A. F. da Silva Filho 1*, I. P. Guimarães 1, E. Dantas 2, L. M. Cocentino 1, D. R. Lima 1, E. Rufino 1

Transcrição

1 Versão online: Comunicações Geológicas (2014) 101, Especial I, IX CNG/2º CoGePLiP, Porto 2014 ISSN: X; e-issn: X Geochemistry and gechronology of syn-collision to syntranscurrence Ediacaran transalkaline granites from the PEAL domain, Borborema Province, NE Brazil Geoquímica e geocronologia de granitos transalcalinos Ediacaranos sin-colisionais a sin-transcorrentes do domínio PEAL, Província Borborema, NE Brasil A. F. da Silva Filho 1*, I. P. Guimarães 1, E. Dantas 2, L. M. Cocentino 1, D. R. Lima 1, E. Rufino LNEG Laboratório Nacional de Geologia e Energia IP Artigo Curto Short Article Abstract: The Borborema Province is the western part of a major late Neoproterozoic mobile belt that extends from Brazil through NW Africa in pre-drift reconstructions. It has been divided into four tectonic domains. The Pernambuco-Alagoas (PEAL) domain occurs along the SE part of the Borborema Province. It contains the largest granitic batholiths of the Borborema Province. Syn-collision, late to post-collision and syn-transcurence suites occur within the northern part of the PEAL domain. Their geochemical signatures show that the studied suites can be divided into; (1) I-type, magnesian series, cordilleran granitoids and (2) A2-type ferroan series granitoids. Geochemical signatures associated to the Nd isotopic signature, points to a Paleoproterozoic crust/mantle component within these rocks, reworked during the Ediacaran amalgamation of West Gondwana. The whole set of data suggest that the studied suites were emplaced during different stages of the Brasiliano orogeny, a transition between active margin and within-plate tectonic setting, which commonly occur at the final stages of an orogenesis. Keywords: Transalkaline, Granites, Neoproterozoic, Gondwana. Resumo: A Província Borborema é a parte oeste de um cinturão móvel que se extende do Brasil até o NW da África em reconstruções pré-drift. Foi dividida em quatro domínios tectônicos. O domínio Pernambuco-Alagoas (PEAL) está localizado na parte sudeste desta província e contém os maiores batólitos graníticos da mesma. Suites sin- e tardi- a pós-colisionais, e suítes sin-transcorrência ocorrem na parte norte do domínio PEAL. Elas apresentam assinaturas geoquímicas que nos permite dividi-las em granitos tipo-i, série magnesiana e cordillerano, e em granitos tipo-a2, da série ferrosa. Assinatura geoquímica associada a assinatura de Nd sugere a existência de componente crustal/mantélica de idade Paleoproterozóica, retrabalhado durante o amalgamento de Gondwana Oeste durante o Ediacarano. O conjunto de dados sugere que as suítes estudadas foram posicionadas durante diferentes estágios da orogênese Brasiliana, numa transição entre ambientes tectônicos de margem ativa para intra-placas. Palavras-chave: Trans-alcalino, Granitos, Neoproterozoicos, Gondwana. 1 Departamento de Geologia, UFPE, Recife, Brazil. 2 Instituto de Geociências, UnB, Brasília, Brasil. * Corresponding author / Autor correspondente: afsf@ufpe.br 1. Introduction The Borborema Province (Fig. 1a) is the western part of a major late Neoproterozoic mobile belt that extends from Brazil through NW Africa in pre-drift reconstructions. This province resulted from convergence and collision among the West African, Congo-São Francisco, and Amazonia land masses about 600 Ma (Van Schmus et al., 2008). It has been divided into four tectonic domains (Van Schmus et al., 2008); Northern, Transverse Zone, Pernambuco-Alagoas (PEAL) and Sergipano domains The PEAL domain occurs along the SE part of the Borborema Province. It contains the largest granitic batholiths of the Borborema Province. The PEAL is comprised by a collage of various units with ages, within the interval 0.52 to 2.40 Ga. The Nd TDM model ages identified within the PEAL domain (Fig. 1b) requires that part of their units protoliths should have a Mesoproterozoic or younger component, although many gneisses also show a Paleoproterozoic or Arquean component. The PEAL domain is an area comprised by three sub-crustal domains (Silva Filho et al., 2014), Garanhuns, Água Branca and Palmares. This paper deals with the geochronological and geochemical characterization of Ediacaran granitic suites (Fig. 1c) from the Garanhuns sub-domain. 2. Geological setting The Garanhuns sub-domain (Fig. 1b) comprises (Silva Filho et al., 2008) Rio Una sequence (garnet-sillimanite-gneisses, migmatized greywackes, quartz-feldspar gneisses and calcsilicates), orthogneisses, tholleiitic metamafics, and granitic plutons. These units show Nd TDM model age between 1.8 Ga and 2.7 Ga. The Garanhuns sub-domain is cut by the sinistral strike-slip shear zones: Capoeiras, Limitão-Caetés and Rio da Chata, and they bounded by Itaíba and Palmares shear zones. Rio da Chata and Palmares shear zones show magnetometric geophysical anomalies. Dioritic plutons also occur along these shear zones.

2 326 A. F. da Silva Filho et al. / Comunicações Geológicas (2014) 101, Especial I, Fig. 1. a) The tectonic domains of the Borborema Province, according to Van Schmus et al. (1995). 1- Cenozoic Sedimentary Covers..b) PE-AL crustal sub-domains according to Silva Filho et al c) Simplified geologic map of the studied area (Fig.1a). 1 - Studied granitoids; 2 - Brasiliano migmatites (Ferreira costa); 3 - Peraluminous granites; 4 - High-K calc-alkaline granitoids; 5 - Supracrustal rocks; 6 - Paleoproterozoic orthogneisses and migmatites; 7 - Shear zones (SZ) and 8- Thrust faults. Fig. 1. a) Os domínios tectônicos da Província Borborema, segundo Van Schmus et al. (1995). 1= Coberturas sedimentares cenozoicas. b) Domínios crustais do PE-A, de Silva Filho et al. (2014). c) Mapa geológico simplificado da área estudada (Fig. 1a). 1 - Granitóides estudados; 2 Migmatitos Brasilianos; 3 - Granitos Brasilianos Peraluminosos; 4 - Ganitóides cálcio alcalinos de alto-k; 5 Metassedimentos; 6 - Ortognaisses e migmatitos Paleoproterozóicos; 7 Zonas de Cisalhamentos e 8 - Falhas de empurrão. 3. Field relationship and petrography Regional surveys within PEAL and Sergipano domains have shown that Brasiliano orogeny is characterized by three stages of deformations, where the main compressional stage (D 2 ) which is related to a flat-lying foliation (S 2 ), was transposed by D 3 deformation that generated a high-angle foliation (S 3 ) and related transcurrence. The study suites, emplaced during operation of the D 2, are called syn-collisional, and they show very penetrative S 2, while those emplaced during late- to postcollision stage show a weak flat-lying foliation. Those emplaced during D 3 are called syn-transcurrence, and they crosscut D 2 structures in the country rocks and does not show planar or linear structures related to D 2. We have classified the studied suites into the following groups Syn-collision suites ( Ma) This group is represented by São Pedro and Jurema suites. The Jurema suite shows elongate shape. It is comprised by tonalites and quartz-diorites, coarse-grained, bearing quartz, plagioclase, alkaline feldspar, biotite and garnet. The São Pedro suite is comprised by stocks, sills and sheets of equigranular tonalites, diorites, sienogranites and quartzdiorites. They are equigranular, medium to fine grained, diorites, quartz-diorites, monzodiorites and tonalites. The mineralogy is comprised by biotite, amphibole, pyroxene, magnetite, sphene, K-feldspar, plagioclase, apatite and quartz. They follow the contact between orthogneisses and Rio Una sequence, but they also occur intruded into the Rio da Chata shear zone, coinciding with a strong magnetometric anomaly, suggesting limit between crustal blocks. Both diorites and sienogranites are intruded within orthogneisses, following a flat-lying S3 foliation. SHRIMP U-Pb in zircon data for these diorites yielded an age of ca. 610 Ma (Brito Neves, personal communication) Late to post-collision suites ( Ma) This group is represented by Alagoinhas and Cachoeirinha suites. The Alagoinhas suite crops out over an area of 80 km 2, bounded by shear zones, and intruded into Paleoproterozoic orthogneisses, and Neoproterozoic metassediments. It shows three main facies, porphyritic granodiorite, porphyritic sienogranite and equigranular sienogranite. The K- feldspar phenocrysts from the porphyritic sienogranite show length up to 5 cm. The matrix is comprised by amphibole, biotite, quartz and plagioclase. Mafic enclaves reach up to dozens meters long and are quite common. They show granodioritic composition and magmatic flow structure. Neves et al. (2005) have identified textures typical of deformation along both borders, east and west, and have related the emplacement of this pluton to the activity of the neighbouring shear zones. TIMS U-Pb in zircon data have been obtained for this suite yielding an age of 581 ± 3 Ma. The Cachoerinha pluton is located in the northern limit of the Garanhuns sub-domain, intruded also into Paleoproterozoic orthogneisses and metassediments. Neves et al. (2005) have showed that it was intruded during shortening, directed to NW. They show composition ranging from quartz-syenites, sienogranites, charnoquites and diorites, slightly porphyritic and isotropic, with perthitic microcline as phenocrysts. Field relationships between syenites and the diorites suggest magma mixing during its evolution. The matrix from the syenites and from the diorites are comprised by biotites, pyroxenes, quartz, opaque, sphene, microcline, zircon, plagioclase, perthite, apatite, epidote and allanite. TIMS U-Pb in zircon data have been obtained for this suite yielding an age of 587 ± 1 Ma.

3 Syn-collision to syn-transcurrence granitoids Syn-transcurrence suites ( ma) The Panelas pluton (Fig. 1c) is intruded into orthogneisses and granites, between Canhotinho and Ribeirão shear zones, both of them sinistral. It is comprised by very coarse grained porphyritic granodiorites, and dioritic enclaves. The granodiorites are comprised by plagioclase phenocrystals, and the matrix is comprised by microcline, quartz, biotite, sphene, plagioclase, epidote, opaque, apatite and zircon. LA-ICPMS U-Pb in zircon data have been obtained for this Panelas pluton yielding an age of 560 ± 4 Ma. Santa Luzia pluton is emplaced close to the Rio da Chata shear zone, and is comprised by biotiteamphibole quartz-monzonites, containing high content of sphene. TIMS U-Pb in zircon data have been obtained, yielding an age of 573 ± 5 Ma. 4. Whole-rock geochemistry Syn-collision suites They are mainly transalkaline suites (Middlemost, 1997) (Fig. 2a), and are ferroan series granites (A-type granites) (Frost et al., 2001) (Fig. 2b). They are K 2 O-rich and most of them fall in the field for shoshonitic suites in the diagram of Peccerillo & Taylor (1976) (Fig. 2c). The samples plot into the field for WPG granites and into the field for VAG + Syn-COLG in the diagrams of Pearce et al. (1984) (Figs. 3a and 3b).The syncollisional suites are very rich in REE, and show a deep Eu anomaly (Fig. 4a). They show a spidergram with a quite flat slope pattern, and troughs at Nb, Ta, Sr, P and Ti (Fig. 4b). Late to post-collision suites They are transalkaline suites according to Middlemost (1997) (Fig. 2a), and are split into the field of ferroan series (A-type granite) and into the field of magnesian series (Cordilleran granites), FeO(FeO+MgO) vs. SiO 2 diagram (Frost et al.,2001) (Figs.2b). They are K 2 O-rich and plot into the field for shoshonitic suites of the Peccerillo & Taylor (1976) (Fig. 2c). They plot into the field for VAG + Syn-COLG in the diagrams of Pearce et al. (1984) (Figs. 3a and 3b).Their REE patterns are characterized by negative Eu anomalies (Fig. 4a). Their spidergrams patterns show a weak fractionation of LILE related to the HREE (Fig. 4b). Syn-transcurrence suites The samples are transalkaline suites in the Middlemost (1997) diagram (Fig. 2a), and plot into the field for A-type granites and into the field for cordilleran granites in the Frost et al. (2001) diagrams (Figs. 2b). They plot only into the field for shoshonitic suites of the Peccerillo & Taylor (1976) (Fig. 2c). The samples plot into the field for WPG granites and into the field for VAG + Syn-COLG in the diagrams of Pearce et al. (1984) (Figs. 2-A and 2-B), suggesting that their syn-collisional geochemical signatures were inherited from their protolith, or that the shear zones to which they are related had also transpressional kynematics. They show a wide REE pattern envelope suggesting that the REE were fractionated during the magmas evolution (Fig. 4a). They show similar spidergrams, which are characterized by high enrichment in LILE, and troughs in the elements Nb, Ta, Sr, P and Ti, and fractionation between LILE and HREE Tb, Tm and Yb (Fig. 4b). Fig. 2. A) The studied granitoids plot in TAS diagram, with fields after Middlemost (1997); B) The composition range of the studied granitoids in the FeOt/(FeOt+MgO) versus SiO2 diagram. Fields of ferroan and magnesian granitoids are from Frost et al. (2001). The fields of RRG+CEUG (anorogenic granitoids), POG (post-orogenic granitoids) and IAG+CAG+CCG (orogenic granitoids) after Maniar & Piccoli (1989); C- The studied granitoids in the K 2 O vs SiO 2 diagram, with fields after Piccerilo & Taylor (1976). Fig. 2. A) Os granitoides estudados plotados no diagrama TAS, com os campos de Middlemost (1997); B) A variação composicional dos granitoides estudados no diagrama FeOt/(FeOt+MgO) versus SiO 2. Os campos dos granitoides ferrosos e magnesianos são de Frost et al. (2001). Os campos de RRG+CEUG (granitoides anorogênicos), POG (granitoides pós-orogênicos) e IAG+CAG+CCG (granitoides orogênicos) de Maniar & Piccoli (1989); C- Os granitoides estudados no diagrama K 2 O vs SiO 2, com os campos de Piccerilo & Taylor (1976).

4 328 A. F. da Silva Filho et al. / Comunicações Geológicas (2014) 101, Especial I, Fig. 3. Trace elements for the study granitoids in the tectonic discriminant diagrams of (A) Pearce (1996) and (B) Pearce et al. (1984). Syn-COLG = syncollisional granites; VAG = volcanic-arc granites; WPG = within-plate granites; ORG = oceanic ridge granites; Post-COLG = post-collisional granites. Symbols as in Figure 1. Fig. 3. Elementos traços para os granitoides estudados nos diagramas discriminantes de (A) Pearce (1996) e (B) Pearce et al. (1984). Syn-COLG = granitos sin-colisionais; VAG = granitos de arco vulcânico; WPG = granitos intra-placa; ORG = granitos de cadeias oceânicas; Post-COLG = granitos pós-colisionais. Símbolos como na figura 1. within these rocks, reworked during the Ediacaran amalgamation of Gondwana; 2) Part of the samples from the syn-collision and syn-transcurrence suites, and the whole set of samples from late to post-collision suites, plot into the field for VAG + Syn-COLG, which has been interpreted as that for granites associated to island arc or to continental arc. However, a WPG signature/component has been identified so far in some studied suites, suggesting they have a component related to an extensional tectonic setting, a component which was inherited from a stage of basin formation. This stage has been described by Silva Filho et al. (2014). The whole set of data, suggest that the studied suites were intruded during the transition between active margin and within-plate tectonic setting of the Brasiliano orogeny. It commonly occurs at the final stages of an orogeny. We recorded a rough correspondence between age of the plutons and chemical signature, where the majority of the younger plutons plots into the A-type field. Fig. 4. A- Chondrite normalized REE plots for the studied granitoids. Chondrite values from Sun (1980). B- Spidergram patterns normalized to the values suggested by Thompson (1982) for the studied granitoids. Fig. 4. A- Os ETR normalizados pelo condrito para os granitoides estudados. Valores de condrito de Sun (1980). B- Padrões Spidergram normalizados para os valores sugeridos por Thompson (1982) para os granitoides estudados. 5. Discussion and conclusions The geochemical signatures divided the studied suites into: (1) I-type, magnesian series, cordilleran granitoids and (2) A 2 -type ferroan series granitoids. Geochemical data associated to the Nd isotopic signatures (Silva Filho et al., 2002) suggest: 1) Paleoproterozoic crust/mantle component Acknowledgments We are indebted to the Brazilian Geological Survey, which supported part of this work, to the CNPq that funded the lab work through various grants, and to Randy Van Schmus for two decades of proficuous cooperation. References Frost, B.R., Barnes, C., Collins, W., Arculus, R., Ellis, D., Frost, C., A chemical classification for granitic rocks. Journal of Petrology, 42, Middlemost, E.A.K., Magmas, Rocks and Planetary Development. Longman, Harlow. 324 p. Neves, S.P., Mariano, G., Beltrão, B.A., Correia, P.B., Emplacement and deformation of the Cachoeirinha pluton inferred through petrostructural studies: Constraints on regional strain fields. Journal of South American Earth Sciences, 19, Pearce, J., Sources and setting of granitic rocks. Episodes, 19, Pearce, T.H., Harris, N.B.W., Tindle, A.D., Trace element discrimination diagrams for the tectonic interpretation of rocks. Journal of Petrology, 25,

5 Syn-collision to syn-transcurrence granitoids 329 Peccerillo, A., Taylor, S.R., Geochemistry of Eocene calcalkaline volcanic rocks from the Kastmonu area, northern Turkey. Contributions to Mineralogy and Petrology, 58, Silva Filho, A.F., Gomes, H.A., Osako, L.S., Guimarães, I.P., Luna, E.B.A., Geologia da Folha Garanhuns. (consultado em 23/3/2014). Silva Filho, A.F., Guimarães, I.P., Van Schmus, W.R., Crustal evolution of the Pernambuco-Alagoas Complex, Borborema Province, NE Brazil; Nd isotopic data from Neoproterozoic granitoids. Gondwana Research, 5, Silva Filho, A.F., Guimarães, I.P., Armstrong. R.A., Rangel da Silva, J.M., Osako, L., Van Schmus, W.R., Cocentino, L.M., SHRIMP U-Pb Zircon Geochronology and Nd Signatures of Supracrustal Sequences and Orthogneisses, Constrains the Neoproterozoic Evolution of the Pernambuco - Alagoas Domain, Southern Part of Borborema Province, NE Brazil. International Journal of Earth Sciences, 103, Thompson, R.N., Magmatism of the British Tertiary volcanic province. Scottish Journal of Geology, 18, Van Schmus, W.R., Brito Neves, B.B., Hackspacher, P., Babinski, M., U/Pb and Sm/Nd geochronolgic studies of eastern Borborema Province, northeastern Brazil, initial conclusions. Journal of South American Earth Sciences, 8, Van Schmus, W.R., Oliveira, E,P., Silva Filho, A.F., Toteu, S.F., Penaye, J., Guimarães, I.P., Proterozoic links between the Borborema province, NE Brazil, and the Central African Fold Belt. Geological Society London Special Publications, 294,