Brazil. J. South Am. Earth Sci. 41, 41–56. Bulanova, G.P., Walter, M.J., Smith, C.B., Kohn, S.C., Armstrong, L.S., Blundy, J., Gobbo, L., 2010. Mineral inclusions in sublithospheric diamonds from Collier 4 kimberlite pipe, Juina, Brazil. subducted protoliths, carbonate melts and primary kimberlite magmatism. Contrib. Mineral. Petrol. 125, 489–510. Carlson, R.W., Araújo, A.L.M., Junqueira-Brod, T.C., Gaspar, J.C., Brod, J.A., Petrinovic, I.A., Holland, M.H.B.M., Pimentel, M.M., Sichel, S., 2007. Chemical and isotopic relationships between peridotite xenoliths and mafic-ultrapotassic rocks from Southern Brazil. Chem. Geol. 242, 418–437. Chmyz, L., Arnaud, N., Biondi, J.C., Azzone, R.G., Bosch, D., Ruberti, E., 2017. Ar-Ar ages, Sr-Nd isotope geochemistry, and implications for the origin of the silicate rocks of the Jacutinga ultramafic-alkaline complex (Brazil). J. South Am. Earth Sci. 77, 286–309. Comin-Chiaramonti, P., Gomes, C.B., 2005. Mesozoic to Cenozoic Alkaline Magmatism in the Brazilian Platform. Editora da Universidadede São Paulo, São Paulo. Conceição, F.T., Bonotto, D.M., 2006. Radionuclides, heavy metals and fluorine incidence

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5. Conclusion New laser-heating 40Ar/39Ar geochronology of single phlogopite crystals from the Tapira alkaline-carbonatite complex suggest at least two distinct periods of magmatism, ~96 for emplacement of the silicic magmas and ~79 Ma for carbonatitic magmatism. The first igneous event was responsible for emplacement of the Silicate Plutonic Series and the second igneous event originated the carbonatite intrusion, which promoted the formation of the metasomatic phlogopites dated in the bebedourites. High-resolution incremental-heating 40Ar/39Ar geochronology, capable of identifying and resolving multiple superimposed magmatic events, can be a powerful technique to resolve the complex superposition of alkaline-carbonatite magmatism in the APIP. More extensive and systematic use of 40Ar/39Ar geochronology in all alkaline complexes in the APIP, applied to various minerals (K-feldspars, phlogopite, K-feldspars, plagioclase, etc.) associated with each magmatic phase, will help to resolve the history of magma emplacement, the changes of magma types with time, the contemporaneity of magmatic events at a regional scale, and the underlying tectonic processes driving 6

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