Geochemistry, Sm-Nd isotopes and SHRIMP U-Pb geochronology of the Morro do Coco Granite (RJ, Brazil): another piece of the post-collisional magmatism of the Ribeira Belt

Abstract

The Morro do Coco granite (MC) is a post-collisional intrusive body that was emplaced at Ribeira Belt, in the Cambro-Ordovician, succeeding the final stages of the Gondwana supercontinent amalgamation. This unit requires more studies, contrasting with similar best investigated occurrences of the central Ribeira Belt. This work presents a detailed investigation of such unit using petrography, Inductively Coupled Plasma (ICP), X-Ray Fluorescence (XRF) whole-rock geochemistry, U-Pb (SHRIMP) geochronology and Sm-Nd isotopes. The rock is a nearly undeformed syenogranite, geochemically classified as a high-K calc-alkaline magnesian, with metaluminous to slightly peraluminous affinity. Strongly light rare earth elements (LREE)-enriched patterns with significant negative Eu anomalies, typical of post-collisional granites, are identified. Similarly to other post-collisional granites of the central Ribeira Belt, U-Pb zircon data provide crystallization age of 496 ± 3 Ma . Sm-Nd isotopic data suggest crustal magma source as indicated by εNd₍₄₉₆₎ of -9, ¹⁴³Nd/¹⁴⁴Nd ratios between 0.511747 and 0.511752 and Mesoproterozoic TDM ages of 1.3 Ga. Based on its geochemical characteristics, morphology and geochronology, this unit is considered here as a member of Nova Friburgo Suite. Geochronological data defined three pulses of magmatism in this part of the orogen during the Cambro-Ordovician (515, 490 and 460 Ma).

KEYWORDS:
Post-collisional; Brasiliano Orogeny; Cambro-Ordovician; Ribeira Belt; Morro do Coco granite

INTRODUCTION

During the Neoproterozoic-Paleozoic, Brasiliano/Pan-African thermotectonic events took place at the margins of the São Francisco paleocontinent (Brazil) and another plate located to the east, possibly the Congo paleocontinent (Heilbron et al. 2000, Brito Neves et al. 2014). The convergence between these two continents and possibly a third one, the Paranapanema block which is presently interpreted as hidden under the Paleozoic Paraná basin in Brazil (cf.Trouw et al. 2000), resulted in a series of collisions that culminated in the amalgamation of the Gondwana supercontinent from ca. 580 to 510 Ma (e.g., Schmitt et al. 2004, Heilbron et al. 2008). At Southeastern Brazil, these orogenic events are presently represented by the Ribeira and Araçuaí Belts (Fig. 1A), which extend for more than 1,400 km along Espírito Santo, Rio de Janeiro, São Paulo and Minas Gerais States.

Figure 1.
Regional maps and tectonic context (modified from Heilbron et al. 2004). (A) Subdivision of the Mantiqueira Province, composed by the Brasilia, Dom Feliciano, Ribeira and Araçuaí Belts and location of Figure 1B area (dashed square). (B) Tectonic map of the Central segment of the Mantiqueira Province: (1) Cenozoic rifts covers; (2) Cretaceous alkaline massifs; (3) Socorro-Guaxupé Nappe (superior nappe); (4) Metasedimentary rocks including Ky-granulites (Inferior nappes); (5) São Francisco Craton basement and autochthonous domain; (6) São Francisco Supergroup; (7) São João Del Rey and Tiradentes formations (autochthonous covers); (8) Andrelandia Group; (9) Juiz de Fora domain (Occidental Terrane); (10) Paraíba do Sul Klippe; (11) Oriental Terrane Gneiss complex; (12) Rio Negro magmatic arc granitoids; (13) Cabo Frio Tectonic Domain Região dos Lagos Ortogneiss; (14) Cabo Frio Tectonic Domain Búzios-Palmital supracrustal sucessions; (15) Embu terrane; (16) São Roque and Açungui terranes; (17) Cambro-Ordivician granitoids: gs- Guarujá-Santos; ai- Anchieta Island; pt- Parati; ma- Mambucaba; mt- Mangaratiba; vd- Vila dois Rios; pb- Pedra Branca; fv- Favela; an- Andorinha; su- Suruí; te- Teresópolis; fr- Frade; nf- Nova Friburgo; cp- Conselheiro Paulino; sr- São José do Ribeirão; sp- São Pedro de Lumiar; sn- Sana; sj- Silva Jardim; cj- Cajú; it- Itaoca; mc- Morro do Coco; ms- Mimoso do sul; sa- Santa Angélica; bu- Buarama; ca- Castelo; pa- Pedra Azul.

The post-collisional phase, considered here as the ongoing geodynamic stage after the maximum plate convergence and the high-pressure metamorphism of this region, precluded “stable” platform conditions that allowed the development of Paleozoic intracratonic basins such as Amazonas and Paraná (Cunha et al. 2007, Milani et al. 2007). Indeed, the post-collisional stage is considered a complex period of large horizontal terrane adjustments along shear zones, associated to crustal extension and uplift, lithospheric delamination, docking and subduction of oceanic plates (e.g., Liégeois 1998). The post-collisional magmatism at Ribeira and Araçuaí Belts encompasses mainly high-K calc-alkaline granitoids, minor dioritic and a single manifestation of true shoshonitic rocks (e.g., Martins et al. 2016).

In this paper, the first study of the Morro do Coco granite (MC) is provided by presenting its field, petrographic and geochemical features, U-Pb zircon geochronology and Sm-Nd isotopic data in order to contribute to the understanding of the post-collisional magmatism that took place in Southeast Brazil during the Cambro-Ordovician period. Previous data of similar occurrences in the same region were also compared and discussed regarding the geographic and age distribution of intrusions with similar age to reconstruct the regional geodynamic scenario during the post-collisional stage of the Gondwana supercontinent amalgamation.

GEOLOGICAL SETTING

The Ribeira Belt (Fig. 1B) is commonly divided into four tectonostratigraphic domains, with rocks verging northwest toward the São Francisco Craton (SFC; e.g., Trouw et al. 2000) namely, from bottom to top:

• Occidental Terrane, the autochthonous domain, composed by intercalations of reworked SFC sheets interleaved with Meso-Neoproterozoic metasedimentary rocks from its passive margin;

• the Embú-Paraíba do Sul Terrane, considered a magmatic arc resulting from the subduction and collision between the São Francisco and the Paranapanema paleocontinents (Trouw et al. 2013). It consists of Paleoproterozoic orthogneiss and metasedimentary rocks, and Neoproterozoic granitoids intrusive in the orthogneisses;

• the Oriental Terrane, comprising paragnaisses intruded by syn-collisional granitoids (Costeiro Domain), Rio Negro Magmatic Arc (ca. 780 to 620 Ma; Tupinambá et al. 2000) and post-collisional granitoid intrusions (ca. 515-454 Ma; Valeriano et al. 2011, Tupinambá et al. 2012, Bongiolo et al. 2016), including MC, focus of this contribution;

• The Cabo Frio Tectonic Domain (CFTD), considered as part of the Congo Craton (e.g., Fernandes et al. 2015). The CFTD constitutes a Paleoproterozoic basement (Região dos Lagos Complex; Fonseca et al. 1979), represented by calc-alkaline orthogneisses of tonalitic to syenogranitic compositions, tectonically interleaved with Neoproterozoic metavolcano-sedimentary successions (Búzios and Palmital; Schmitt et al. 2004).

The main periods of thermotectonic activity recorded at the Ribeira Belt are:

• ca. 790-620 Ma, pre-collisional stage, including subduction and development of magmatic arcs such as Rio Negro and Serra da Bolívia (Tupinambá et al. 2012; Heilbron et al. 2013);

• 620-605 Ma, collisional stage between the Embú-Paraíba do Sul microcontinent and the São Francisco Craton passive margin (Trouw et al. 2013) that originated the southern Brasilia Belt;

• 605-550 Ma, collisional stage between a continental arc and the Oriental Terrane, representing the main collisional event registered at the Ribeira Belt (Heilbron and Machado 2003);

• 540-510 Ma, the docking of the Cabo Frio Terrane against the Oriental Terrane in the Buzios Orogeny (Schmitt et al. 2004);

• 510-440 Ma, when post-collisional magmatism took place in the Oriental Terrane (Valeriano et al. 2016, Schmitt et al. 2004, Bongiolo et al. 2016).

This complete history of subductions and orogenies recorded at Ribeira Belt resembles a long-term maintenance of high temperatures and low cooling rates at the active (upper) plate. Thus, Ribeira Belt is considered a hot orogen (e.g., Bento dos Santos et al. 2010, 2015, Valeriano et al. 2016), which might have implications on the post-collisional magmatism observed throughout its entire area.

The Cambro-Ordovician magmatism at the Ribeira-Araçuaí belts

Several occurrences of post-collisional Cambro-Ordovician granitoids, and fewer records of intermediate and basic rocks (De Campos et al. 2004, Valeriano et al. 2017) are found at the Ribeira and Araçuaí belts (Fig. 1B). Valeriano et al. (2011, 2016) summarize the occurrence and characteristics of the 15 largest granitic intrusions in the Central Ribeira Belt. Based on geochronological, compositional and structural characteristics, Valeriano et al. (2011, 2017) individualized two groups of post-collisional intrusions:

• the Cambrian Suruí Suite (ca. 510 Ma);

• the Ordovician Nova Friburgo Suite (ca. 485 Ma).

At the Central Ribeira Belt, other smaller post-collisional bodies are known, such as the Cajú granite (454 ± 5 Ma; Bongiolo et al. 2016); the São Pedro de Lumiar granite (~490 Ma; Mendes et al. 2011); the Mambucaba granite (492 ± 15; Machado et al. 1996) and the MC granite, focus of this study.

Cambro-Ordovician intrusions also occur at the northern area where there is a transition between the Ribeira and the Araçuaí belts, like the Santa Angélica (513 ± 8 Ma; Söllner et al. 2000), Mimoso do Sul and Buarama (495 ± 5 Ma and 480 ± 4 Ma, respectively; Söllner et al. 2000) and Várzea Alegre (499 ± 5 Ma; Mendes et al. 2005) bimodal intrusive bodies. Post-collisional granitic intrusions have also been identified at the southern Ribeira Belt, such as the Guarujá-Santos granites (497 ± 7 Ma; Janasi et al. 2015) and the Anchieta Island (500 ± 6 Ma; Azevedo Sobrinho et al. 2011).

The geographic distribution of these intrusions along a NE/SW relatively restricted corridor of about 80 km width inside the Oriental Terrane (Fig. 1B) raises ideas about the geodynamic models responsible for their emplacement, which Valeriano et al. (2016) suggest to be a response to break-off of the subducted lithospheric slab and production of localized post-collisional magmatism along a narrow zone in the upper plate.

ANALYTICAL METHODS

Sampling and petrography

Geological mapping and sampling were carried out at the MC region (Fig. 2). The granite and its host rocks were sampled prioritizing fresh rocks and scattering in the mapped area. Eight thin sections of MC granite were studied under transmitted light petrography at the Universidade Federal do Rio de Janeiro (UFRJ). Modal mineral distribution was obtained for all samples by counting at least 600 points per thin section. The plagioclase composition was petrographically calculated using the a-normal extinction-angle method (cf.Hibbard 1995).

Figure 2.
Geological Map of the Morro do Coco Granite and geographic location of the samples used in this work.

Lithogeochemistry

Nine whole-rock samples from MC granite (^{Suppl. Tab. 1A}) were analyzed by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) to evaluate major elements plus MnO, BaO, Cr₂O₃ and SrO, and by ICP Mass Spectrometry (ICP-MS) to determine trace elements at the ALS laboratory, Canada. Samples were digested by Lithium tetraborate fusion prior to nitric acid dissolution, before analyses. Duplicate and blank samples were used to control the analyses’ accuracy and precision, respectively. Loss on Ignition (LOI) was obtained by heating the samples at 1,000ºC for over an hour. Five other samples were analyzed by X-Ray Fluorescence (XRF) (Philips PW2400 with Rh-bearing tube) for major and some trace elements in the XRF laboratory at UFRJ. Duplicate analyses showed comparable results between the laboratories. All results were recalculated to anhydrous totals after Fe-oxidation adjustment to obtain the Fe₂O₃/FeO ratios from the Fe₂O_3(t) measured (cf.Middlemost 1989) using the IgRoCS software (Verma and Rivera-Gómez 2013). Adjusted data were used to calculate CIPW norms and were plotted on multidimensional tectonic discrimination diagrams for acidic rocks using the TecDIA software of Verma et al. (2015). Other chemical classification, geotectonic discriminant and elemental diagrams developed by several authors were plotted using the GeoChemical Data toolkit 5.0 (GCDkit; Janoušek et al. 2006).

Zircon U-Pb geochronology

In-situ high resolution SHRIMP dating on zircon crystals was performed on one sample (CLR-02C) from MC granite. Zircon crystals were concentrated by magnetic and heavy liquid separation prior to handpicking under binocular microscope. The sample was mounted on adhesive tape, placed on epoxy resin, and polished. TEMORA 1 zircon (²⁰⁶Pb/²³⁸U age of 416.78 ± 0.33 Ma; Black et al. 2003) was used as standard. Zircon grains were assessed under cathodoluminescence (CL) imaging to determine its internal domains and to select the spots for analysis. Isotopic data was acquired by a SHRIMP IIe at the Geochronology Research Center (CPGeo; Universidade de São Paulo). Softwares used at the CPGeo were LabVIEW 8.5 and SHRIMP SW 2.1. Raw data reduction was done using SQUID 1.06 Excel macro (Ludwig 2001), and concordia diagrams were plotted using ISOPLOT 4.15 (Ludwig 2012). U concentration was calibrated against the standard crystal SL13 with 238 ppm of U (Claué-Long et al. 1995, Sato et al. 2014). Common lead corrections were made to ²⁰⁴Pb (cf.Stacey and Kramers 1975). The uncertainties of isotopic ratios and ages were reported at the 1% level, and weighted mean ages were reported as 95% confidence limits (Sato et al. 2014).

Whole-rock Sm-Nd isotopes

Whole-rock Sm-Nd isotope analyses on two samples ­(CLR-02C and CLR-07C) were carried out by ICP-MS (Thermo Neptune) at CPGeo, following the procedures described by Sato et al. (1995). Prior to ICP-MS analysis, Sm and Nd concentrations were obtained through isotope dilution, with the addition of a ¹⁴⁹Sm and ¹⁵⁰Nd combined spike, followed by chemical digestion by HF, HNO₃ and HCl in Savillex™ vessels. Chromatographic separation of Sm and Nd was achieved in two steps, starting with AG50W-X8 resin to separate Rare Earth Elements (REE) from the bulk sample, followed by Sm and Nd separation using LaNthanides (LN) resin. Isotopic ¹⁴³Nd/¹⁴⁴Nd ratios were normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 (DePaolo 1981). ¹⁴⁷Nd/¹⁴⁴Nd ratios and eNd_(t) values were calculated according to Hamilton et al. (1983). Analytical blank concentration was 31pg and the mean value for ¹⁴³Nd/¹⁴⁴Nd of JNDi standard from July to May 2014 was 0.512097 ± 0.000005. Resulting data was plotted using ISOPLOT 4.15 (Ludwig 2012).

FIELD ASPECTS AND PETROGRAPHY

MC granite is a tabular plutonic body that occupies an area of about 45 km² at northern Rio de Janeiro State, Brazil (Fig. 2). It is intrusive in paragneiss of the São Fidélis Unit and ortogneiss of the Angelim Suite, both part of the Neoproterozoic Costeiro Domain of the Central Ribeira Belt (Fig. 1B). São Fidélis Unit comprises a supracrustal sequence of the Costeiro Domain, consisted of paragneiss metamorphosed under amphibolite facies P-T conditions (Eirado et al. 2017). In the studied area, it occurs as garnet-biotite-sillimanite gneiss, cropping out at lower topographic areas. The Angelim Suite ortogneiss is a group of intrusive plutonic rocks of predominant tonalitic composition in the supracrustal São Fidélis Unit (Tupinambá et al. 2007). It is represented in the studied area by a garnet-hornblende-biotite gneiss, cropping out at higher topographies.

In the field, MC occurs mostly as in situ blocks and boulders. The rock is macroscopically very homogeneous throughout the studied area. Its modal proportions plot in the field of syenogranites in the Quartz, Alkali feldspar, Plagioclase (QAP) diagram (^{Suppl. Tab. A2 and A3}).

Petrographically, MC is a grey, leucocratic, equigranular, medium-grained (3 mm in average) rock composed essentially by quartz (27%), alkali-feldspar (28%), biotite (16%) and plagioclase (10.5%). Allanite, titanite, apatite, opaque minerals and rare rutile are the accessory phases. Chlorite and sericite are the main secondary minerals (Fig. 3). Quartz occurs as subhedral to anhedral crystals, varying from 0.5 to 3 mm, and locally show undulose extinction (Figs. 3G and 3H); alkali-feldspar occurs as subhedral to euhedral crystals varying between 0.5 to 4.5 mm, often showing Carlsbad and tartan twinning and alteration to sericite; recrystallization of alkali-feldspar subcrystals is also observed locally (Figs. 3K and 3L).

Figure 3.
Photomicrographs of Morro do Coco. Each pair of photomicrographs is shown as parallel and crossed polars, respectively. (A) and (B) General aspects of mineralogy and texture of the granite; (C) and (D) Zircon crystal on detail; (E) and (F) Yellow epitode crystal with allanite core; (G) and (H) Detail on quartz crystal showing undulose extinction; (I) and (J) Detail on plagioclase crystal and myrmekitic texture on plagioclase near alkali-feldspar; (K) and (L) Detail on alkali-feldspar subcrystal in larger alkali-feldspar crystal; (M) and (N) Detail on quartz crystal in alkali-feldspar.

Biotite is light to dark brown, subhedral to euhedral varying from 0.1 to 1 mm and usually occurs as inclusions in quartz. Alteration to chlorite and opaque minerals is observed. Plagioclase crystals are anhedral to subhedral, with grain size ranging from 0.2 to 2 mm, and generally show polysynthetic twinning. Myrmekitic texture is locally observed in plagioclase crystals rimmed by alkali-feldspar (Figs. 3I and 3J). Alteration to sericite can be observed on plagioclase, whose measured plagioclase composition is compatible with andesine (An_34-38).

Titanite occurs as light brown, subhedral to anhedral crystals of 0.5 mm on average. They may present alteration to opaque minerals within crystal borders and fractures. Epidote occurs as prismatic, with yellow color and pleochroism, subhedric to euhedric, 0.5 to 1 mm crystals with allanite cores (Figs. 3E and 3F). Zircon crystals are subhedral to anhedral, measuring up to 0.3 mm and occur mostly included within quartz and biotite crystals (Figs. 3C and 3D) and apatite occurs as submillimetric rounded crystals.

RESULTS

Whole rock geochemistry

Whole-rock major and trace elements analyses were performed on 14 MC granite samples (Tab. 1). SiO₂ content varies from 68 to 72 wt.%, Al₂O₃ concentrations are between 14 and 15 wt.%, K₂O has high concentrations of up to 6 wt.%, and CaO and Na₂O have average contents of 2 to 2.8 wt.%, respectively. Average Fe₂O_3(t) content is 2.9 wt.%.

Table 1.
Representative geochemical analyses of Morro do Coco granite by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) (major elements), Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (trace elements) and X-Ray Fluorescence (XRF) (major and trace elements).

	Combined ICP-AES and ICP-MS									XRF
	CLR-02A	CLR-02C	CLR-03A	CLR-03C	CLR-07C	CLR-80C	CLR-86C	CLR-87A	CLR-X	CLR-02	CLR-03	CLR-07	CLR-80	CLR-89
SiO2	68.30	69.00	68.70	68.50	68.40	70.10	71.10	71.00	72.00	67.96	67.87	70.52	68.32	70.32
TiO2	0.77	0.75	0.80	0.76	0.95	0.72	0.47	0.42	0.42	0.80	0.75	0.44	0.71	0.44
Al2O3	14.55	14.85	14.80	14.65	14.50	14.50	13.95	13.80	13.95	15.25	15.39	14.42	15.13	14.82
Fe2O3	2.98	2.95	3.03	2.98	3.61	2.78	2.84	2.66	2.71	3.14	3.03	2.78	2.66	2.65
MnO	0.02	0.02	0.03	0.02	0.04	0.03	0.03	0.03	0.03	0.02	0.02	0.03	0.03	0.03
MgO	0.93	0.95	0.93	0.91	1.02	0.83	0.58	0.51	0.51	0.97	0.94	0.55	0.82	0.53
CaO	2.16	2.17	2.22	2.17	2.19	2.11	1.80	1.66	1.69	2.08	2.03	1.55	1.98	1.79
K2O	5.51	5.80	5.87	5.56	5.56	5.67	5.95	5.95	6.05	5.72	5.81	6.20	5.94	5.99
Na2O	3.03	3.06	2.95	3.09	2.77	2.98	2.51	2.44	2.49	3.17	3.22	2.45	3.07	2.78
P2O5	0.20	0.19	0.22	0.20	0.26	0.17	0.11	0.10	0.11	0.19	0.18	0.09	0.15	0.09
SrO	0.06	0.06	0.05	0.05	0.04	0.03	0.02	0.02	0.02	-	-	-	-	-
BaO	0.25	0.26	0.26	0.24	0.22	0.18	0.13	0.12	0.12	-	-	-	-	-
L.O.I	0.96	0.83	0.77	0.87	0.89	0.93	0.87	0.83	0.84	0.71	0.79	0.91	0.74	0.67
Total	99.72	100.89	100.63	100.00	100.45	101.03	100.36	99.54	100.94	100.01	100.03	99.94	99.55	100.11
ASI*	0.98	0.97	0.97	0.97	1.00	0.97	1.01	1.02	1.02	1.00	1.01	1.06	1.00	1.04
K*	0.54	0.55	0.57	0.54	0.57	0.56	0.61	0.62	0.62	0.54	0.54	0.62	0.56	0.59
CaO/Na2O	0.71	0.71	0.75	0.70	0.79	0.71	0.72	0.68	0.68	0.66	0.63	0.63	0.64	0.64
Rb	205.00	206.00	245.00	203.00	238.00	230.00	246.00	242.00	255.00	76.00	74.00	187.00	171.00	182.00
Ba	2180.00	2270.00	2300.00	2130.00	1925.00	1555.00	1140.00	1005.00	1070.00	1933.00	1873.00	1196.00	1518.00	1195.00
Sr	506.00	512.00	487.00	486.00	386.00	327.00	207.00	186.00	197.50	426.00	389.00	62.00	275.00	168.00
Th	68.00	57.00	60.20	77.60	60.20	75.80	99.50	107.50	114.00	-	-	-	-	-
U	1.92	1.90	2.04	2.08	3.37	2.44	2.01	2.56	2.72	-	-	-	-	-
Zr	646.00	617.00	684.00	632.00	823.00	622.00	435.00	375.00	402.00	407.00	382.00	275.00	340.00	286.00
Ga	25.60	25.20	25.70	25.80	27.40	26.20	22.60	22.30	23.50	5.00	7.00	5.00	6.00	6.00
Nb	10.10	9.60	11.50	10.90	16.40	11.60	17.70	16.70	19.10	17.00	17.00	13.00	16.00	14.00
Ni	-	-	-	-	-	-	-	-	-	35.00	33.00	34.00	35.00	33.00
Co	-	-	-	-	-	-	-	-	-	9.00	10.00	11.00	11.00	11.00
Zn	-	-	-	-	-	-	-	-	-	28.00	24.00	28.00	31.00	24.00
V	46.00	46.00	48.00	46.00	49.00	39.00	24.00	20.00	23.00	69.00	69.00	51.00	68.00	51.00
Cr	20.00	10.00	20.00	10.00	10.00	20.00	< 10	20.00	20.00	12.00	38.00	3.00	35.00	23.00
Sn	2.00	3.00	3.00	3.00	4.00	2.00	2.00	2.00	3.00	-	-	-	-	-
La	229.00	199.50	234.00	272.00	201.00	205.00	232.00	229.00	241.00	-	-	-	-	-
Ce	434.00	381.00	439.00	511.00	396.00	403.00	460.00	453.00	478.00
Pr	42.90	38.20	43.90	50.80	42.30	40.80	47.20	47.20	49.80	-	-	-	-	-
Nd	133.00	122.00	136.50	159.50	141.50	131.00	151.00	150.50	159.00	-	-	-	-	-
Sm	14.35	14.25	15.15	18.00	17.05	15.00	19.35	19.45	20.60	-	-	-	-	-
Eu	2.01	1.98	2.12	2.19	2.33	1.63	1.70	1.60	1.74	-	-	-	-	-
Gd	6.13	5.90	6.67	7.34	7.50	6.22	9.42	9.54	10.15	-	-	-	-
Tb	0.61	0.59	0.66	0.76	0.84	0.68	1.00	1.01	1.06	-	-	-	-	-
Dy	2.35	2.35	2.71	3.04	3.55	2.66	4.15	3.77	3.97	-	-	-	-	-
Ho	0.37	0.34	0.42	0.46	0.56	0.43	0.59	0.55	0.57	-	-	-	-	-
Er	0.84	0.83	0.97	0.96	1.27	0.90	1.32	1.09	1.19	-	-	-	-	-
Tm	0.10	0.09	0.10	0.13	0.17	0.11	0.15	0.13	0.15	-	-	-	-	-
Yb	0.57	0.43	0.67	0.61	0.94	0.59	0.82	0.75	0.79	-	-	-	-	-
Lu	0.07	0.07	0.09	0.07	0.12	0.09	0.11	0.11	0.11	-	-	-	-	-
Y	9.10	9.30	10.60	10.90	15.00	10.30	15.20	13.50	14.50	14.00	14.00	16.00	14.00	15.00
Cs	1.26	0.87	1.50	0.90	1.36	0.85	0.83	1.20	1.23	-	-	-	-	-
Ta	0.70	0.80	0.90	0.80	1.10	1.00	1.00	0.70	1.80	-	-	-	-	-
Hf	15.60	14.20	15.60	15.00	19.30	15.40	12.10	10.90	11.20	-	-	-	-	-
Rb/Sr	0.41	0.40	0.50	0.42	0.62	0.70	1.19	1.30	1.29	-	-	-	-	-
Ba/Rb	10.63	11.02	9.39	10.49	8.09	6.76	4.63	4.15	4.20	-	-	-	-	-
La/Yb(N)	270.86	312.79	235.46	300.62	144.16	234.25	190.75	205.85	205.67	-	-	-	-

• Major (wt.%) and selected trace element (ppm); ASI* = Al₂O₃/CaO + Na₂O+K₂O molar ratio; K*= K/Na + K molar ratio.

MC granites are characterized by high Ba (~1,700 ppm), Zr (~500 ppm), Th (~75 ppm) and light rare earth elements (LREE) (La 225 ppm and Ce ~435 ppm), and medium Sr contents (~340 ppm). Nonetheless, heavy rare earth elements (HREE) show small concentrations such as Yb (0.7 ppm) and Lu (0.1 ppm).

The CIPW normative mineralogy (^{Suppl. Tab. A4}) corresponds to quartz (ca. 23-29 wt.%), orthoclase (ca. 33-37 wt.%), plagioclase (ca. 28-36 wt.%), and lesser amounts of hyperstene, magnetite, ilmenite, apatite and normative corundum, which is compatible with petrographic results.

The studied granites are high-K calc-alkaline, magnesian to ferroan and metaluminous to slightly peraluminous (Fig. 4). In the K₂O vs. SiO₂ diagram (Fig. 4B), MC samples plot in the shoshonite series field. However, they are considered here as high-K calc-alkaline rocks, as they do not meet all the requirements to be classified as a true shoshonite (cf.Morrison 1980, Campbell et al. 2014), such as:

• Na₂O/K₂O > 1 at 55 wt% SiO₂;

• negative or absent correlation between K₂O vs. SiO₂ at SiO₂ > 57 wt%;

• La/Yb_N between 8 and 46.

Figure 4.
Geochemical classification diagrams for whole rock analyses of Morro do Coco Granite. (A) TAS (Cox et al. 1979); (B) K₂O vs. SiO₂ (Peccerillo and Taylor 1976); (C) and (D) geochemical classification diagrams (FeO_t/FeO_t + MgO vs. SiO₂ and Na₂O + K₂O-CaO vs. SiO₂; Frost et al. 2001); (E) A/NK vs. A/CNK diagram (Shand 1943).

Using multielemental diagram normalized to the average crust (Weaver and Tarney 1984; Fig. 5A), MC samples display moderate enrichment in large ion litophile elements (LILE) such as Rb, Th and K and are less enriched in high field strength elements (HFSE) such as P and Ti. Negative Sr, Ba, P, Nb, Ti and Zr anomalies would indicate fractionation of plagioclase, apatite, titanite and zircon. The chondrite-normalized REE pattern of Boynton (1984; Fig. 5B) is strongly fractionated, with (Ce/Yb)_N ratios ranging from 109 to 229 and significant negative Eu anomalies (Eu/Eu* = 0.37 to 0.66).

Figure 5.
Multielemental and rare earth elements (REE) patterns for Morro do Coco granite samples. (A) Multielemental diagram normalized to average crust (Weaver and Tarney 1984); (B) REE pattern diagram normalized for chondrite (Boynton 1984).

In binary diagrams using silica as differentiation index, major elements present negative correlation for Al₂O₃, CaO, MgO, Na₂O, TiO₂, P₂O₅ and Fe₂O_3(t). Positive correlations only occur for K₂O and K₂O/Na₂O (Fig. 6). Minor and trace elements (Fig. 7), exhibit negative SiO₂ correlation for Sr, Zr and Ba and positive correlations for Y and Nb. Other elements such as La show random distribution. Nb/Zr vs. Nb binary diagram, used to identify crystal fractionation and different magma sources (Sial and McReath 1984), shows two different clusters for MC samples (Fig. 8) and two evolution lines of fractional crystallization.

Figure 6.
Binary diagrams using SiO₂ as differentiation index for major elements of Morro do Coco granite samples.

Figure 7.
Binary diagrams using SiO₂ as differentiation index for selected minor and trace elements of Morro do Coco granite samples.

Figure 8.
Nb/Zr versus Nb binary diagram plot of Morro do Coco samples. The horizontal lines represent process of fractional crystallization. The two clusters may indicate different magmatic sources.

Geotectonic classification

A series of geotectonic classification diagrams for acidic rocks were created by Verma et al. (2012, 2013) based on linear discriminant analysis of natural logarithm transformed ratios of major (m3) and immobile trace elements (tacid). The plot of MC in these function-based multidimensional diagrams indicates the rock has collisional affinity on the major elements-based diagrams (m3; Fig. 9), and collisional affinity trending to arc setting on the immobile trace elements-based diagrams (tacid; Fig. 10).

Figure 9.
Multidimensional geotectonic classification diagram for acidic rocks based on major elements (m3, Verma et al. 2012) for Morro do Coco samples and a comparison with other post-collisional granitic intrusions found in Ribeira-Araçuaí Belts. Data from: Almeida (2010; Suruí), Valeriano (2012; Pedra Branca), Ribeiro (2006; Silva Jardim), Teixeira (2010; Parati), Azevedo Sobrinho et al. (2011; Anchieta Island), Junho (1992; Teresópolis), Potratz and Valeriano (2017; Itaoca), Bongiolo et al. (2016; Cajú).

Figure 10.
Multidimensional geotectonic classification diagram for acidic rocks based on immobile trace elements (tacid, Verma et al. 2013).

In addition, this classification provides a helpful statistical evaluation of the probability the rock has to belong to a specific geotectonic setting (Tab. 2). For the m3 diagrams, MC has a final probability of 83% to collisional setting, 16% to intraplate setting (CR+OI) and 1% to arc setting (CA). Tacid diagrams probabilities are 75% to collisional setting, 23% to arc setting and 2% to intraplate setting.

Table 2
Geotectonic setting probabilities based on the application of multidimensional diagrams to acidic rocks of Morro do Coco Granite.

Magma type, Figure name	Figure type	Total number of samples	Number of discriminated samples
			Arc			CR+OI [x ± s] [p_CR+OI] Θ	Col [x ± s] [p_Col] Θ
			IA+CA [x ± s] [p_IA+CA] Θ	IA [x ± s] [p_IA] Θ	CA [x ± s] [p_CA] Θ
Acid; Verma et al. (2012); log-ratios of all major elements	(IA+CA-CR-Col)	14	0 (0)	---	---	0 (0)	14 [0.864 ± 0.074] (0.6729-0.9931)
	(IA-CA-CR)	14	---	0 (0)	1 (0.6912)	13 [0.744 ± 0.102] (0.5708-0.8993)	---
	(IA-CA-Col)	14	---	0 (0)	0 (0)	---	14 [0.9745 ± 0.0155] (0.9456-0.9980)
	(IA-CR-Col)	14	---	0 (0)	---	0 (0)	14 [0.855 ± 0.063] (0.6885-0.9849)
	(CA-CR-Col)	14	---	---	0 (0)	0 (0)	14 [0.804 ± 0.092] (0.5695-0.9869)
Diagrams based on log-ratios of major elements	{Σn} {Σprob} [%prob]	70	{0} {0} [0%]	{0} {0} [0%]	{1} {0.6912} [1%]	{13} {9.6726} [16%]	{56} {48.9627} [83%]
Acid; Verma et al. (2013); log-ratios of immobile trace elements	(IA+CA-CR+OI-Col)	9	0 (0)	---	---	0 (0)	9 [0.730 ± 0.161] (0.5400-0.9522)
	(IA-CA-CR+OI)	9	---	0 (0)	8 [0.99999 ± 0.00000] (1.0000-1.0000)	1 (0.7504)	---
	(IA-CA-Col)	9	---	0 (0)	1 (0.5528)	---	8 [0.734 ± 0.186] (0.5182-0.9670)
	(IA-CR+OI-Col)	9	---	0 (0)	---	0 (0)	9 [0.9957 ± 0.0126] (0.9621-1.0000)
	(CA-CR+OI-Col)	9	---	---	0 (0)	0 (0)	9 [0.767 ± 0.155] (0.5582-0.9618)
Diagrams based on log-ratios of immobile trace elements	{Σn} {Σprob} [%prob]	45	{0} {0} [0%]	{0} {0} [0%]	{9} {8.5527} [23%]	{1} {0.7504} [2%]	{35} {28.3084} [75%]

• IA: island arc; CA: continental arc; IA+CA: combined island and continental arcs, i.e., arc setting; CR: continental rift; OI: ocean island; CR+OI: combined continental rift and ocean island, i.e., within-plate (WP) setting; Col: collisional setting; Θ: the probability values for samples are represented by e.g., (pIA+CA): probability for the combined island and continental arc setting in the first diagram; [pCol]: probability for the collision setting in the diagrams; −: ± 1 standard deviation of the probability estimates for all samples discriminated in a given tectonic setting; these are reported in [], the values are rounded mostly following the indications put forth by Verma (2005); the final rows give a synthesis of results as {Σn}, {Σprob} and [%prob], where {Σn} is the total number of samples or data points plotting in all five diagrams which is reported in the column of total number of samples, whereas the sum of samples plotting in a given tectonic field is reported in the respective tectonic field column; {Σprob}: the sum of probability values for all samples plotting in a given tectonic field, reported in the respective tectonic field column; [%prob]: the total probability of a given tectonic setting expressed in percent after assigning the probability of IA + CA to IA and CA (using weighing factors explained in Verma and Verma 2013).

On more traditional geotectonic classification diagrams such as Rb vs. Y+Nb, Nb vs. Y, Rb vs. Ta+Yb and Ta vs. Yb of Pearce et al. (1984; Fig. 11), MC plots within the limit between syn-collisional granite (syn-COLG) and volcanic arc granite (VAG). Also, most of the samples plot in the post-collisional granite field (post-COLG; Pearce 1996) within the Rb vs. Y+Nb diagram (Fig. 11A).

Figure 11.
Pearce et al. (1984) granite geotectonic discriminant diagrams. (A) Rb vs. Y+Nb diagram (including field for post-collisional granites from Pearce 1996); (B) Nb vs. Y; (C) Rb vs. Ta + Yb; (D) Ta vs. Yb.

U-Pb geochronology

Seventeen SHRIMP spots on zircon crystals from one MC sample (CLR-02C) were analyzed (Tab. 3). Crystals are prismatic, some with sub-rounded rims, and their size range from 125 to 260 µm in length and 25 to 50 µm in width, with a length/width ratio varying from 10:3 on elongated crystals to 2:1 on sharp ones. Concentric or irregular oscillatory zoning can be observed for most crystals under CL imaging, interpreted as indicating their igneous origin (Fig. 12A).

Table 3.
Analytical results of sensitive high resolution ion microprobe (SHRIMP) U-Pb zircon data for the Morro do Coco Granite.

Grain spot	U (ppm)	Th (ppm)	232 Th/ 238 U	Common 206Pb (%)	Rad 206Pb (ppm)	207 Pb/ 235 U	error (%)	206 Pb/ 238 U	error (%)	207 Pb/ 206 Pb	error (%)	error corr	206 Pb/ 238 U Age (Ma) 204 corr	error (1s)	207 Pb/ 206 Pb Age (Ma) 204 corr	error (1s)	Disc (%)
MC01-1.1	94	110	1.21	0.41	7.4	0.74	3.4	0.0914	1.6	0.0587	3.0	0.467	564.0	8.5	557.5	64.7	-1
MC01-2.1	169	332	2.03	0.28	11.1	0.61	3.9	0.0763	2.7	0.0579	2.9	0.674	474.1	12.2	526.7	63.9	11
MC01-3.1	128	239	1.92	0.29	8.8	0.63	3.4	0.0796	1.6	0.0574	3.0	0.458	493.8	7.4	508.1	66.5	3
MC01-4.1	237	72	0.32	0.00	19.6	0.79	1.8	0.0960	1.5	0.0597	1.0	0.827	591.1	8.3	594.4	21.7	1
MC01-5.1	219	227	1.07	0.03	15.2	0.64	2.6	0.0807	1.7	0.0577	2.0	0.645	500.2	8.1	516.6	43.6	3
MC01-6.1	132	216	1.68	0.47	9	0.63	3.5	0.0791	2.1	0.0581	2.8	0.597	490.5	9.9	533.1	61.5	9
MC01-7.1	142	258	1.87	-0.07	9.6	0.62	4.3	0.0785	1.6	0.0568	4.0	0.372	487.1	7.5	485.0	88.4	0
MC01-8.1	219	305	1.44	1.05	15.5	0.64	4.3	0.0812	1.5	0.0573	4.0	0.349	503.4	7.3	504.6	89.6	0
MC01-9.1	232	566	2.52	0.50	15.1	0.62	3.7	0.0753	2.0	0.0593	3.1	0.540	468.2	8.9	576.6	67.0	23
MC01-10.1	128	266	2.14	0.48	8.7	0.64	2.8	0.0784	1.5	0.0589	2.3	0.554	486.8	7.2	563.4	50.5	16
MC01-11.1	155	293	1.96	0.23	10.6	0.64	2.3	0.0793	1.5	0.0588	1.7	0.670	492.0	7.2	559.5	36.5	14
MC01-12.1	145	298	2.12	0.51	10.3	0.65	3.4	0.0824	1.5	0.0575	3.0	0.448	510.5	7.5	509.8	67.1	0
MC01-13.1	179	397	2.29	0.43	12.1	0.62	3.1	0.0785	1.5	0.0571	2.7	0.490	487.1	7.1	495.5	59.1	2
MC01-14.1	180	396	2.28	0.43	12.5	0.64	3.0	0.0805	1.6	0.0581	2.5	0.528	498.8	7.5	532.5	55.1	7
MC01-15.1	209	447	2.21	-0.09	14.2	0.65	2.0	0.0791	1.6	0.0601	1.2	0.813	490.6	7.6	605.8	24.9	23
MC01-16.1	200	370	1.91	0.76	13.6	0.64	3.9	0.0783	1.5	0.0593	3.6	0.386	486.0	7.1	578.7	78.6	19
MC01-17.1	451	70	0.16	0.01	31.7	0.65	1.7	0.0819	1.4	0.0572	0.8	0.876	507.3	7.1	500.4	17.6	-1

Figure 12.
Geocrhonology of Morro do Coco granite. (A) Cathodoluminescence images from zircon crystals of Morro do Coco granite; (B) statistical plot of all ages of analyzed spots with error bar of 2σ. Only the spots represented by black bars were used to calculate the concordant age; (C) U-Pb sensitive high resolution ion microprobe (SHRIMP) Concordia diagram showing only nine selected spots.

Statistical plot of all spots analyzed are shown in Figure 12B. Spots 1.1 and 4.1 yielded ²⁰⁶Pb/²³⁸U age of 564 and 591 Ma, respectively, being assigned as a xenocrystal, probably derived from the host rocks. Aside from these two plots, others were excluded due to high discordance degree or low Th/U ratios. Eight plots (2.1, 3.1, 5.1, 6.1, 7.1, 8.1, 13.1, 14.1) have ²⁰⁶Pb/²³⁸U ages between 474 and 507 Ma and yielded a concordant age of 496 ± 3 Ma with mean weighted standard deviation (MWSD) concordance of 0.63 (Fig. 12C), which we interpret as the crystallization age for the MC granite.

Sm-Nd isotopes

Sm-Nd results for two samples (CLR-02C and CLR-07C) of the MC granite are presented in Tab. 4. The samples show ¹⁴³Nd/¹⁴⁴Nd ratios of 0.511752 and 0.511747, εNd_(496Ma) ~ -9, and T_DM model age of 1.3 Ga (Fig. 13). Data suggest Mesoproterozoic age for parental magma extraction from its source.

Table 4.
Sm-Nd isotopic data for the Morro do Coco Granite.

Sample	Sm (ppm)	Nd (ppm)	147 Sm/ 144 Nd	error (2s)	(143Nd/144Nd)0	εNd(0)	(143Nd/144Nd)t	εNd(t)	TDM (Ga)	t (Ga)
CLR-02C	16	150	0.067880	0.00040	0.511752	-17.3	0.511532	-9.15	1.35	0.5
CLR-07C	16	113	0.064385	0.00038	0.511747	-17.4	0.511538	-9.03	1.32	0.5

Figure 13.
εNd vs. time (Ga) evolution diagram for two samples (CLR-02C and CLR-07C) of the Morro do Coco granite.

DISCUSSION

The generation of MC in the late Cambrian is related to the post-collisional geodynamics that took place at the final Brasiliano orogeny, during the waning stages of the Gondwana supercontinent amalgamation. Based on the acquired data, the following sections will provide details about its geochemical and geotectonic significance and its correspondence to other similar occurrences in the Ribeira and Araçuaí belts.

Geochemistry of Morro do Coco granite

Whole-rock geochemical data classifies MC as a high-K, calc-alkaline magnesian syenogranite with metaluminous to slightly peraluminous affinity (aluminum saturation index - ASI between 0.97 and 1.06; Fig. 4, Tab. 1). Other important geochemical characteristics of the MC granite are:

• high K₂O concentrations (up to 6 wt.%, Fig. 4B) in K₂O vs. SiO₂ diagram;

• high K₂O + Na₂O contents (around 9 wt.% Tab. 1);

• great LREE enrichment and negative Eu anomalies on the chondrite-normalized REE pattern (Fig. 5B);

• strongly negative εNd₍₄₉₆₎ of ca. -9, indicating crustal melting source.

Still, Mesoproterozoic T_DM ages of 1.3 Ga obtained for MC (Fig. 13, Tab. 4) are similar to model ages obtained for intermediate and mafic rocks from Várzea Alegre (Medeiros et al. 2000) and syn-collisional calc-alkaline granitoids from São Paulo and Minas Gerais States (Janasi et al. 1997), which they interpret as being related to an important crust-forming event for the area, possibly in a subduction environment associated to lithospheric mantle metasomatism. Further, magnesian granitoids are related to partial melting of continental crust under oxidizing (and probably wet) conditions (Osbourn 1959, Frost et al. 2001).

Variation diagrams using SiO₂ as differentiation index support that fractional crystallization took part in MC magma evolution as they present negative correlations for Al₂O₃, CaO, MgO, Na₂O, Fe₂O_3(t) and TiO₂ and a positive one for K₂O. The depletion in CaO (and Sr) and the enrichment of K₂O (and Rb) with increasing SiO₂ contents accounts, respectively, for plagioclase fractionation and alkali-feldspar crystallization (Figs. 6 and 7). Negative Eu anomalies (δEu up to 0.35; Fig. 5B) also ascribe the removal of plagioclase from the melt by crystal fractionation or the partial melting of a source rock leaving plagioclase in the residue (cf.Wilson 1989). The two horizontal trend lines in the Nb vs. Nb/Zr binary plot (Fig. 8) are also interpreted as result of the fractional crystallization process.

Fractional crystallization is considered an important process in the history of similar post-collisional rocks in the region, but are certainly combined, in a higher or lesser degree, with mixing between mantle-derived magmas and crustal melts (e.g., Wiedemann et al. 1987, Mendes et al. 1997, 2005). This combined magma source is not evidenced in MC, as demonstrated by its petrographic and geochemical (e.g., lack of association with basic rocks and slightly peraluminous affinity).

A comparison of Morro do Coco with similar occurrences

By comparing the chemical composition of MC with other similarly evolved (SiO₂ contents) post-collisional rocks of the Ribeira-Araçuaí belts (^{Suppl. Tab. A5}), they are clearly akin. Mean values of SiO₂ (70 wt.%), Al₂O₃ (14.4 wt.%), Fe₂O_3(t) (2.7 wt.%), CaO (1.8 wt.%) Na₂O (3.1 wt.%) and K₂O (5.8 wt.%) of the other intrusions are very alike to the concentrations of major elements in MC (Tab. 1). Only the younger Cajú granite, presenting slightly higher SiO₂ contents (up to 73 wt.%), has lower Fe₂O_3(t) contents (1.2 wt.%)_.

The plot of geochemical data of post-collisional granites of the Central Ribeira Belt in the major-elements based multidimensional tectonic discrimination diagrams of Verma et al. (2012; Fig. 9) show they were all formed at a collisional setting, but one group (Parati, Silva Jardim, Anchieta Island, Teresópolis, Itaoca and MC) plot closer to the collisional/continental rift fields transition, while the other (Suruí, Pedra Branca and Nova Friburgo) plot scattered within the collisional field (Fig. 9A to C). Also, a different chemical behavior between these two groups is observed in the last diagram (Fig. 9E), where the latter demonstrates higher arc setting affinity. This variability may reflect heterogeneity of their magmatic sources, as there is no direct correlation between age or Suite and these geochemical signatures.

Trace-elements-based tectonic classification diagrams corroborate that MC was formed at a collisional/post-collisional setting but, in this case, some samples show arc affinity (Figs. 10 and 11), as noted by Pearce (1996). As stated by Frost et al. (2001), the trace-elements compositions of post-collisional granitoids are highly controlled by melting source and crystallization history, rather than the tectonic environment, as they can be derived from different types of sources depending on the composition of the thickened crust. Thus, this arc-related geochemical signature of MC could be inherited from the melting source.

Incipient deformation is mostly described in the Suruí Suite (Valeriano et al. 2017). Undulose extinction in quartz crystals and development of myrmekite in plagioclase (Fig. 3G-3J) may indicate solid-state deformation (cf.Vernon et al. 2004) in the MC granite. Also, alkali-feldspar recrystallization (Fig. 3K-3L) indicates P-T arrangement of at least 480ºC at 4 kb conditions (Voll 1976, Altenberger et al. 1987) during MC emplacement. This is the expected temperature for the study area in the post-collisional stage of the Ribeira orogen at 495 Ma, using cooling rates of 10ºC/Ma after the metamorphic peak at 525 Ma (ca. 780°C and 9 kb; Schmitt et al. 2004) near the study area. Thus, incipient deformation observed in MC and other post-collisional intrusion of the Ribeira orogeny might represent the rheological behavior of their magmas under advanced crystallization (35-65%; e.g., Fernandez and Gasquet 1994).

Post-collisional age span in southeastern Brazil

The current subdivision of the Cambro-Ordovician granitic intrusions of the Ribeira Belt into two Suites (i.e., Suruí and Nova Friburgo) suggests two pulses of magmatism during the post-collisional stage of the Brasiliano orogeny. The Suruí and Nova Friburgo Suites are distinguished over age, structural and geomorphological criteria. According to Valeriano et al. (2017), Suruí Suite (~510 Ma) occurs mostly as stocks and batholiths and show superimposed weak ductile deformation; while Nova Friburgo Suite (~485 Ma) granitoids are generally represented by tabular intrusions without clear signs of ductile deformation.

Nevertheless, by comparing these ages and extending this comparison to the South Ribeira and Araçuaí belts, a broader distribution can be observed if we consider analytical errors (Fig. 14, Tab. 5). In Fact, only Pedra Branca, Suruí, Santa Angélica and Pedra Azul granitoids clearly cluster as older representants of this magmatism, and Cajú as the youngest intrusion (ca. 454 Ma; Bongiolo et al. 2016). The other intrusions span intermediately, at ca. 495-475 Ma, which could either indicate three pulses of magmatism (515, 495 and 455 Ma) or a continuous event from 515 to 475 Ma with an interval of 20 Ma to a second pulse at ca. 455 Ma.

Table 5.
Geochronological data compilation of some of the Cambro-Ordovician intrusive bodies in the Ribeira and Araçuaí belts.

Unit	Suite	Age	Error	Method		Reference
Guarujá-Santos	-	497	7	U-Pb SHRIMP	zircon	Janasi et al. (2015)
Anchieta	-	499.7	5.9	LA-ICPMS	zircon	Azevedo Sobrinho et al. (2011)
Parati	Suruí	500	-	U-Pb	zircon	Ludka et al. (2006)
Pedra Branca	Suruí	513	5	ID-TIMS	zircon	Heilbron and Machado (2003)
Vila Dois Rios	Suruí	?	-	-	-	Fernandes (2000); Eirado et al. (2006)
Suruí	Suruí	512	7	LA-ICPMS	zircon	Valeriano et al. (2011)
Silva Jardim	Suruí	505	2	LA-ICPMS	zircon	Moraes (2009)
Mangaratiba/Mambucaba	Nova Friburgo	492	15	ID-TIMS	titanite	Machado et al. (1996)
Favela	Nova Friburgo	482	6	LA-ICPMS	zircon	Heilbron and Machado (2003)
Andorinha	Nova Friburgo	490	9	LA-ICPMS	zircon	Valeriano et al. (2011)
Teresópolis/Frades	Nova Friburgo	481	6	LA-ICPMS	zircon	Valeriano et al. (2011)
Nova Friburgo	Nova Friburgo	489	4	LA-ICPMS	titanite	Valeriano et al. (2011)
Conselheiro Paulino	Nova Friburgo	?	-	-	-	Pacheco (2010)
São José do Ribeirão	Nova Friburgo	?	-	-	-	Mendes et al. (2002)
Sana	Nova Friburgo	491	10	LA-ICPMS	zircon	Valeriano et al. (2011)
São Pedro de Lumiar	Nova Friburgo	490	-	Inferred	-	Mendes et al. (2011)
Itaoca	Nova Friburgo	476	2	ID-TIMS	monazite	Neto et al. (2014)
Morro do Coco	Nova Friburgo	496	3	U-Pb SHRIMP	zircon	This work
Cajú	-	454	5	U-Pb SHRIMP	zircon	Bongiolo et al. (2016)
Mimoso do Sul	-	495	5	U-Pb	zircon	Söllner et al. (2000)
Santa Angélica	-	513	8	U-Pb	zircon	Söllner et al. (2000)
Buarama	-	480	4	U-Pb	zircon	Söllner et al. (2000)
Pedra Azul	-	536	31	Whole-rock Rb-Sr	-	Platzer (1997)
Várzea Alegre	-	499	5	ID-TIMS	zircon	Mendes et al. (2005)

• SHRIMP: sensitive high resolution ion microprobe; LA-ICPMS: laser ablation inductively coupled plasma mass spectromete; ID-TIMS: thermal ionization mass spectrometry isotopic dilution:

Figure 14.
Plot of the geochronological data compilation for the Cambro-Ordovician intrusive bodies at the Ribeira-Araçuaí belts from Southern Ribeira Belt to Southern Araçuaí Belt. The Parati and São Pedro de Lumiar error bars are inferred. The Pedra Azul has a very imprecise age due to the methodology used (whole-rock Rb-Sr). Data References are shown in Table 5.

The MC granites have crystallization age of 496 ± 3 Ma, similarly to those of the Nova Friburgo Suite crystallization (ca. 485 Ma; Fig. 14, Tab. 5). Based on geochemical characteristics, age and tabular geometry, MC is interpreted here as belonging to the Nova Friburgo Suite.

Post-collisional Ribeira vs. Araçuaí belts and current geodynamic models

The petrographic and geochemical characteristics of MC are indicative of granites formed by processes associated to delamination of overthickened crust (Frost et al. 2001), which is one of the most accepted geodynamic models proposed for the Cambro-Ordovician magmatism at the Ribeira-Araçuaí belts (Heilbron and Machado 2003, Pedrosa-Soares et al. 2008). Break-off of subducted oceanic lithosphere is also proposed as a triggering mechanism for this post-collisional magmatism (Söllner et al. 1987, 2000). In fact, both processes could have taken place synchronously. Other possible mechanism is the onset of a hotspot due to destabilization of the asthenospheric mantle after orogenic collapse, proposed by De Campos et al. (2016) to explain post-collisional bimodal intrusive bodies such as Mimoso do Sul, Buarama and Várzea Alegre (ca. 499 to 480 Ma; Söllner et al. 2000, Mendes et al. 2005) at the Araçuaí Belt.

These models highlight extensive melting of the lower continental crust driven by asthenospheric mantle uplift and the production of mantle-derived tholeiitic magmas, which is not observed in MC, as it comprises a homogenous granitic body with no evidence of associated mafic rocks. Either way, these continental scale processes could have provided the necessary heat to partially melt the continental crust and generate MC.

Post-collisional intrusions in Southern Ribeira Belt (e.g., Anchieta Island quartz-monzonite; Azevedo Sobrinho et al. 2011) and in Northern Ribeira, and Southern Araçuaí belts transition zone, Espírito Santo State (e.g., Mimoso do Sul, Santa Angélica, Castelo, Pedra Azul and Várzea Alegre) are interpreted to result from the interaction between mantle-derived magmas and partial melting of the lower crust (De Campos et al. 2004, 2016, Azevedo Sobrinho et al. 2011). Regarding the latter, De Campos et al. (2016) emphasize the complexity and uniqueness of their internal structures and the broadness of magma mixing/mingling evidences, and that they represent the roots of these plutons exposed by deep erosion levels combined with high vertical exposures, which is supported by geobarometric studies that point to intrusions depths around 20 km for these rocks (> 6 kb; Mendes et al. 1997, Medeiros et al. 2001).

However, MC and other Cambro-Ordovician intrusive bodies emplaced at the central Ribeira Belt, Rio de Janeiro State, seem to represent a shallower portion of the continental crust, with “undeformed” high-K calc-alkaline to alkali-calcic mainly metaluminous granites and lesser amounts of quartz-monzonites (e.g., Suruí, Pedra Branca, Parati, Silva Jardim, Vila Dois Rios, Nova Friburgo, Mangaratiba, Favela, Andorinha, Teresópolis, Sana, São Pedro, Itaoca, etc.). Post-collisional pegmatites that crop out in the CFTD and are related to the Cajú granite were emplaced at shallow crustal levels, between 1.7 to 3.5 km (Bongiolo et al. 2016).

Most of these granitoids carry gabbroic, dioritic and quartz-dioritic microgranular enclaves (Valeriano et al. 2016) which could be related to primitive magma sources, but no mafic enclaves/xenoliths were observed in MC so far. Despite the occurrence of scarce structures interpreted as magma mingling (e.g., Mendes et al. 2002, Pacheco 2010, Valeriano et al. 2017), the post-collisional granitoid intrusions of the central Ribeira Belt do not display clear signs of interaction with more primitive (basic) magmas as it is observed in the Araçuaí Belt.

Considering that the CFTD was formed by a west-dipping subduction prior to the Buzios orogeny, this event could have provided extra heating of the upper plate (Costeiro Domain). Thus, the more significant crustal contribution concentrated at the central Ribeira Belt, along with the larger number of known post-collisional intrusions in this sector (Fig. 1), could be related to their proximity to the CFTD. The alignment of post-collisional magmatism along a NE/SW-trending corridor through the Ribeira Belt is concordant with preexisting NE/SW dextral shear zones that could have supported the ascent of post-collisional magmatism after the Búzios orogeny (Schmitt et al. 2016, Martins et al. 2016).

CONCLUSIONS

Based on the presented investigation, our study of the post-collisional MC Granite indicates that:

• The MC granite is a homogeneous, nearly undeformed, tabular syenogranite that was emplaced at the final stages of continental amalgamation at the central Ribeira Belt, during the late Cambrian. U-Pb zircon ages yields a crystallization age of 496 ± 3 Ma;

• As MC has no evident field or petrographic association to basic/ultrabasic rocks and is classified as an evolved high-K calc-alkaline rock, it is reasonable to assume a major magmatic crustal source, which is stressed by the strong negative εNd value;

• MC exhibits negative correlations of Al₂O₃, CaO, MgO, Fe₂O_3(t) and TiO₂ along with positive correlation of K₂O with increasing SiO₂ contents, strong Eu anomalies, and Nb/Zr vs. Nb plots favoring the fractional crystallization process;

• The comparison of MC with other Cambro-Ordovician granitic intrusions of the central Ribeira Belt present similar geochemical characteristics. Based on geochemistry, morphology and geochronology we classify MC as belonging to the Nova Friburgo Suite;

• The age distribution of the main post-collisional magmatism in Central Ribeira Belt spans between 513 to 454 Ma and could be divided into three main pulses of magmatism: 515, 495 and 455 Ma.

ACKNOWLEDGEMENTS

F.R.A. Bione thanks the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the MSc. scholarship in Brazil. This work was mostly funded by the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) (grant #E-26/111.752/2010 - APQ1). The Programa de Apoio à Pós-Graduação (PROAP) is also acknowledged for financing part of the geochemical analyses. The authors also acknowledge the Centro de Tecnologia Mineral (CETEM) for all the support during sample preparations and two anonymous reviewers of the Brazilian Journal of Geology for their comments that improved the quality of the manuscript.

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ARTICLE INFORMATION

• 1
  Manuscript ID: 20190010.

Supplementary material

Supplementary data associated with this article can be found in the online version: Supplementary Table A1, Supplementary Table A2, Supplementary Table A3, Supplementary Table A4 and Supplementary Table A5.

Publication Dates

• Publication in this collection
  12 Sept 2019
• Date of issue
  2019

History

• Received
  19 Feb 2019
• Accepted
  24 May 2019