The Barreiro suite in the central Ribeira Belt (SE-Brazil): a late Tonian tholeiitic intraplate magmatic event in the distal passive margin of the São Francisco Paleocontinent

Abstract

New geochemical, U-Pb, Lu-Hf and Sr-Nd data from the Barreiro Suite metabasites in comparison with enclaves within the distal Andrelândia Group and the orthogranulites of the Juiz de Fora Complex are presented. Geochemical data suggest intraplate setting, with high and low-TiO₂ , TDM_Nd ages between 1.80 and 1.41 Ga, negative ΕNd_t and (⁸⁷ Sr/⁸⁶ Sr)i between 0.714 and 0.747. Results contrast with part of the Juiz de Fora Complex enclaves, with island arc tholeiites- calcalkaline basalts (IAT-CAB) geochemical signatures, TDM_Nd ages between 2.58 and 2.16 Ga, positive ΕNd_t values and (87Sr/86Sr)_i between 0.700 and 0.712. U-Pb data for the Barreiro Suite yielded a crystallization age of 766 ± 13 Ma and a metamorphic overprint of 619 ± 6 Ma. The results indicate three episodes of mafic magmatism in the Occidental terrane of the Ribeira Belt. The two older episodes are related to Rhyacian arc evolution (ca. 2.2 to 2.1 Ga) and to the Statherian (ca. 1.7 Ga) tectonics, and occur only within the Juiz de Fora Complex, while the younger ca. 766 Ma episode constrains the timing of distal passive margin evolution. An important implication is that these late Tonian metabasic rocks could have been a source of detrital zircons for the sedimentation along the distal Andrelândia basin.

KEYWORDS:
Andrelândia Group; U-Pb; Sm-Nd; Lu-Hf; within-plate basalts

INTRODUCTION

Tholeiitic mafic rocks are very common in many different tectonic environments, ranging from continental and oceanic plateaus, continental rifts to passive margins and convergent tectonic settings such as magmatic arcs (Pearce & Cann 1973, Pearce et al. 1975, Pearce 1983, 1987, Wilson 1989). In high-grade metamorphic terrains, the original tectonic setting is commonly unclear because recrystallization and deformation obscure primary characteristics. In such terrains, the geochemical and isotopic signatures, and the age of mafic rocks are important tools for the determination of their tectonic setting. Geochronological dating also provides additional constraints on the age of genetically-related high-grade metasedimentary units in orogenic belts.

The Ribeira Belt, southeast Brazil, comprises a Neoproterozoic and Early Paleozoic network of orogenic belts that resulted from the amalgamation of Western Gondwana (Fig. 1A ), evolving through episodes of diachronous tectonic accretions and continental collisions in Neoproterozoic and Cambrian times. Together with the Araçuaí Belt (Heilbron et al. 2004, 2008, 2017a, Pedrosa-Soares et al. 2008, Alkmim et al. 2017), the Ribeira Belt makes up a 300 km wide and 600 km long deeply eroded orogenic system that runs roughly parallel to the southeastern coast of Brazil (Fig. 1B).

Figure 1.
(A) Ribeira Belt in the scenario of South America: 1: Andean Belt; 2: Patagonia terrane; 3: Phanerozoic cover; 4: Cratons; 5: Neoproterozoic belts; 6: São Francisco craton; (B) tectonic organization of SE Brazil: 1: Phanerozoic cover; 2-4: Ribeira Belt units; 2: Apiaí terrane; 3: reworked passive margins; 4: arc-related terranes; 5: Brasília Belt.

The external zone of this orogenic system comprises the reworked Neoproterozoic passive margin of the São Francisco paleocontinent. Because of intense deformation and metamorphism related to the Brasiliano collage, the metasedimentary units together with syn-depositional magmatic rocks crop out as high-grade gneisses, tectonically interleaved with highly-deformed basement slivers during the collisional episodes.

The Macaúbas and Andrelândia groups (Paciullo et al. 2000, Pedrosa Soares et al. 1998, 2000) represent the main Neoproterozoic passive margin sedimentary sequences of the Araçuaí and Ribeira belts. They are mainly siliciclastic, and depositional age constraints are mostly based on U-Pb ages of detrital zircons from quartzitic rocks (Valeriano et al. 2004, Valladares et al. 2004, 2008, Belém et al. 2011, Westin & Campos Neto 2013, Degler et al. 2017, Frugis & Campos Neto 2018). Results obtained suggest São Francisco craton source rocks, and the main constraint for the maximum possible sedimentation age of these units is the youngest detrital zircon, ranging in age from Tonian to Cryogenian ages. In several studies, the Cryogenian zircons are interpreted reflecting a shift from passive margin to active tectonic setting, with provenance from approaching magmatic arcs (Campos Neto et al. 2011, Belém et al. 2011, Degler et al. 2017). The major problem is that most of these studied units are metamorphosed to upper amphibolite to granulite facies, so many of these young zircons shows complex textures, with internal cores with different ages and compositions, suggestive of metamorphic reworking at high temperature that could lead to Pb loss. Therefore, another tool for constraining the depositional age of these high-grade siliciclastic rocks is the age of syn- or post-depositional magmatic rocks.

This study aims to contribute to the understanding of the development of the distal passive margin of the São Francisco paleocontinent by presenting new geological, geochemical and geochronological data for the Barreiro Suite (Heilbron et al. 2012, 2017b). This recently described unit includes tholeiitic mafic rocks that occur within the distal segment of the Andrelândia basin. However, a further toll for constraining the depositional age of these high-grade siliciclastic rocks is the age of syn or post-depositional magmatic rocks. We also present a comparison of geochemical and Sm-Nd and Sr isotope data from metabasic rocks intercalated in the Andrelândia and Macaúbas sequences with data for basic enclaves and layers that occur within the basement units such as the Juiz de Fora Complex.

TECTONIC SETTING: THE ANDRELÂNDIA GROUP IN THE DISTAL SÃO FRANCISCO PASSIVE MARGIN

The study area is located in the central segment of the Neoproterozoic-Cambrian Ribeira Belt (Almeida et al. 1981, Heilbron et al. 2004). Following the tectonic subdivision of Heilbron et al. (2008, 2017a), the Ribeira Belt is divided into four tectono-stratigraphic terranes that sequentially docked against the southeastern portion of the São Francisco paleocontinental passive margin (Fig. 2). From west to east, these terranes are the Occidental Terrane (reworked SFC margin); the Paraíba do Sul-Embú Terrane, docked at ca. 620-595 Ma; the Oriental Terrane, docked at ca. 605-565 Ma; and the Cabo Frio Terrane, docked at ca. 535-510 Ma.

Figure 2.
Tectonic map of central Ribeira Belt, modified from Heilbron et al. (2017a). The green rectangle shows the studied area.

The Occidental Terrane, the focus of this work, is regarded as the reworked passive margin of the São Francisco paleocontinent and comprises three major litho-tectonic units:

• reworked Paleoproterozoic basement and bimodal Mesoproterozoic magmatic rocks (Heilbron et al. 1998, 2010, Noce et al. 2007, Degler et al. 2018);

• a Neoproterozoic metasedimentary sequence described originally as the Andrelândia Group (Paciullo et al. 2000);

• syn- to late-collisional Neoproterozoic granitoid rocks (Fig. 2).

Studies carried out in the proximal zone of the Neoproterozoic passive margin (Paciullo 1997, Paciullo et al. 2000, Ribeiro et al. 1995) proposed a subdivision of the Andrelândia Group into two major sequences. The basal Carrancas Sequence (informal designation) comprises psammitic banded gneisses with quartzites, pelitic schist and amphibolite intercalations (A1 + A2), followed by a regressive succession of quartzites bearing characteristic green muscovite (A3), succeeded by a transgressive succession of graphite-rich grey schists and feldspathic quartzites (A4). The overlying Serra do Turvo Sequence consists of pelitic schists and gneisses (A6), followed by the uppermost unit of plagioclase-rich schists and gneisses (A5). In the distal segment of the passive margin, where our study is located, only the basal unit (A1 + A2 equivalent?) and a pelitic unit of paragnais and schist (A6) are present. In the recent geological map of Rio de Janeiro State (Heilbron et al. 2017b), the name of Raposos Group was adopted for this distal portion of the Andrelândia Group. Because of intense tectonic shuffling with basement rocks of the Juiz de Fora Complex, and metamorphic conditions up granulite facies, the reconstruction of the architecture of this distal basin is difficult.

Major attempts to determine the depositional age of the Andrelândia Group were based on detrital zircons in metasedimentary rocks and on few Sm-Nd model ages of intercalated metabasic rocks (Heilbron et al. 1989, Frugis & Campos Neto 2018). Most data point a Tonian onset of sedimentation after ca. 1.0-0.9 Ga (Valeriano et al. 2004, Valladares et al. 2004. On the other hand, younger Neoproterozoic zircons with ages of ca. 680 Ma for the A5 unit have been attributed to provenance from magmatic arcs during the orogenic stage (Belém et al. 2011, Frugis & Campos Neto 2018, Westin & Campos Neto 2013), indicating a shift to an active setting. It is important to stress that high-pressure granulite facies metamorphism between 640 and 600 Ma hampers precise determination of the minimum age of sedimentation (Coelho et al. 2017, Heilbron et al. 2017a, Trouw et al. 2013).

Metabasic rocks are interlayered with the A1 + A2 and A6 units and normally parallel to the primary sedimentary bedding. They vary from fine-grained and foliated amphibolites to coarse-grained mafic granulites. In the high-pressure metamorphic facies, garnet + clinopyroxene + quartz is a very common assemblage in the lower Andrelândia nappe system. Relics of retro-eclogite have been described by Silva et al. (2002), Trouw et al. (2000, 2013), Campos Neto (2000), and by Coelho et al. (2017). In the upper nappe system, referred to Heilbron et al. (2000) as the Juiz de Fora crustal duplex system, (garnet) + orthopyroxene + clinopyroxene + quartz is the most common assemblage (Duarte et al. 2003).

Gonçalves & Figueiredo (1992) studied mafic rocks that occur near the town of Andrelândia, while Marins (2000) presented geochemical data for mafic rocks located between Conservatória and Barra do Piraí. Both studies report intraplate to MORB-like signatures. A few Sm-Nd model ages of ca. 1.0 Ga were also reported for mafic rocks near Andrelândia town (Heilbron et al. 1989).

MAFIC ROCKS OF THE STUDY AREA: GEOLOGY AND PETROGRAPHY

Centimetric to decametric enclaves of metabasic rocks are interlayered within rocks of the Neoproterozoic Andrelândia Group and the orthogranulites of the Juiz de Fora Complex.

The Barreiro Suite is dominated by metabasic bodies shown in the 1:100.000 geological maps of Santo Antônio de Pádua (Heilbron et al. 2012) and Ubá (Noce et al. 2003) sheets. A decametric-scale mafic body was described within the orthogranulites of the Juiz de Fora Complex. Initially, our work was conducted to date this mafic occurrence. However, our detailed geological mapping shows that the Barreiro suite also occur within the Andrelândia metasedimentary rocks and within the ca. 620-595 Ma syn-collisional garnet charnockites (diatexites) of the Salvaterra suite (Duarte et al. 2013) (Fig. 3). The major mafic bodies occur in the western part of the mapped area, as enclaves within this syn-collision Brasiliano granitoid suite. For comparison, these minor lenses within the other country rocks were sampled.

Figure 3.
Geological map of the studied area with the location of the analysed enclaves from the country rocks.

The contact relationships with country rocks are different in the two situations. The lenses in the garnet charnockite, especially the centimentric ones, show textural evidence of partial digestion, suggesting that they are refractory material that survived partial melting.

Mafic lenses within the Andrelândia Group and the Juiz de Fora Complex

The mafic lenses (Figs. 3 and 4) have centimetre to decametre dimensions and display at least two types of relationships with country rocks. Some of the largest bodies display very sharp contacts either with the metasedimentary rocks of the Andrelândia Group, or with the orthogranulites of the Juiz de Fora Complex. On the other hand, centimetric scale enclaves within the orthogranulites show diffuse contacts possibly indicating that they have a common magmatic origin (Fig. 5).

Figure 4.
Geological map of the Barreiro Suite, with the location of the analysed rocks. All samples gave the same prefix code RE-CE.

Figure 5.
Outcrop images of country rocks of the Barreiro Suite. (A) Garnet-biotite gneiss of the Andrelândia Group; (B) hand sample of Opx-bearing granitoid of the Salvaterra Suite; (C) contact between the leuco-charnockite of the Juiz de Fora Complex and the grey foliated granite of the Bonfim Suite; (D) stretched mafic granulite included in the ortho-granulites of the Juiz de Fora Complex.

Mafic enclaves are granulite facies rocks, containing ortho and clinopyroxene, and locally garnet (Fig. 5). Hornblende, biotite and plagioclase also occurs, besides accessory phases such as ilmenite, zircon and apatite. Textures vary from granoblastic to mylonitic, compatible with those of the country rocks.

The Barreiro Suite

In the larger (up to 50 m) lenses of the Barreiro Suite (Fig. 4), two lithofacies were observed, consisting respectively of medium to fine- and coarse-grained rocks (Fig. 6). Because of the quality of the outcrops, mainly as quasi in-situ mega-boulders, it was not possible to clearly observe the relationships between these two facies. In the field, both occurs as dark gray rocks with a subtle foliation. However, under the microscope, there is a granonematoblastic texture where a visible foliation is defined by the preferential orientation of the mafic minerals. Deformational textures such as incomplete twinning of feldspars, undulose extinction and polygonal granoblastic contacts are commonly observed. The coarse-grained facies rocks locally contain plagioclase porphyroblasts, possibly representing relics of magmatic megacrysts.

Figure 6.
(A) Hand sample and (B) photomicrograph of the coarse-grained variety of metabasic rocks of the Barreiro Suite; (C) hand sample and (D) photomicrograph of the fine-grained variety of the metabasic rocks of the Barreiro Suite; (E) general overview of the Barreiro Suite outcrops with dispersed boulders.

The main mineralogy of both facies is represented by orthopyroxene, clinopyroxene, dark hornblende, plagioclase and opaque phases, mainly ilmenite. Biotite and hornblende also occur along the margins of the pyroxenes, suggesting formation by retrogressive reactions. Apatite, quartz and zircon are the common accessory phases.

Geochemistry

Sampling and analytical procedures

Nineteen samples of the metabasic rocks of the Barreiro suite and 23 samples of mafic lenses within Andrelândia Group and Juiz de Fora Complex rocks were selected for geochemical analyses (Tab. 1). Samples were crushed and milled at the Laboratório de Preparação de Amostras (LGPA) of the Universidade do Estado do Rio de Janeiro (UERJ). Activation Laboratories (Ontario, Canada) performed major and trace element analysis, including Rare Earth Elements (REE). Details of the analytical techniques used by this laboratory are presented at www.actlabs.com. Treatment of the data was carried out using the GeoChemical Data ToolKIT (GCDkit) software of Janoušek et al. (2006).

Table 1.
Geochemical data of the metabasic rocks

Geochemical Data: Major Elements
Sample	type	Unit	SiO2	Al2O3	FeO	Fe2O3t	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	LOI	Total
RE-CE-15	HTiO2	BS	53.51	14.66	11.57	12.86	0.274	4.24	9.2	1.65	0.25	2.608	1.35	0.18	100.8
RE-CE-16	HTiO2	BS	52.05	13.95	13.98	15.54	0.313	2.97	7.05	3.41	1.16	3.03	0.91	-0.02	100.4
RE-CE-19	HTiO2	BS	48.74	15.92	12.67	14.08	0.215	5.19	8.19	3.2	0.74	3.117	0.64	0.46	100.5
RE-CE-30B	HTiO2	BS	48.65	15.08	12.2	13.56	0.248	5.96	8.61	2.84	0.6	3.767	1.08	-0.28	100.1
RE-CE-2A	HTiO2	BS	49.19	14.46	12.05	13.39	0.223	4.73	7.92	3.31	0.98	2.437	0.5	1.5	98.65
RE-CE-12	HTiO2	BS	54.48	13.75	12.33	13.7	0.236	2.93	6.66	3.23	1.7	2.815	0.89	-0.1	100.3
RE-CE-13B	HTiO2	BS	51.45	15.6	12.12	13.47	0.267	4.51	7.61	3.48	0.93	2.762	0.63	0.04	100.7
RE-CE-27C	HTiO2	BS	51.02	15.41	12.57	13.97	0.223	3.34	6.75	3.46	2.18	2.832	0.76	0.29	100.2
RE-CE-28C	HTiO2	BS	49.5	17.16	10.53	11.7	0.193	5.91	8.45	3.53	1.11	2.238	0.38	0.54	100.7
RE-CE-29	HTiO2	BS	50.58	16.31	11.23	12.48	0.204	4.82	7.77	3.54	1.46	2.42	0.58	0.46	100.6
RE-CE-30A	HTiO2	BS	50.05	15.32	12.99	14.44	0.225	4.01	7.53	3.09	1.6	3.013	0.78	0.3	100.4
RE-CE-70B	HTiO2	BS	47.98	16.31	13	14.45	0.22	4.84	7.98	3.13	0.71	2.948	0.7	-0.21	99.05
RE-CE-3A	LTiO2	BS	51.81	16.42	9.76	10.85	0.181	5.76	8.56	3.4	0.74	1.919	0.36	0.96	101
RE-CE-03B	LTiO2	BS	48.67	17.22	9.65	10.72	0.188	7.45	9.79	3.03	0.48	1.586	0.28	1.06	100.5
RE-CE-25	LTiO2	BS	51.99	16.83	9.71	10.79	0.174	5.49	7.53	3.59	1.55	1.86	0.33	0.62	100.8
RE-CE-68	LTiO2	BS	57.34	15.8	9.3	10.34	0.157	3.84	6.13	3.39	0.63	1.776	0.41	0.3	100.1
RE-CE-3C	LTiO2	BS	49.41	17.39	9.51	10.57	0.174	6.63	9.3	3.14	0.86	1.679	0.32	0.98	100.4
RE-CE-11	LTiO2	BS	51.24	17.26	9.89	10.99	0.18	5.62	8.69	3.25	0.63	1.764	0.39	0.69	100.7
RE-CE-149	LTiO2	BS	52.7	16.94	10.19	11.32	0.186	6.03	7.76	3.38	0.68	1.724	0.3	-0.07	100.9
MI-BR-39	LTiO2	AND	48	13.88	12.76	14.18	0.21	7.3	11.88	1.41	0.54	1.386	0.1	-0.13	98.75
MI-BE-87B	LTiO2	AND	50.84	15.02	8.9	9.89	0.16	9.57	7.8	2.65	1.87	1.426	0.46	0.67	100.4
PA-BE-133B	LTiO2	AND	49.01	19.04	8.98	9.98	0.156	5.67	8.8	3.47	1.22	1.697	0.29	0.56	99.88
MI-BR-54	LTiO2	JF	40.45	12.52	14.06	15.62	0.301	21.58	6.92	0.51	0.31	0.348	0.09	0.68	99.32
MI-VIII-15	LTiO2	JF	46.8	9.98	9.57	10.64	0.155	21.07	8.98	1.28	0.19	0.335	0.03	1.28	100.7
SP-BR-16	LTiO2	JF	48.84	13.79	13.35	14.84	0.206	6.64	10.85	2.78	0.93	1.435	0.1	0.28	100.7
MI-II-62	LTiO2	JF	49.81	15.24	12.09	13.44	0.172	6.63	9.98	3.22	0.95	0.974	0.14	0.14	100.7
MI-V-40	LTiO2	JF	46.14	14.95	11.12	12.36	0.25	8.31	12.6	2.64	1.17	1.058	0.28	1.04	100.8
MI-BL-27	LTiO2	JF	51.01	14.8	9.37	10.41	0.167	7.58	11.9	1.92	0.65	0.934	0.12	0.88	100.4
PA-BE-146	LTiO2	JF	50.82	13.84	10.88	12.09	0.233	7.76	10.11	2.69	0.71	1.146	0.41	0.11	99.93
PA-BE-136	LTiO2	JF	50.85	15.32	9.75	10.83	0.163	7.41	7.6	3.17	2.39	1.024	0.48	0.09	99.31
SP-BR-06A	LTiO2	JF	50.07	14.36	11.58	12.87	0.185	7.25	8.52	2.82	1.19	0.948	0.23	1.19	99.65
PA-BE-137A	LTiO2	JF	51.27	10.61	10.13	11.26	0.203	12.84	8.85	2.04	2.32	0.629	0.39	0.24	100.7
SP-BR-06C	LTiO2	JF	42.48	18.52	10.95	12.17	0.138	6.24	12.23	2.44	1.44	1.585	1.05	0.99	99.28
SP-BR-17B	LTiO2	JF	49.22	14.16	12.62	14.02	0.187	7.02	11.23	2.33	0.68	1.356	0.11	0.02	100.3
MI-BR-19C	LTiO2	JF	48.56	13.65	13.49	14.99	0.217	6.59	10.03	2.64	1.3	1.557	0.12	0.69	100.3
MI-BR-19E	LTiO2	JF	51.26	14.41	12.26	13.62	0.265	5.4	5.28	2.33	3.35	1.538	0.13	1.35	98.93
PA-BE-92D	LTiO2	JF	44.68	12.52	14.75	16.39	0.247	11.73	8.2	1.56	3.7	0.343	0.04	1.46	100.9
MI-BE-83A	LTiO2	JF	52.16	16.3	9	10.01	0.165	4.99	7.32	3.64	1.42	1.621	0.32	1.2	99.15
MI-IX-03B	HTiO2	JF	51.62	16.41	9.79	10.88	0.156	5.22	7.89	3.15	1.79	2.157	0.86	0.27	100.4
MI-VIII-65	HTiO2	JF	46.78	18.07	9.6	10.67	0.166	4.72	9	3.39	2.57	2.245	1.15	0.57	99.35
MI-III-18	HTiO2	JF	49.11	16.98	11.45	12.72	0.2	5	8.63	3.23	0.68	2.467	0.81	0.07	99.89
MI-BR-56	HTiO2	JF	51.05	16.62	11.4	12.67	0.201	4.03	8.09	3.37	1.48	2.465	1.13	-0.24	100.9
Geochemical Data: Trace Elements
Sample	Type	Unit	Sc	Be	V	Ba	Sr	Y	Zr	Cr	Co	Ni	Cu	Zn	Ga	Ge	As	Rb	Nb
RE-CE-15	HTiO2	BS	26	5	252	230	593	44	354	200	30	60	20	160	24	2	6	5	29
RE-CE-16	HTiO2	BS	30	2	250	1089	553	49	342	20	34	30	40	180	22	1	< 5	8	28
RE-CE-19	HTiO2	BS	30	1	262	657	610	34	174	30	44	30	40	120	21	1	< 5	3	19
RE-CE-30B	HTiO2	BS	31	1	347	446	569	34	159	70	34	30	30	130	18	1	< 5	7	23
RE-CE-2A	HTiO2	BS	27	2	225	775	507	39	287	20	39	20	30	140	21	2	< 5	8	20
RE-CE-12	HTiO2	BS	28	2	227	1364	528	52	430	< 20	30	< 20	30	170	23	2	< 5	16	28
RE-CE-13B	HTiO2	BS	30	2	211	677	573	40	317	50	38	30	30	130	21	2	< 5	15	20
RE-CE-27C	HTiO2	BS	28	2	224	1605	498	50	545	< 20	31	< 20	30	140	23	1	< 5	24	31
RE-CE-28C	HTiO2	BS	27	2	224	299	556	30	221	40	42	< 20	30	90	20	1	< 5	15	15
RE-CE-29	HTiO2	BS	27	2	229	1147	622	36	573	< 20	38	20	30	110	20	1	< 5	29	19
RE-CE-30A	HTiO2	BS	30	2	261	1377	553	48	510	< 20	36	< 20	30	140	22	1	< 5	13	27
RE-CE-70B	HTiO2	BS	27	2	239	732	619	37	299	< 20	45	< 20	30	140	26	2	< 5	6	20
RE-CE-3A	LTiO2	BS	26	2	207	466	626	20	153	70	37	30	40	100	20	1	< 5	9	12
RE-CE-03B	LTiO2	BS	28	1	180	353	585	23	92	100	42	40	20	80	17	1	< 5	3	9
RE-CE-25	LTiO2	BS	25	2	187	437	537	30	225	40	37	30	30	90	19	1	< 5	67	13
RE-CE-68	LTiO2	BS	23	2	170	372	427	28	152	30	29	< 20	20	100	25	2	< 5	5	18
RE-CE-3C	LTiO2	BS	28	1	212	504	627	29	177	140	37	60	30	80	18	1	< 5	9	11
RE-CE-11	LTiO2	BS	27	2	220	711	646	27	257	30	37	30	30	100	19	1	< 5	5	12
RE-CE-149	LTiO2	BS	24	1	171	427	641	20	81	60	38	40	30	90	21	2	< 5	6	3
MI-BR-39	LTiO2	AND	41	< 1	369	12	146	19	57	150	64	90	30	120	19	2	< 5	7	4
MI-BE-87B	LTiO2	AND	26	2	186	1475	711	26	228	590	44	230	60	90	17	2	< 5	57	14
PA-BE-133B	LTiO2	AND	25	2	189	441	587	24	180	40	35	30	20	60	20	2	< 5	34	3
MI-BR-54	LTiO2	JF	14	< 1	75	327	114	12	13	200	114	450	60	280	9	1	< 5	4	1
MI-VIII-15	LTiO2	JF	33	< 1	186	22	35	19	20	2020	70	870	< 10	50	8	2	< 5	< 2	1
SP-BR-16	LTiO2	JF	39	< 1	392	64	131	31	73	90	56	90	10	120	20	2	< 5	8	4
MI-II-62	LTiO2	JF	36	< 1	253	207	264	20	87	90	49	140	30	110	20	2	< 5	5	4
MI-V-40	LTiO2	JF	45	2	331	399	286	16	94	410	45	90	< 10	120	19	2	< 5	21	3
MI-BL-27	LTiO2	JF	40	< 1	285	181	202	18	80	310	42	90	90	70	17	2	< 5	19	6
PA-BE-146	LTiO2	JF	43	1	170	1164	643	32	102	490	35	110	40	100	18	2	< 5	16	< 1
PA-BE-136	LTiO2	JF	26	2	192	1141	739	22	139	280	35	80	< 10	100	19	2	< 5	55	< 1
SP-BR-06A	LTiO2	JF	47	3	213	165	324	58	188	210	42	100	10	310	23	2	< 5	15	13
PA-BE-137A	LTiO2	JF	30	1	160	880	464	25	146	1000	42	260	< 10	110	15	2	< 5	72	< 1
SP-BR-06C	LTiO2	JF	32	< 1	389	256	765	34	60	30	50	< 20	60	80	22	1	< 5	32	5
SP-BR-17B	LTiO2	JF	40	< 1	358	221	276	21	77	130	67	130	140	130	19	2	< 5	4	4
MI-BR-19C	LTiO2	JF	43	1	419	134	250	30	81	110	54	80	30	150	22	2	< 5	12	10
MI-BR-19E	LTiO2	JF	41	3	200	690	272	83	100	90	40	50	30	190	27	3	< 5	235	47
PA-BE-92D	LTiO2	JF	27	3	180	352	53	23	32	720	54	420	< 10	280	19	2	< 5	157	9
MI-BE-83A	LTiO2	JF	24	2	183	488	494	26	239	< 20	33	20	20	90	19	2	< 5	34	11
MI-IX-03B	HTiO2	JF	19	< 1	237	1070	673	22	148	70	36	60	30	90	18	2	< 5	35	9
MI-VIII-65	HTiO2	JF	28	4	253	1521	1392	39	252	30	15	< 20	10	120	24	2	< 5	75	23
MI-III-18	HTiO2	JF	30	1	266	613	639	36	187	< 20	37	30	30	130	22	2	< 5	9	20
MI-BR-56	HTiO2	JF	29	2	235	1252	674	49	655	40	29	30	30	140	22	2	< 5	16	31
Geochemical data: Trace and Rare Earth Elements (REE)
Sample	Type	Unit	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Hf	Ta	Pb	Th	U
RE-CE-15	HTiO2	BS	67	142	17.8	75.1	15.3	4.42	12.7	1.8	9.5	1.8	4.9	0.65	4	0.57	7.7	1.6	7	4.5	1.6
RE-CE-16	HTiO2	BS	52.2	114	15.1	65.2	13	3.77	11.5	1.7	9.5	1.8	5.3	0.76	4.8	0.7	6.9	1.5	6	0.8	0.2
RE-CE-19	HTiO2	BS	28.8	64	8.58	36.7	8	2.57	7.3	1.1	6.5	1.3	3.4	0.49	3	0.43	3.7	1	< 5	0.2	< 0.1
RE-CE-30B	HTiO2	BS	36.7	81.8	10.9	44.9	9.1	2.08	7.8	1.1	6.2	1.2	3.1	0.42	2.5	0.38	3.4	1.3	< 5	0.8	0.1
RE-CE-2A	HTiO2	BS	35.3	74.5	9.61	40.5	8.6	2.71	9	1.3	7.4	1.5	4.2	0.58	3.6	0.56	5.9	1	7	0.4	0.2
RE-CE-12	HTiO2	BS	59.2	130	17.1	70.8	14.7	3.66	13.1	1.9	10.8	2.1	5.8	0.79	4.9	0.72	8.9	1.5	7	0.5	0.2
RE-CE-13B	HTiO2	BS	35.8	77.5	10.1	42.8	9.3	2.61	9.1	1.3	7.7	1.5	4.4	0.6	3.7	0.55	6.1	1.1	6	0.6	0.2
RE-CE-27C	HTiO2	BS	59.4	125	16.1	65.4	13	3.8	11.3	1.7	9.7	1.8	5.2	0.72	4.3	0.64	10.1	1.4	9	0.9	0.2
RE-CE-28C	HTiO2	BS	20.1	44.4	5.88	25.6	6.1	2.15	5.6	0.9	5.5	1.1	3.1	0.46	2.9	0.42	4.6	1	< 5	1.2	0.4
RE-CE-29	HTiO2	BS	40.8	84.4	10.5	42	8.4	2.83	7.6	1.1	6.9	1.3	3.8	0.53	3.3	0.5	10.1	1	6	0.8	0.2
RE-CE-30A	HTiO2	BS	53.5	114	14.8	60.2	12.1	3.56	11	1.6	9.2	1.8	4.9	0.7	4.3	0.64	9.7	1.3	8	1	0.2
RE-CE-70B	HTiO2	BS	37.5	81.7	10.8	47.2	10.1	3.1	9.6	1.6	8.6	1.6	4.5	0.67	4	0.59	6.2	1.2	5	0.3	0.1
RE-CE-3A	LTiO2	BS	24.9	50.6	6.51	26.5	5.5	1.83	5.1	0.7	4.3	0.8	2.3	0.33	2.1	0.31	3.4	0.6	6	0.5	0.1
RE-CE-03B	LTiO2	BS	18.1	38.8	5.08	21.5	4.9	1.87	5	0.8	4.4	0.9	2.6	0.36	2.2	0.31	2.3	0.5	< 5	0.5	0.1
RE-CE-25	LTiO2	BS	24.9	50.8	6.53	27	5.8	1.96	6	1	5.9	1.1	3.3	0.46	3	0.44	4.6	0.7	8	2.9	0.8
RE-CE-68	LTiO2	BS	34.9	73.9	9.61	38.5	8	2.02	7.6	1.3	6.8	1.2	3.3	0.51	3	0.38	3.7	1.1	11	4.1	0.9
RE-CE-3C	LTiO2	BS	27.2	60.4	7.89	32.6	7.1	2.01	6.5	1	5.9	1.2	3.3	0.45	2.8	0.41	4	0.5	< 5	0.9	0.2
RE-CE-11	LTiO2	BS	35.2	67.1	8.42	35.1	6.9	2.29	6.6	1	5.4	1.1	3.1	0.43	2.7	0.37	5.5	0.4	7	0.5	0.1
RE-CE-149	LTiO2	BS	17.2	35.8	4.69	20.3	4.2	1.84	4.1	0.7	3.7	0.7	2	0.3	1.9	0.28	1.5	0.7	< 5	0.1	< 0.1
MI-BR-39	LTiO2	AND	6.4	17	2.52	12	3.4	1.13	3.7	0.7	4	0.8	2.3	0.34	2.1	0.34	1.6	0.4	< 5	1.6	0.2
MI-BE-87B	LTiO2	AND	59.4	111	12.7	45.2	7.9	2.07	5.5	0.8	4.5	0.9	2.5	0.38	2.4	0.37	4.1	0.6	9	9.9	1.2
PA-BE-133B	LTiO2	AND	18.4	38	4.87	20.7	4.8	1.61	4.6	0.8	4.6	0.9	2.6	0.41	2.5	0.38	3.6	0.9	< 5	2.3	0.4
MI-BR-54	LTiO2	JF	8.2	5	2.12	8.7	2.1	0.7	2.2	0.4	2.5	0.5	1.3	0.2	1.3	0.21	0.4	0.1	< 5	0.4	0.1
MI-VIII-15	LTiO2	JF	12.1	4.2	3.76	15.9	3.9	1.11	3.3	0.6	3.7	0.7	2	0.31	2	0.28	0.5	< 0.1	< 5	0.4	0.2
SP-BR-16	LTiO2	JF	22.3	16.7	6.05	25.2	6.3	1.99	6.5	1.1	6.6	1.3	3.5	0.5	3.2	0.49	1.9	0.3	< 5	0.4	0.1
MI-II-62	LTiO2	JF	11.7	22.6	3.12	12.8	3.2	0.99	3.7	0.6	3.9	0.8	2.4	0.38	2.4	0.33	2.2	0.3	< 5	0.6	0.1
MI-V-40	LTiO2	JF	15.3	33.6	4.41	18.6	4.3	1.22	3.9	0.6	3.1	0.6	1.8	0.28	1.8	0.25	2	0.3	8	1.2	0.3
MI-BL-27	LTiO2	JF	10	21	2.76	12.1	2.9	0.97	3.1	0.5	3.2	0.6	1.8	0.26	1.7	0.24	1.8	0.3	< 5	1.8	0.4
PA-BE-146	LTiO2	JF	31.9	65.5	8.65	36.9	8	2.25	7.4	1.1	6.4	1.2	3.4	0.51	3	0.44	2.6	0.5	< 5	1.4	0.2
PA-BE-136	LTiO2	JF	23.7	53.3	7.34	31.3	6.7	1.66	5.5	0.8	4.4	0.8	2.4	0.37	2.2	0.3	3.1	0.5	8	0.1	0.1
SP-BR-06A	LTiO2	JF	39	104	15.1	66.5	16	2.26	13.3	2.1	11.2	2	5.4	0.76	4.5	0.68	4.1	0.9	6	2.7	1.6
PA-BE-137A	LTiO2	JF	23.3	56.7	8.08	34.6	7.4	1.59	5.9	0.8	4.5	0.9	2.4	0.4	2.5	0.36	3.1	0.6	< 5	0.3	0.3
SP-BR-06C	LTiO2	JF	22.4	57.8	8.87	42.6	10	2.61	9.2	1.4	7.3	1.4	3.8	0.51	3	0.46	1.9	0.3	< 5	1.7	0.7
SP-BR-17B	LTiO2	JF	5.4	13.2	2.16	10.7	3.2	1.13	3.8	0.7	4.2	0.8	2.4	0.35	2.3	0.37	1.8	0.3	< 5	0.3	< 0.1
MI-BR-19C	LTiO2	JF	24.9	55.9	6.86	25.7	6	1.77	5.9	1.1	6.2	1.2	3.3	0.5	3.2	0.5	2.2	0.7	7	1.1	0.1
MI-BR-19E	LTiO2	JF	53.5	155	23.3	99.4	23.1	2.06	19.1	3.1	17	3.2	8.8	1.34	8.4	1.28	3.1	2.1	12	4.8	0.3
PA-BE-92D	LTiO2	JF	10.3	23.8	3.22	13.2	3.4	1.13	3	0.6	3.6	0.8	2.4	0.39	2.5	0.4	0.9	0.3	< 5	2.5	0.6
MI-BE-83A	LTiO2	JF	27.5	55.7	6.91	28.1	5.9	1.85	5.1	0.8	4.8	0.9	2.6	0.39	2.5	0.37	4.2	0.6	7	1.4	0.3
MI-IX-03B	HTiO2	JF	27.6	60.4	7.96	34.5	6.6	1.89	5.7	0.8	4.3	0.8	2.1	0.3	1.8	0.26	2.7	0.5	< 5	1	0.2
MI-VIII-65	HTiO2	JF	60.8	132	17.3	73.9	14.9	3.52	10.8	1.6	7.7	1.3	3.6	0.52	3.1	0.42	4.9	1.1	8	4.2	1
MI-III-18	HTiO2	JF	33.2	76	10.3	45.7	9.4	2.51	7.9	1.2	6.9	1.3	3.5	0.56	3.3	0.44	3.5	0.9	< 5	0.3	< 0.1
MI-BR-56	HTiO2	JF	62.6	139	18.8	76.6	15.2	3.02	12.6	1.8	9.6	1.7	4.8	0.73	4.5	0.63	11.8	1.2	8	0.9	0.1

RESULTS

The rocks of the Barreiro suite (Tab. 1) range from gabbros to diorites, with silica contents between 47.98 and 54.48%, MgO 2.97-7.45%, and TiO₂ 1.72-3.77. They define a subalkaline tholeiitic series that can be subdivided into high (> 2.2%) and low-TiO₂ (< 2.2%) groups (Fig. 7). Both geochemical groups are recognized in both the coarse-and fine-grained varieties of the Barreiro suite. As expected, the High-TiO₂ group displays relatively higher contents of P₂O₅, REE and high field strengh elements (HFSE) elements such as Zr, Ba and Y. The low-TiO₂ group is less evolved or less contaminated as expressed by higher MgO and CaO contents and lower FeO_t contents. The chondrite-normalized REE diagrams (Fig. 7) display enrichment in total content of REE and the negative Eu\Eu* anomalies of the High-TiO2 group. In all tectonic discriminant diagrams (Fig. 8), both groups consistently suggest intraplate continental (CT and E-MORB) to passive margin (E-MORB) environments, with both continental tholeiites and E-MORB signatures. The plosts involving the high-field strength elements, Tb, Ta, Th and Nb (Figs. 8E, 8F) seem to rule out extensional back-arc settings.

Figure 7.
Geochemical data for the rocks of the Barreiro Suite and mafic enclaves within the country rocks: (A) binary TiO₂ × SiO₂ diagram with subdivision into two groups; (B) chemical classification of the analysed rocks in the TAS diagram of Cox et al. (1989); (C) chemical classification in the R1 × R2 diagram of De La Roche et al. (1980); (D) AFM diagram of Irvine and Baragar (1971); (E) chrondrite-normalized spidergram of the High-TiO₂ and Low-TiO₂ groups of the Barreiro Suite; (F) chrondrite-normalized spidergram including alkaline basic enclaves in the Andrelândia Group and Juiz de Fora Complex. Both spidergrams were normalized using the chondrite composition of Boynton (1984).

Figure 8.
Plot of analyzed sample in tectonic diagrams: (A) Zr/4- 2Nb-Y triangular diagram of Meschede (1986); (B) Zi × Ti of Pearce (1982); (C) TiO2 x 10 MnO × 10 P2O5 triangular diagram of Mullen (1983); and (D) Ti x V diagram of Shervais (1982), (E) triangular diagram of Cabanis and Thiéblemont (1988), in Xia and Li (2019); (F) normalized binary diagram of Scanni (2015) (normalized by Sun & McDonough 1989)

On the other hand, as expected, the mafic enclaves included in both the Andrelândia metasedimentary rocks and the orthogranulites of the Juiz de Fora Complex display heterogeneous geochemical signatures, suggesting different tectonic environments. Using the geochemical data, the enclaves of the country rocks were tentatively separated into four groups (Figs. 7 and 8):

• Group 1 with tholeiitic intraplate signatures, mostly with low-TiO₂ contents, with the exception of a single sample. These enclaves occur both within the orthogranulites of the Juiz de Fora complex and in the metasedimentary rocks of the Andrelândia group. Their geochemical signature is very similar to that obtained from the low-TiO₂ rocks of the Barreiro Suite;

• Group 2 comprises low-TiO₂ samples with E-MORB compositions;

• Group 3 comprises intraplate alkaline mafic rocks, which occur in both the cover and basement associations;

• Group 4 represents the most common signatures for the mafic enclaves within the Juiz de Fora Complex, comprising tholeiitic to calcalkaline compositions (IAT and to CAB), indicating arc-related environments. As mafic rocks of this group crop out only within the orthogranulites of the Juiz de Fora complex, we suggest that they belong to the convergent Rhyacian history of the basement.

Enclaves of groups 1 to 3 (both Andrelândia Group and Juiz de Fora Complex) and all Barreiro Suite samples plot in the intraplate to passive margin extensional settings (Fig. 8). On the other hand, the Group 4 of the Juiz de Fora Complex (CAB to IAT convergent tectonic settings) is probably related with the Rhyacian history of the basement (Fig. 8).

U-PB GEOCHRONOLOGY AND HF ISOTOPES OF THE BARREIRO SUITE

In order to constrain the age of the intraplate magmatism of the Barreiro Suite, sample MI-BR-68, a diorite, was selected for U-Pb analysis. This sample produced the largest number of recovered zircons suitable for geochronological investigation.

Preparation and analytical procedures

The sample preparation involved crushing (ca. 30 kg) and manual panning, followed by density and magnetic separation in the LGPA laboratory of the UERJ. Zircons typically show pale brown to pink colors, with short prismatic habit (2:1 to 3:1 aspect ratios), and less commonly with sub-equant shapes. Scanning electron microscope cathodoluminescence images were obtained at the MULTILAB facilities of the UERJ. Most of the grains display complex zoning patterns, rounded bright overgrowths and resorption textures that, with typical soccer ball grains, are typical of high-grade metamorphism (Rubatto 2017).

U-Pb isotopic data were obtained at the Geochronology Laboratory of the Universidade de Brasília (UnB) using a Thermo-Finnigan Neptune multicollector inductively coupled plasma mass spectrometer connected to a “New Wave 213 mm Nd-YAG” laser beam with spot diameters of 30 to 50 µm, and frequency of 6 to 10 Hz. Bracketing technique of analyses, described in detail by Bühn et al. (2009), involved the use of the JG1, Temora 2 and FC1 standards. For the standard and sample analyses, isotope ratio measurements consist of blocks of 40 cycles, each with 1.049 sec duration.

Analyses with either errors higher than 10% for isotope ratios, ²⁰⁴Pb contents higher than 0.5%, or discordance above 5%, were not considered in the age calculations. Analyses with ²⁰⁶Pb/²⁰⁴Pb ratios higher than 3,000 indicate negligible common Pb contents, obviating the need for common Pb correction. When necessary, the correction is made with the in-house spreadsheet of Bühn et al. (2009), following Stacey and Kramers (1975). Age calculations and plots were performed with the Isoplot V.3 software of Ludwig (2003).

The Lu-Hf analyses were performed at the University of Notre Dame (USA), using a UP193 nm laser ablation system coupled to a NuPlasma II Multi-Collector ICPMS. The Lu-Hf spots were located as the same zircon texture domains of the U-Pb spots. Analytical conditions were laser spots of 50 µm diameter, 60 s of ablation after a run of 30 s for background, pulse of 5 Hz corresponding to an energy fluence of ~10 J/cm². Methodology followed the protocol of Simonetti and Neal (2010), using the Plesovice, BR266 and 1500 standards. Mass interference of ¹⁷⁶Lu with ¹⁷⁶Hf was corrected using the ¹⁷⁶Lu/¹⁷⁵Lu ratio of 0.02655 (International Union of Pure and Applied Chemistry 1998), and the decay constant of ¹⁷⁶Lu of 867 × 10^-11 (Söderlund et al. 2004). Chondritic ratios used for ε_Hf calculations were ¹⁷⁶Hf/¹⁷⁷Hf = 279718, ¹⁷⁶Lu/¹⁷⁷Hf = 0.0336 (Bouvier et al. 2008), and present-day depleted mantle ratios of ¹⁷⁶Hf/¹⁷⁷Hf = 0.28325 and ¹⁷⁶Lu/¹⁷⁷Hf = 0.0388 (Griffin et al. 2000; updated by Andersen et al. 2009).

U-Pb results

Analysis of sample MI-BR-68 produced 45 spots (Tab. 2), nine of which were discarded due to poor analytical parameters such as isotope ratios errors higher than 10%, ²⁰⁴Pb contents higher than 0.5%, or discordance above 5%.

Table 2.
Summary of U-Pb zircon data obtained by inductively coupled plasma mass spectrometry (LA-ICPMS) (Mi-BR-68) - ZIRCON

Spot/	Th ppm	U ppm	Pb ppm	Th/U	ƒ -206	Ratios							Age (Ma)						Discordance %
						207 Pb*/ 235 U	1σ	206 Pb*/ 238 U	1σ	Rho	207 Pb*/ 206 Pb*	1σ	206 Pb/ 238 U	1σ	207 Pb/ 235 U	1σ	207 Pb/ 206 Pb	1σ
Z-01	176	360	41	0.49	0.0009	0.81869	3.63	0.09884	3.10	0.85	0.06008	1.89	608	19	607	22	606	11	0
Z-02	132	96	14	1.38	0.0031	1.03905	4.70	0.12048	2.83	0.60	0.06255	3.75	733	21	723	34	693	26	-6
Z-03	71	298	41	0.24	0.0007	1.17151	2.42	0.13013	1.77	0.73	0.06529	1.66	789	14	787	19	784	13	-1
Z-04	150	350	39	0.43	0.0010	0.88157	2.65	0.10567	1.98	0.75	0.06051	1.77	648	13	642	17	622	11	-4
Z-05	57	77	10	0.75	0.0027	0.81597	5.96	0.09855	4.61	0.77	0.06005	3.77	606	28	606	36	605	23	0
Z-06	200	346	38	0.58	0.0007	0.81093	3.33	0.09780	2.82	0.85	0.06014	1.77	602	17	603	20	608	11	1
Z-07	12	66	17	0.18	0.0019	2.91376	2.67	0.23946	2.16	0.81	0.08825	1.56	1384	30	1385	37	1388	22	0
Z-08	45	333	44	0.14	0.0006	1.17653	2.83	0.13035	2.33	0.82	0.06546	1.60	790	18	790	22	789	13	0
Z-09	41	687	70	0.06	0.0005	0.79912	3.71	0.09649	3.35	0.90	0.06007	1.58	594	20	596	22	606	10	2
Z-10	66	155	18	0.43	0.0018	0.85099	4.27	0.10161	3.17	0.74	0.06074	2.86	624	20	625	27	630	18	1
Z-11	22	1005	96	0.02	0.0003	0.78038	3.72	0.09518	3.34	0.90	0.05947	1.64	586	20	586	22	584	10	0
Z-12	26	168	21	0.16	0.0012	1.07514	3.36	0.12121	2.42	0.72	0.06433	2.33	738	18	741	25	753	18	2
Z-13	25	40	8	0.63	0.0026	0.84299	6.07	0.10107	4.51	0.74	0.06049	4.06	621	28	621	38	621	25	0
Z-14	30	46	8	0.65	0.0048	1.18654	6.45	0.13132	4.74	0.73	0.06553	4.38	795	38	794	51	791	35	0
Z-15	140	217	24	0.65	0.0012	0.80498	3.64	0.09731	2.91	0.80	0.06000	2.18	599	17	600	22	603	13	1
Z-16	52	276	30	0.19	0.0010	0.81484	4.23	0.09851	3.84	0.91	0.05999	1.77	606	23	605	26	603	11	0
Z-17	79	351	38	0.23	0.0010	0.81989	3.67	0.09884	3.24	0.88	0.06016	1.73	608	20	608	22	609	11	0
Z-18	14	331	32	0.04	0.0015	0.82274	3.71	0.09891	3.24	0.87	0.06033	1.80	608	20	610	23	615	11	1
Z-19	39	69	8	0.58	0.0025	0.82829	7.43	0.09977	5.68	0.76	0.06021	4.78	613	35	613	45	611	29	0
Z-20	82	62	9	1.33	0.0044	1.16436	5.89	0.12907	4.55	0.77	0.06543	3.75	783	36	784	46	788	30	1
Z-21	67	455	42	0.15	0.0009	0.91080	2.90	0.10738	2.25	0.78	0.06152	1.83	658	15	657	19	657	12	0
Z-22	56	173	16	0.33	0.0023	0.81295	4.73	0.09820	3.68	0.78	0.06004	2.96	604	22	604	29	605	18	0
Z-23	45	297	26	0.15	0.0012	0.81363	3.83	0.09815	3.04	0.79	0.06013	2.33	604	18	604	23	608	14	1
Z-24	51	92	12	0.55	0.0025	1.03636	7.44	0.11878	6.17	0.83	0.06328	4.15	724	45	722	54	718	30	-1
Z-25	2	28	2	0.08	0.0119	0.72127	19.74	0.08933	14.3	0.72	0.05856	13.7	552	79	551	109	551	75	0
Z-26	151	183	70	0.83	0.0004	7.44485	1.85	0.39657	1.39	0.76	0.13616	1.21	2153	30	2166	40	2179	26	1
Z-27	9	17	6	0.51	0.0023	7.24563	6.67	0.39303	4.15	0.62	0.13370	5.23	2137	89	2142	143	2147	112	0
Z-28	44	156	15	0.29	0.0024	0.94416	4.48	0.11023	3.69	0.82	0.06212	2.55	674	25	675	30	678	17	1
Z-29	69	115	12	0.60	0.0026	0.89659	5.08	0.10611	3.70	0.73	0.06128	3.49	650	24	650	33	649	23	0
Z-30	44	338	30	0.13	0.0009	0.81589	3.90	0.09824	3.41	0.88	0.06023	1.89	604	21	606	24	612	12	1
Z-31	29	121	9	0.25	0.0016	0.84257	5.18	0.10108	3.44	0.66	0.06046	3.87	621	21	621	32	620	24	0
Z-32	21	99	9	0.22	0.0024	1.08145	5.02	0.12212	3.27	0.65	0.06423	3.81	743	24	744	37	749	29	1
Z-33	60	166	14	0.36	0.0018	0.83414	4.90	0.10011	4.31	0.88	0.06043	2.33	615	26	616	30	619	14	1
Z-34	45	241	20	0.19	0.0008	0.84907	4.00	0.10167	3.26	0.82	0.06057	2.31	624	20	624	25	624	14	0
Z-35	57	263	24	0.22	0.0014	0.83449	3.34	0.10021	2.76	0.83	0.06039	1.88	616	17	616	21	618	12	0
Z-36	7	627	51	0.01	0.0005	0.83278	3.30	0.10023	2.81	0.85	0.06026	1.72	616	17	615	20	613	11	0
Z-37	29	87	8	0.34	0.0030	0.83633	6.24	0.10043	5.03	0.81	0.06040	3.69	617	31	617	38	618	23	0
Z-38	262	202	28	1.30	0.0013	0.92605	3.73	0.10856	3.04	0.81	0.06187	2.17	664	20	666	25	670	15	1
Z-39	79	140	16	0.57	0.0018	0.82444	4.94	0.09918	4.02	0.81	0.06029	2.87	610	24	611	30	614	18	1
Z-40	60	178	19	0.34	0.0021	0.82405	3.89	0.09920	2.92	0.75	0.06025	2.56	610	18	610	24	613	16	0
Z-41	19	95	11	0.21	0.0032	0.87459	5.18	0.10374	4.08	0.79	0.06114	3.19	636	26	638	33	644	21	1
Z-42	180	326	31	0.56	0.0008	0.83788	2.81	0.10108	2.38	0.85	0.06012	1.50	621	15	618	17	608	9	-2
Z-43	44	138	13	0.32	0.0018	0.83850	4.82	0.10095	3.57	0.74	0.06024	3.24	620	22	618	30	612	20	-1
Z-44	369	312	39	1.19	0.0011	0.83308	2.67	0.09968	2.24	0.84	0.06062	1.45	613	14	615	16	626	9	2
Z-45	236	542	56	0.44	0.0009	0.87582	2.55	0.10566	2.07	0.81	0.06012	1.48	647	13	639	16	608	9	-7

• ¹Sample and standard are corrected after Pb and Hg blanks; ²²⁰⁷Pb/²⁰⁶Pb and ²⁰⁶Pb/²³⁸U are corrected after common Pb presence. Common Pb assuming ²⁰⁶Pb/²³⁸U - ²⁰⁷Pb/²³⁵U concordant age; ³ƒ-206 denotes the percentage of ²⁰⁶Pb that is common Pb; ⁴²³⁵U = 1/137.88*Utotal; ⁵Standard GJ-1; ⁶Th/U = ²³²Th/²³⁸U *0.992743; ⁷All errors in the table are calculated 1 sigma (% for isotope ratios, absolute for ages) samples marked in bold letters are discarded.

All zircons plot along the Concordia between ca. 780 Ma and ca. 610 Ma (Fig. 9), perhaps suggesting ion diffusion under high-temperatures of the regional granulite facies metamorphism. It also confuses distinction between igneous and metamorphic zirons on the basis of zircon textures and Th/U (Rubatto 2017). However, based on the bimodal pattern of the probability density plot of 207 Pb/206 Pb ages (Fig. 9), a group of 8 zircons with concentric oscillatory zoning and relatively high Th/U ratios (Rubatto 2017) yielded a Concordia age of 766 ± 13 Ma, interpreted as the age of reference for the mafic magmatism. The cluster of younger zircons, with relatively lower Th/U ratios, yielded a Concordia age of 619 ± 6 Ma, interpreted as the best estimate for the age of the metamorphic overprint. This latter age is comparable with the early metamorphic ages reported for the Ribeira Belt (Bento dos Santos et al. 2007, Machado et al. 1996, Heilbron & Machado 2003). Field observations support this interpretation, such as the presence of thin leucocratic intrusive veins and partial digestion of the mafic enclaves of the Barreiro Suite within the syn-collisional granitoid rocks.

Figure 9.
LA_ICPMS U-Pb data for sample MI-BR-68 of the Barreiro Suite. (A) Concordia diagram for all analysed zircons, indicating the Mesoproterozoic and Paleoproterozoic inheritance; (B) relative probability plot of ²⁰⁷Pb/²⁰⁶Pb ages showing the bimodal pattern; (C) calculated crystallization age of 766 ± 3 Ma with selected images of analysed zircons; (D) calculated high-grade metamorphic overprint age of 619 ± 6 Ma.

Three inherited zircons, with ages of ca. 1.38 Ga, 2.14 Ga and 2.15 Ga, reflect the basement associations of the Occidental terrane of the Ribeira Belt, confirming the intraplate setting of the Barreiro Suite as indicated by geochemical criteria.

Lu-Hf results

Lu-Hf analyses (Tab. 3) were performed on the same sample as the U-Pb analyses. Zircon grains were selected for the investigation of crystallization age, metamorphic overprint and inheritance. Measured ¹⁷⁶Hf/¹⁷⁷Hf ratios vary between 0.281927 and 0.282559, while ¹⁷⁶Lu/¹⁷⁷Hf ratios are below 0.0015.

Table 3.
Lu-Hf data of sample MI-BR-68

Sample	U-Pb age (Ma)	Sample (present-day ratios)			Sample (initial ratios)			T DM (Ma)
		176 Hf/ 177 Hf	± 2SE	176 Lu/ 177 Hf	176 Hf/ 177 Hf (t)	eHf(t)	± 2SE
MI-BR-68_01(z36)	616	0,281927	0,000018	0,000018	0,281927	-16,62	0,47	1796
MI-BR-68_02(z37)	617	0,282125	0,000021	0,000165	0,282123	-9,66	0,49	1538
MI-BR-68_03(z16)	606	0,282354	0,000014	0,000237	0,282351	-1,83	0,07	1230
MI-BR-68_04(z19)	613	0,282402	0,000013	0,000499	0,282397	-0,06	0,00	1172
MI-BR-68_05(z17)	608	0,282294	0,000014	0,000028	0,282294	-3,82	0,12	1305
MI-BR-68_06(z03)	789	0,282328	0,000014	0,001451	0,282306	0,69	0,01	1307
MI-BR-68_07(z20)	783	0,282338	0,000016	0,001000	0,282323	1,17	0,05	1277
MI-BR-68_08(z14)	795	0,281983	0,000015	0,000702	0,281972	-10,98	0,52	1752
MI-BR-68_09(z12)	738	0,282218	0,000014	0,000977	0,282204	-4,08	0,10	1442
MI-BR-68_10(z15)	599	0,282497	0,000015	0,000411	0,282493	3,02	0,09	1040
MI-BR-68_11(z10)	624	0,282496	0,000014	0,000389	0,282491	3,53	0,11	1042
MI-BR-68_12(z07)	1388	0,282066	0,000014	0,000165	0,282061	5,62	0,09	1617
MI-BR-68_15(z28)	733	0,282558	0,000018	0,000664	0,282548	8,02	0,23	964
MI-BR-68_16(z42)	621	0,282335	0,000015	0,000108	0,282334	-2,10	0,05	1252
MI-BR-68_18(z22)	604	0,282425	0,000017	0,000332	0,282422	0,62	0,02	1136
MI-BR-68_21(z09)	594	0,282105	0,000013	0,000090	0,282103	-10,86	0,37	1562
MI-BR-68_22(z31)	621	0,282146	0,000015	0,000053	0,282146	-8,77	0,30	1504

Zircons with late Tonian ages, interpreted as crystallization ages, yield a broad spectrum of T_DM Hf model ages, as expected for intraplate basaltic rocks, ranging from ca. 1.75 to 0.96 Ga with predominantly positive εHf_{(t = 766)} values between + 5.61 and + 0.24. Two grains yielded negatives εHf_{(t = 766)} values of -10.97 and of -4.07 (Fig. 10).

Figure 10.
Hf evolution diagram for the analysed zircons. The blue ellipse highlights data for zircons defining the crystallization age, while the red ellipse shows the data for metamorphic zircons. A single datum for the Mesoproterozoic inherited zircon grain is presented in yellow. Selected images with both the U-Pb and Lu-Hf targets and Th-U ratios are presented below. As expected for high-grade metamorphism the analysed zircons do not have low Th/U ratios.

On the other hand, most of the Ediacaran metamorphic zircons and overgrowths, with U-Pb ages between 624 and 594 Ma, yielded Hf T_DM model ages between 1.80 Ga and 1.04 Ga, with negative εHf_{(t = 766)} values between -10.86 and -1.82. The metamorphic zircons are interpreted as having grown from in situ melting films within the basic rocks, the preservation of primary ¹⁷⁶Hf/¹⁷⁷Hf compositions in metamorphic zircon can be usually attributed to subsolidus recrystallization (Zeh et al. 2010), as strongly suggested by very complex textures detected on the cathodoluminescence images. Tedeschi et al. (2018) obtained similar results for high-temperature granulites in the Southern Brasília belt.

The single grain with age of ca. 1.39 Ga represents inheritance, with a Hf T_DM model age of 1.62 Ga with εHf_{(t = 1.62)} of +5.61, suggesting juvenile sources.

Sm-Nd AND Sr ISOTOPES

Analytical procedures

Thirteen samples of the Barreiro suite and from mafic enclaves within Juiz de Fora Complex and Andrelândia Group country rocks were selected. Samples preparation involved fragmentation with a jaw crusher and milling at the LGPA-UERJ.

Sm-Nd and Sr isotopic data were obtained by at the Laboratory Geochronology and Radiogenic Isotopes (LAGIR) of the UERJ, where chemical procedures are carried out in clean rooms using purification of Milli-Q^® water and PA Merck^® acids purified by repeated sub-boiling distillation. Between 25 and 50 mg of pulverized rock samples were subjected to digestion in Savillex^® vessels on a hot plate, after the addition of proportional amounts of a double ¹⁴⁹Sm-¹⁵⁰Nd tracer, using a mixture of concentrated HF and HNO₃ 6N for 3 days, followed by further digestion with HCl 6N for 2 days. Separation of Sr and REE used cation exchange following conventional techniques with Teflon columns filled with Biorad AG50W-X8 resin (100-200 mesh) in HCl medium. For the separation of Sm and Nd from the other REE, a secondary column was used with the Eichrom LN-B-25S (50-100 µm) resin.

After evaporation Sm, Nd and Sr samples were separately loaded onto previously degassed Re filaments assembled in double filament mounts, using H3PO4 as ionization activator.

Isotope ratios were measured with a Thermo Scientific TRITON in static mode using up to 8 Faraday collectors. The measured isotope ratios are reported with absolute standard errors (2σ) below 0.00001. The measured ratios were normalized respectively to the natural constant ratios of ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, ¹⁴⁷Sm/¹⁵²Sm = 0.5608 and ⁸⁸Sr/⁸⁶Sr = 8.3762. The average ¹⁴³Nd/¹⁴⁴Nd ratio of repeated measurements of the JNdi-1 reference material (Tanaka et al. 2000) is 0.512098 ± 0.000006 (n = 322). The average ⁸⁷Sr/⁸⁶Sr ratio of the NBS-987 reference material (Wise & Waters 2007) is 0.710239 ± 0.000007 (n=158). Repeated analyses of the BCR and AVG rock reference materials from the United States Geologic Survey yield ¹⁴⁷Sm/¹⁴⁴Nd ratios with reproducibility within 1% (Valeriano et al. 2008). Neodymium (T_DM) model ages were calculated using the depleted mantle model of De Paolo (1981).

Sm-Nd and Sr isotopes results

The location of samples is shown in Figures 3 and 4, and the analytical results are presented in Table 4. Two isotopic groups can be discriminated. The majority of samples, including all those from the Barreiro suite, part of those from the Juiz de Fora Complex, and one single sample occurring within the metasedimentary rocks of Andrelândia-Raposos group yielded late Paleoproterozoic to Mesoproterozoic T_DM model ages, between ca. 1.80 Ga and 1.50 Ga. The εNd_t values obtained are -8 to -3, and initial ⁸⁷Sr/⁸⁶Sr ratios between 0.7017 and 0.7092. This range of values suggests different degrees of contamination from country rocks, as indicated by inherited zircons with old U-Pb ages.

Table 4.
Sm-Nd and Sr isotopic data of the metabasic rocks

Sample	Unit	Sm	Nd	Rb*	Sr	143 Nd/	abs	147 Sm/	87 Sr/	abs	t	ε	f	143 Nd/	ε	TDM	87 Sr/
		ppm	ppm	ppm	ppm	144 Nd	st error	144 Nd	86 Sr	st error	(Ma)	0 Ma	(Sm/Nd)	144 Nd (t)	(t)	Ga	86 Sr/ (t)
RE-CE-03A	Barreiro LTi	6.2	31.1	9	626	0.511899	0.000005	0.1200	0.707183	0.000006	766	-14.4	-0.38972	0.511296	-6.9	1.76	0.7067
RE-CE-149	Barreiro LTi	4.6	21.5	6	641	0.512056	0.000008	0.1296	0.709232	0.000008	766	-11.4	-0.34095	0.511405	-4.8	1.69	0.7089
RE-BR-68	Barreiro LTi	8.1	39.2	5	427	0.512018	0.000007	0.1257	0.707539	0.000004	766	-12.1	-0.36087	0.511387	-5.1	1.68	0.7072
RE-CE-2A	Barreiro HTi	9.9	46.6	8	507	0.512094	0.000005	0.1286	0.706950	0.000009	766	-10.6	-0.34611	0.511448	-4.0	1.61	0.7065
RE-CE-13B	Barreiro HTi	10.4	48.7	15	573	0.512049	0.000004	0.1288	0.706041	0.000005	766	-11.5	-0.34529	0.511402	-4.9	1.68	0.7052
RE-CE-28C	Barreiro HTi	5.7	24.4	15	556	0.512569	0.000004	0.1406	0.704435	0.000010	766	-1.4	-0.28519	0.511862	4.1	0.97	0.7036
RE-CE-29	Barreiro HTi	9.3	46.7	29	622	0.512088	0.000006	0.1209	0.706583	0.000007	766	-10.7	-0.38529	0.511481	-3.3	1.50	0.7051
RE-CE-30A	Barreiro HTi	8.3	41.7	13	553	0.512048	0.000006	0.1205	0.708587	0.000007	766	-11.5	-0.38746	0.511443	-4.1	1.55	0.7078
RE-CE-30B	Barreiro HTi	10.5	53.5	7	569	0.511861	0.000005	0.1188	0.709596	0.000004	766	-15.2	-0.39591	0.511264	-7.6	1.80	0.7092
RE-CE-70B	Barreiro HTi	10.0	47.9	6	619	0.512091	0.000003	0.1267	0.705819	0.000009	766	-10.7	-0.35601	0.511454	-3.8	1.58	0.7055
RE-CE-27C	Barreiro HTi	14.8	74.3	24	498	0.512070	0.000006	0.1204	0.707729	0.000006	766	-11.1	-0.38794	0.511465	-3.6	1.52	0.7062
RE-CE-13B	Barreiro HTi	10.3	48.6	15	573	0.512040	0.000009	0.1288	0.706054	0.000009	766	-11.7	-0.34542	0.511394	-5.0	1.70	0.7052
RE-CE-19	Barreiro HTi	8.5	39.7	3	657	0.512091	0.000005	0.1291	0.705865	0.000006	766	-10.7	-0.34367	0.511443	-4.1	1.62	0.7057
MI-BE-87B	Andrelândia	8.2	48.6	57	711	0.511970	0.000006	0.1204	0.707192	0.000008	766	-13.0	-0.38794	0.511365	-3.7	1.41	0.7047
MI-BR-39	Andrelândia	3.0	9.6	7	146	0.512692	0.000007	0.1870	0.715332	0.000037	766	1.1	-0.04929	0.511753	2.0	1.69	0.7138
MI-III-18	Juiz de Fora WPB	10.0	48.4	9	639	0.512059	0.000008	0.1245	0.707242	0.000006	766	-11.3	-0.36717	0.511433	-4.2	1.60	0.7068
PA-BE-137A	Juiz de Fora IAT	8.0	38.8	72	464	0.511425	0.000009	0.1254	0.715975	0.000002	2400	-23.7	-0.36247	0.509441	-1.7	2.58	0.7004
MI-BR-19C	Juiz de Fora IAT	6.1	26.5	12	250	0.511734	0.000005	0.1386	0.717248	0.000008	2400	-17.6	-0.29543	0.509542	0.3	2.43	0.7124
MI-VIII-15	Juiz de Fora IAT	4.2	17.2	2	35	0.512047	0.000008	0.1496	0.712324	0.000009	2400	-11.5	-0.23962	0.509681	3.0	2.16	0.7066

• Rb*-Values from geochemical data

Sample RE-CM-28C, with highest Mg number, shows less contaminated isotopic signatures, probably reflecting the primary composition of the mafic magma with low SiO₂ and high MgO contents. The young T_DM age of 0.97 Ga, ε_{t =} +4.1 and initial ⁸⁷Sr/⁸⁶Sr 0.7036 of this sample characterize it as the most primitive sample of the Barreiro Suite (Fig. 11).

Figure 11.
Sm-Nd isotopic data for selected analysed samples. (A) Nd evolution diagram; (B) εnd_(i) × ⁸⁷Sr/⁸⁶Sr_(i) diagram for the samples of Barreiro Suite and a few mafic enclaves from the Andrelândia Group and Juiz de Fora Complex.

Three mafic enclaves with geochemical signatures of convergent tectonic settings (IAT/BAB or CAB), found in the Juiz de Fora Complex, yielded T_DM ages between 2.58 Ga and 2.16 Ga, with ε_t between +3.0 and -1.7. These samples display initial ⁸⁷Sr/⁸⁶Sr ratios between 0.7004 and 0.7124. These results reflect an older convergent juvenile accretion history of the Juiz de Fora complex, as previously reported by Heilbron et al. (2010) and Degler et al. (2018).

DISCUSSION

Two tectonic settings in the studied area indicated by metabasic rocks

The data presented above, summarized in Table 5, suggest late Tonian (ca. 766 Ma) extensional intraplate magmatism both in the cover and in the basement of the distal segment of the São Francisco paleocontinent passive margin (Andrelândia Group).

Table 5.
Summary of the new data obtained for mafic rocks of the Barreiro Suite and mafic enclaves of Andrelândia Group and Juiz de Fora Complex.

Mafic groups		U-Pb age (Ga)	TDM(Nd) (Ga)	εNdt	87 Sr/ 86 Sr(i)	εHft and TDM(Hf)
High TiO2 Barreiro Suite and few mafic enclaves	Intraplate		1.80 to 1.50	-8 to -3	0.7017 to 0.7092
Low TiO2 Barreiro suite and enclaves of Juiz de Fora Complex and Andrelândia Group	Intraplate	0.77	1.80 to 1.50	-8 to -3	0.7017 to 0.7092	1.75 to 0.96 Ga, +5.61 to +0.24
Most primitive sample of the Barreiro Suite (RE-BR-28)	Intraplate		0.97	+4	0.7036
Low TiO2 enclaves of the Andrelândia group and Juiz de Fora Complex	Morb		1.80 to 1.50	-8 to -3	0.7017 to 0.7092
Low TiO2 enclaves of Juiz de For a complex	IATand CAB	2.2 to 2.07	2.58 to 2.16	+3 to -1.7	0.7004 to 0.7124

The development of the Andrelândia basin in the proximal zone started in the Mesoproterozoic to Neoproterozoic transition, as indicated by the youngest detrital zircons of ca. 0.9 Ma (Valeriano et al. 2004, Belém et al. 2011) and T_DM of interlayered metabasic rocks of ca. 1.0 Ga (Heilbron et al. 1989, Frugis & Campos Neto 2018). The new data presented here indicate that the evolution of the distal passive margin lasted at least until late Tonian times.

This late Tonian within-plate magmatism displays within plate basalts (WPB) signatures, with both high-TiO₂ and low-TiO₂ groups, with a few samples showing relatively primitive MORB-like signatures. Mafic magmatic rocks intrude both basement rock associations (Juiz de Fora Complex), as well the cover of distal Andrelândia passive margin (Figs. 12 and 13).

Figure 12.
Composite stratigraphic chart for the Andrelândia basin, including proximal (autochthonous and Andrelândia domains) and distal (Andrelândia and Juiz de Fora domains), modified from Ribeiro et al. (2013) and Heilbron et al. (2017a). Amphibolite data are from this work, Frugis & Campos Neto (2018) and Heilbron et al. (1989). Younger detrital zircon was compiled from Valladares et al. (2004, 2008), Ribeiro et al. (2013), Degler et al. (2017), Frugis & Campos Neto (2018).

Figure 13.
A comparison of new data and previously published data for mafic rocks in the region of the Andrelândia deformed basin units. Proximal data are compiled from Paciullo (1997) and Gonçalves and Figueiredo (1992), compared with our data for the distal part of the basin (Barreiro Suite) and the mafic enclaves of both the Andrelândia Group and the Juiz de Fora Complex. (A) TiO₂ × MgO diagram showing that both proximal and distal samples could be subdivided into the same groups, and (B) plot in the Zi-Ti tectonic diagram of Pearce (1982), with green, blue, yellow and pink fields to show respectively, the intraplate High- and Low-TiO₂ groups, MORB-like samples and a few older mafic enclaves of the Juiz de Fora Complex with arc-related signatures.

A few samples detected only in the Juiz de Fora Complex display convergent tectonic signatures (IAT to CAB) and older T_DM model ages, indicating that they probably belong to the Paleoproterozoic history of the basement.

Both associations show evidence of Neoproterozoic Brasiliano overprinting, with high deformation textures and granulite facies metamorphism between ca. 619 (this work) and 565 Ma (Duarte 2003, Heilbron & Machado 2003, Bento dos Santos et al. 2007, Degler et al. 2017).

Correlation with previous data reported for the Andrelândia basin

Previous works have reported combined geochemical and geochronological data for the metabasic rocks interlayered with the metasedimentary rocks of the Andrelândia Basin. In the most proximal region, between Madre de Deus de Minas and Andrelândia towns, southern Minas Gerais State, Gonçalves & Figueiredo (1992) and Paciullo et al. (2000) obtained very similar results, with low- and high-TiO₂ amphibolites, pointing to intraplate to MORB-like signatures, suggestive of syn-sedimentary magmatism. These authors suggested that the magmatism evolved in time from high-TiO₂ intraplate rocks in the lower units to MORB signatures in the upper units of the basin. Marins (2000) also reported the same geochemical groups for the amphibolites interlayered within the distal portion of the Andrelândia basin (Raposos Group). The comparison between all the available geochemical data is presented in Figure 13 and provides additional evidence of a major episode of intraplate to E-MORB tholeiitic magmatism between 0.9 Ga and 0.77 Ga associated with the development of the southeastern and eastern São Francisco paleocontinent passive margin.

Tectonic implications for the Neoproterozoic passive margin evolution within the Araçuaí-Ribeira orogenic system

The geochronology data presented herein provide constraints to the timing of evolution of the distal segment of the Andrelândia basin (Raposos Group) from ca. 1.0 Ga to ca. 766 Ma (Fig. 12). This age interval agrees well with the evolution of the Macaúbas Group, located to the north, in the Araçuaí Belt. Other reported coeval intraplate magmatic episodes are the Pedro Lessa suite, the Northern Espinhaço mafic dykes and the Salto da Divisa A-type granites, with ages of 910 to 850 Ma, as well as correlative bimodal volcanic rocks and A-type granites in Africa with ages of 930 to 910 Ma (Pedrosa-Soares & Alkmim 2011, Pedrosa-Soares et al. 2000, 2016).

Published ages between ca. 735 Ma and 675 Ma, from the Southern Bahia Alkaline Province, and from the La Louila felsic volcanism in southwest Gabon (≤ 713 Ma) constrain the timing of deposition of the top units of the Macaúbas Group and African correlatives (Pedrosa-Soares & Alkmim 2011). This data indicates that the extensional history of the São Francisco eastern passive margin could have lasted until Cryogenian times, coeval with the development of outboard magmatic arcs eastern wards (Heilbron et al. 2008, 2017a).

CONCLUSIONS

The combination of the data presented herein with previously reported studies for the metabasic rocks of the Ribeira Belts reveals that at least three different episodes of mafic magmatism can be detected in the distal part of the Andrelândia basin and associated basement complexes. The two older episodes are represented by dismembered metabasic rocks found within the orthogranulites of the basement Juiz de Fora Complex and related to the Rhyacian arc (ca. 2.2 to 2.1 Ga) and to Statherian (ca. 1.7 Ga) intraplate magmatism. The youngest episode occurs as dismembered metabasic bodies that intruded both the cover and the basement association, with a late Tonian age of ca. 766 Ma, and clearly displays intraplate signatures associated with the evolution of the distal passive margin (Fig. 13).

An important implication of the characterization of this Late Tonian to Early Cryogenian intraplate metabasic rocks is that they must have shed detrital zircons that have been found sparsely in previous studies on the distal segments of the Andrelândia basin. We suggest that the existence of detrital zircons originated from intraplate magmatic events should be considered in future provenance studies of metasedimentary units of Neoproterozoic succession around the São Francisco craton, rather than necessarily originating from approaching magmatic arcs, as they have been commonly interpreted.

Acknowledgements

The authors are grateful to CPRM-PRONAGEO, CNPq and FAPERJ for research funding, and to the technicians of the laboratories from Brasília (UnB-Brazil), LAGIR (UERJ-Brazil) and Notre Dame (USA) universities. Monica Heilbron and Marcela Lobato were granted respectively Senior Visiting Professor and Sandwich PhD CAPES scholarships. Claudio Valeriano acknowledges a scholarship from CNPq (Estágio Senior no Exterior). We also would like to thank the generous revisions of Brendan Murphy, Robert Pankhurst and a third anonymous reviewer, which helped to improve an earlier version of the manuscript.

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ARTICLE INFORMATION

• 1
  Manuscript ID: 20180129.

Publication Dates

• Publication in this collection
  20 May 2019
• Date of issue
  2019

History

• Received
  20 Nov 2018
• Accepted
  19 Feb 2019