Geological and isotopic characteristics of granites from the Western Pernambuco-Alagoas Domain : implications for the crustal evolution of the Neoproterozoic Borborema Province Caracterização geológica e isotópica dos granitos do Domínio Pernambuco-Alagoas Oeste : implicações na evolução crustal da Província Borborema

Manuscrito ID: 30073. Recebido em: 28/12/2013. Aprovado em: 07/10/2014. ABSTRACT: The Western Pernambuco-Alagoas Domain is a complex tectonic domain of the Southern part of the Neoproterozoic Borborema Province. It borders the Northern margin of São Francisco Craton. U-Pb and Sm-Nd data discussed in this work show that, in the Western Pernambuco-Alagoas Domain, large volumes of granitic rocks of various ages related to different tectonic events are recognized. The Cariris Velhos Event is represented by Lobo (974 ± 8 Ma) and Rocinha (956 ± 2 Ma) orthogneisses; whereas Paleoproterozoic and Archean basement rocks are represented by Fulgêncio Orthogneiss (1,996 ± 8 Ma), Riacho Seco Gneissic-migmatitic Complex (1,992 ± 27 Ma), and orthogneisses of Entremontes Complex (2,734 ± 11 Ma). Six groups of granitoids are recognized: Brasiliano granitoids (Ediacaran-Cryogenian), with Mesoto Paleoproterozoic model ages; Cariris Velhos granitoids (Tonian) yielding Mesoproterozoic model ages; Paleoproterozoic granitoids from Pernambuco-Alagoas Domain, with Neoarchean to Paleoproterozoic Nd model ages; Paleoproterozoic granitoids from Riacho Seco Nucleus, represented by Riacho Seco Gneissic-migmatitic Complex, with Archean model ages; Archean granitoids represented by the rocks of Entremontes Complex; and granitoids emplaced in the São Francisco Craton displaying Paleoproterozoic to Archean model ages. Prominent among the new data obtained is the Paleoproterozoic age of Riacho Seco Gneissic-migmatitic Complex, which is formed by Archean basement reworking. The new data reveal the presence of Paleoproterozoic and Archean basement inliers and a large volume of Cariris Velhos granitoids in the Western Pernambuco-Alagoas Domain. The orogen, therefore, involved extensive reworking of older blocks, possibly including parts of São Francisco-Congo Craton.


INTRODUCTION
The Borborema Province (Almeida et al. 1977), in Northeastern Brazil, underlies an area of approximately 450,000 km 2 , corresponding to the Western portion of the extensive Brasiliano-Pan African orogenic system formed by a convergence of the West Africa/São Luis and San Francisco-Congo Cratons (Fig. 1). The study area is located in the Pernambuco-Alagoas Domain (PEAL) and its subdomain, the Riacho Seco Nucleus (RSN) in the Southern Borborema Province. The PEAL is located between the E-W Pernambuco Lineament to the North and the Sergipano Belt, and São Francisco Craton to the South. This is one of the crustal blocks comprising the Southern Subprovince of Van Schmus et al. (2011) or the Southern Domain of Brito Neves et al. (2000), which are part of the Neoproterozoic orogenic system along the Northern margin of São Francisco Craton.
The subdomain RSN was previously considered a small Archean block within the younger PEAL. The correlation between PEAL with other domains of the Borborema Province is still unclear, as well as the ages of their main units and tectonic events recorded in its rocks. In this study, we present U-Pb zircon ages, Nd isotopes, and geological and geochemical data of metagranites from the PEAL. Five rock units were studied and dated by U-Pb zircon dating, using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS): Entremontes Complex, Fulgêncio orthogneiss, Riacho Seco Gneissic-migmatitic Complex, Lobo and Rocinha orthogneisses. The types of analysis that each geologic unit were subjected are summarized in Tab. 1. Some isotope analyzes of the granitoids from São Francisco Craton were performed to compare with the bodies studied in the PEAL. The new data clarified several aspects of the geology of the western segment of PEAL and its role in the tectonic evolution of the Borborema Province along the Northern margin of São Francisco Craton.

ANALYTICAL PROCEDURES
Several outcrops were described and samples were collected, some of them were selected for petrographic, geochemical, and isotopic studies (Tab. 1). The most representative bodies to understand the geological setting were subjected to more detailed studies. The methodology of this study is explained as follows: ■ The petrographic analysis helped defining the composition and metamorphic grade of the main units. ■ Major and trace element contents were determined to establish the main group of granitoids (Entremontes Complex, Fulgêncio and Rocinha orthogneisses). Data obtained in this study were added to the geochemical ones of Salgueiro and Parnamirim Brazil Geological Service (CPRM) maps. All samples were prepared (by fusion with lithium metaborate and aqua regia digestion), and their major and trace element analyses performed by SGS Laboratories Geosol Ltda., using ICPMS technology. ■ The isotopic study was the main goal of the investigation, with analyses of whole rock (Sm-Nd) and zircon grains (U-Pb). Zircon concentrates were separated from approximately five-kilogram samples. The concentration of heavy minerals was carried out by panning, which was followed by magnetic separation and final selection under a binocular microscope. Zircon grains were arranged in mounts made of epoxy resin that were polished to expose inside the grains. They were imaged using a scanning electron microscope (SEM) equipped with cathode luminescence (CL) and backscattered electron (BSE) detectors in order to select the best spots for mineral analyses. The U-Pb evaluations were carried out with a Thermo-Finnigan multi-collector LA-ICP-MS installed at the Geochronology Laboratory of University of Brasília (samples RF-111,  and at the Laboratory of Isotope Geology of the Federal University of Rio Grande do Sul (samples RF-179 and RF-323). Ages were calculated using the ISOPLOT 3.0 software (Ludwig 2000). Twenty to 40 spot analyses were carried out for each sample. The Sm-Nd isotopic analyses were performed at both mentioned university laboratories and both followed the method described by Gioia and Pimentel (2000). In this procedure, approximately 50 mg of the sample is mixed with a powdered tracer solution ( (Trompette 1994). In paleogeographic reconstructions, the province extends from Central and Northeastern Brazil to West Africa through the Neoproterozoic orogenic areas of Cameroon, Nigeria, Niger, Algeria, Togo, and Benin ( Fig. 1). This Province includes a complex system of high-temperature crustal scale shear zones that separate the province into tectonic domains, and they also control the emplacement of granites during the Brasiliano/Pan-African orogeny (Caby et al. 1991;Jardim de Sá 1994;Vauchez et al. 1995). The province has been studied for decades, and different authors developed evolution models and configuration of their different domains (Almeida et al. 1981;Brito Neves 1983;Brito Neves et al. 1982Jardim de Sá 1994;Sial 1986;Neves 2003;Santos et al. 2010;Van Schmus et al. 1995Ganade de Araújo et al. 2014. In this paper, the internal divisions of the Borborema Province used are based on models put forward by Van Schmus et al. (1995, 2008, Brito Neves et al. (2000) and Santos et al. (2000Santos et al. ( , 2010. They subdivided the province into three sub-provinces (Fig 1) The Northern Sub-province is exposed to the North of Patos Shear Zone, and consists mainly of Paleoproterozoic basement (including some Archean nuclei) partially covered by Neoproterozoic supracrustal rocks and intruded by Brasiliano plutonic rocks. The Central Sub-province is exposed between Patos and Pernambuco shear zones, and it is mainly characterized by the NE-SW to EW transcurrent faults system. The Southern Sub-province, confined between Pernambuco Shear Zone and São Francisco Craton, includes Sergipano and Riacho do Pontal supracrustal belts and Pernambuco-Alagoas Domain, which can be divided into Western and Eastern segments, separated by Tucano-Jatobá Cretaceous sedimentary basin (Angelim & Kosin 2001), as seen in Fig. 1.

Archean and Paleoproterozoic rock units
Archean rock units are not widespread in Borborema Province and they form small inliers or nuclei among Paleoproterozoic terrains, such as São José do Campestre Massif comprising rocks with U-Pb zircon ages between 3.4 and 2.7 Ga (Dantas et al. 1998;2004); Troia-Pedra Branca Block, where orthogneisses of Cruzeta Complex gave U-Pb zircon ages between 2.85 and 2.64 Ga (Fetter 2000); and Entremontes Complex with orthogneiss dated by U-Pb zircon data as 2.734 ± 11 Ga (Cruz 2013). Paleoproterozoic basement in the Borborema Province includes rocks formed during three major events, with ages at 2.35, 2.15 and 2.0 Ga (Dantas 1997;Neves et al. 2006;Souza et al. 2007). On the African counterpart (Congo Craton), Lerouge et al. (2006) have reported SHRIMP U-Pb zircon ages of 2,066 and 2,044 Ma for the emplacement of granitoids and of ca. 2050 and 1985 Ma for metamorphic events. These ages from African and South-American counterparts represent a segment of the Eburnean-Trans-Amazonian orogeny that resulted in the formation of a possible Paleoproterozoic supercontinent.
The Cariris Velhos Event (Santos 1995) was originally defined as an orogenic event forming an extensive metamorphic belt, inserted within Alto Pajeú Domain (Brito Neves et al. 1995;Santos et al. 2010). Other authors (Bittar 1998;Neves 2003) argue, however, that Cariris Velhos Event represents an extensional event associated with continental rifting, which formed rocks dated between 1.0 and 0.9 Ga. This latter interpretation was based on the geochemical nature of volcanic and plutonic rocks, as well as on the absence of metamorphic zircon ages in the range between 1.0 and 0.9 Ga.

Brasiliano Orogeny
The Brasiliano Orogeny (650 -500 Ma) affected the whole Borborema Province and was responsible for the regional deformation and medium-to high-grade metamorphism, for the generation and emplacement of large volumes of granite throughout the province, and also for the development of continental-scale transcurrent shear zones, which is one of the main characteristics of the province. Two main models have been used to describe the geodynamic evolution of Borborema Province during the Neoproterozoic. In one of them, the Borborema Province is considered the product of an accretionary event involving the collage of several allochthonous terrains Santos et al. 2004). The other model suggests an intra-plate setting in which reworking of pre-existing Archean-Paleoproterozoic crust and deposition of Proterozoic volcano-sedimentary sequences took place in an intra-continental environment (Neves 2003;Neves et al. 2014;. A new model proposed by Ganade de Araújo et al. (2014), based on some aspects of the mentioned ones, suggests that the Borborema Province underwent two major collisional events, one between 620 and 610 Ma and another between 590 and 580 Ma.

Brasiliano magmatism
The Brasiliano magmatism is symbolized by a series of granitoids with different types and dimensions. Most of the Neoproterozoic granite genesis is coeval with the development of large shear zones, which exerted strong tectonic control on the emplacement of the granitoids (Caby et al. 1991;Vauchez et al. 1995). The older Neoproterozoic plutons (640 -610 Ma) are more strongly deformed, suggesting the pre-to syn-tectonic nature of intrusions. Younger plutons (590 -570 Ma), on the other hand, are mostly undeformed and are therefore considered to be late-to post-tectonic intrusions. All schemes of classification of the Neoproterozoic granitic magmatism in NE Brazil (e.g. Almeida et al. 1977;Sial 1986;Guimarães et al. 1998)

Western Pernambuco-Alagoas Domain
The PEAL is an East-West oriented tectono-stratigraphic block of the Southern part of Borborema Province (Van Schmus et al. 2008). It was originally defined as a massif formed by Archean to Paleoproterozoic gneisses and migmatites, intruded by Brasiliano granitic bodies (Brito Neves et al. 1982). PEAL (Fig. 2) is now considered a complex tectonic domain including units of various ages (Van Schmus et al. 1995;Oliveira et al. 2006;Cruz 2013;Cruz et al. In press). Two main units in the Western segment of the domain are recognized (Santos 1995;Medeiros & Santos 1998;Medeiros 2000): Cabrobó Complex comprising metavolcanosedimentary and metasedimentary sequences metamorphosed under amphibolite facies conditions, with local migmatites, and Belém do São Francisco Complex formed mainly by granitic-granodioritic gneisses and migmatites with remnants of supracrustal rocks and mafic lenses. U-Pb dating on zircon of granodiorite gneiss of Belém do São Francisco Complex indicated the age of 2,074 ± 34 Ma .
The metamorphic complexes of PEAL are intruded by variably deformed pre-to post-tectonic Neoproterozoic plutonic rocks. Recent U-Pb zircon data produced during mapping projects of the Brazilian Geological Survey, as well as by the present work suggest that Belém do São Francisco Complex may be divided into different units: Neoarchean Entremontes Complex, Orosirian Fulgêncio Orthogneiss, and Lobo and Rocinha orthogneisses related to Tonian Cariris Velhos Event. Besides the wide range of ages found in PEAL rocks, the structural complexity of the area is also noteworthy with thrust sheets in contact with São Francisco Craton and Sergipano and Riacho do Pontal belts. Transcurrent zones along the contact with the Transversal Zone to the North are recognized.
The RSN (Santos 1995;Angelim & Kosin 2001) has been considered the exposure of older (Archean) rock units enclosed within PEAL. It may be subdivided into two major units (Cruz et al. In press): Riacho Seco Gneissic-migmatitic Complex that is made up of gneissic rocks, generally migmatized, including remnants of medium-to high-grade supracrustal rocks, and Riacho Seco Metasedimentary Complex represented by schists and gneisses of medium to high metamorphic grade, calcium-silicates rocks, and marbles. A Rb-Sr isochron on migmatites indicates an age of 2.9 Ga for these rocks (Mascarenhas & Garcia 1989), whereas a Sm-Nd (T DM ) model age for Riacho Seco orthogneisses is ca. 3.1 Ga (Angelim & Kosin 2001). Both ages suggest an overall Archean age for the RSN rocks. Such nucleus is partially surrounded by the Cabrobó Complex along a contractional shear zone. In the South, it is in contact with São Francisco Craton through the sinistral transcurrent Riacho Seco Shear Zone.

Entremontes Complex
The Entremontes Complex (Cruz 2013) consists mainly of granitic and minor granodioritic gneisses, as well as of some local amphibolite outcrops. The petrographic analyses of the granitic gneiss show that the main mafic mineral is amphibole (hornblende and hastingsite), followed by biotite, and minor garnet and clinopyroxene (hedenbergite). Migmatism and compositional banding are common, as well as milonitic structures as mineral stretching. The metamorphic conditions reached the medium grade (recrystallization of K-feldspar+biotite+amphibolite), with retrogression to lower grade (biotite recrystallized). The geochemical analyzes resulted in high SiO 2 contents (74 to 70%), as   (Boynton 1984), the patterns show fractionation between HREE and LREE and negative Eu anomalies (Fig. 4A). The primordial mantle normalized spidergram (Wood et al. 1979) presents negative anomalies of Ba, Nb, Ta, U, and Sr as well as strong negative Ti anomaly and marked enrichment in Tb and Y (Fig. 4D), some of these characteristics are also viewed in the ORG normalized spidergram (Pearce 1984), as seen in Fig. 4G. In Pearce's (1994) diagram ( Fig. 5A), samples fall in the post-collisional to intra-plate field and according to the criteria (A1/A2) established by Eby (1992), they fall in the WPG+ORG and A2 field, indicative of A-type granites derived from mixed sources (Figs. 5B to 5D). The Entremontes Complex, as indicated by chemical data, is related to A-type magmatism most likely in a post-collisional setting.

Paleoproterozoic granitoids
Paleoproterozoic granites of PEAL in the studied area include granitic migmatites and orthogneisses found in the vicinities of Orocó, Pernambuco State (Orocó Migmatites), and Fulgêncio Orthogneisses; those from RSN comprising granitic migmatite exposed along São Francisco River near Orocó city, Pernambuco state (Caraíbas migmatites), and Riacho Seco Gneissicmigmatitic Complex; and São Francisco Craton granitoids including a pink alkaline migmatite near Riacho Seco and a biotite gneiss with sillimanite and garnet found near Santa Maria da Boa Vista, Pernambuco.

Fulgêncio Orthogneiss
This is a porphyroblastic coarse-grained migmatitic orthogneiss that contains discontinuous bands of concentrations of feldspar augen. In outcrops located to the South, the augen texture predominates, and structures like banding are scarce. Other structural features are shear bands with predominant dextral sense, mineral stretching lineation oriented North to South and low-to medium-angle foliation dipping to the West. Biotite is the main mafic mineral, accessory minerals are apatite, titanite, zircon, allanite, and opaque minerals. The recrystallization of K-feldspar and presence of metamorphic amphibole indicate upper greenschist to amphibolite facies, with retrogression to greenschist facies indicated by biotite recrystallization. An additional petrographic facies of these orthogneisses is found at the NW portion of the exposure area, consisting of fine-grained banded rocks. Inter-fingering of the two textural facies was found in some outcrops.
The geochemical analyzes show SiO 2 contents ranging between 65 and 74% (Tab. 3) and present high-K calcalkaline and metaluminous to peraluminous nature (Figs. 3A to 3C). The REE diagram (Boyton 1984), as seen in Fig. 4B, shows strong fractionation between LREE and HREE and negative Eu anomalies. In the primordial mantle-normalized spidergram (Wood et al. 1979), as in Fig. 4E, negative anomalies of Ba, Nb, U, Ti and Y are observed, some of them are also seen in the ORG normalized spidergram (Pearce et al. 1984), Fig. 4H, suggesting the presence of subduction-related metasomatized mantle in their genesis. Very low concentrations of Sr indicate a marked fractionation of plagioclase in the original magma. In the tectonic discrimination diagram of Pearce et al. (1994), Fig. 5A, these samples correspond to post-collisional granitic composition, close to the boundary between syn-collisional and volcanic arc granite fields. In Eby (1992) diagrams, the samples demonstrate characteristics of late-to post-collisional tectonic setting with a probably mixed magma source (Figs. 5B to 5D).

Riacho Seco Gneissic-migmatitic Complex
This complex consists of orthogneisses with remnants of supracrustal rocks. It comprises the basement rocks of RSN, in this subdomain the supracrustal sequence is represented by Riacho Seco Metasedimentary Complex. The main rock types identified are reddish granitic orthogneiss with biotite and subordinately granodioritic orthogneiss with titanite and hornblende. They show subtle to well-developed compositional banding by different proportions of mafic minerals. Some outcrops are migmatitic locally mafic gneisses and amphibolite lenses are also present. Mineral textures (K-feldspar porphyroblasts) and paragenesis (recrystallized K-feldspar+biotite+amphibole) seen in the felsic gneisses suggest an amphibolite-facies metamorphism. The garnet-diopside-amphibole paragenesis, locally recognized, suggests that the unit might have reached the granulite facies with subsequent retrogression to amphibolite/upper greenschist facies.

Lobo Orthogneiss
L o b o O r t h o g n e i s s d e f i n e d b y B r i t o a n d Freitas (2011) Figure 4. Multi-elemental diagrams: (A to C) REE diagrams (Boynton 1984); (D to F) Expanded diagrams normalized by primordial mantle (Wood et al. 1979); (G to I) Expanded diagrams normalized by ORG (Pearce et al. 1984) in order -Entremontes Complex, Fulgêncio Orthogneiss, and Rocinha Orthogneiss samples.
garnet-muscovite-biotite granitic to granodioritic orthogneisses, locally containing K-feldspar augens and mafic xenoliths. They exhibit low angle foliation, with mineral stretching lineation generally oblique or perpendicular to the foliation plane. Metamorphic conditions were of the upper greenschist facies indicated by the presence of K-feldspar and biotite recrystallized and garnet. Compositionally, it is metaluminous to peraluminous and of medium-K calcalkaline nature.

Rocinha Orthogneiss
Rocinha Orthogneiss established by Cruz and Accioly (2013) consists of coarse-to medium-grained porphyroclastic to porphyroblastic granite, and the main mafic mineral is biotite, commonly displaying mylonitic features and metamorphosed under upper greenschist-to amphibolite-facies conditions indicated by deformation and recrystallization of K-feldspar and biotite. The geochemical analyzes show high SiO 2 (Tab. 4) contents (77 to 68%), K calc-alkaline to alkaline nature and metaluminous to slightly peraluminous chemical characteristics (Figs. 3A to 3C). Patterns in the primordial mantlenormalized (Wood et al. 1979) multi-element diagram (Fig. 4F) presents negative Nb anomalies, characteristically found in magmas sources modified by subduction, and also pronounced negative Sr and Ti anomalies and Tb and Y enrichment, a Ta-Nb depletion is observed in the Ocean Ridge Granites (ORG) -normalized spidergram (Pearce et al. 1984), as in Fig. 4I. The chondrite-normalized REE diagram (Boynton 1984), Fig. 4C, reveals flat HREE patterns and strong negative Eu anomalies, which are features similar to those of rocks formed in intra-plate settings. In the trace element diagrams of Pearce (1996) and Eby (1992), Figs. 5A to 5D, these rocks are similar to post-collisional/intra-plate rocks and A-type (A2) granites. The chemical signatures of Rocinha Orthogneiss suggest that the original magmas were formed in a post-collisional setting.

Brasiliano granitoids
Some additional granite bodies correlated to the Brasiliano Cycle were investigated in this study: Cryogenian-Ediacaran intrusions, among these are the equigranular medium-to fine-grained with muscovitebiotite or biotite-amphibole migmatitic granitic body with metasedimentary xenoliths exposed in Ibó, in Bahia state (Ibó migmatites); and the grey amphibole-biotite granitic augen-gneiss of Chorrochó and Abaré, Bahia state (Chorrochó augen-gneiss).

U-Pb Data
New U-Pb data were acquired for this investigation. The results were used in combination with data obtained for the Salgueiro and Parnamirim mapping projects carried out by the Brazilian Geological Survey (CPRM) and they are available at http://geobank.sa.cprm.gov.br.

Entremontes Complex
The zircon grains from the sample of amphibole -bearing granitic gneiss dated in this study (RF-179 -Tab. 5) are prismatic with rounded edges, and they have an average aspect ratio of 3:1 (Fig. 6A) Pearce (1996); (B and C) by Eby (1992). Discriminate diagram of A-type granitoids (Eby 1992 Table 3. Geochemical data from Fulgêncio Orthogneiss.    zircon are homogeneous and these were analyzed with spots concentrated in the cores. The U-Pb analyses indicate a crystallization age of 2,734 ± 11 Ma (Fig. 6B).

Fulgêncio Orthogneiss
The investigated sample is a granitic gneiss (sample RF-243), as in Tab. 6. The BSE and CL images show abundant zircon crystals with concentric oscillatory zoning and thin (between 10 to 50 mm) overgrowths (Figs. 7A to 7C). The U-Pb results indicated an age of 1,996 ± 8 Ma (Fig. 7D). No significant differences were found between the ages of core and rims of zircons crystals analyses. Fulgêncio Orthogneiss has, therefore, a Paleoproterozoic granite protolith, which is slightly younger than others generated during Paleoproterozoic orogenesis within Borborema Province.

Riacho Seco Gneissic-migmatitic Complex
Riacho Seco Gneissic-migmatitic Complex sample is a medium-grained equi-granular biotite granite-gneiss (sample RF-111). Zircons from this sample are rather heterogeneous in terms of shape and size, with elongated prismatic crystals, as well as rounded ones with concentric zoning. Overgrowths and corrosion features are therefore recognized (Figs. 8A to 8D). The U-Pb results are complex and difficult to interpret due to the variety of ages found for the different zircon populations (Tab. 7). Three major age groups are identified: 1,992 ± 27; 2,461 ± 24 and 2,704 ± 12 Ma (Figs. 9A to 9C), the first two discords show Neoproterozoic lower intercept ages (548 ± 76 Ma and 559 ± 40 Ma). In homogeneous crystals, Paleoproterozoic ages predominate, whereas in the heterogeneous crystals Archean ages are more seen with a few exceptions. Two zircon grains analyzed have differences in ages between core and rim, with Archean age in the core (2,526 to 2,711 Ga) and Paleoproterozoic age on the edge (2,003 to 2100 Ga). The age patterns of Riacho Seco Complex suggest that its igneous protoliths are the product of anatexis of Archean continental crust indicated by the abundant presence of inherited archean zircons.

Lobo Orthogneiss
Lobo Orthogneiss (Brito & Freitas 2011), recognized during Salgueiro mapping project, was dated by the U-Pb LA-ICPMS method on zircon at 994 ± 25 Ma (Brito & Marinho 2012      ( Fig. 10B) interpreted as the crystallization age of the protolith. Zircons showing Stenian periods are also observed in this sample. The crystals are predominantly prismatic; some grains show homogenous texture and narrow zoned edges and others show oscillatory concentric zoning, no significant differences ages were found between spots on core and rim (Fig. 10A). Data indicate that the granite/granodiorite protolith of Lobo Orthogneiss crystallized during the Cariris Velhos Event and probably suffered shear deformation during the Brasiliano Orogeny.

Rocinha Orthogneiss
Sample RF-323 of Rocinha Orthogneiss (Cruz & Accioly 2013) was investigated. The zircon grains are prismatic, with rounded edges and aspect ratios ranging from 2:1 to 4:1. The elongated grains have reddish color and some inclusions, whereas the smaller ones tend to be pink. The BSE images show predominance of homogeneous grains, although some thin overgrowths are observed (Fig. 11A). Some crystals showing cores and wider "porous" rims may suggest an alteration process (Figs. 9C to 9D). One sample collected from the type locality of Rocinha Orthogneiss is dated (RF-323) (Tab. 9), and the analyses indicate a concordia age of 956 ± 2 Ma (Fig. 11B).

Nd Isotopes
Sm-Nd analyses were carried out on 23 whole-rock samples (Tab. 10, Fig. 12 Figure 10. U-Pb data from sample RF-270 for the Lobo Orthogneiss. (A) Backscatter images for some of the analyzed zircon grains, some grains show homogenous texture and narrow zoned edges, and others present oscillatory concentric zoning, no significant differences in age were found between spots on core and rim; (B) U-Pb zircon age Concordia diagram.

DISCUSSION
The Entremontes Complex and metaplutonic bodies from RSN show Archean U-Pb and Sm-Nd ages, similar to rock units identified in the Congo and São Francisco Cratons (Bizzi et al. 2003;Teixeira et al. 2010), implying a correlation with the cratonic rocks.
Such Complex presents U-Pb zircon age of 2,734 ± 11 Ma and Sm-Nd model age of 3.16 Ga and e Nd (T) of -3.8. These data imply anatexis of, or contamination with, older continental crust in the granitoid protholith genesis. The Entremontes Complex may be considered an allochthonous strip or basement inlier within the Western PEAL (Cruz 2013). One possibility to be considered is that it would be a fragment of the São Francisco Craton strongly deformed and displaced by the Brasiliano tectonics. An alternative hypothesis is that it may represent a microplate accreted to the Northern margin of São Francisco Craton during the Brasiliano orogeny.
The geochemical data reveal characteristics that are typically associated with intra-plate magmatism; however, others are common in subduction related granitoids. The Entremontes Complex, as indicated by the chemical data, is related to A-type magmatism most likely in a post-collisional setting during the end of Archean. Other units, in Borborema Province and São Francisco Craton, show similar ages and characteristics (Dantas et al. 2004;Teixeira 1997). The Entremontes Complex is the oldest rock unit in the PEAL.
The crystalline basement of the RSN represented by Riacho Seco Gneissic-migmatitic Complex presents a complex evolution, with Archean (2,704 ± 12 Ma) and Paleoproterozoic (1,992 ± 27 and 2,461 ± 24) zircons, besides lower intercept ages (548 ± 76 Ma and 559 ± 40 Ma). Despite the array of zircon populations, the petrographic evidence clearly points to an igneous origin of these rocks. The Th-U ratios and images of zircon grains were not conclusive to separate the different zircon population ages in igneous or metamorphic. The age pattern suggests that igneous protoliths crystallized in the Paleoproterozoic are the result of reworking of Archean crust, with the unit also suffering the effects of the Brasiliano cycle. Other hypothesis would be the RSN rocks are Archean and metamorphosed with partial melting during the Paleoproterozoic. Two additional localities in the Riacho Seco gneissic-migmatitic Complex had samples analyzed (Brito Neves, personal communication). Additional data confirm the complexity of geological processes, which those rocks had gone through. Both samples presented three main age groups: 3.4, 2.65, and 2.0 Ga; 3.2, 2.55, and 2.1 Ga. The RSN is located close to the boundary between the craton and PEAL, and it is difficult to establish which of the two tectonic domains it belongs to. Due the complexity showed by the data acquired, further studies are needed to better define this unit.
Most Paleoproterozoic rocks in Borborema Province have ages between 2.3 and 2.0 Ga Neves 2003;Van Schmus et al. 2008). Fulgêncio Orthogneiss, however, has the age of 1.9 Ga, indicating a later stage in the Paleoproterozoic orogeny, a notion that is reinforced by their chemical characteristics associated with post-collisional setting. Few basement areas found in the Borborema Province have similar ages (Neves et al. 2014).
The U-Pb analyzes of zircon in Rocinha Orthogneiss indicate a concordia age of 956 ± 2 Ma, slightly younger compared with the average age of Cariris Velhos granite known in literature (990 -960 Ma), including Lobo Orthogneiss (Brito & Marinho 2012). The Sm-Nd isotopic data yielded a model age value of 1.4 Ga and e Nd (T) of -0.8, which represents contamination with, or anatexis of, older crustal material (Cruz & Accioly 2013).
The large volume of Cariris Velhos granites (Tonian-Stenian) is remarkable. Its counterparts in Africa are still unknown, however these rocks are abundant in the Alto Pajeú Domain of the Central Sub-province further to the North (Santos et al. 2010;Guimarães et al. 2012), and therefore Cariris Velhos Belt is extended to the Western PEAL. A variety of granitic intrusions attributed to the Cariris Velhos Event has chemical characteristics similar to volcanic arc granites as well as to A-type post-collisional granites (Cruz & Accioly 2013;Santos et al. 2010;Kozuch 2003), suggesting that the Cariris Velhos event involved subduction process and does not represent purely an extensional episode.
Brasiliano plutons with Mesoproterozoic model ages are exposed in the Eastern PEAL, such as Águas Belas Pluton (Silva Filho et al. 2010). These bodies may be correlated with Brasiliano plutons in the Western part of the domain (Tab. 8).
The granitic-gneissic rocks in the Western PEAL with ages ranging from 1.0 to 0.6 Ga (Brasiliano and Cariris Velhos plutons) show TDM periods between 1.2 and 1.8 Ga. These model ages are quite similar to those found in the Central African Fold Belt, in which TDM ages range from 1.0 to 1.8 Ga, mainly in the Adamawa-Yade Domain (Van Schmus et al. 2008).

CONCLUSIONS
Based on U-Pb and Sm-Nd isotopic data gathered in this study, it was possible to divide the granitoids of the study area (covering approximately 70% of the area of the Western Pernambuco-Alagoas Domain) into six main groups: ■ Brasiliano granites (Cryogenian-Ediacaran), with model ages ranging from Paleoproterozoic to Mesoproterozoic and chemical features similar to those of the high-K calc-alkaline series; ■ Cariris Velhos granites (Tonian), presenting Mesoproterozoic model ages and chemical characteristics ranging from alkali-rich to calc-alkaline. This group includes Lobo (974 ± 8 Ma) and Rocinha (956 ± 2 Ma) orthogneisses; ■ Paleoproterozoic granites from PEAL, with model ages ranging between Neoarchean and Paleoproterozoic and high-K calc-alkaline chemical nature, including Fulgêncio Orthogneiss (1,996 ± 8 Ma); ■ Paleoproterozoic granites from RSN, with Archean model ages, mainly represented by Riacho Seco Gneissic-migmatitic Complex (1,992 ± 27 Ma); ■ Archean granites from PEAL, represented by rocks of the Entremontes Complex with U-Pb age of 2,734 ± 11 Ma and model age of 3.2 Ga; ■ São Francisco Craton granites with model ages ranging from Mesoarchean to Paleoproterozoic periods.
The Orosirian Fulgêncio Orthogneiss is exposed in an extensive area of the domain, and constitutes a very important exposure of older sialic basement rocks in the Western PEAL area. The basement of RSN (subdomain within PEAL) is formed mainly of Archean rocks reworked by Paleoproterozoic tectonic. Data acquired showed that the Western PEAL rocks were affected by four distinct tectonic events, starting with an Archean event, followed by a Paleoproterozoic, and subsequently affected by two others during the Neoproterozoic (Cariris Velhos and Brasiliano Cycle).