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Gondwana Research 23 (2013) 380–389

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Isotope geochemistry of Paleoproterozoic metacarbonates from Itatuba, Borborema Province, Northeastern Brazil: Evidence of marble melting within a collisional suture Roberto Ventura Santos a,⁎, Edilton J. dos Santos b, João Adauto de Souza Neto b, Luis Christian Montreuil Carmona c, Alcides Nóbrega Sial d, Luis Henrique Mancini a, Lauro Cézar Montefalco de Lira Santos a, Gilzênia Henrique do Nascimento b, Lucas UpaDafubigin Santos Mendes b, Emerson Marcello Ferreira Anastácio b a

Instituto de Geociências, Universidade de Brasília (UnB), CEP 70910-900, Brasília, DF — Brazil Dept. de Geologia, Universidade Federal Pernambuco (UFPE), 50.740-530, Recife (PE), Brazil c Dept. de Geologia, Centro de Ciências, Universidade Federal do Ceará (UFC), 60.455-780, Fortaleza (CE), Brazil d NEG-LABISE, Dept. de Geologia, UFPE, C.P. 7852, Recife, 50670-000, Brazil b

a r t i c l e

i n f o

Article history: Received 12 June 2011 Received in revised form 13 February 2012 Accepted 15 April 2012 Available online 5 May 2012 Handling Editor: E. Tohver Keywords: Carbon isotopes Paleoproterozoic Marbles Collisional suture Borborema Province

a b s t r a c t Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions. Both rocks may present similar texture and foliation to the host rock, or even cross cutting field relationships, which could be interpreted either as igneous or high-grade metamorphosed marble. Diagnostic criteria are even more complex when there is evidence of melting of the metasedimentary carbonate rock, such as has been described in the Himalayas and in the Eastern Ghats, India. In the Alto Moxotó Terrane, a high-grade gneissic domain of the Borborema Province, Northeastern Brazil, there are metacarbonates associated with banded gneisses and different metaplutonic rocks. Field evidence indicates the absence of other metasedimentary rocks associated with these marbles, thus suggesting that these carbonates were separated from other siliciclastic metasedimentary rocks. The presence of marble also suggests that it may represent the initial stage of a crustal carbon recycling into the mantle. These marbles present many field similarities to carbonatites (e.g., fluid-flow structure) and, together with metagranites and metamafic intrusions, may represent a major collisional tectonic suture. A detailed study of the carbon, oxygen and strontium isotopic composition of these marbles is presented. This study aims to identify the origin of the different isotopic components. It is argued that these rocks were subjected to temperature and pressure conditions that were sufficiently high to have melted them. The isotopic data presented here support this interpretation and indicate the mixing of two components: (i) one characterized by radiogenic Sr isotopes and mantle-like carbon isotopes, which is associated with the gneissic and mafic rocks, and (ii) another characterized by low 87Sr/86Sr ratios and highly positive δ13C values. Available geochemical data for the upper Paleoproterozoic indicate that the 87Sr/86Sr ratio of ocean water, varying between 0.7050 (2.25 ± 0.25 Ga) and 0.7047 (1.91 Ga), falls within the lower range of the samples from Itatuba and thus reinforces the interpretation that these marbles are sedimentary-derived and were partially contaminated by interaction with the host gneissic and mafic rocks. © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction Deformed carbonatites are rarely described in the literature because such igneous rocks in general occur in stable tectonic environments such as in old cratonic areas. Examples of linear and deformed carbonate-bearing rocks showing petrographic and geochemical appearance similar to carbonatites have been described in the literature and have been interpreted either as chemically modified marbles or as deformed carbonatites emplaced in mobile belts (Fourcade et al.,

⁎ Corresponding author. E-mail address: [email protected] (R.V. Santos).

1996; Lapin et al., 1999; Lentz, 1999; Burke et al., 2003; Attoh et al., 2007; Chakhmouradian et al., 2008). The problem is even more complex, because both intrusive carbonate rocks formed by melting or by tectonic extrusion of ductile carbonates, and deformed carbonatites may present similar textural features such as foliation and crosscutting field relationships with the host rock. Diagnostic criteria fail when there is no evidence of melting of the metasedimentary carbonate rock, such as those described in the Eastern Ghats, India (Bhowmik et al., 1995; Le Bas et al., 2002) and the Himalayas (Liu et al., 2006). There may be, however, an important link between carbonatites and marbles, because there is increasing evidence that carbon is recycled into the mantle by subduction. For instance, increase in

1342-937X/$ – see front matter © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2012.04.010

R.V. Santos et al. / Gondwana Research 23 (2013) 380–389

381

Fig. 1. Geologic scheme of the Borborema Province showing the main structures and trend of mafic rocks. PSZ = Patos Shear Zone; PESZ = Pernambuco Shear Zones; TAPT = Alto Pajeú Terrane; AMT = Alto Moxotó Terrane; RCT = Rio Capibaribe Terrane. Modified from Santos and Medeiros (1999).

strontium and lead isotopic ratios of carbonatite over time has been related to an increase in the radiogenic character of the carbonatite mantle source due to crustal recycling (Baker, 1996). Evidence of crustal carbon recycling into the mantle is also recorded in diamonds, which, in contrast to carbonatites, can preserve the record of different mantle regions as they grow (Cartigny et al., 1998; Thomassot et al., 2009). The Borborema Province, NE Brazil, is part of the West Gondwana Margin and is characterized by complex geological/geotectonic domains that evolved from the Archaean to the Neoproterozoic (Brito-Neves et al., 2000a,b; Van Schmus et al., 2011; Amaral et al., 2012). These subdomains are made of Paleoproterozoic basement and Neoproterozoic supracrustal rocks bounded by continental-scale shear zones (Vauchez et al., 1995; Brito-Neves et al., 2000a,b; Santos et al., 2002; Van Schmus et al., 2011). These terrains were welded together during the Neoproterozoic (Vauchez et al., 1995; Brito-Neves et al., 2000a,b; Sá et al., 2002), during which most of the Paleoproterozoic tectonic and deformational histories

were obliterated. In the Itatuba area, located in the Domínio da Zona Transversal of the Borborema Province, occur Paleoproterozoic metacarbonate rocks associated with retroeclogitized metamafic–ultramafic rocks and plagioclase-bearing orthogneisses (Bimodal Suite) hosted by a gneissic complex (Fig. 1) (Almeida et al., 1997; Carmona, 2006). These rocks are located within the Alto Moxotó Terrane, which has been interpreted as a major collisional Paleoproterozoic tectonic suture by Santos et al. (2010). In this paper, a detailed study of the carbon, oxygen, strontium and neodymium isotopic composition of these metacarbonates is presented. Previous investigation of C and O isotopes conducted by Carmona (2006) in various carbonate rocks of this area was inconclusive because the isotopic C and O data fall in a field between sedimentary carbonates and carbonatites. The new isotopic data indicate that these carbonates have been separated from the associated siliciclastic metasedimentary rocks, thus suggesting that it may represent the initial stage of a crustal carbon recycling into the mantle.

Fig. 2. Geological map of the Itatuba area, NE Brazil.

382


2. Geological setting The study area is situated in the Alto Moxotó Terrane (AMT), an important Paleoproterozoic fragment of the Borborema Province that consists of high-grade metaplutonic and metasedimentary rocks (Brito-Neves et al., 2000a,b; Van Schmus et al., 2011) (Fig. 1). In the Itatuba area, the dominant rocks are orthogneisses and migmatites of the Floresta Complex, in addition to minor metasedimentary rocks of the Sertânia Complex. The intrusive MalhadaVermelha bimodal suite is represented by metamafic–ultramafic rocks and plagioclase-bearing orthogneisses as well as by lenses of metacarbonate rocks that occur in minor amounts. The metamafic–ultramafic end member of the MalhadaVermelha bimodal suite often contains small concentrations of Fe–Ti ore (Santos and Medeiros, 1999; Santos et al., 2004; Carmona, 2006). The area has a main ENE–WSE foliation trend and has two main blocks separated by a contractional shear zone (Rodrigues, 2008). The NNW block is characterized by an amphibolite-rich unit including amphibole-biotite gneiss, biotite gneiss and stromatic migmatite with local occurrences of mafic retrograde eclogite (Almeida et al., 2009). The SSE block is essentially formed by a subordinate migmatite unit with occurrences of gneiss, amphibolite and pyroxenite with Fe–Ti ore.

Recent investigation has revealed that metacarbonates occur in the matrix of apolymictic breccia, are strongly deformed and are hosted by gneissic rocks (Fig. 2) (Carmona, 2006). They exhibit igneous features and intrusive field relationships, such as brecciated bodies, magmatic banding, and flow structures, similar to carbonatites elsewhere (Fig. 3A, B, D, E and G). The fragment dimensions vary from a few centimeters to over 15 cm long, and they are mostly metagranites in composition (Fig. 3F), anorthosite and metamafic rocks (Fig. 3C). Most fragments are from mafic and gneissic rocks from the host bimodal suite. Rocks of lamprophyric nature, alkaline gabbros and alkaline Fe-hastingsite syenogranites occur and are associated with the marbles, denoting a typical association of anorogenic magmatism. The absence of metasedimentary siliciclastic rocks associated with metacarbonate rock led previous studies to suppose that they were deformed carbonatites (Santos et al., 2007; Souza-Neto et al., 2009). Pyroxenite xenoliths and pyroxene aggregates are commonly found associated to the metacarbonate rocks. In general, these rocks are well-preserved and show limited evidence of reaction with the carbonate host rock. Millimetric to centimetric fragments of microcline, plagioclase and quartz are also common in the matrix and usually do not show evidence of metasomatism, thus reinforcing the low reactivity of the matrix as well as its strong

Fig. 3. Field features of Itatuba marbles showing the dimensions of the clasts (A, B and C), the flow structures (B and D) and the limited reactivity of the magma (C and F). Intercalation of marble and granite (E), and banding structures marked by concentration of silicate minerals (G).

R.V. Santos et al. / Gondwana Research 23 (2013) 380–389 Table 1 δ13C, δ18O and 87Sr/86Srdata for carbonate samples from the Itatuba region. Samples analyzed at UFPE (*) and at UnB (**). Sample δ13CVPDB δ18OVPDB (‰) (‰) ** * * * ** * * * * * ** ** ** ** ** ** ** ** ** ** ** ** * * ** * ** * * ** * * * * * * ** ** ** ** **

305-B 8.8 CC-106 7.7 CC-109 5.8 CC-11 5.5 Q5/1 CC-11B 7.2 CC-129 7.5 CC6.8 134A CC-226 6.1 CC-253 5.9 CC4.7 253B CC4.8 269A CC5.4 269B CC6.5 275A CC6.6 275B CC− 0.8 275C CC7.0 275E CC5.7 293A CC7.6 293B CC6.2 293D CC6.9 293F CC6.2 293G CC7.4 293H 2.1 CC298A CC2.4 298B CC2.9 298E CC-3 3.8 CC7.2 302A CC8.0 305B CC5.5 308A CC5.6 308B CC4.5 308C CC2.5 308E CC4.1 30A CC1.9 313A CC6.9 316A CC1.2 321A CC-324 6.1 CC5.5 324A-1 CC5.6 324A-2 CC5.5 324A-3 CC6.3 324B-1

87

Sr/ Sr

Sample

86

− 14.9 − 17.5 − 17.5 − 20.9

Metacarbonate Metacarbonate Metacarbonate Metacarbonate

− 25.0 − 17.9 − 19.0

Metacarbonate Metacarbonate Metacarbonate

− 29.7 − 19.1 − 17.8

Metacarbonate Metacarbonate Metacarbonate

− 19.1

0.70583 Metacarbonate with clasts of gneiss

− 20.2

0.70695 Metacarbonate

− 17.8

0.70497 Grayish metacarbonate

− 17.8

0.70432 Metacarbonate with clasts of gneiss

− 20.6


− 17.0

0.7061

Metacarbonate

− 14.9

Grayish metacarbonate

− 15.3


− 15.2

0.7046

− 15.4

0.70464 Banded metacarbonate

− 17.8


− 15.9


Grayish metacarbonate

− 21.2

Metacarbonate

− 18.7

Metacarbonate

− 17.7

Metacarbonate

− 18.7 − 15.6

Metacarbonate Metacarbonate

− 16.5

Metacarbonate

− 14.6

Metacarbonate

− 13.6

Metacarbonate

− 16.4

Metacarbonate

− 19.1

Metacarbonate

− 18.3

Metacarbonate

− 18.4

Metacarbonate

− 17.3

Metacarbonate

− 20.0

Metacarbonate

− 14.6 − 15.8 − 16.0

Metacarbonate 0.70538 Banded metacarbonate with millimetric clasts of granitic rocks 0.70528

− 16.0

0.7056

− 16.1

0.70498 Contact between the metacarbonate and a mafic fragment

383

Table 1 (continued) Sample δ13CVPDB δ18OVPDB (‰) (‰) ** CC6.2 324B-2 ** CC4.8 324B-3 ** CC4.2 324B-4 ** CC6.2 324C 5.7 ** CC324D ** CC6.3 324E * CC6.2 324G * CC4.6 324H * CC-38 3.7 * CC-3A 4.0 * CC-78 7.0 * CC-90 6.3 * CC-91 5.0 ** CC-93 − 3.3 * CC-94 7.7

87

Sr/ Sr

Sample

86

− 16.2

0.70512

− 16.3

0.70555

− 16.0

0.70597

− 15.7


− 15.8

0.7055

− 15.9

0.70508 Metacarbonate

− 16.3

Metacarbonate

− 16.0

Metacarbonate

− 17.8 − 18.7 − 13.2 − 17.8 − 16.5 − 17.3 − 15.8

Metacarbonate Metacarbonate Metacarbonate Metacarbonate Metacarbonate Metacarbonate Metacarbonate

Metacarbonate

crushed/cataclastic behavior. However, skarns are commonly found associated with the marbles or orthogneiss or as fringes around metabasic inclusions and syn-plutonic dikes within the metacarbonate. The carbonate matrix of the breccias is dominantly calcitic (45– 80%) and usually also contains diopside (5–30%) and olivine (0– 10%). Accessory minerals include hornblende, tremolite–actinolite, clinozoisite–epidote, phlogopite, apatite, allanite, titanite, hercynite, scapolite, garnet, opaque minerals and the occasional riebeckite, which is probably related to associated alkaline gabbros. Recent geochronological investigation (Santos et al., 2004, 2010) shows that the metacarbonates are Orosirian in age. U–Pb SHRIMP zircon data of the host metamafic–ultramafic and orthogneissic rocks vary from 2180 to 1953 Ma and their Nd TDM model ages are compatible with an intrusive juvenile episode cutting a gneissic– migmatitic complex of 2.35 Ga. Eclogite metamorphism affecting the intrusive suite is probably associated with a collisional episode that was redeformed by a late transcurrent episode and is documented by minor intrusive Brasiliano granites, well constrained by a U–Pb SHRIMP zircon age of ca. 587 Ma. 3. Sampling and analytical procedures Samples were taken from selected outcrops and, in general, are well preserved and show weak signs of weathering. Samples of carbonates for carbon, oxygen, and strontium isotopes were extracted using a hand-drill that made possible detailed sampling, including profiles across the contact between the marble and the host rock. A total of 57 samples were analyzed for C and O isotopes, and 23 samples for Sr isotopes. For oxygen and carbon isotopes obtained at the University of Brasília, Brazil, an aliquot of approximately 300 μg of each sample was placed in glass vials that were subsequently submitted to a He flush at 72 °C. After flushing, the aliquots were attacked with concentrated phosphoric acid, and the CO2 released was analyzed for carbon and oxygen isotopes in a Delta V Advantage connected to a Gas Bench II apparatus from the Geochronos Laboratory of the University of Brasília. Analyses of NBS 18 during the period of this study yield an average value of −5.11‰ for δ 13CVPDB and 23.06‰ for δ 18OVPDB. Another set of carbonate samples was analyzed at NEG-LABISE, Federal University of Pernambuco, Brazil, by using the conventional digestion method (McCrea, 1950). The δ 13CVPDB and δ 18OVPDBvalues were

384


Fig. 4. Detailed δ13C and δ18O in marbles and carbonate-bearing samples from Itatuba. Figures A, C, D, E and G are medium-grained marbles with millimeter-size silicate fragments. Figure B represents the transition contact between marble and a basic fragment, along which there is a marked variation of δ18O that is also shown in Fig. 8. Note the presence of gneissic fragments in Figures C and E and the texture of the large gneissic fragment shown in C. Figure F shows gneiss with disseminated carbonate.

measured on cryogenically cleaned CO2 (Craig, 1957) in a triple collector SIRA II mass spectrometer. The values obtained for the standard NBS-20 in a separate run against BSC internal standard yielded δ 13CVPDB = −1.05‰, and δ 18OVPDB = − 4.22‰, in close agreement with the values reported by the US National Bureau of Standards (−1.06‰ and −4.14‰, respectively). All carbon and oxygen isotopes are referred to VPDB. For the 87Sr/ 86Sr analysis, 50 mg of carbonate powder samples were weighed into Teflon beakers and digested in weak acetic acid to dissolve only the carbonate fraction and avoid leaching Rb from the non-carbonate constituents of the samples. 87Sr/ 86Sr ratios were measured using a Neptune, Thermo MC-ICP-MS, at the Geochronos Laboratory of the University of Brasília. Analyses of the NBS 987

standard conducted during the course of this work yielded an average value of 0.710230 ± 8 (1σ). Uncertainties in individual analyses are better than 0.01% (2σ). For Nd isotopes, whole-rock sample dissolution was conducted in Teflon Savillex beakers or Parr-type Teflon bombs. Sm and Nd extraction from whole-rock powders followed the technique of Richard et al.(1976), in which the separation of the REE as a group using cation-exchange columns precedes reversed-phase chromatography for the separation of Sm and Nd, using columns loaded with HDEHP (di-2-ethylhexeyl phosphoric acid) supported on Teflon resin. Sm and Nd samples were loaded onto Re evaporation filaments of a double filament assembly. Sm and Nd isotopic analyses were performed using a Finnigan MAT-262 multi-collector mass spectrometer in static


385

Fig. 5. δ18O versus δ13C for marbles from Itatuba. Note that most samples present δ13C values higher than 4‰.

mode. Uncertainties for Sm/Nd and 143Nd/ 144Nd ratios are considered to be better than ±0.1% (2σ) and 0.005% (2σ), respectively, based on repeated analyses of international rock standards BCR-1 and BHVO-1. 143 Nd/ 144Nd ratios were normalized to a 146Nd/ 144Nd ratio of 0.7129. Nd procedure blanks were smaller than 100 pg.

4. Results The isotopic values of carbon, oxygen and strontium are presented in Table 1 and in Figs. 4 to 7. In general, these isotope ratios exhibit a narrow variation range. For instance, most samples present δ 13C values ranging from 4‰ to 7‰, which are higher than most marbles derived from marine carbonates (Fig. 5). Only one sample presented a value near 0‰. In terms of strontium isotope ratios, the majority of samples were concentrated between 0.704 and 0.707. Two samples had isotopic ratios above 0.709. It is also evident in Fig. 6 that there is a negative correlation between the carbon and strontium isotopic ratios. Though less clear, a similar feature is also observed in δ 18O versus 87Sr/ 86Sr for the same samples (Fig. 7). It should be emphasized that the same sample that presents a high 87Sr/ 86Sr ratio (0.71056) also presents the lower δ 13C (−0.83‰) and δ 18O (−20.61‰ VPDB) values. Neodymium isotope data are presented in Table 2 and Fig. 8. Samples have TDM model ages varying between 2.24 and 3.15 Ga, thus indicating recycled materials with a quite heterogeneous source.

5. Discussion 5.1. Field relationships Carbonate rocks from Itatuba present features that suggest that they were emplaced as magma. Arguments that support this conclusion include (1) the presence of fluid-flow structures, some of which surround fragments within the carbonate rocks (Fig. 2A, B); (2) the brecciated nature of the carbonate rocks in which fragments of different rocks are all mixed together (e.g., gneiss, mafic rocks and silicate-bearing rock), thus revealing that they were transported for a significant distance; and (3) the convolute fold structure, some of which are dismembered, emphasizing the plastic behavior of the rocks. The above features resemble a carbonatite, which is a carbonate magma usually associated with breccias and explosive volcanism. Since Angico do Dias carbonatite (Silva et al., 1988; Antonini et al., 2003) and Hoggarfenites and carbonatites (Fourcade et al., 1996) have been discovered, Itatuba has been considered a strong candidate (Santos et al., 2002). However, a few attributes raise questions about this interpretation. Carbonatites are usually fluid-rich magmas that are accompanied by strong alkali metasomatism, which produces rocks known as fenites. In general, fenitization affects the carbonatite host rock and is often associated with the formation of Na- and Kbearing minerals, such as phlogopite, albite, aegirine, and other minerals. This process occurs even in deeply emplaced complexes, such as

Fig. 6. δ13C versus 87Sr/86Sr diagram suggesting the mixture of two main components: a radiogenic and low-δ13C component, and a less-radiogenic and high-δ13C component interpreted as closer to the primary carbonate.

386


Fig. 7.

87

Sr/86Sr versus δ18O for Itatuba marbles.

values as high as 7‰, as is the case of Itatuba. One should note, however, that metamorphic reaction between carbonate and silicate under high temperature conditions (>270 °C) could not explain the high δ 13C values observed in Itatuba. Under these conditions the CO2 produced by the reaction between these minerals is enriched in 13 C relative to the remaining carbonate, thus leading to a carbonate residuum with δ 13C values lower than the initial carbonate (Valley, 1986). Even for a marble derived from limestone, these carbon values are significantly high and have been reported only in carbonate rocks formed in specific sedimentary environments or during certain geologic time intervals. Considering that these rocks have an age of 2.2 Ga and that they were deformed at 1.9 Ga, it is argued that marbles from Itatuba may be derived from Paleoproterozoic limestones that preserved the positive carbon excursion of the Lomagundi event (Schidlowski et al., 1976; Schidlowski, 1988, 2001). Based on the field observations, it is argued that these rocks were subjected to temperature and pressure conditions high enough to be melted. The isotopic data presented here support this interpretation and indicate that the negative trend observed in Figs. 6 and 7 represents the mixing of two different reservoirs. One characterized by radiogenic Sr isotopes and mantle-like carbon isotopes that are associated with the gneissic and mafic rocks sampled by the magmatic marble; and another characterized by low 87Sr/ 86Sr ratio and highly positive δ 13C values as shown in Fig. 9. Available geochemical data for the upper Paleoproterozoic indicate that the 87Sr/ 86Sr ratio of ocean water varied between 0.7050 (2.25 ± 0.25 Ga) and 0.7047 (1.91 Ga) (Veizer et al., 1992). These values fall within the lower range of the samples from Itatuba, thus reinforcing the interpretation that these marbles are sedimentary derived rocks that were partially contaminated by interaction with the host gneissic and metamafic–ultramafic rocks. Hence, one reservoir would be Paleoproterozoic carbonates, here represented by sample CC-275A,

Jacupiranga, in which carbonatite dikes crosscutting the mafic host rock form a centimetric size reaction zone composed of phlogopite and carbonate layers (Santos and Clayton, 1995). Fenitization or any other kind of register of metasomatism is not evident in Itatuba. The contact between the molten carbonate and fragments of acid and basic rocks is usually very well preserved (Fig. 3C). Minerals that may be associated to fenitization, such as riebeckite, are commonly associated to intrusive alkaline granites. Carbonatite magmas are believed to have very low viscosity (Treiman and Schedl, 1983; Dobson et al., 1996). In general, breccias associated with carbonatite are related to explosive events and display angular fragments. In the case of breccias related to shattering, the fragments are usually monolithic. On the other hand, breccias related to the explosive movement of fluid or magma are usually polymict because they sample different host rocks along their way to the surface. In Itatuba, most breccias are matrix supported and present fragments of different rocks, thus indicating that they are polymict. However, there is no evidence of an explosive event, but of a slow moving magma that sampled fragments of the host rock (Fig. 8G). This observation is supported by the spherical to subspherical shape of most fragments within the carbonate magma and by the large amount of small fragments dispersed in the matrix (Fig. 4A, C and E). 5.2. Isotope and trace-element geochemistry The isotopic geochemistry of carbonate rocks from Itatuba may not be reconciled with the expected geochemistry of most carbonatites. Because of its highly enriched fluid phase, carbonatites are usually submitted to strong post-magmatic alteration, which significantly affects the primary isotopic composition of these rocks. Although carbonatites may have a wide range of carbon, oxygen and strontium isotope ratios, there is no reported carbonatite with δ 13C

Table 2 Sm and Nd isotope data for Itatuba samples. Sample

Sm (ppm)

Nd (ppm)

147

CC CC CC CC CC CC CC

0.614 1.794 5.76 3.076 2.326 1.405 2.246

2.806 8.544 18.251 11.362 12.073 8.313 9.354

0.1322 0.127 0.1908 0.1637 0.1165 0.1022 0.1452

11 275F2 275 B2 274 B8 274 B5 274 B6 274 B7

Sm/144Nd

143

Nd/144Nd (± 2SE)

0.511705 ± 8 0.511302 ± 14 0.512974 ± 7 0.512026 ± 16 0.511463 ± 17 0.511392 ± 12 0.511755 ± 27

εNd (0)

TDM (Ga)

Sample

− 18.21 − 26.06 6.54 − 11.94 − 22.92 − 24.3 − 17.22

2.5 3.08 – 3.15 2.47 2.24 2.88

Metacarbonate Metacarbonate Fragment of gneiss Metacarbonate Metacarbonate Metacarbonate Metacarbonate


387

Fig. 8. Nd evolution curves for Itatuba rocks indicating that these rocks are made of juvenile and older crustal materials.

and the other the continental crust itself. These diagrams also show that different samples from an outcrop may present quite distinct isotope ratios (e.g. CC-275B and E), indicating isotope heterogeneity within a meter scale. The Nd evolution curve for AMT samples of this work is presented in Fig. 7. The TDM values range between 2.24 and 3.08 Gy, thus indicating that these rocks were formed from reworked Paleoproterozoic and Archaean crustal materials. These evolution curves are quite similar to those reported by Santos et al. (2004), who also obtained Nd model ages ranging from Paleoproterozoic to Archaean for supracrustal rocks of the Sertânia Complex and Alto Moxotó Terrane. Trace-element geochemistry data of Itatuba metacarbonates were reported by Carmona (2006) in his doctorate thesis. Fig. 10 compares the REE patterns of these rocks with average carbonatite (Nelson et al., 1988) and crustal values (Gromet et al., 1984), showing that Itatuba rocks do not present the enrichment in light rare earths elements commonly observed in carbonatites. Fig. 11 compares trace-element data from Itatuba with carbonatites worldwide (Le Bas, 1999) and average composition of limestone (Locke and Butler,

1993) showing again that the metacarbonates are depleted trace elements compared to carbonatites. In general, samples from Itatuba have trace element patterns that resemble NAAS and PASC, presenting lower elemental concentrations. 5.3. Geotectonic implications The Borborema Province is composed of terranes with different ages and origins that were amalgamated during the Brasiliano Cycle (Santos, 1996; Brito-Neves et al., 2000a,b). The geological history of these terranes and their relationship with similar African terranes still remain unraveled. The Alto Moxotó Terrane is one of these terranes and is mostly composed of Paleoproterozoic rocks with a minor amount of Archaean blocks. Previous studies have proposed the existence of a suture zone in the region, which is in agreement with the occurrence of the metacarbonate magma described here. For instance, tholeiitic metavolcanic rocks have been described in AMT and interpreted as island arc basalts related to a suture zone (Beurlen et al., 1992; Almeida et al., 1997). More recently,

Fig. 9. Carbon, oxygen and strontium isotopic variation across a 5 cm contact between marble and a mafic fragment. Note the homogeneous behavior of the oxygen isotopes and the inverse behavior of the carbon and strontium isotopes. Picture at the right side shows the sampling location across the profile.

388


Fig. 10. REE of metacarbonates from Itatuba compared with average carbonatites reported by Nelson et al. (1988), as well as North American Sedimentary Composite-NASC (Gromet et al. 1984) and Post Archaean Australian Sediment (PAAS). The REE were normalized to chondrite using Boyton (1984) data.

petrographic/textural and field evidence led Almeida et al. (2009) to propose that metamafic rocks that outcrop in the same region of this study might have been metamorphosed under eclogite facies and later retrograded to amphibolite and greenschist facies. Previous studies (e.g., Toteu et al., 2001) that attempted to reconstruct the pre-drift layout of the South American (Trompette, 1997) and African continents place the Borborema province in Northeast Brazil adjacent to the Precambrian basement of Cameroon. These reconstructions are based on paleogeographic and geochronologic studies, which show the existence of comparable crust formation events (Van Schmus et al., 2011). They are also reinforced by the existence of eclogite outcrop on both sides of the Atlantic, such as those described by Almeida et al. (2009) and by Schenk et al. (2006), which mention the existence of these rocks in the Nyong Complex at the SW corner of the Central African Fold Belt.

mostly composed of gabbro, gneiss and anorthosite, thus indicating the absence of sedimentary-derived rocks. These clasts are immersed in a carbonate matrix that shows flow structures and presents limited reactivity with the silicate-bearing rocks. The carbon isotopic compositions of these rocks range between − 3.3‰ and + 8‰ and are distinct from those of carbonatites. Detailed centimetric scale sampling for isotope studies suggests a mixture of host rock and marble, where the former is more radiognenic and depleted in 13C and the latter less radiogenic and enriched in 13 C. It is argued that these metacarbonates were deformed in their way to a subduction zone that is probably along a Paleoproterozoic suture that has counterparts both in South America and Africa.

6. Conclusions

The authors would like to thank CNPq — Conselho Nacional de Desenvolvimento Científico e Tecnológico (National Council for Scientific and Technological Development) for the financial support granted to RVS (CNPq 482018/2007-0).

Metacarbonates from the Itatuba region are strongly deformed and associated with metamorphosed igneous rocks. Clast fragments are

Acknowledgments

Fig. 11. Trace-elements of Itatuba metacarbonates compared with carbonatites (Le Bas, 1999) and the average composition of limestone (Locke and Butler, 1993).


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