U-Pb ( LA-ICP-MS ) of detrital zircon and whole rock Nd and geochemical constraints on the provenance , depositional age and tectonic setting of the metasedimentary Piriá Basin , northern Brazil : implications for the evolution of the Gurupi Belt

A Bacia do Piria (Formacao Piria) e um rifte com forma de hemi-graben desenvolvido sobre rochas pre-cambrianas do Cinturao ­Gurupi. O conteudo litologico se distribui em quatro litofacies que ocorrem interdigitadas: (1) arcoseos e grauvacas com niveis de pelitos, (2) siltitos e pelitos laminados, (3) arcoseos com estrutura hummocky, e (4) ­conglomerado oligomitico. Esse conjunto se formou em leques aluviais (conglomerados) e em sistema fluvial (arcoseos, grauvacas, siltitos e pelitos) que se estabeleceu e evoluiu a medida que a subsidencia avancou e sofreu anquimetamorfismo e deformacao tectonica muito leve. Analises U-Pb em zircao detritico estabelecem idade maxima para a sedimentacao em 591 Ma e indicam fontes detriticas com idades muito variaveis, do Neoproterozoico ao ­Arqueano. As fontes principais sao do periodo Riaciano, que representa o principal periodo de formacao de crosta continental no Fragmento ­Cratonico Sao Luis e no embasamento do Cinturao Gurupi. Fontes importantes do ­Neoproterozoico foram identificadas no segmento oriental da bacia. Fontes com idades desconhecidas na regiao foram tambem identificadas. ­Combinados, os dados U-Pb em zircao detritico, os dados geoquimicos e de isotopos de Sm-Nd em rocha total, e petrograficos revelam proveniencia principal a partir de rochas felsicas e intermediarias proximais e de fontes sedimentares retrabalhadas. Em conjunto, os resultados indicam que a Formacao Piria se depositou em bacia pos-orogenica relacionada com o estagio final do ciclo orogenico Brasiliano, responsavel pelo soerguimento do Cinturao Gurupi.


INTRODUCTION
From the end of the Brasiliano orogenic cycle (Ediacaran-Cambrian transition), Paleozoic intracratonic basins and Mesozoic to Cenozoic sedimentary covers formed during the various stages (transition, stabilization and reactivation) of evolution of the South American Platform (Almeida et al. 2000).The Piriá Basin (Piriá Formation; Costa et al. 1977), which overlies Precambrian rocks of the Gurupi Belt and São Luís cratonic fragment in northern Brazil (Fig. 1), is likely associated with the post-orogenic and/or transition stages.
The spatial distribution of the Piriá Basin has recently been reviewed (Klein & Sousa 2012, Lopes & Klein 2014), but fundamental aspects about basin evolution remain unconstrained.These include the lithological content and sedimentary setting of deposition, the depositional age and sources of the sediments, the influence of metamorphism and deformation, and the tectonic meaning of the basin.Because of the lack of fossil record and radiometric dating in the basin, Ediacaran to Cambrian times are in general inferred for the sedimentation (Abreu et al.This will be based on U-Pb and Pb-Pb detrital zircon ages, and whole-rock Sm-Nd and geochemical data.

GEOLOGICAL SETTING
It is beyond the scope of this paper to discuss in detail the stratigraphy of the study region.For details, the reader is referred to recent geological maps (Klein et al. 2008a, Klein and Lopes 2011, Klein & Sousa 2012, Lopes & Klein 2014).Only the major tectonic domains and associations are discussed here: 1. São Luís cratonic fragment, 2. Gurupi Belt, and 3. Phanerozoic sedimentary covers (Fig. 1).
The Gurupi Belt is interpreted as a NW-SE-trending mobile belt developed at the south-southwestern margin of the São Luís cratonic fragment during the Neoproterozoic and early Cambrian (Almeida et al. 1976, Klein et al. 2005, 2012), as part of the widespread Brasiliano orogenic cycle.The existence of tectonic activity during these era/periods is documented by a series of Rb-Sr and K-Ar ages (see primary references in Klein et al. 2005).The basement unit consists of an Archean metatonalite (2594 ± 3 Ma), but most of the lithological framework of the belt comprises rock units with ages that are similar to those found in the São Luís cratonic fragment, which likely represent the reworking of rocks formed in the same Rhyacian event that produced the rocks that form the present-day cratonic area.These rocks include gneisses (Itapeva Complex, 2158 to 2167 Ma), the metavolcano-sedimentary Chega Tudo Formation (2160 to 2148 Ma), a metasedimentary sequence of unknown age, and minor bodies of amphibolites (2150 ± 8 Ma) (Palheta et al. 2009, Klein & Lopes 2011, Klein et al. 2012 and references therein).A prominent feature of the belt is the series of plutons of collision-and post-collision-type peraluminous and potassic granites to quartz-syenites formed at 2100 ± 15 Ma (Palheta et al. 2009, Klein et al. 2012).Few Neoproterozoic intrusions have been recognized to date.Notwithstanding, these intrusions are markers of different stages of evolution of the Gurupi Belt. 1.The Boca Nova Nepheline Syenite (732 ± 7 Ma, Klein et al. 2005) likely indicates the opening of the basin (rift) that was subsequently involved in the Neoproterozoic-Ediacaran orogeny; 2. the metamorphosed calc-alkaline Caramujinho microtonalite (624 ± 16 Ma, Klein & Lopes 2011), whose tectonic meaning is still uncertain, and 3. the collision-type Ney Peixoto granite (549 ± 4 Ma, Villas & Sousa 2007, Palheta et al. 2009).
The metasedimentary (passive margin?) sequences, which include the Gurupi Group and the Cabeça de Porco Formation, are interpreted as Neoproterozoic in age (Lopes & Klein 2014).The Piriá Formation, which is the subject of this paper, overlies the Gurupi Group and precedes the deposition of the Paleozoic intracratonic Parnaiba Basin.

ANALYTICAL PROCEDURES
U-Pb LA-ICP-MS analyses were undertaken at the Laboratório de Estudos Geocronológicos, Geodinâmicos e Ambientais of the Universidade de Brasília (UnB), Brasília, Brazil.The analyses followed procedures described in detail in Bühn et al. (2009).Concentrates of zircon were obtained by crushing the rock and then sieving and panning.Zircon crystals were hand-picked under a binocular microscope, mounted in epoxy resin, and polished with diamond paste; a conductive gold-coating was applied just prior to analysis.At UnB, the analyses were performed with a Thermo Finnigan Neptune multicollector inductively coupled plasma mass spectrometer with an attached New Wave 213 μm Nd-YAG solid state laser.The acquisition followed a standard -sample bracketing technique with four sample analyses between a blank and a GJ-1 zircon standard.The accuracy was controlled using the standard TEMORA-2 or UQZ.Raw data were reduced using an in-house program and corrections were done for background, instrumental mass-bias drift and common Pb, as described in Bühn et al. (2009).The ages and probability plots were calculated using ISOPLOT 3.0 (Ludwig 2003).Analyses were preceded by Cathodoluminescence and/or Backscattered Electron imagery done at UnB.
Zircon dating by the Pb evaporation method (Kober 1986) was conducted at the Laboratório de Geologia Isotópica (Pará-Iso) of the Universidade Federal do Pará, Belém, using the double filament array.Isotopic ratios were measured in a FINNIGAN MAT 262 mass spectrometer, and data were acquired in the dynamic mode using the ion-counting system of the instrument.For each step of evaporation, a step age is calculated from the average of the 207 Pb/ 206 Pb ratios.When different steps yield similar ages, all are included in the calculation of the crystal age.Common Pb corrections were made according to Stacey and Kramers (1975), and only blocks with 206 Pb/ 204 Pb ratios higher than 2500 were used for age calculations. 207Pb/ 206 Pb ratios were corrected for mass fractionation by a factor of 0.12% per a.m.u, given by repeated analysis of the NBS-982 standard, and analytical uncertainties are given at the 2σ level.
Whole rock Sm-Nd analyses were undertaken at the UnB and UFPA laboratories, and the analytical procedures for Sm-Nd analyses are described in Gioia and Pimentel (2000).Fifty mg of whole rock powders were mixed with a 149 Sm/ 150 Nd spike and dissolved in Savilex vessels.The Sm-Nd separation used cation exchange Teflon columns with Ln-Spec resin, then Sm and Nd were deposited in Re filaments, and the isotopic ratios were determined on a FINNIGAN MAT 262 mass spectrometer using the static mode.The Nd data were normalized to a 146 Nd/ 144 Nd ratio of 0.7219 and uncertainties in the Sm/Nd and 143 Nd/ 144 Nd ratios were about 0.4% (1σ) and 0.005% (1σ), respectively, based on repeated analysis of the BHVO-1 and BCR-1 standards.The crustal residence ages were calculated using the values of DePaolo (1981) for the depleted mantle (T DM ).
Whole rock powders were analyzed at the Acme Analytical Laboratories Ltd. in Vancouver, Canada, by Inductively Coupled Plasma Emission Spectrometry (ICP-ES) for major elements and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for trace elements, including the rare-earth elements, after fusion with lithium metaborate/tetraborate and digestion by diluted HNO 3 .Loss on ignition (LOI) was determined by weight difference after ignition at 1,000°C.Blank analyses were always below the minimum detection limit for each element, and the analytical protocol included the analysis of the reference materials (standards) CSS, DS7, SO-18, and OREAS76A.

General aspects
According to Lopes and Klein (2014), the Piriá Formation crops out in three basin segments that cover older units of the Gurupi Belt and of the São Luís cratonic fragment (Fig. 1).The thickness of the basin is unknown.The main and largest basin segment forms a hemi-graben with the main axis oriented to the north-south direction.This segment is limited to the west by a grossly north-south-trending, high-angle normal fault with associated conglomerate beds.The normal fault was displaced by orthogonal transfer faults (Figs. 2 and 3).The east-limiting normal fault is more continuous, dips with shallower angles and lack conglomerates associated to the fault plane.Limited paleocurrent information indicates preferential flow direction to the northwest and southwest.The western and smallest segment of the basin lies discordantly over the metasedimentary Cabeça de Porco Formation.The eastern segment has normal faulted contacts with granitoids of the basement, and the northern portion of this segment was displaced by post-sedimentation normal or strike-slip faults.

Lithological content
Historically, the Piriá Formation has been described as composed of arkose, subarkose, greywacke and subordinate conglomerate, siltstones with thin marble layers, and shale (Abreu et al. 1980, Costa 2000, Truckenbrodt et al. 2003).Some lines of evidence indicate that the rocks of the Piriá Formation underwent very low degree of metamorphism (anquimetamorphism), which includes (Costa 2000, Truckenbrodt et al. 2003, and this study): 1. the high degree of diagenesis, 2. presence of neoformed epidote in sandstones, 3. the large amount of sericite in pelites, 4. presence of local cleavage, and 5. presence of local polygonal texture.
Therefore, the prefix "meta" is implicit in the following descriptions of the Piriá rocks.
Only the conglomerate facies has been separated in the geological map by Lopes and Klein (2014).Syn-rift basal units, which are commonly observed in similar basins elsewhere, have not observed in the Piriá Basin.These include siliciclastic sequences formed in response to early fault displacements, in subaerial settings, with water supply enough to sustain fluvial systems (e.g., Prosser 1993).quartz (60%), feldspars (35%, predominantly plagioclase) and minor amounts of opaque minerals, zircon, garnet, epidote, sericite, biotite, chlorite and tourmaline.Epidote comprises detrital grains and grains formed from alteration of plagioclase, indicating anquimetamorphism (Fig. 4B).The dissemination of chlorite gives the greenish color to the rocks.Some portions of this facies have more than 15% greenish, slightly oriented matrix composed of quartz and chlorite and represent greywackes.These are well-sorted, fine-to very fine-grained rocks with sub-angular grains of quartz (30%), feldspars (40%) and lithic fragments (<11%).Detrital and neoformed epidote is common.The shales are brownish and greenish, fine-grained and laminated rocks, which are unevenly crosscut by thin quartz veinlets.Dykes of mafic rocks and carbonate-quartz-epidote small veinlets are locally observed, indicating possibly hydrothermal alteration.
Lithofacies sp: laminated siltstones and pelites this lithofacies is observed in the central-southern portion of the main basin segment.The fine-grained rocks show greenish grey to reddish colors and plane-parallel to slightly undulated lamination, which is characterized by the alternation of silt and clay layers (Fig. 4C).Tiny sericite flakes are visible only under the microscope.Very fine grained layers of subarkose and arkose occur interstratified within the layers of siltstone and pelites.The poorly sorted arkose/subarkose is composed of subangular grains of quartz (64 to 95%), K-feldspar and plagioclase (2 to 32%), and up to 11% of lithic fragments.The matrix totalizes 10% of the rocks and comprises quartz and muscovite.Well rounded feldspar grains indicate possible reworking of the associated arkoses.The transition between siltstones/pelites and arkose/subarkose is gradational.

Lithofacies ah: arkose with hummocky stratification
This lithofacies crops out in the western and main segments of the basin.It is composed of greenish grey, fine-to medium-grained and moderately sorted arkoses.The rocks show a variety of sedimentary structures, including plane-parallel stratification with tangential lamination at the top and base of the foresets, stratification truncated by wave-ripples, hummocky cross-stratification (Fig. 4D), large-scale cross-bedding, and local convolute structures (Fig. 4E), which are typical of subaqueous environment.Thickening of the arkose layers toward the top of the sedimentary package is observed locally.Coarse-grained and micro-conglomeratic sandstones are also common.The sandstones are well-sorted rocks composed of subrounded to angular grains of quartz (92%), feldspars (5%), and lithic fragments.Well-preserved detrital grains of epidote are commonly observed, indicating proximal source areas.

Lithofacies cg: oligomictic conglomerate
this lithofacies crops out along the fault that limits the western margin of the main basin segment (Figs. 2 and 3).The conglomerates are predominantly matrix-supported rocks, with clasts of quartz and rarely of metamorphic rocks.The quartz clasts are 1 to 3 cm large, subangular and set in a matrix composed predominantly of quartz and muscovite, and minor biotite and fine-grained rock fragments (Fig. 4F).Grains of muscovite are usually larger than those of other matrix minerals.Layers composed of mottled silt and clays occur as intercalations in the conglomerate package and show sharp, rarely gradational contacts.X-rays analysis detected the presence of abundant smectite and subordinate kaolinite, quartz and mica.

Major and trace elements
Whole rock major and trace elements results are presented in Table 1.The samples show large variability in    the major element chemical composition and comprise predominantly litharenite and greywacke, and subordinately arkose, subarkose and Fe-sand (Fig. 5).No geographic distribution pattern is observed in this variation.Al 2 O 3 shows negative correlation (r < -0.9) with SiO 2 , indicating that most of the silica is present as quartz.
The strong (r >0.9) to moderate (r = 0.63 to 0.75) positive correlation with MgO and Na 2 O, and with Fe 2 O 3 and CaO, respectively, might be explained by the Piriá sediments being controlled by the abundance of Fe-Mg oxides/silicates, feldspars and clays, with progressive dilution by increasing quartz contents.The rare earth elements (REE) are enriched with respect to the chondrite, and most of the samples show distribution similar to that of the upper continental crust (UCC), which is characterized by fractionation between light and heavy elements, and a weakly negative Eu anomaly (Fig. 6A).The large ion lithophile (LIL) and high field strength (HFS) elements (including some REE) are variably enriched when compared to primitive mantle values and also follow (especially the HFSE) the composition of the upper continental crust (Fig. 6C).This pattern is characterized by negative breaks in Nd, P and Ti, and by positive anomalies of Pb.The Piriá samples, however, show larger enrichment in Zr than that showed by the UCC.Three samples (EK31, EL34, EL43) do not follow the REE-LILE-HFSE patterns (Figs.6B and 6D, neihter the zircon and monazite accumulation trends.

Source-area weathering
The chemical index of alteration (CIA; Nesbitt & Young 1982, Nesbitt 2003) values range from 62 to 69, and plot above the feldspars join in the ACNK diagram (Fig. 7A), which indicates only moderate degree of sourcearea weathering.In addition, the samples plot between the granite and average shale compositions, suggesting conversion of feldspar to clays.No linear weathering trend is observed.On the other hand, the index of compositional variability (ICV; Cox et al. 1995, Potter et al. 2005) shows variance (Fig. 7B) that might be attributed    (1993) and Pettijohn (1975).(B) Plot of chemical index of alteration versus the index of chemical variability, after Cox et al. (1995) and Potter et al. (2005).Average rock values according to LaMaskin et al. (2008, and references therein).PAAS is post-Archean Australian average shale (Taylor & McLennan 1985).
CIA: chemical index of alteration; ICV: index of chemical variability.
to differences in weathering or variation in the composition of the source rocks.Considering the large chemical variation, and the small range of CIA values, it is likely that the composition of the source rocks has played a more important role than the chemical weathering.

U-Pb LA-ICP-MS results
Isotopic results were obtained for a greywacke (sample EL37A) from the main segment of the Piriá basin and for a subarkose (sample EK31) from the western segment.Sampling locations are shown in Figure 2, and the isotopic results of 115 zircon crystals (out of 127) that show less than 10.0% discordance and low common Pb (f 206 below 3.0%) concentrations are presented in Table 2.
Zircon crystals from sample EL37A are nearly all euhedral to subhedral; they present inclusions, rare fractures and lack evidence of sedimentary transport.The main internal feature is the oscillatory zoning (Fig. 8A).Although rare core/rim zoning can be observed, the very low metamorphic grade of the sample and the Th/U ratios indicate that they represent events recorded in the (magmatic) source.The 207 Pb/ 206 Pb apparent ages range from 2103 ± 8 to 2234 ± 16 Ma, and all but three ages fall within the 2120 to 2180 Ma interval, forming a nearly unimodal distribution, with a peak at 2140 Ma (Fig. 9).
The crystals from sample EK31 vary widely in shape and size.Fractures and inclusions are common.Most are subrounded but of low sphericity, with oscillatory or sector zoning; some are elongated and show well-preserved prisms and pyramids, suggesting little transport, and some are fragments of larger broken crystals with oscillatory zoning (Fig. 8B).The 207 Pb/ 206 Pb apparent ages and 206 Pb/ 238 U ages (for zircons younger than 1000 Ma) spread between 563 ± 9 and 2656 ± 12 Ma, and show polymodal distribution, with three main populations peaking at 591, 2047 and 2134 Ma (Fig. 9).Several statistically subordinate populations (up to 3 crystals each) occur at 635, 935, 1100, 1290, 1530, 1960, 2045, 2204, 2356, 2450 and 2656 Ma.The youngest concordant crystal (0.8%) yielded an age of 591 ± 8 Ma.

Pb-evaporation results
The 207 Pb/ 206 Pb ages obtained by the Pb-evaporation technique must be considered only as a minimum age of the detrital zircon, given that the degree of discordance cannot be evaluated (Kober 1986).Despite this limitation, the data presented here (Table 3) showed to be useful.The two samples comprise a conglomerate of the main segment of the basin (RL10) and the Fe-sandstone of the eastern segment (BP02).

Sm-Nd results
The whole rock Sm-Nd concentrations and isotopic ratios are shown in Table 4, with calculated values for the epsilon parameter and the depleted mantle model ages (T DM ).Samples show low REE contents, with Sm ranging from 0.63 to 6.51 ppm, and Nd between 3.23 and 29.9 ppm.The Sm/Nd ratios are mostly normal for clastic sedimentary rocks (e.g., McLennan & Hemming 1992), except for the high value (and high 147 Sm/ 144 Nd ratio) showed by sample EL42A.Model ages vary from 1.55 to 2.09 Ga (with a higher T DM age of 2.42 Ga for sample EL42A), and the εNd values calculated back to 540 Ma range from -5.8 to -15.4.

Depositional age
One zircon in sample BP02 (02/04) show an age of 407 ± 10 Ma, which is isolated and quite distinct from the populations age found in this sample.Since we cannot evaluate the degree of discordance of this analysis, and considering only the U-Pb results, a maximum depositional age for the Piriá Formation is indicated by the well-constrained peak at 591 Ma (Fig. 9), which is also the age of the youngest concordant zircon.This age is compatible with the geological evidence, which lead previous interpretations to establish the Ediacaran-Cambrian transition as the time of sedimentation (Abreu et al. 1980, Klein et al. 2005, Lopes & Klein 2014).The timing of rifting and the beginning of sedimentation (fault-related conglomerate deposition) remain uncertain.Considering that the collision-type Ney Peixoto Granite intruded at 549 ± 4 Ma, that the Piriá Formation presents anquimetamorphism, which is not present in the sedimentary rocks from the intracratonic Parnaíba Basin, and that no fossil record has been detected so far in the Piriá Formation, which differs from the fossil-rich Parnaíba Basin, the main   2.
stage of sedimentation in the Piriá Basin took place probably in the Cambrian period.

Sedimentary setting
Airborne magnetic geophysical data and field evidence (Lopes & Klein 2014) strongly indicate that the Piriá Basin (at least its better characterized central segment) corresponds to a hemi-graben type rift, which is bounded by normal faults.Considering the depositional age, this rift was formed by extensional tectonics that followed the end of the Brasiliano orogeny that built up the Gurupi Belt.The presence of the conglomerate facies associated with the western normal fault (Fig. 2) suggests deposition in alluvial fans, with debris flows associated with the high relief imparted by the bounding fault slope.All the other facies are likely related to the flexural portion of the basin.Accordingly, the syn-rift basal units were deposited by fluvial systems near the conglomerate unit, and with continued subsidence, this fluvial system migrated to the central portion of the rift.The evolution of this fluvial system took place in arid/semi-arid conditions, free of chemical weathering, which permitted the preservation of feldspars and detrital epidote.Truckenbrodt et al. (2003) suggested lacustrine or shallow marine setting for the deposition of the Piriá Formation.However, we understand that the presence of isolated structures that indicate storm deposited sediments (hummocky cross-stratification) is equivocal with respect to the definition of transitional to coastal sedimentary setting, and may only represent periods of instability of the subaqueous environment, such as flow variations within the fluvial canal.As such, we interpret the arkosic lithofacies (Ap and Ah) to be related to the fluvial canals, whilst the siltstone lithofacies is associated with low energy flow portions of the flood plains of the fluvial system.

Provenance
In a previous work, based on the heavy minerals content of sandstones of the Piriá Basin, mainly kyanite, staurolite, amphibole, epidote, Truckenbrodt et al. (2003) have suggested that proximal medium grade metamorphic rocks were the main detrital sources.These correspond to the Santa Luzia do Pará and Gurupi Group, according to the present day knowledge (Lopes & Klein 2014).This is partially valid, but the abundance of plagioclase-rich arkose and greywacke indicates felsic and intermediate rocks as major sources.
Immobile/less mobile elements and their elemental ratios are useful indicators of potential source of sediments (felsic, mafic, sedimentary), and of sedimentary recycling or hydraulic sorting.According to Fralick et al. (2009), in bivariate plots of immobile elements (e.g., Zr-Nb), samples that come from similar sources should align along a straight line that passes through the origin.This is not the case of the sediments of the Piriá Formation (Fig. 10A), indicating multiple sources.In addition, the Th-Ce and Ti-V relationships (Figs.10B and 10C) indicate sources with components enriched in Th and Ti and/or hydraulic enrichment in heavy minerals that contain these elements, such as monazite and Fe-Ti oxides (magnetite, titanite), respectively.Furthermore, hydraulic sorting is also suggested by the high Zr/Sc ratios when compared to the Th/Sc ratio (Fig. 11), indicating zircon addition (McLennan et al. 1993), which is consistent with the high whole-rock Zr concentration (Table 1).Figure 11 also shows that most samples of the Piriá Formation plot on the compositional field of potential Rhyacian source rocks occurring in the São Luís cratonic fragment and Gurupi Belt.In the same line, several other elements and elemental ratios indicate predominance of evolved felsic to intermediate igneous and recycled sedimentary sources for the sediments of the Piriá Formation (Fig. 12).
The cumulative age probability distribution diagrams (Fig. 9) show that the main zircon sources are Rhyacian rocks (or they reworked and erosional products) with ages similar to those found in orogenic rock associations of the São Luís cratonic fragment (2240( -2009 Ma; Ma;Klein et al. 2008b).The Sm-Nd data (Fig. 13) additionally support this interpretation.This is not surprising, since most of the units that crop out in the study region are Rhyacian in age.
Two other important sources have age peaks at 605 and 1517 Ma (Fig. 9).The age range of the youngest source (590 -645 Ma) falls within the known interval of development of the Gurupi Belt (732 -549 Ma).This range of ages also suggests that more orogenic felsic rocks might be present in the region than it is known to date, or, alternatively, that the sources might be located in adjacent Neoproterozoic orogens, such as the western portion of the Borborema Province (e.g., Ganade de Araújo et al. 2012).The source units for zircons with ages around 1500 Ma, which have been recorded in the conglomerate and subarkose samples, are more enigmatic.Rocks with this age are not known in the study region and are very rare in the Borborema Province (see Ganade de Araújo et al. 2012, Amaral et al. 2015).Furthermore, detrital zircons of this age in Cretaceous sediments and modern river sands from Gurupi-São Luís and Borborema are also very rare (Knudsen et al. 2015).These ages might represent unknown cryptic Mesoproterozoic rocks in the study area, or reworking of older sedimentary sources and distant provenance.In this regard, although these ages have been reported for sedimentary rocks of the Volta Basin in the West African Craton  The fields of potential Rhyacian source rocks from the Gurupi Belt and São Luís cratonic fragment were drawn with data from Klein et al. (2008bKlein et al. ( , 2009Klein et al. ( , 2012)).(Kalsbeek et al. 2008) (930, 1120(930, , 1290(930, and 1965 Ma) Ma) are also unknown in the region and might have also came from adjacent terranes.
The probability density plots (Fig. 9) of the different samples suggest that the Piriá Basin is a very heterogeneous basin with respect to their sources and paleogeography.Furthermore, differences in the age distribution of different samples suggest that the central (main sector) and eastern sectors might represent isolated segments or even sub-basins.The data of the greywacke sample EL37A (both restricted age range and the presence of crystals with little or no transport abrasion) indicate that their sediments came from a proximal source, probably in a micro-basin with a certain degree of confinement.In addition, the Nd data of this sample, showing T DM of 2.09 Ga, which is slightly  Rudnick & Gao (2004).
younger than the younger concordant zircons, indicate that the proximal source is very juvenile and that it possibly contains mafic rocks that do not contribute with zircons to detritus.On the other hand, the major peak of ~ 1500 Ma identified in the sample RL10 suggests that a terrain of this age was the major supplier at this area of the basin, but it had little expression, or is not even identified in other samples.Sample BP02 also highlights the differences in provenance of the central and eastern segment, since it shows the most important age population with ages between 526 and 680 Ma, which is only seen as a minor peak in sample EK31 from the small western segment.In summary, petrographic, whole-rock geochemistry and Nd data, and detrital zircon geochronology indicate that felsic to intermediate Rhyacian rocks of the São Luís cratonic fragment and Gurupi Belt were the main sources of sediments that filled the central segment of the Piriá Basin.Sedimentary recycling of zircon has also been important, and little Neoproterozoic sediments appear to have contributed with the sedimentary budget in this segment, which is in keeping with the present day knowledge about the evolution of the Gurupi Belt (see Klein et al. 2012, Klein & Lopes 2011).However, Neoproterozoic sources were of major importance in the eastern segment.A number of unknown sources are present in the zircon record and deserve further investigation.

Implications for tectonic evolution
The Ediacaran-Cambrian boundary in the South American platform corresponds to the period of closure of the Brasiliano/ Pan-African cycle of orogenies and beginning of the crustal extension and lithosphere thinning (i.e., the transition period, Brito Neves 2002) that gave rise to rifting and formation of intracratonic basins.Basins formed in this period are considered to represent either (1) precursor rifts that underwent slight inversion before the initiation of deposition of the intracratonic basins, such as the Jaibaras Basin, which preceded the formation of the Parnaíba Basin (Oliveira & Mohriak 2003), or (2) late-to post-orogenic basins (molasse?)still related to the end of the Brasiliano cycle of orogenies, for instance, the Camaquã Basin in the Sul-Riograndense Shield (Paim et al. 2014).
The rocks of the Piriá Formation (Basin) show anquimetamorphism, which is recorded by the replacement of feldspar clasts by epidote and chlorite, and by the replacement of the pelitic portion of the greywackes matrix by chlorite.In addition, limited deformation is evidenced by minor foliation of the matrix of greywackes, post-sedimentary undulations of sandstone and siltstone strata, and the presence of quartz veins and, locally, epidote-carbonate veinlets.In consequence, we interpret the Piriá Basin as a post-orogenic basin related to the final stages of evolution of the Gurupi Belt.

CONCLUSIONS
Based on rock associations and their field characteristics and relationships, and in whole-rock geochemistry, detrital zircon geochronology and Nd isotopes, the following conclusions can be drawn with respect to the Piriá Basin (Formation): 1.The maximum depositional age is 591 Ma, which places the formation in the Ediacaran-Cambrian boundary.2. The sedimentary setting comprises fluvial systems, including alluvial fans, plain floods and fluvial canals.3. Rhyacian felsic to intermediate rocks and recycled sedimentary rocks are the main sources of the sediments that filled especially the main segment of the basin, which likely come from erosion of rock units of the São Luís cratonic fragment and its reworked margin within the Gurupi Belt. 4. Neoproterozoic (526 -680 Ma) sources were important for the eastern segment of the basin, despite the poverty of known outcropping sources of this age in the region, and it is also recorded as a subordinate source in the western segment.5.A Mesoproterozoic source of ca.1500 Ma is well-represented in the conglomerate from the base of the central segment of the Piriá.Rocks of this age are not known so far in the study region.(2005( , 2008b( , 2009( , 2012( ), Palheta et al. (2009)), Klein & Lopes (2011).The Precambrian upper crust and arc rocks boxes are from McLennan & Hemming (1992).6.A series of sediment sources with ages that do not match with those found in the São Luís cratonic fragment and Gurupi Belt are also present and their origin remain uncertain.These might represent reworked sedimentary sources, provenance from local sources that were eliminated from the stratigraphic record by erosion or that underlie the exposed terranes, or provenance from sources located outside the study region (Amazonian Craton, Borborema Province?). 7. The basin is associated to the post-orogenic stage of evolution of the Gurupi Belt.

Figure 2 .
Figure 2. Simplified geological map of the study area (adapted from Lopes & Klein 2014 and Klein & Sousa 2012), with location of samples used in geochronological and Nd analyses.

Figure 3 .
Figure 3. Schematic block diagram depicting the spatial relationships of the Piriá Basin, structural features and lithofacies subdivision.

Figure 4 .
Figure 4. Field and microscopic aspects of rocks of the Piriá Formation.(A) Arkose of the Ap lithofacies with plane-parallel lamination.(B) Photomicrograph (crossed polarizers) of the arkose showing epidote crystals around detrital quartz.(C) Laminated siltstone of the Sp lithofacies.(D) Arkose of the Ah lithofacies with hummocky stratification.(E) Convolute structures in arkose.(F) Conglomerate with quartz pebbles.

Figure 5 .
Figure 5.Chemical classification of the rocks of the Piriá Formation (Herron 1988).

Figure 6 .
Figure 6.(A, B) Chondrite-normalized rare earth elements and (C, D) primitive mantle-normalized diagrams for the rocks of the Piriá Formation.Normalization is according to Boynton (1984) and Sun & McDonough (1989), respectively.The thick dashed line stands for the Upper Continental Crust (data from Rudnick & Gao 2005).Samples are separated into two sets for each normalization (A -C and B -D) for clarity (see text for discussion).

Figure 8 .
Figure 8. Cathodoluminescence (EL37) and backscattering electron (EK31) images of typical detrital zircon crystals from the Piriá Formation.Scale bars are 300 mm in A, and 200 mm in B, and the spots are numbered as in Table2.

Figure 9 .
Figure 9. Cumulative age probability plots for detrital zircon of the Piriá Formation.The grey bars show the time interval of known magmatic activity in the São Luís cratonic fragment (2240 -2009 Ma) and Gurupi Belt (732 -549 Ma).

Table 1 .
Major and trace elements chemical composition of samples from the Piriá Formation.
nd: not determined

Table 2 .
U-Pb isotopic results from detrital zircon of the Piriá Formation.

Table 3 .
Isotopic results obtained from the Pb-evapororation technique in detrital zircon from the Piriá Formation.

Table 4 .
Whole rock Sm-Nd data for metasedimentary rocks of the Piriá Formation.
10 Figure 10.Chemical plots for sedimentary rocks of the Piriá Formation, for evaluation of single or multiple sources and hydraulic sorting (see Fralick et al. 2009).(A) Zr-Nb, (B) Ce-Th, and (C) V-TiO 2 diagrams.
, they are also rare and are unlikely sources of the Piriá zircons.Minor Siderian and Archean ages (2350, 2450 and 2660 Ma) are present in inherited zircon from Rhyacian granitoids of the basement of the Gurupi Belt (Palheta et al. 2009, Klein et al. 2012) and may additionally suggest an underlying crust of this age.Other subordinate ages Provenance of the Piriá Basin, Gurupi Belt Brazilian Journal of Geology, 46(Suppl 1): 123-144, June 2016 Figure 12.Bivariate plots for provenance evaluation.(A) Zr versus TiO 2 , (B) Al 2 O 3 versus TiO 2 , (C) La/Sc versus Co/Th, (D) discriminant function 1 and 2, from Roser & Korsch (1988), (E) Hf versus La/Th (after Floyd & Leveridge 1987); the cross stands for the Upper Continental Crust of