Contributions to the petrography , geochemistry and geochronology ( UPb and Sm-Nd ) of the Paleoproterozoic effusive rocks from Iricoumé Group , Amazonian Craton , Brazil

Resumo. CONTRIBUTIONS TO THE PETROGRAPHY, GEOCHEMISTRY AND GEOCHRONOLOGY (U-Pb AND Sm-Nd) OF THE PALEOPROTEROZOIC EFFUSIVE ROCKS FROM IRICOUME GROUP, AMAZONIAN CRATON, BRAZIL . O extremo sul do escudo das Guianas no craton Amazonico hospeda excelentes exposicoes de rochas efusivas acidas a intermediarias de idade Paleoproterozoica, inseridas no Grupo Iricoume, as quais exibem notavel estado de preservacao de texturas e estruturas. As rochas acidas sao riolitos a riodacitos porfiriticos de granulacao fina a media, com alto teor em SiO 2 e alcalis, bem como elevados conteudos de Rb, Zr, Nb+Ta, La+Ce e 10 4 Ga/Al, com baixos conteudos de Fe 2 O 3tot , TiO 2 , CaO, Sr e Co. Exibem composicao subalcalina, metaluminosa a peraluminosa, alem de atributos geoquimicos compativeis ao magmatismo do tipo A. Em diagramas de ambiencia tectonica, essas rochas se posicionam sobre o campo pos-colisional a intra-placa. Por outro lado, as rochas intermediarias sao basalto andesiticos a andesitos porfiriticos de granulacao fina, relativamente enriquecidos em  TiO 2 , Al 2 O 3 , Fe 2 O 3total , MgO, CaO, Sr e Co, com baixo conteudo de SiO 2 , K 2 O,  Rb, Zr, Nb+Ta, La+Ce. Exibem composicao subalcalina, metaluminosa e se posicionam sobre o campo de basalto intra-placa. O conteudo em SiO 2 revela uma associacao bimodal marcada por uma discordância entre rochas intermediarias (SiO 2 67%). A idade U-Pb de cristalizacao das rochas acidas, obtida em cristais de zircao por LA-MC-ICPMS e de 1.882 ± 11 Ma. Os dados isotopicos Sm-Nd obtidos para os diferentes tipos de rochas revelaram idade modelo Nd T DM entre 2.59 – 2.16 Ga e valores de eNd(t) entre -5.78 to 0.03, indicando que a evolucao magmatica foi dominantemente relacionada ao retrabalhamento de crosta continetal Paleoproterozoica (crosta Transamazonica) durante o periodo Rhyaciano-Sideriano, com algum grau de mistura com magmas derivados do manto ou contaminacao por crosta Arquena. As informacoes petrograficas, geoquimicas e geocronologicas apresentadas neste trabalho sugerem um ambiente pos-colisional a intra-placa para o vulcanismo Iricoume, sob a um sistema complexo de caldeiras com varios condutos eruptivos, bem como eventual colpaso e resurgencia de magmas, envolvendo elevacao da crosta inferior e geracao de magmas basalticos em um regime extensional.

The Iricoumé-Mapuera magmatic association consists of effusive and pyroclastic rock units of acid to intermediate composition forming the Iricoumé Group, which is intruded by geochemically similar alkali-rich granites known as the Mapuera Intrusive Suite.The northeastern region of Amazonas state (Fig. 1) displays excellent exposures of the Iricoumé-Mapuera association, which, during the last decade, have become the object of scientific research focusing on the petrographic, geochemical and geochronological characteristics (Valério 2006(Valério , 2011;;Ferron 2006;Pierosan 2009;Marques 2011;Costa 2010).These studies have demonstrated that the Iricoumé-Mapuera association is formed by A-type magmatism with metaluminous to weakly peraluminous compositions, with geochemical affinity towards within-plate to post-collisional tectonic setting.The available geochronological data, mainly those obtained from the zircon 207 Pb/ 206 Pb evaporation method, indicate crystallization ages between 1879 and 1897 Ma.However, Nd-Sr-Hf isotopic data are still limited or nonexistent.Tab. 1 summarizes the isotopic information available for the Iricoumé volcanic rocks in the southern Guyana shield.
In this study, we present new data on the petrography, whole-rock geochemistry and geochronology (U-Pb and Sm-Nd) of effusive rocks from the Iricoumé Group in the northeastern region of Amazonas state, and compare them with the information available in the literature.Their petrogenesis and the geodynamic evolution associated with the Paleoproterozoic volcanic processes in the Amazonian craton.

ANALYTICAL METHODS
The petrographic investigations were performed at the microscopy laboratory of the Department of Geosciences, Universidade Federal do Amazonas (DEGEO-UFAM).The samples were chosen and prepared for geochemistry and isotopic (Sm-Nd) analyses at the laboratories of the DEGEO-UFAM, using conventional crushing in a jaw mill and in agate pot.Geochemical analyses were undertaken at the ACME Analytical Laboratories, Vancouver, Canada.The samples were analyzed for major elements (SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3tot , MnO, MgO, CaO, Na 2 O, K 2 O and P 2 O 5 ) by Inductively Coupled Plasma-Emission Spectrometry and for trace and rare-earth elements by Inductively Coupled Plasma-Mass Spectrometry (ICPMS).
The isotopic analyses (U-Pb and Sm-Nd) were carried out at the isotope geology laboratories of the Universidade de Brasília, by applying the analytical procedures described below: ■ U-Pb analyses by LA-MC-ICPMS followed the analytical procedure described by Bühn et al. (2009).Zircon concentrates were extracted using conventional gravimetric and magnetic separation techniques.The zircon grains were selected under a binocular microscope to obtain fractions of similar size, shape and colour.For in situ U-Pb isotopic analysis, hand-picked zircon grains were mounted in epoxy mounts and polished to obtain a smooth surface.The samples were mounted in an in-house, specially designed laser cell and loaded into a New Wave UP213 Nd:YAG laser (λ = 213 nm), linked to a Thermo Finnigan Neptune Multi-collector ICPMS.Helium was used as the carrier gas and mixed with argon before entering the ICP.
The laser was run at a frequency of 10 Hz and energy of ~ 100 mJ/cm 2 with a spot size of 30 µm for U-Pb dating and 40 µm for Hf isotopic analyses.Plotting of U-Pb data and age calculations were performed using ISOPLOT 3.0 (Ludwig 2003), and errors for isotopic ratios are presented at 1σ level.■ Sm-Nd isotopic analyses followed the method described by Gioia & Pimentel (2000).Whole-rock powders (ca.50 mg) were mixed with a 149 Sm-150 Nd spike solution and dissolved in Savillex capsules.Sm and Nd extraction of whole-rock samples followed conventional cation exchange techniques, using Teflon columns containing  phosphoric acid (HDEHP) supported on PTFE powder).Sm and Nd samples were loaded on re-evaporation filaments of double filament assemblies and the isotopic measurements were carried out on a multi-collector Finnigan MAT 262 mass spectrometer in static mode.Uncertainties for Sm/Nd and 143 Nd/ 144 Nd ratios are better than ± 0, 2% (2σ) and ± 0.003% (2σ), respectively, based on repeated analyses of international rock standards BHVO-1 and BCR-1.143 Nd/ 144 Nd ratios were normalized to 146 Nd/ 144 Nd of 0.7219 and the decay constant (λ) used was 6.54 × 10 −12 .

GEOLOGICAL SETTING
The the Silurian-Devonian to Cretaceous sedimentary cover of the northern edge of the Amazon Basin (Fig. 1).The Paleoproterozoic basement includes gneisses, migmatites, meta-granites, amphibolites and charnockites comprising the so-called Jauaperi/Anauá Complex.The geochronological data on the basement rocks are still inconclusive and vary from 2.02 to 1.86 Ga (Santos et al. 2002;Reis et al. 2003;CPRM 2006;Almeida 2006).In the study area, the nature of these metamorphic rocks is controversial.Some geologists believe that they represent products of contact metamorphism produced by Paleoproterozoic Mapuera Intrusive Suite, represented by the A-type São Gabriel batholith, but others suggest that they are simply the exposure of Paleoproterozoic basement rocks (Fig. 1C).Four Paleoproterozoic (1900 -1782 Ma) involving I-and A-type magmatic events are recognized in the region (Fig. 1B) as follows: (1) the Água Branca Intrusive Suite (1900 -1890 Ma) is characterized by an expanded granite series that varies from predominantly porphyritic to seriate-textured monzogranites and granodiorites to diorites.These rocks are metaluminous to peraluminous, calc-alkaline with high-K I-type geochemical signature (Araújo Neto & Moreira 1976;Almeida 2006;CPRM 2006;Almeida & Macambira 2007;Valério 2006Valério , 2011;;Valério et al. 2006Valério et al. , 2009Valério et al. , 2012)); (2) the Iricoumé Group (1897 -1881 Ma) comprises effusive rocks and pyroclastic flow associations.These rocks are metaluminous to weakly peraluminous, subalkaline with high-K and A-type affinity; (3) the Mapuera Intrusive Suite (1889 -1871 Ma) is represented by monzogranite, syenogranite and alkalifeldspar granite stocks.These rocks are weakly peraluminous, subalkaline with high-K, and have geochemical characteristics similar to A-type granites.Due to the geochemical and geochronological similarities between felsic volcanic rocks of the Iricoumé Group and granitic rocks of the Mapuera Suite, these rocks have been interpreted as a coeval volcano-plutonic association and inserted as part of the regional units known as the Uatumã Supergroup (Oliveira et al. 1975;Araújo Neto & Moreira 1976;Veiga et al. 1979;Schobbenhaus et al. 1994;Costi et al. 2000;CPRM 2006;Valério et al. 2009Valério et al. , 2012;;Ferron et al. 2006Ferron et al. , 2010;;Pierosan et al. 2011aPierosan et al. , 2011b;;Berreto et al. 2013); and (4) the Madeira Intrusive Suite (1834 -1782 Ma) occurs in the Pitinga Sn-mine area and is composed by biotite-syenogranite, alkali-feldspar rapakivi, hipersolvus and albite granite stocks bearing cryolite-zircon-cassiterite-pyroclore-columbite-xenotime mineralization.These rocks are peraluminous to metaluminous, rarely peralkaline, and also present geochemical and mineralogical characteristics of A-type granites (Horbe et al. 1991;Fuck et al. 1993;Lenharo 1998;Costi et al. 2000Costi et al. , 2005;;Ferron et al. 2006Ferron et al. , 2010)).The Paleoproterozoic geological record is completed by the volcano-sedimentary cover of the Urupi Formation and by the ca.1.79 Ga old mafic-tholeitic rocks of the Quarenta Ilhas Formation (CPRM 2006).In addition, a magmatic event marked by gabbroic intrusions is also recognized in the region but has not been studied in detail yet (Souza & Nogueira 2009).
The Meso-to Neoproterozoic geological context is represented by the alkaline magmatism of the Seringa Formation (1.07 Ga) and by siliciclastic sedimentary sequence of the Prosperança Formation (CPRM 2006;Cunha et al. 1994;Nogueira 1999), while the Phanerozoic Trombetas and Javari Groups represent the Amazon sedimentary basin (Cunha et al. 1994).In addition, Mesozoic diabase dikes and Cenozoic laterite cover occur in the area as well (CPRM 2006;Horbe et al. 2001).

Effusive Rocks of the Iricoumé Group
The Iricoumé Group exposures are partially covered by dense rainforest, thick soil cover, by the lake of the Balbina hydroelectric dam, as well as by restricted access areas (e.g.indigenous reserves, Fig. 1B).However, in the farms and agricultural settlement areas, as well as in the mining fronts, excellent exposures of the Iricoumé volcanism are observed, with remarkable preservation of original igneous textures and structures.Geology, 44(1): 121-138, March 2014 The Paleoproterozoic effusive rocks of the Iricoumé Group

Petrography
The acid rocks are rhyolites to rhyodacites, which exhibit red to pink-gray colour, fine to medium-grained porphyritic texture marked by feldspars and quartz-phenocrysts embedded in a fine-grained matrix.Amygdales and fractures filled by late-to post-magmatic minerals, such as epidote, chlorite, carbonate and quartz, are also recognized.Occasionally, the outcrops display magmatic layering with sense of flow to SW (Fig. 2A).The absence of vitroclastic and lithoclastic textures, associated with myrmekite features locally present in these rocks, indicates that these are sub-aerial to aerial lava-flows.However, in some outcrops displaying subhorizontal to curvy-planar layering, where occasional pumice, lithic and fiamme are observed, suggesting that these represent ignimbrite-flows modified by high-temperature rheomorphic process (e.g.Pioli & Rosi 2005;Andrews & Branney 2011), which makes it difficult to distinguish between ignimbrite-flows and lava-flows.
In general, the acid rocks present modal contents of alkali-feldspar, plagioclase (oligoclase, An 26-30 ) and quartz-phenocrysts varying from 15 to 60 vol.%.They display euhedral to subhedral shapes, with lengths ranging from 0.3 to 3 mm, and commonly exhibit corrosion and resorption features, undulating extinction and are low to moderately fractured (Fig. 2B and C).Bulbous-type myrmekite is a common occurrence in the rims of larger crystals.Quartz-feldspar groundmass presents fine-grained phaneritic to aphanitic textures, with crystals usually oriented in the magmatic-flow circumventing the phenocrysts.They also have subhedral crystals of hornblende, biotite, titanite, zircon, hematite, magnetite, pyrite, ilmenite, allanite and apatite as accessory.Hornblende and biotite crystals are partially chloritized as phenocrysts shapes with lengths ranging from 0.3 to 0.8 mm in the finer groundmass.Frequently, ilmenite crystals involved by a thin rim of rutile are observed, and may probably represent the product of Fe-Ti excess exsolved from the ilemenite structure during temperature decrease or oxygen fugacity variation (e.g.Haggerty 1991).
The intermediate rocks are andesite to trachy-andesite, which exhibit green to green-greyish colour, fine-grained porphyritic texture highlighted by plagioclase-phenocrysts embedded in a fine-grained to aphanitic matrix.The outcrops also display subhorizontal layering marking the magmatic flow and frequent amygdales filled by late-to post-magmatic minerals, such as epidote, chlorite, carbonate and quartz (Fig. 2D).The phenocrysts, ranging from 25 to 50 vol.%,are mainly plagioclase (andesine, An 38-46 ), and rare alkali-feldspar.Plagioclase-phenocrysts have euhedral to subhedral prismatic shapes, with lengths ranging from 1 to 15 mm usually forming a glomeroporphyritic texture (Fig. 2E).They exhibit albite-carlsbad twinning, are slightly fractured and have corroded edges, moderate decalcification stage marked by epidote and white-mica generation.The groundmass has a fine phaneritic texture and is composed of short lath-shaped plagioclase crystals with subophitic to trachytic texture oriented along the magmatic-flow, circumventing the phenocrysts.They also have partially chloritized hornblende and biotite, as well as pyroxene crystals partially replaced by fibrous amphibole (actinolite-tremolite) form interstitial mineral arrangement.Quartz, titanite, magnetite, ilmenite, pyrite, chalcopyrite and apatite are present as accessory (Fig. 2F).

Whole-Rock Geochemistry
Whole-rock compositions for acid to intermediate rocks of the Iricoumé Group are shown in Tab. 2 and are discussed below.
The acid rocks of the Iricoumé Group have been recognized as A-type (Valério et al. 2009;Ferron et al. 2010;Pierosan et al. 2011a).In the study area, these rocks also present geochemical attributes compatible with A-type magmatism, such as relatively high alkalis (Na 2 O + K 2 O), Zr, Th, Nb and La + Ce contents, as well as high 10 4 Ga/Al = 2.35 -2.90 ratios (Collins et al. 1982;Clemens et al. 1986;Whalen et al. 1987;Eby 1992).On the Zr and Zr + Nb + Ce + Y versus 10 4 Ga/Al diagrams, the samples plot in the A-type granite fi eld (Fig. 4A and 4B), and on the Nb-Y-Ce diagram, the samples plot in the A 2 -subtype fi eld (Fig. 4C), with Y/Nb ratios > 1.2, indicative of magmas emplaced during extensional collapse according to Eby (1992).Th ese geochemical characteristics are consistent with the information obtained about the Iricoumé volcanism in the Pitinga mining district, located about 150 km to the north of the study area (Fig. 1B).versus Nb/Y diagrams, these rocks tend to plot in andesite to trachy-andesite fi elds (Fig. 3B and 3C).On the A/NK versus A/CNK diagram, this sample plots in the metaluminous fi eld (Fig. 3D).
In the multi-element spidergrams, these rocks also demonstrate a pattern similar to upper continental crust, marked by positive Rb, Th , K, La, Ce, Nd, Sm, Zr and Hf anomalies and negative Ba, Nb, Ta, Sr, P and Ti anomalies (Fig. 3F, e

Geochronology (U-Pb and Sm-Nd)
U-Pb and Sm-Nd data for the volcanic rocks of the Iricoumé Group are listed in Tabs. 3 and 4, and are discussed below.
Th e rhyolite SVP3C sample (810915/9823114 UTM coordinates) was chosen for U-Pb by LA-MC-ICPMS isotopic analyses and a total of 28 zircon crystals were handpicked.Zircon crystals are euhedral to subhedral, pale yellow to pale pink, zoned and displaying short-to long-prismatic shape (180 -500 µm), with few to moderate micro-inclusions or fractures.Typical fi ne oscillatory zoning may be observed (Fig. 5).Th e U-Pb results obtained for 21 analyzed zircon crystals allowed the calculation of a discordia line indicating an upper intercept age of 1882 ± 11 Ma, which is here interpreted as the crystallization age of the rhyolite (Fig. 5).Th ese data are consistent with the Pb-Pb interval age (1881 -1896 Ma) known for the Iricoumé volcanism in this region (Tab. 1;e.g. Macambira et al. 2002;Valério 2006;Ferron 2006;Berreto et al. 2013).
As all samples showed 147 Sm/ 144 Nd ratios in the range of 0.10 -0.12, these data were useful for calculation of T DM model ages.For the acid rock samples, the ε Nd (t = 1882 Ma) values ranged from -4.43 to 0.03, while Nd T DM model ages ranged from 2.59 to 2.16 Ga, and fractionation indexes (f Sm/Nd ) between -0.38 and -0.51.Th e f Sm/Nd values were calculated using the Goldstein et al. (1984)    model ages are between 2.44 and 2.13 Ga.The fractionation indexes (f Sm/Nd) are between -0.34 and -0.45.This data is also consistent with the Sm-Nd data known for Iricoumé volcanism in this region (Tab.1).
The Nd T DM model age values for all samples are between 2.59 and 2.16 Ga.The ε Nd (t) values from -4.78 to 0.03 indicate that the magmatic evolution was dominantly produced by related ensialic reworking of older Paleoproterozoic crust, with some degree of mixing of the felsic magmas with Paleoproterozoic (1.88 Ga) mantle-derived magmas.The diagram ε Nd versus time summarizes the Nd evolution patterns provided by the rocks samples investigated (Fig. 6), and it suggests the existence of two isotopically distinct groups: (1) the group (C6604, SVP2E, SVP3A, SVP3B) display ε Nd (t) values close to zero, suggesting a more contribution juvenile; and (2) the group (SVP3C1, GC3735, SVP4A) have ε Nd (t) values that are substantially more negative, between -3.4 and -5.5, indicating increased participation of crustal material.
These Sm-Nd isotopic data are interpreted as indicative of reworking, during the Orosian, of a previous Rhyacian or Siderian juvenile continental crust (Transamazonian crust crust-forming event), or with probable contamination by Archean crust (Marques et al. 2007).

DISCUSSION
The volcanic rocks of the Iricoumé Group present some important geochemical differences especially on aluminum saturation index (A/NK versus A/CNK), K 2 O/Na 2 O ratio, alkali contents (Na 2 O + K 2 O), as well as Zr + Nb + Ce + Y and contents.On Harker diagrams, the SiO 2 contents clearly indicate the bimodal nature of the magmatism with a compositional gap between intermediate (SiO 2 < 57.7 wt%) and acid rocks (SiO 2 > 67 wt%), without delineation of geochemical common trends, suggesting different evolution and magmatic sources (Fig. 7).The progressive increase of SiO 2 is followed by decrease in Al 2 O 3 , Fe 2 O 3tot., MgO, CaO, TiO 2 , P 2 O 5 and Sr contents, indicating that plagioclase, hornblende, pyroxene, biotite and apatite have an important role in progressive fractional crystallization, especially in intermediate rocks.On the other hand, Na 2 O, K 2 O and Rb contents tend to show a positive correlation with increase of SiO 2 , suggesting action of the plagioclase and alkali-feldspar fractionation, mainly in acid rocks.Ta, Nb, Zr, Ce and Y behave as incompatible elements with their contents increase towards to acid rocks, probably likely to alkali-feldspar fractionation or to accumulation of accessory minerals.
The inverse correlation between the less-and more-differentiated rocks on the Rb versus Sr diagram is also noteworthy (Fig. 8), suggesting that plagioclase and alkali-feldspar were the main minerals fractionated during the magmatic evolution.On Hf + La + Ce versus Zr diagram (Fig. 8), note a positive correlation marked by progressive enrichment of HFS elements towards more differentiated rocks, which is the result of fractionation from the accumulation of incompatible elements likely to accessory minerals structure, such as zircon, allanite, apatite and titanite.These petrogenetic characteristic are consistent with the data available for Iricoumé volcanism in the Pitinga mining district (Ferron et al. 2010;Pierosan et al. 2011a).
Although typical basaltic composition rocks have not been registered in this region, it is possible that some rocks classified here as andesites are originally basalts modified by later hydrothermal process or amygdaloidal texture presence.This is suggested by the contents of some elements that are considered to be unusual for typical andesite, such as variation in MgO, slightly depleted Na 2 O, high K 2 O, CaO and LOI on whole-rock geochemistry (Tab.1).Therefore, it is possible that some basaltic rocks had their original chemical composition modified and moved towards the andesitic composition.For further evaluation of crustal contamination, Zr/Y versus Nb/Y and Zr/Y versus Ce/Y ratios of studied andesite samples along with average crust and primordial mantle are plotted (Fig. 8).The samples plot on a trend very close to average crust, which suggests crustal contamination.This pattern associated with negative ε Nd (t) data indicates that these andesites are probably derived either from a modified mantle source, or the original magmas were contaminated with older crustal rocks during ascent and crystallization.
With respect to the tectonic setting of origin, the multicationic R1 versus R2 values diagram indicate that the acid rocks are roughly distributed between late-orogenic and post-orogenic, whereas on the Rb versus Y + Nb diagram, the samples plot on the post-collisional or within-plate domain (Fig. 9A and 9B).This geochemical characteristic is also consistent with the data available for Iricoumé felsic rocks in the Pitinga mining district (Ferron et al. 2010;Pierosan et al. 2011a).On the other hand, for intermediate rocks tectonic discriminant diagrams applied to basalts, such as Nb*2 versus Z/4 versus Y ternary (Meschede 1986) and Zr/Y versus Zr (Rollinson 1993), were used.The samples plot in the within-plate basalt fields (Fig. 9C and 9D).
Several geological mechanisms have been suggested to explain the association between A-type granite/rhyolite with basalt in post-collisional or in within-plate tectonic settings (Whalen et al. 1996;Creaser et al. 1991;Turner . 1992;Dall'Agnol et al. 1994;Bonin 2007).In the southernmost Guyana shield has been recognized that I and A-type magmatism took place between 1.92 and 1.86 Ga in distinct tectonic settings.During the initial stages, the I-type magmatism is represented by the Água Branca Intrusive Suite emplaced in syn-to post-collisional uplift setting (Valério 2006(Valério , 2011;;Valério et al. 2006Valério et al. , 2009)).Th is was followed by a more-evolved stage, represented by the Iricoumé-Mapuera association emplaced in within-plate or post-collisional tectonic settings as volcano-plutonic ring systems associated to an extensional event (Valério et al. 2009;Ferron et al. 2010;Pierosan et al. 2011a).
Other authors (e.g.Almeida 2006;Almeida et al. 2007), on the other hand, suggest that the emplacement of the Água Branca and Mapuera suites occurred during the same tectonic stage in response to asthenosphere rise producing mantle-derived magmas which promoted extensive re-melting of the lower crust during underplating.Th e ε Nd (t) values presented in this paper suggested mixing of two-magmas sources (mantle-and crustal-derived magmas) for the Iricoumé volcanism.Pierosan et al. (2009) suggest that the slab-breakoff tectonic model could have been involved to explain the participation of two magmatic sources during a post-collisional setting, promoting asthenosphere uplift and leading to the melting of the metasomatized overriding mantle lithosphere (e.g.Davies & Blanckenburg 1995).Another alternative tectonic model would involve mantle plume and lithospheric delamination (e.g.Saunders et al. 1992;Elkins-Tanton 2005;Pirajno 2007;Pirajno & Hoatson 2012), but the lack of additional isotopic (Sm-Nd, Sr-Sr, Lu-Hf ) and petrological data, as well as seismic tomography investigations, still inhibit the better understanding of the tectonic evolution for the region.
In the southernmost Guyana shield, the extensive Iricoumé volcanism is associated with caldera system complex, identified mainly in the Pitinga mining district (Ferron et al. 2002;Pierosan et al. 2011b), which displays several eruptive conduits, as well as collapse structures, favouring the larger production of pyroclastic  material (Souza et al. 2007).On the basis of the study by Bryan (2007) and Bryan & Ernst (2008) proposals, discussed by Klein et al. (2012), the high magma volume produced at a short time registered in this region is compatible with Silicic Large Igneous Province (SLIP).

CONCLUSIONS
T h e Ir i c o u m é v o l c a n i s m i n t h e s o u t h e r nmost Guyana shield consists of a bimodal association comprising andesite/basalt to rhyolite components.The acid rocks are subalkaline with high-K, metaluminous-to-peraluminous compositions, geochemically compatible to A-type magmatism emplaced in post-collisional to within-plate tectonic settings.The intermediate rocks are subalkaline with medium-to high-K contents, metaluminous nature and geochemically compared with within-plate and post-collisional uplift toleiitic basalts.
The U-Pb age presented here for the rhyolites (1882 ± 11 Ma) is consistent with the Pb-Pb age already reported for the Iricoumé volcanic rocks in this region.The T DM model ages (2.59 -2.16 Ga) and ε Nd (t) ( -5.43 -0.The Paleoproterozoic effusive rocks of the Iricoumé Group (Rhyacian or Siderian) juvenile continental crust with some contribution from mantle-derived basaltic magmas.It is also possible that there was some contamination by Archean crust.
The data presented here supports the within-plate or post-collisional tectonic setting proposed for the Iricoumé volcanism, most likely to crustal uplift and melting promoted by the emplacement of basalt magma in an extensional regime.This extensive volcanic event was associated with caldera system complex with the eventual collapse and magma resurgence during a short-time interval, comparable to other SLIP in other parts of the world.

Figure 2 .
Figure 2. Macroscopic and microscopic features of the eff usive rocks of the Iricoumé Group.(A) Magmatic fl ow layering with sense of fl ow to SW in acid rocks; (B and C) porphiritic texture in thin section on rhyolites and rhyodacites, respectively; (D) amygdales and fractures fi lled by late-to post-magmatic minerals in andesites; (E and F) porphyritic to glomeroporphyritic micro-texture with amygdales in andesites.

Figure 5 .
Figure 5. Images transmitted light microscopy images of zircon and concordia diagram rhyolite sample SVP3C showing a crystallization age of 1882 ± 11 Ma.

Figure 6 .
Figure 6.ε Nd (0) versus time (Ga) diagram applied to acidic and intermediary effusive rocks from Iricoumé Group.Prepared in accordance to depleted mantle model of De Paolo (1981).

Figure 7 .Figure 8 .
Figure 7. Harker-type diagrams applied to acidic and intermediary eff usive rocks from Iricoumé Group (blank circle = acid rocks and fi lled circle = intermediate rocks).
-plate basalts C: mid-ocean ridge basalts (MORB) D: MORB and volcanic-arc basalts E: MORB and within-plate basalts Zr : within-plate alkali basalts AII and C: within-plate tholeiite basalts B: P-type MORB basalts D: N-type MORB basalts C and D: volcanic arc basalts C 03) indicate that the magmatic evolution was preferentially related to reworking of older Paleoproterozoic 134 Brazilian Journal of Geology, 44(1): 121-138, March 2014

Table 1 .
Isotopic resume available for the Iricoumé volcanism in the southern Guyana shield 124Brazilian Journal of
equation.For the intermediate rock samples, for which zircon ages are not available, the age of the rhyolite was assumed as their crystallization age, based on stratigraphic correlations.Th e ε Nd (t = 1882 Ma) between -3.74 and 0.80, whereas T DM 129 Brazilian Journal of Geology, 44(1): 121-138, March 2014

Table 4 .
Sm-Nd data for the effusive rocks of the Iricoumé Group