The Macururé Complex ( Sergipano Belt , NE Brazil ) in southern Alagoas state : Geology and geochronology

*Corresponding author Sérgio P. Neves E-mail address: serpane@hotlink.com.br


Introduction
The Borborema Province of northeastern Brazil is one of the several Brasiliano/Pan-African belts formed during the assemblage of West Gondwana (Van Schmus et al. 2008;Neves 2015 and references therein).The province is divided in three subprovinces by the Patos and Pernambuco dextral shear zone systems (Fig. 1a).The Southern Subprovince is further subdivided in the Pernambuco-Alagoas (PEAL) Domain and the Sergipano and Riacho do Pontal belts.The Sergipano Belt has a triangular shape and is limited by the PEAL Domain in the north and by the São Francisco Craton in the south, being separated in western and eastern sectors by the Tucano-Jatobá rift system (Fig. 1a).The Sergipano Belt has been divided in several lithostratigraphic domains (Davison and Santos 1989;D'el-Rey Silva 1999;Oliveira et al. 2006Oliveira et al. , 2010) ) (Fig. 1b).The Macururé Domain is the largest unit and has been the subject of several studies in recent years.However, most of them have been conducted in the state of Sergipe (Bueno et al. 2009;Oliveira et al. 2010, 2015a, 2015b, Conceição et al. 2016;Lisboa et al. 2019).In this paper, we report the results of geological mapping, petrographic and structural studies and geochronological dating of an area in the middle portion of the Macururé Domain in the state of Alagoas (Figs. 1b and 1c).With this data, we discuss the structural style, metamorphic grade, the age of deposition and the provenance of the metasedimentary rocks in order to contribute to the understanding of the geological evolution of the Sergipano Belt.
The Poço Redondo-Marancó and Canindé domains are located in the northwestern portion of the Sergipano Belt (Fig. 1b), the latter being separated from the PEAL Domain by the Jacaré dos Homens contractional shear zone (Mendes et al. 2009(Mendes et al. , 2012)).Another domain, named Rio Coruripe, was proposed for the northeastern portion of the Sergipano Belt (Silva Filho and Torres 2002;Mendes et al. 2009Mendes et al. , 2012) ) but it is debated if it represents the high-grade counterpart of the Macururé Domain (Oliveira et al. 2006) or the eastward continuation of the Canindé Domain (Neves et al. 2016).The Poço Redondo-Marancó Domain consists of migmatitic orthogneisses and augen gneisses and of a supracrustal sequence comprising metasedimentary rocks interlayered with amphibolites, meta-andesites, metadacites, metarhyolites and serpentinites (Davison and Santos 1989;Carvalho 2005;Silva Filho 2006).The Canindé Domain comprises metavolcano-sedimentary sequences and variably deformed and metamorphosed peridotitic, gabbroic, dioritic and granitic rocks (Davison and Santos 1989;Mendes et al. 2009;Oliveira et al. 2010).The mesosome of migmatites from the Poço Redondo-Marancó Domain yielded crystallization ages ranging from 980 to 960 Ma and the largest augen gneiss body has crystallization age of 952 ± 2 Ma (Carvalho 2005)  The Macururé domain constitutes most of the central and northern parts of the Sergipano Belt.It consists of metasedimentary rocks of the Macururé Complex, which surrounds basement rocks of the Jirau do Ponciano Dome (Fig. 1b).The Jirau do Ponciano Dome consists of tonalitic to granodioritic high-grade orthogneisses (Jirau do Ponciano Complex) interleaved with metavolcano-sedimentary rocks of the Nicolau-Campo Grande Complex (Mendes et al. 2009(Mendes et al. , 2012)).The Macururé Complex include quartzites, metapelites and metarhythmites.This complex was metamorphosed dominantly under amphibolite facies conditions (Silva et al. 1995), and intruded by several granitic stocks (Bueno et al. 2009;Oliveira et al. 2015b;Conceição et al. 2016;Lisboa et al. 2019) during the Brasiliano Orogeny.Zircon U-Pb dating of orthogneiss samples of the Jirau do Ponciano Complex yielded dominant crystallization ages in the interval 2.04-2.06Ga (Spalletta and Oliveira 2017).These ages are similar to crystallization ages of amphibolite and metarhyolite samples from the Nicolau-Campo Grande Complex (2.05-2.07Ga; Lima et al. 2019).U-Pb dating of detrital zircon grains from metasedimentary rocks of the Macururé Complex showed age clusters mainly in the intervals 1050-930 Ma and 2100-1900 Ma (Oliveira et al. 2006(Oliveira et al. , 2010(Oliveira et al. , 2015a)).The granitoids emplaced into the Macururé Complex yielded crystallization ages ranging from 630 to 570 Ma (Bueno et al. 2009;Oliveira et al. 2015b;Conceição et al. 2016;Lisboa et al. 2019).

Study area
The study area is situated in the eastern portion of the Sergipano Belt, northeastward of the city of São Brás, State of Alagoas (Fig. 1c).Orthogneisses belonging to the Jirau do Ponciano Dome occur in its northern portion.They are represented by muscovite-bearing granitic gneisses, with local intercalation of amphibolite lenses, and by migmatized orthogneisses with amphibole-bearing mesosome.The lithological associations of the Macururé Complex include muscovite quartzite (Fig. 2a), banded metasedimentary rocks (metarhythmites; Fig. 2b) and mica schist (Fig. 2c).Quartzite occurs at the southern contact of the orthogneissic basement with the Macururé Complex, forming the base of the metasedimentary cover, and as map-scale (Fig. 1c) to thin lenses intercalated with the banded metasedimentary rocks.Pelitic-psammitic metasedimentary rocks are the most abundant lithologies.The pelitic bands are composed of quartz, biotite, muscovite and garnet whereas the psammitic layers are dominated by quartz ± feldspar.The banded appearance gives place to a more homogeneous texture in fine-to medium-grained pelitic schists, which show the same mineral assemblage of the pelitic bands of the metarhythmites.Calc-silicate rocks composed of quartz, plagioclase, amphibole, epidote and quartz may occur as thin, centimeter-to decimeter-thick bands in these rocks (Fig. 2d).A small body of porphyritic granite intrudes the metasedimentary sequence in the west part of the area.

Structure and metamorphism
In the study area, the Macururé Complex shows a relatively simple geological structure.The centimetric to decimetric alternating layers of metapelites and metapsammites in the metarhythmites represent the original bedding (S 0 ) and can be related to the Tc-Td-Te or Td-Te intervals of classic Bouma turbidites (Silva et al. 1995).The main foliation is usually parallel to the bedding and shows low to moderate dip to the southwest (Fig. 3a).This foliation is interpreted to be of second generation (S 2 ) because intrafolial, tight to isoclinal folds defined by micas, so indicating existence of a previous metamorphic foliation (S 1 ), are locally observed (Fig. 3b).In places where S 2 is oblique to S 0 , its dip is always steeper (Fig. 3c).Mineral lineations, sometimes defined by kyanite, plunge mainly to the southwest (Figs.1c, 3a and 3d).Stretching lineations are only rarely observed and kinematic shear criteria were only found near the city of São Brás (Fig. 1c).There, the foliation dips to the northeast and en-echelon quartz veins (Fig. 3e) and C'-type shear bands (Fig. 3f) indicate top-to-thesouthwest tectonic transport.Poles to S 2 define a broad girdle whose axis plunges 23° to S72°W (Fig. 3a), which coincides with the orientation of hinge lines of crenulations formed in a later deformation phase.
In schists and metarhythmites, garnet is ubiquitous as an accessory mineral phase (Fig. 4a) and centimetric porphyroblasts of staurolite and kyanite (Figs.2c and 3d), sometimes together (Fig. 4b), are common.Syntectonic growth of kyanite during development of S 2 is attested by its orientation parallel to L 2 , with no sign of superposition of a previous foliation (Figs.3d and 4c).Garnet may show an internal foliation oblique to the external foliation, indicating early-S 2 growth.Staurolite usually occurs as elongate poikiloblasts with internal foliation continuous with S 2 (Fig. 4d), but in places the internal foliation is slightly curved, indicating early-S 2 growth.

U-Pb geochronology
Two samples were chosen for the geochronological study, one from a kyanite-bearing garnet biotite muscovite schist and one from a muscovite quartzite, but the first did not yield zircon grains.The analyzed sample (MAC-02) is from a quartzite collected at coordinates 10°06'52.7"S;36°49'23.8"W(Fig. 1c).Zircon grains were separated using conventional techniques.U-Pb zircon ages were obtained by laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the Université de Montpellier, France, following the procedure detailed in Neves et al. (2016).
The U-Pb results are shown in Table 1 and in Fig. 5. Fortyone grains were analyzed, all of which show oscillatory zoning and high Th/U ratios (mostly above 0.5) suggestive of derivation of igneous (or metaigneous) sources (e.g., Kirkland et al. 2015).Of these, 25 grains yielded discordance < 5% and are the only discussed.The concordant data define three age groups.The three populations of zircon grains have similar morphologies, being dominated by elongate grains with rounded corners (Fig. 5).

Discussion
The dominant flat-lying foliation in the study area suggests development associated with thrust tectonics.The absence of mylonitic belts and the rarity of stretching lineations indicates distributed non-coaxial deformation, without strain localization.The local presence of shear criteria indicating top-to-the-southwest tectonic transport (Figs. 3e, f) agrees with the he overall vergence of the Sergipano Belt towards the São Francisco Craton (Brito Neves et al. 1977;Jardim de Sá et al. 1992;D'el-Rey Silva 1999;Oliveira et al. 2010).In this context, the southwestward dip of the main foliation in the study area is attributed to later folding.In places where S 2 is oblique to S 0 , its dip is always steeper (Fig. 3c), indicating that the study area may be located in the normal limb of an inverted macroscopic fold with northeastern vergence.This inference is consistent with location of the study area south of the Jirau do Ponciano Dome, which is shown on regional maps as an inverted antiform (Mendes et al. 2009(Mendes et al. , 2012;;Lima et al. 2019).Oliveira et al. (2010) proposed a time gap between S 1 and S 2 based on observations in the central portion of the Macururé Domain.In contrast, S 1 and S 2 are here interpreted as resulting from progressive deformation, giving the absence of any metamorphic discontinuity, as previously discussed by Silva et al. (1995).The mineral assemblage muscovite+b iotite+garnet±kyanite±staurolite (Fig. 4b) indicates medium pressure and temperature conditions (amphibolite facies) typical of Barrovian metamorphism.The detrital zircon ages of sample MAC-02 (Fig. 5) are similar to other dated samples from the Macururé Complex (Van Schmus et al. 2011;Oliveira et al. 2006Oliveira et al. , 2010Oliveira et al. , 2015a)), which revealed no post-Tonian zircon grains (Fig. 6).The dominant latest Mesoproterozoic to early Neoproterozoic ages show that the main sources of the detritus were formed during the Cariris Velhos event (Brito Neves et al. 1995).This event was first documented in the Alto Pajeú Domain of the Central Subprovince (Brito Neves et al. 1996;Santos et al. 2010;Van Schmus et al. 2011;Guimarães et al. 2012) but was later recognized in the PEAL Domain (Brito et al. 2008;Cruz et al. 2014;Da Silva Filho et al. 2014), Marancó-Poço Redondo Domain (Carvalho 2005;Oliveira et al. 2010), and Riacho do Pontal Belt (Caxito et al. 2014).Although the nature of the Cariris Velhos event (if orogenic or extensional) is still debated, the dominance of bimodal metavolcanic rocks and of metagranitoids with A-type signature favors an intraplate origin for the magmatism (Neves 2003;Guimarães et al. 2011Guimarães et al. , 2012Guimarães et al. , 2016)).Given the wide distribution of the Cariris Velhos-related rocks, it is not possible to ascertain if zircon grains of the dominant age group were sourced from proximal rocks (Marancó-Poço Redondo Domain) or had more distal provenance.The absence of zircon grains younger than c. 900 Ma in our and in previously analyzed samples could indicate that deposition of the Macururé Complex occurred at the end or shortly afterward the Cariris Velhos event.Alternatively, it may simply be that younger sources were either not available for erosion at the time of deposition or were not present along the drainage system.
The most obvious source for zircon grains in the age interval 1.95-2.05Ga is the Jirau do Ponciano Dome, which shows a similar age range (Spalletta and Oliveira 2017;Lima et al. 2019), although it is somewhat surprising the comparatively small number of analyses in this age range given the proximity of available source rocks.The provenance of zircon grains with ages around 1.1 Ga (Fig. 5) is more enigmatic.The age of 1091±13 Ma recorded in a metavolcanic rock from the Riacho Gravatá Complex of the Alto Pajeú Domain (Guimarães et al. 2012) is so far the only one documented in central and southern Borborema Province.Guimarães et al. (2012) interpreted this age as related to an early stage of rifting preceding the Cariris Velhos event.Similarly, detrital zircon grains from the Sopa-Brumadinho Formation of the Espinhaço Supergroup falls within the interval between 1080±16 and 1240±20 Ma (Chemale Jr. et al. 2012).They are tentatively interpreted as resulting from erosion of A-type source rocks during the initial rifting stage corresponding to deposition of the basal units of the Upper Espinhaço Group (Chemale Jr. et al. 2012).Therefore, it is possible that the c. 1.1 Ga zircon population records localized extensional events at the late Mesoproterozoic in the Borborema Province and/or São Francisco Craton.

Conclusion
The Macururé Domain in southern Alagoas state is constituted by Paleoproterozoic orthogneisses that outcrop in the northern part of the study area and by quartzite, metarhythmite and micaschist, which are the dominant lithotypes.The southwestern dipping flat-lying foliation was developed under amphibolite facies conditions, with the paragenesis muscovite+biotite+garnet±kyanite±stauro lite indicating medium P-T (Barrovian) metamorphism.U-Pb analyses of detrital zircon grains point to the dominance of early Tonian rocks (1.0 to 0.94 Ga) as the main sources of the sediments, with a smaller contribution of Paleoproterozoic (c.2.0 Ga) and Late Mesoproterozoic (c.1.1 Ga) sources.

Figure 1 -
Figure 1 -Geotectonic setting and geographic location of the study area.(a) Sketch showing the subdivision of the Borborema Province in Northern (NS), Central (CS) and Southern (SS) subprovinces, highlighting the Sergipano (SB) and Riacho do Pontal (RP) belts bordering the São Francisco Craton.Shear zone systems: PaSZ, Patos; EPSZ, East Pernambuco; WPSZ, West Pernambuco.(b) Simplified geological map of the area outlined in (a) showing the subdivision of the eastern portion of the Southern Subprovince according to Mendes et al. (2009).Rectangle shows location of the study area.(c) Geological map of the study area.Inset shows its location in the State of Alagoas.
. The detrital zircon population in the metasedimentary rocks from the Canindé Domain is dominated by Neoproterozoic grains, amongst which the youngest have ages ranging from 708 to 663 Ma (Nascimento 2005; Oliveira et al. 2015a).

Figure 3 -
Figure 3 -Structural aspects of the study area.(a) Poles to the main foliation (S2) and their Kamb contours (2 sigma intervals), and mineral lineations plotted on equal-area (lower hemisphere) projections.The great circle is fitted to the distribution of the foliation poles and the square indicates its pole (251°, 23°).(b) Intrafolial isoclinal folds.(c) Mica foliation oblique to bedding.(d) Porphyroblasts of kyanite defining a mineral lineation.(e, f) Kinematic shear criteria.(e) En-echelon quartz veins.(f) C'-type shear bands.

Figure 5 -
Figure 5 -Probability and Concordia plots for LA-ICP-MS zircon analyses from sample MAC-02 and representative backscattered electron (BSE) images of analyzed grains.
The oldest group comprises five analyses with 207 Pb/ 206 Pb ages ranging from 1954±28 Ma (2s) to 2046±38 Ma (2s).The second group comprises four analyses that yielded Late Mesoproterozoic ages around 1100 Ma.The remaining 16 analyses are of latest Mesoproterozoic to earliest Neoproterozoic age, showing a large age peak at c. 985 Ma.The youngest grain has a 206 Pb/ 238 U age of 939±36 Ma (2s), which is considered as the maximum age for deposition.

Table 1 .
U-Pb LA-ICP-MS analyses of zircon grains from muscovite quartzite (sample MAC-02) of the Macururé Complex.