Lithogeochemistry and geochronology of the subalkaline felsic plutonism that marks the end of the Paleoproterozoic orogeny in the Salvador – Esplanada belt , São Francisco craton ( Salvador , state of Bahia , Brazil )

Studies conducted over the last decade concerning the rocks that underlie the municipality of Salvador have shown a complex geological history with a great diversity of medium- to high-grade metamorphic lithotypes, deformed in several phases and frequently cut by tabular mafic dykes and irregular granitic bodies. The latter, which were the subject of this study, frequently outcrop along the coastline of Salvador and are classified petrographically as monzo-syenogranites. They are classified as subalkaline and peraluminous according to their geochemical data, and stand out for being enriched in light rare earth elements and having a strong negative Europium (Eu) anomaly. These rocks are produced from anatectic melts 
or through the interaction of mantle-derived magmas with crustal materials. The negative values of eNd(t) (-6.08) corroborate with the crustal character and in the diagrams of tectonic ambience, they are plotted in the field of post-tectonic granites. The Sm-Nd model age (TDM) around 2.9 Ga indicates a neoarchean source for these lithotypes, whereas their U-Pb zircon age (LA-ICPMS) of 2,064 ± 36 Ma is similar to the U-Pb (SHRIMP) and Pb-Pb (evaporation) ages for late-tectonic granites of the Itabuna–Curaca–Salvador belt. Considering the results of recent studies in the area of Salvador, the monzo- syenogranites can be interpreted as late-tectonic intrusions, since they are affected by dextral shear zones correlated with the last stage of deformation registered in the granulites of Salvador.


INTRODUCTION
The relationships between magmatic bodies and deformational events are useful to unravel the complex interactions between tectonics and the processes of generation and emplacement of magmas.Thus, granitoid bodies are good tracers of the rheological evolution of host rocks, as well as of stress fields and kinematics (Druguet et al. 2008).These rocks are present in different crustal levels at various scales, from large granitic plutons to small anatectic granitic veins (leucosome) in migmatitic terrains.
The granulitic rocks that outcrop in Salvador, state of Bahia, Brazil, are located nearby the confluence of two important tectonic macro-units of the São Francisco craton (SFC; Almeida 1977): the first one, with N45° trends, corresponds to the Salvador-Esplanada belt (SEB) of Barbosa and Dominguez (1996), and the second one, oriented N10°, corresponds to the Itabuna-Salvador-Curaçá belt (ISCB) of Barbosa andSabaté (2002, 2004) (Fig. 1).Both units show a complex evolutionary history (Barbosa and Dominguez 1996, Barbosa and Sabaté 2002, 2004, Delgado et al. 2002), which makes it difficult to establish precise geotectonic models and the connection between these two units.The granitoids that occur in the ISCB can be classified as (i) syntectonic, contemporary to the formation of the belt and to the crustal thickening (~2.1 Ga), and (ii) post-tectonic, associated to sinistral transcurrent faults related to the peak of granulitic metamorphism and orogenic collapse (~2.07 Ga) (Barbosa et al. 2008).
This study had the objective of placing the granitic bodies and veins that outcrop in Salvador within the regional tectonic context.In addition, the petrographic, petrochemical, geochronological, and isotopic data of these rocks are presented and discussed, aiming to contribute to the knowledge of their tectonic environment.

ANALYTICAL PROCEDURES
Twelve whole-rock granite analyses of major and trace elements were carried out at the laboratories of GEOSOL and Geology and Surveying Ltd., and are reported in Tab. 2. Major (SiO 2 , Al 2 O 3 , FeO (t), MgO, CaO, TiO 2 , P 2 O 5 , and MnO) and trace (V, Rb, Ba, Sr, Ga, Nb, Zr, Y, and Th) elements were analyzed by X-ray fluorescence, and rare earth elements were determined using inductively coupled plasma mass spectrometry (ICP-MS).Na 2 O and K 2 O contents were determined using atomic absorption spectrometry.
The procedures described by Peucat et al. (1999) were followed to analyze the Nd whole-rock isotopic compositions.
The values were adjusted to the pattern of Nd AMES standard, which provided a mean 143 Nd/ 144 Nd ratio of 0.511896 ± 7, with an error of 0.0015%.The model ages (T DM ) were calculated using values of ε Nd +10 for the current depleted mantle and 147 Sm/ 144 Nd ratio of 0.2137, assuming a radiogenic linear growth starting at 4.54 Ga.
Several U-Pb (zircon) analyses were carried out for the monzo-syenogranite sample SG-10G at the Laboratoire Magmas et Volcans -Université Blaise Pascal in Clermont-Ferrand, France, using the in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) method as described by Hurai et al. (2010).The error measured for each analysis (ratios and ages) is presented at the level of 1σ.For the calculation of 207 Pb/ 206 Pb weighted mean ages, a confidence limit of 95% was considered.The errors in the discordant intercept ages are presented at the level of 2σ and were calculated using the software Isoplot (Ludwig 2001).
The 207 Pb/ 206 Pb dating method through evaporation of lead from zircon monocrystals, developed by Kober (1986), was used for the same SG-10G sample by means of successive heating stages in a thermal ionization mass spectrometer (TIMS), using the Finnigan MAT 262 mass spectrometer of the Laboratoire de Géosciences Rennes-CNRS, France.
All ages were calculated using decay constants and abundance of the isotopes listed by Steiger and Jäger (1977).

REGIONAL GEOLOGICAL SETTING
In the portion of the SFC that outcrops in the state of Bahia, high-grade metamorphic rocks occur in the area of Itabuna-Ilhéus, in the south, and in the area of Curaçá, in the north, comprising the Itabuna-Salvador-Curaçá block (ISCB) of Barbosa andSabaté (2002, 2004).These rocks, at the current level of erosion, are the roots of an orogenic belt, with N-S orientation and Paleoproterozoic age of around 2.1 Ga (Peucat et al. 2011) (Fig. 1A).The southern part of this belt consists of at least four groups of tonalite/trondhjemite, three of them with Archean ages between 2.6 and 2.7 Ga (Peucat et al. 2011), and one Paleoproterozoic group (~2.2 Ga) (Barbosa andSabaté 2004, Peucat et al. 2011) (Tab. 1).Subordinately, charnockitic bodies of Archean age also occur, as well as bands of supracrustal rocks and gabbros, and basalts related to ocean floor or back-arc basins (Teixeira 1997).In addition, intrusions of granulitized monzogranites of shoshonitic affinity are also found (Barbosa 1990).The northern area of the ISCB essentially consists of tonalite-trondhjemite-granodiorite (TTG) orthogneisses of 2.7 Ga (Figueiredo 1989 (Melo et al. 1995), as well as mafi c-ultramafi c rocks that form the so-called São José do Jacuípe Suite, also of Archean age (Silva et al. 1997).Th e whole ISCB is intruded by syenites dated at 2.08 -2.09Ga (Conceição et al. 2003, Oliveira et al. 2004), and syn-and post-tectonic granites intrusions dated around 2.06 Ga (Silva et al. 2002, Barbosa et al. 2008).Th is entire crustal segment was strongly aff ected by Paleoproterozoic tectonics with all its lithotypes plunged into granulite facies metamorphism (Barbosa & Sabaté 2002, 2004).Th e SEB of Barbosa and Dominguez (1996) consists of high-grade metamorphic rocks, which are roughly N45° aligned (Fig. 1B).Th ese lithotypes underlie Salvador, in Bahia, and extend up to Boquim, in the state of Sergipe.A large portion of the northeastern part of the belt is covered by Tertiary deposits of the Barreiras Formation and, by Quaternary sediments, and in the southwestern part, by the sedimentary rocks of the Recôncavo-Tucano Mesozoic basin.
Geologically, the area of Salvador (Fig. 2) was subdivided by Barbosa and Dominguez (1996) into three major domains: (i) the Alto de Salvador, which is a horst of granulitic rocks (Barbosa et al. 2005); (ii) the Recôncavo sedimentary basin, which is limited, eastwards, by the Salvador   New U-Pb zircon ages in the Salvador-Esplanada belt fault system; and (iii) the Atlantic Coastal Margin, composed of Tertiary and Quaternary deposits of unconsolidated sediments (Dominguez et al. 1999).Despite the lack of outcrops, the studies carried out by Barbosa et al. (2005), focusing on the western area of the horst, showed a great diversity of ortho-and para-derived metamorphic lithotypes of high-and medium-metamorphic grade, which are polydeformed and frequently cut by mafic dykes and irregular monzo-syenogranitic bodies.Souza et al.
Regarding the ductile deformations, at least three stages of continuous deformation were recorded by Barbosa et al. (2005) on the granulitic lithotypes, which are the host rocks of the monzo-syenogranitic bodies and veins in question.The main structures of the first stage comprised recumbent folds with subhorizontal axes, which were isoclinally refolded during the second phase.These isoclinal folds have subvertical axial planes and subhorizontal axes.The third phase includes transcurrent dextral shear zones, subparallel to the axial surfaces of the isoclinal folds, which are coeval with the second deformational phase and produce mineral stretching lineations parallel to their axes.U-Pb monazite ages (in situ LA-ICPMS) indicate that the third deformational phase occurred at 2,064 ± 9 Ma (Souza 2013).
Numerous faults and fractures cut the granulitic rocks.The most significant are those oriented N60° -N90°, associated with the intrusion of mafic dykes, and those oriented N120° -N160°, where tabular bodies and monzo-syenogranitic veins were placed.In addition to these, faults oriented N30° have also been registered, and the Iguatemi and Jardim de Alah faults (Fig. 2) exhibit a general N40° orientation.
Although Souza et al. (2010 and references therein) recently presented important results on the granulitic rocks, various geological problems still need to be solved.One of them, which shows the generation and age of the felsic magmatism of the SEB, will be dealt with in this study.

PETROGRAPHICAL AND GEOCHEMICAL ASPECTS
The monzo-syenogranitic dykes and veins outcropping along the coast of Salvador (Fig. 2) fill fractures in various directions.Two dominant directions present are as follows: 1. Fractures oriented N60° -N90°:The dykes exhibit fineto medium-grained texture, are moderately deformed, and present folds and boudinage when affected by the late dextral shear zones correlated to the third stage of deformation of Barbosa et al. (2005).Mafic dykes were observed with the same orientation.In the outcrop of the Paciência Beach (SG-10F) for example, interpenetrations of mafic material in felsic material and vice versa were identified, characterizing a mingling-type heterogeneous physical mixing of basaltic and granitic magma (Walker & Skelhorn 1966, Wiebe 1991) (Figs.3A and B). 2. Fractures oriented N40° -N70°: The dykes present medium-to coarse-grained texture and occur as vertical and subvertical bodies, with thickness ranging from 0.5 to 2 m and keeping abrupt contacts with their host rocks (Fig. 3C).Irregular bodies are also found, with various thicknesses and diffuse contacts.On the edge of some of the granitoid dykes, in contact with the host rocks, pegmatitic facies were observed, probably related to late magmatic-hydrothermal processes.
The geochemical data of the major elements allowed classifying them as subalkaline and peraluminous (Fig. 4A and B), characterizing their origin as formed by the partial fusion of a crustal source (Chappell & White 1974, White & Chappell 1977).They are acidic rocks, with SiO 2 contents ranging from 69 to 73% (Tab.2).The Harker (1909) type binary diagrams show magmatic differentiation trends, in which TiO 2 , Na 2 O, MgO, Sr, Zr, and Ba are compatible with fractional crystallization whereas K 2 O and Rb are incompatible (Fig. 5).Furthermore, as shown in Fig. 6, these lithotypes are enriched in light rare earth elements (LREE) (136 < La N < 602) and present a strong negative Europium (Eu) anomaly.The geochemical data of their trace elements, represented in discrimination diagrams of tectonic environment, are not conclusive.Most of them are located in the fields of intraplate and syncollision granites of Pearce et al. (1984), but some samples plot in the field of post-collision granites of Pearce (1996) (Figs. 7A and 7B).

ISOTOPIC GEOCHEMISTRY AND GEOCHRONOLOGY
The SG-10G sample of a monzo-syenogranite vein corresponds to a representative part of the felsic portion of the mingling-type mixture observed at Paciência Beach (Fig. 3B).

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Brazilian Journal of Geology, 44(2): 221-234, June 2014 Jailma Santos de Souza-Oliveira et al.One sample (SG-10G) of the monzo-syenogranitic bodies was selected for U-Pb zircon dating.Th is sample was intrusive into the ortho-derived granulites, at the Rio Vermelho district in the city of Salvador (Figs. 2 and 3).Dated zircon grains are large (spot size in Fig. 8 is ca.30 µm) and they are mainly clear, euhedral, elongated, and without any visible inherited core (see TL images of grains 1.1 and 2.1 in Fig. 8).Th ey are typical of high-temperature types (S19-S24, Fig. 8, after Pupin (1980)).Th e protolith of these granulites were dated at 2,561 ± 7 Ma and the granulite facies were 2,089 ± 11 Ma by U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon ages (Silva et al. 1997).
U-Pb zircon ages obtained using ICP-MS data are concordant to slightly discordant and defi ne an intercept age at 2,064 ± 36 Ma, with the mean of 207 Pb/ 206 Pb ages of 2,067 ± 13 Ma in 21 analyses.Th ese ages are similar to the Pb-Pb zircon TIMS evaporation age also obtained in this study (i.e., 2,064 ± 6 Ma) and are interpreted as the age of crystallization of the granites.Th ree grains that exhibit zoned cores (see grains 8.1 and 19.1 in Fig. 8) showed 207   ages of ca.2.3 -2.4 Ga, which were interpreted as an inheritance from the surrounding Archean basement.Whole-rock Sm-Nd elemental and isotopic data obtained from the same sample were as follows: Sm = 22.9219 ppm; N d = 1 3 4 .6 9 5 7 p p m ; 147 Sm / 144 N d = 0 . 1 0 2 8 ; 143 Nd/ 144 Nd = 0.5110.These data provided a model age (T DM ) of 2.859 Ga, and ε Nd(0) = -30.9.Considering the 2.07 Ga crystallization age, the parameter ε Nd(2.07Ga) results in the value of -6.08, suggesting an important participation of a crustal component in the formation of these granites.

DISCUSSIONS AND CONCLUSIONS
The data obtained in this study from the monzo-syenogranitic bodies and veins intrusive into the granulites of Salvador allow us to classify them as subalkaline    (Pearce et al. 1984).The ellipse corresponds to the fi eld of post-collision granites of Pearce (1996).

A B
On the basis of the present results and the recent studies performed by Souza (2013), the monzo-syenogranites can be positioned as late-to post-tectonic granites, since they exhibit deformation related to the late dextral shear zones dated at 2,064 ± 9 Ma (in situ LA-ICPMS monazite) of Paleoproterozoic orogenesis in the SEB.
Moreover, taking into account the age of 2,083 ± 4 Ma attributed to the metamorphism of the ISCB (Peucat et al. 2011), the geochemical and geochronological data found in this study are compatible with the geodynamic model proposed by Barbosa andSabaté (2002, 2004), as well as with the tectonic evolution of the northeastern part of the ISCB proposed by Oliveira et al. (2010).Similar to our conclusion, these geodynamic models include some peraluminous types as late-tectonic granites, which are undeformed or rather weakly deformed and cut through the older granulitic rocks and exhibit negative values of ε Nd(T) between -13 and -5.

Figure 2 .
Figure 2. Simplifi ed geological map of the area with the location of the studied samples.

Figure 3 .
Figure 3. Association of the monzo-syenogranitic veins with mafi c dykes in fractures and faults with orientation of N60° -N90°.(A) Panoramic view of Paciência Beach.(B) Detail of a physical mixing of basaltic and granitic magma (mingling).(C) Verticalized monzo-syenogranitic body with pegmatoid texture and abrupt contacts with its host rocks.

Figure 5 .
Figure 5.Chemical variation diagrams of Harker (1909) for major and trace elements of the monzo-syenogranitic bodies of Salvador.