Arc accretion and crustal reworking from late Archean to Neoproterozoic in Northeast Brazil

New systematic Nd isotope and U-Pb geochronology data were applied to Precambrian rocks of northeastern Brazil to produce a crustal-age distribution map for a small basement inlier (1,500 km2). The results support episodic crustal growth with five short periods of crustal formation at ca. 2.9 Ga, 2.65 Ga, 2.25 Ga, 2.0 Ga, and 0.6 Ga. Based on the frequency histogram of U-Pb zircon ages and Nd isotopic data, we suggest that about 60% of the continental crust was formed during the Archean between 2.9 Ga and 2.65 Ga. The remaining 40% of crust was generated during the Rhyacian to Neoproterozoic (~2.0–0.6 Ga). This overall continental growth is manifested by accretionary processes that involved successive accretions surrounding an older core, becoming younger toward the margin. Strikingly, this repetitive history of terrane accretion show a change from lithospheric peeling dominated accretionary setting during the late Archean to a more, modern-day akin style of arc-accretion during the Proterozoic. Similar tectonic processes are observed only in large continental areas (>1,000,000 km2) as in the North American continent basement and in the Amazonian Craton.

migmatites in the central area display higher Th and K concentrations (Fig. 1D), followed by an abrupt reduction of these elements in the inner rim orthogneiss, and again high contents in the outer rim granite, reflecting distinct geological compartments from west to east. In addition, based on integrated analysis of structural pattern, ternary gamma-spectrometric map (Fig. 1D) and thorium anomaly map, we suggest that shear zone systems define major terrane boundaries. The Portalegre Lineament corresponds to a 20-40 km wide shear zone that separates the Rio Grande do Norte and Jaguaribe domains (Fig. 1C). The Paraú Lineament divides the west part of the Rio Grande do Norte domain into the distinct eastern Itajá and western Campo Grande blocks.
The Campo Grande Block consists of migmatitic gneisses that display multiple phases of partial melting 38 . These migmatites comprise Archean tonalitic gneisses that contain granitic Proterozoic leucosomes and alkali granite dikes. The mafic-ultramafic rocks comprise amphibolites and pyroxenites that are present as boudinaged bodies within the Archean migmatitic complex, which are further oriented parallel to the leucosomal layers of the host migmatites 39 . The overall outcrop pattern suggests that these mafic-ultramafic rocks were originally emplaced as dykes, intruding the host migmatitic gneisses. The ultramafic pyroxenites show relict cumulate texture, and re-equilibration to cummingtonite-grunerite-rich rocks, with varying proportions of chlorite, serpentine and magnetite. Amphibolites comprise massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase + clinopyroxene in symplectitic texture, typical of retrograded high-pressure rocks 39 . The Itajá Block is composed of Paleoproterozoic K-feldspar-rich orthogneiss, and wehrlite intrusions that occur as elongated boudins (<100 m) in the host orthogneiss; minor amphibolite and supracrustal rocks also appear. Neoproterozoic pegmatite and alkaline granite intrusions make up almost 20-30% of both blocks.
Spatial Pattern of Ages based on the Nd Evidence for Diachronous Crustal Accretion. The evolution of the Campo Grande Block involves at least seven thermal-tectonic events (Supplementary Table 1). The first magmatic event remains recorded in 2.98 to 2.91 Ga old tonalitic paleosome ( Fig. 2A), which constitutes the central core of the block. All zircon crystals from tonalite samples are prismatic (100 to 300 μm), with Th/U ratios from 0.125 to 0.583 and internal zonation ( Fig. 2A), all typical features of magmatic crystals 40 . The 2.9 Ga calc-alkaline magma represents a rare record of this age 41 , particularly in West Gondwana 42,43 . Inherited zircon cores of 3311 ± 52 Ma suggest a Paleoarchean crust as protolith source for the 2.9 Ga magmatism. The second partial melting event is represented by 2.65 Ga alkaline leucosome (e.g., ADE-23 sample) with thick K-feldspar-rich layers from the central portion of the strongly migmatized Archean core. Forty-four prismatic zircon crystals from this sample yield a Neoarchean Discordia age of 2651 ± 19 Ma. The 2.0 Ga and ca. 600 Ma zircon cores and rims are recorded in the migmatites. For example, the ADE-12 granitic migmatite sample yielded only 2.0 Ga prismatic zircon grains, while most of ca. 600 Ma Neoproterozoic ages are obtained in the overgrowth rims from the Archean migmatite zircon cores.
The clinopyroxene-garnet amphibolite lenses show the same 2.69-2.65 Ga age range, interpreted as the crystallization age of the protolith, also based on internal zonation, morphology and high Th/U ratio in zircon cores (Fig. 2B). The well-rounded (50 to 100 μm) zircon grains from amphibolite samples show zonation from core to rim, with well-defined rims, showing low to very bright luminescence (Fig. 2B), therefore indicating subsequent resorption and recrystallization 40 . The amphibolite samples also have 2.0 Ga well-rounded zircon crystals with zoned cores followed by outermost CL-bright overgrowths (Fig. 2B) possibly due to a subsequent event. The ca. 600 Ma Neoproterozoic homogeneous zircon grains, without internal zonation (Fig. 2B), are also recorded in the amphibolites. This confirms that the 2.65 Ga tholeiitic intrusions were subsequently torn apart during 2.0 Ga and 600 Ma tectonothermal events and are now present as isolated boudins. The 2.9 Ga inherited zircon grains were captured or assimilated by 2.65 Ga tholeiitic magma during its ascent and emplacement in the 2.9 Ga host tonalite basement 39 . Furthermore, as there are no fine-grained felsic veins intruding the amphibolite, it is unlikely that the 2.65 Ga zircon grains obtained in the mafic lenses came from other sources 39 .
The ultramafic pyroxenite lenses contain 2.7-2.6 Ga Archean and 2.3 Ga Paleoproterozoic zircon crystals with distinct morphological features like oscillatory zoning (Fig. 2C), typical of magmatic zircon 39 . Furthermore, pyroxenite samples present 2.0 Ga and ca. 600 Ma zircon grains that show varying degrees of rounding or absorbed borders (Fig. 2C), similar to features described in zircon grains from ultramafic rocks in high-grade metamorphic terrains 44,45 . The supracrustal rocks, such as garnet-biotite gneiss (AT-23 sample), bear 2.7-2.6 Ga zircon cores, suggesting that Neoarchean tonalite and tholeiitic rocks were the main provenance (Fig. 2D). The majority of these Archean zircon cores from the supracrustal sample display 2.2 to 2.0 Ga overgrowth rims (Fig. 2D). Besides, a few 2.46 to 2.44 Ga old zircon grains obtained in the Archean tonalite and supracrustal rocks suggest restricted Siderian magmatism.
In the eastern portion of the Campo Grande Block, the K-feldspar-rich alkali granite magmatism of 2.23-2.18 Ga Rhyacian age generated a large volume of magmatic rocks (Fig. 2E). Lastly, the K-feldspar-rich (20-30%) granitic plutons make up the western limit of the study area. The feldspar crystals develop a strong foliation parallel to the transcurrent shear zone. Neoproterozoic granites emplaced along the Portalegre shear zone (ADE-13 sample) have elongated prismatic zircon grains (3:1) that yield a crystallization age of 604 ± 12 Ma (Fig. 2F). On the other hand, granitic intrusions sampled in the central portion of the Campo Grande Block (e.g., AT-23 sample) show prismatic zircon crystals (2:1) crystallized at 566 Ma (Supplementary Table 1; Fig. 2F).
The T DM model ages and ε Nd(t) values of migmatite samples support a complex history for the study area (Table 1,  The pyroxenites display heterogeneous Nd isotopic data ( Table 1). The 2.6 Ga old pyroxenite samples display positive and negative ε Nd(t) values with T DM model ages between 2.6 and 3.2 Ga, whereas 2.3 Ga old pyroxenites show positive ε Nd(t) values with restrict T DM model ages of 2.29-2.37 Ga (Fig. 3E). The younger T DM model ages of 1.4 and 2.0 Ga with strongly negative ε Nd(t) values may suggest metamorphic alteration in the Sm-Nd isotopic  www.nature.com/scientificreports www.nature.com/scientificreports/ system during Proterozoic times (Figs. 3E, 2C). Furthermore, we suggest that the negative values of ε Nd(t) and older T DM for the 2.65 Ga ultramafic rocks may reflect enriched sources or crustal assimilation. The supracrustal protoliths have T DM model ages of 3.6 to 2.6 Ga with positive to negative ε Nd(t) values for the 2.65 Ga crystallization age and negative ε Nd(t) values (Fig. 3D) during Paleoproterozoic events. The 2.2 Ga K-feldspar-bearing augen orthogneisses display a Nd isotopic signature characterized by negative (-8.0) to positive (+5.0) ε Nd(t) values and T DM between 2.3 and 3.1 Ga (Fig. 3C), indicating a Rhyacian calc-alkaline magmatism with crustal reworking and juvenile sources contributions. Lastly, the Neoproterozoic granites present strongly negative ε Nd(t) values (-20.57 and -14.25) with relatively younger T DM model ages of 2.10 and 2.39 Ga (Table 1).
T DM model ages and ε Nd(t) values support a complex history for the Campo Grande Block (Fig. 3A-H). Whole-rock Nd isotope results indicate that the isotope system preserved the protolith source signature despite of crustal reworking and high-grade metamorphic events that affected the Archean core. Paleoproterozoic ages appear in the 1.95 Ga granitic leucosome generation and 2.0 Ga metamorphic overgrowth zircon rims on Neoarchean zircon cores from the ultramafic and supracrustal protolith rocks inside the Archean core. The Rhyacian orthogneisses from the eastern portion and 2.0 Ga granitic leucosome from the Archean central portions display similar T DM model ages and ε Nd(t) , meaning that both K-feldspar-rich alkaline magmatism and crustal anatexis have similar sources. Nevertheless, crustal reworking was intense in the eastern block area, practically obliterating the Archean protolith record. A second high-grade metamorphic event -the seventh recorded event -is indicated by 614-593 Ma old zircon grains and rims around the Archean zircon cores from the amphibolite samples 39   www.nature.com/scientificreports www.nature.com/scientificreports/ that the Paleoproterozoic crust is the main protolith source for the Neoproterozoic alkali granitic magmatism. That is, on the outermost overgrowths of the Archean dome the reworking process is dominant when compared to the core (Fig. 3G). The progressive decrease in T DM model ages from the core (3.7 Ga) towards the margins (2.1 Ga) of the block, integrated with structural, thorium anomaly map, and U-Pb zircon age patterns suggest accretionary processes for the continental growth (Fig. 3H). Thus, Nd isotope evolution reflects the crustal growth www.nature.com/scientificreports www.nature.com/scientificreports/ from the Archean core protolith, following extensive Paleoprotezoic juvenile accretion and reworking, as well as Neoproterozoic crustal magmatism at the outer rim.
In contrast, the Itajá Block only records two events of magma generation (Supplementary Table 2). The first event is represented by orthogneisses that were formed at 2.23 Ga (Fig. 2H), displaying negative to weakly positive ε Nd(t) values (Fig. 3F) and T DM model ages between 2.2 and 2.7 Ga (Table 1). Clinopyroxenites and wehrlites, crystallized at 2.19 Ga (Fig. 2G), with positive ε Nd (t) values (Fig. 3F), intruded these orthogneisses, indicating juvenile tholeiitic magmatism. Therefore, alkali granitic and ultramafic magmatism took place in a short time interval of ~40 Ma (2.23 to 2.19 Ga), similar to the reported events in the Lajes Block 44 , which is exposed 40 km eastwards, separated from the Itajá Block by the Neoproterozoic Seridó intracontinental fold belt (Fig. 1C,D). Furthermore, inherited zircon grains of Siderian age (ca. 2.32 Ga) are recorded in the host orthogneiss from the Itajá area. The intense Rhyacian reworking obliterated the possible older sources (Fig. 2F,G). Therefore, a genetic correlation with the Archean core of the Campo Grande Block is unclear (Fig. 3G). Nevertheless, it is indisputable that the protolith sources are dominantly Neoarchean, as suggested for the Lajes Block 45 . crustal Reworking and terrain Docking. The integration of all Nd isotope and U-Pb zircon age patterns allowed the establishment of limits and genetic correlations between the crustal fragments that form the Campo Grande and Itajá blocks (Fig. 4A-D). Our results support that 2.9 Ga and 2.7-2.6 Ga Archean crustal reworking and minor 2.2 Ga Paleoproterozoic juvenile mantle were the primary sources for the continental growth through accretionary mechanisms 5,15,16,19,45,46 . The first rim around the Archean core seems to engulf the core migmatites  www.nature.com/scientificreports www.nature.com/scientificreports/ in a circular shape (Fig. 4A-D). This geometry is feasible via a 2.9 Ga domal fashion of tonalitic magmatism that engulfed the Archean core. However, the subsequent events may have occurred due to terrane accretionary mechanisms. Therefore, our results may indicate a change in the mechanism of continental evolution, namely dome formation at 2.9 Ga to terrane accretion starting at 2.7 Ga.
Based on the frequency histogram of the U-Pb zircon ages and the area mapped, it is suggested that at least 30-40% of the Campo Grande Block was already formed at 2.9 Ga (Fig. 3G, H). After 2.9 Ga, there was an increase in the rate of continental crust growth, probably due to subduction-like processes and peeling-off driven convergent settings 12,13,20 . Therefore, the accretionary orogenic collage derived from a complex diversity of protolith sources 47,48 , as described in this study (Fig. 4A-F). That is, the continental evolution is complex and includes several components of different scale, composition, and age 10,15,47,48 . Thermal and compositional contrasts between continental and oceanic lithosphere lead to subsidence processes by plate tectonics 49,50 . The subsidence of oceanic crust allowed the efficient mechanical coupling of the microcontinents and remnant magmatic arcs in the orogenic wedge 2,3 . In this scenario, magmatic arc formation is probably the most important mechanism to maintain the continental crust reservoir 18,19 . Paleoproterozoic 2.25-2.18 Ga high-K calc-alkaline magmatism may represent a thermal weakening zone that allowed the reworking and juvenile magmatism 11,18 . In the Borborema Province, Paleoproterozoic arc magmatism represents a more significant period of crustal growth within the South American continent 19 , similar to the study area. Thus, terrain accretion and partial melting mainly in the root of the magmatic arc setting from 2.2 Ga promote the differentiation and growth of the continental crust 5,15,51 .
The preservation of the felsic continental block between 2.9 to 2.2 Ga in the Borborema Province may mark the transition and initiation of plate tectonics, implying a higher consumption of mafic crust during Proterozoic physical mechanisms of accretion compared to late Archean processes. One possibility would be crustal reworking via lower mafic crustal peeling-off (e.g. delamination) during continent-continent convergence 15,16 . Despite the significant increase in isotopic studies, late Archean reworking and recycling processes remain largely unknown 15,16 . Therefore, a different style of plate tectonics and subduction possibly occurred during the early Archean, with transitional physical mechanisms between the late Archean and the Phanerozoic-style. However, any model that calls upon fractionation of a single magmatic event or process to produce continental crust is unrealistic 51 .

conclusions
Nd isotope data and U-Pb geochronology within the distinct terrains provide constraints for the succession of magmatic and metamorphic phases that resulted in continental accretion of heterogeneous rocks from 2.9 Ga to ca. 566 Ma ago in northeast Brazil. These led to the assembly of the Rio Grande do Norte domain. The Campo Grande Block represents high-grade metamorphic terrains with multiple partial melting, meta-ultramafic, and metamafic lenses that record polyphase metamorphism, magmatism, and intense shearing. Our data bear evidence that the distribution and nature of the continental crust reflect the secondary processes of reworking. The age succession associated with the geochemical patterns of the Precambrian evolution of the Campo Grande Block highlights the importance of the accretionary dynamics for the continental growth. The accretionary process is cyclic and repeated in space and time, allowing the continental growth to start by Mesoarchean to Neoarchean crustal peeling-off driven lithospheric convergence to Proterozoic magmatic arc accretion. When the events ended at the Neoproterozoic (ca. 566 Ma), the Archean to Paleoproterozoic Campo Grande and Rhyacian Itajá complexes amalgamation in the center of West Gondwana was concluded.

Methods
Geological Mapping and Petrography. Geological mapping was undertaken in the Campo Grande area with the purpose of investigating the gneiss-migmatite complex. Geological mapping was supported by geochemical, geophysical and petrographic investigations. Systematic thin sections cut relative to foliation were obtained from representative samples from outcrops of migmatite, orthogneiss, ultramafic and supracrustal rocks. The petrography was done at the Microscopy Laboratory of the Institute of Geosciences of Universidade de Brasília (Brazil). U-pb isotopes. Zircon grains from samples were separated by conventional procedures and magnetic separator after concentration by hand panning. U-Pb isotopic analyses were performed on zircon grains using a Thermo-Fisher Neptune High Resolution Multicollector Inductively Coupled Plasma Mass Spectrometer (HR-MC-ICP-MS) coupled with a Nd:YAG UP213 New Wave laser ablation system at the Laboratory of Geochronology of Universidade de Brasília. U-Pb analyses on zircon grains were carried out by the standard-sample bracketing method 52 , using the GJ-1 standard zircon 53 in order to quantify the amount of ICP-MS fractionation. The tuned masses were 238, 207, 206, 204 and 202. The integration time was 1 second and the ablation time was 40 seconds. A 30 µm spot size was used and the laser setting was 10 Hz and 2-3 J/cm 2 . Two to four unknown grains were analyzed between GJ-1 analyses. 206 Pb/ 207 Pb and 206 Pb/ 238 U ratios were time corrected. The raw data were processed off-line and reduced using an Excel worksheet 54 . During the analytical sessions, the zircon standard 91500 55 was also analyzed as an external standard.
Common 204 Pb was monitored using the 202 Hg and ( 204 Hg + 204 Pb) masses. Common Pb corrections were not done due to very low signals of 204 Pb (<30 cps) and high 206 Pb/ 204 Pb ratios. Reported errors are propagated by quadratic addition [(2SD 2 + 2SE2)1/2] (SD = standard deviation; SE = standard error) of external reproducibility and within-run precision. External reproducibility is represented by the standard deviation obtained from repeated analyses (~1.1% for 207 Pb/ 206 Pb and up to ~2% for 206 Pb/ 238 U) of the GJ-1 zircon standard during the analytical sessions, and the within-run precision is the standard error calculated for each analysis. Concordia