Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial I, 265-267
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
Evidences of Rhyacian extensional event in the Central Domain of the Borborema Province, NE Brazil Evidências de evento extensional Riaciano, na Provincia Borborema, NE Brasil I. P. Guimarães2*, E. G. Silva1, A. F. da Silva Filho2, R. Armstrong3, C. Neysi Almeida4 Artigo Curto Short Article © 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: Granitic gneisses (Fazenda Salvador) and metamafic to meta-ultramafic rocks occur as intrusions in granodioritic to granitic orthogneisses, in the Central Domain of the Borborema Province, NE Brazil. The Fazenda Salvador orthogneisses are coarse to very coarse grained, porphyroclastic to equigranular and locally migmatized. Enclaves of dioritic compositions are common, usually showing crenulated to lobate contacts with the augen gneisses. Metabasite and meta-ultramafic rocks including amphibolite, garnet amphibolite and garnet pyroxenite, were recorded in the studied area. They occur as lens and pods within the migmatized granodioritic orthogneisses. The Fazenda Salvador orthogneisses are slightly peraluminous, ferroan and show geochemical signatures of alkaline granitoids. They show REE patterns characterized by (Ce/Yb)N ratios ranging from 111.84 to 8.65 and Eu negative anomalies (Eu\Eu* = 0.38 – 0.81). The spidergram patterns are characterized by trough at Sr, Nb, P and Ti. The metamafic rocks are tholeiitic basalts metamorphosed under high-P granulitic conditions. They show high TiO2/K2O ratios, similar to those recorded in ocean tholeiitic basalts. The metabasites and Fazenda Salvador orthogneisses have similar U-Pb zircon SHRIMP ages, 2095 ± 4 Ma and 2094 ± 8 Ma respectively, which are interpreted as the protholith crystallization ages. Our data suggest an extensional event of Rhyacian age in the Central Domain of the Borborema Province. Keywords: Borborema Province, Orthogneiss, U-Pb zircon ages, Extension. Resumo: Gnaisses graníticos (Fazenda Salvador) e rochas metamáficas e meta-ultramáficas ocorrem intrudidas em gnaisses granodioríticos a graníticos no domínio central da Província Borborema, NE Brasil. Os ortognaisses Fazenda Salvador são grossos a muito grossos, porfiroclástos a equigranular, localmente migmatizados. Encraves de composição diorítica ocorrem comumente mostrando contatos crenulados e lobados com os ortognaisses encaixantes. Metabasitos e rochas meta ultramáficas incluindo anfibolitos, granada anfibolitos e granada piroxenitos foram identificados na área estudada. Ocorrem como pods e lentes dispersas em ortognaisses migmatizados de composição granodiorítica. Os orthognaisses Fazenda Salvador são fracamente peraluminosos, ferrosos e mostram assinatura geoquímica de granitos alcalinos. Os padrões de ETR normalizados em relação aos valores do condrito, são caracterizados por razões (Ce/Yb)N variando de 111,84 a 8,65 e anomalias negativas de Eu (Eu\Eu* = 0,38 – 0,81).Os padrões spidergrams são caracterizados por depressões em Sr, Nb, P e Ti. Os metabasitos e rochas meta ultramáficas são basaltos toleiticos metamorfisados em condições de pressões levadas no fácies granulito. Eles mostram elevadas razões TiO2/K2O, semelhantes as observadas em basaltos toleíticos oceânicos. Os
metabasitos e os ortognaisses Fazenda Salvador tem idades U-Pb em zircão por SHRIMP semelhantes 2095 ± 4 Ma e 2094 ± 8 Ma respectivamente, as quais foram interpretadas com a idade de cristalização do protólito. Os dados apresentados sugerem um evento extensional de idade riaciana, no domínio Central da Província Borborema. Palavras-chave: Província Borborema, Ortognaisse, Idades U-Pb em zircão, Extensão. 1
Universidade Federal de Pernambuco Pós-Graduação em Geociências –Universidade Federal de Pernambuco 3 Australian National University 4 Universidade Federal do Rio de Janeiro * Corresponding author / Autor correspondente:
[email protected] 2
1. Introduction The Borborema Province consists of gneissic and migmatitic basement complexes, mostly formed during the Paleoproterozoic (2.0–2.2 Ga), partially covered by Neoproterozoic metasedimentary and metavolcanic rocks. Lens and pods of metabasites enclosed by Paleoproterozoic orthogneisses occur in a narrow belt that extends NE-SW for ~180 km, in the south limit of the socalled Alto Moxotó Terrain (Santos et al., 1997). The Brasiliano event affected the entire province and it was responsible by development of large scale shear zone, metamorphism under high temperature and low pressure conditions, and intense granitic magmatism. In this work, we present and discuss geochemical and U-Pb SHRIMP zircon data for the Fazenda Salvador augen gneiss and metabasite to meta-ultramafic rocks (Fig. 1), intruded in the east part of the Central Domain of the Borborema Province. 2. Geologic aspects The Fazenda Salvador orthogneisses comprise coarse to very coarse grained augen to equigranular gneisses, sometimes migmatized, including locally enclaves of diorite. The enclaves until preserve some magmatic features, as crenulated
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to lobate contacts with the enclosing granodioritic, tonalitic to granitic orthogneisses, which have protholith crystallization ages ranging from 2.3–2.2 Ga. Leucogranites of possible Neoproterozoic ages were mapped as dykes, isolated or as swarms, cutting the Fazenda Salvador orthogneisses. The studied orthogneisses were mainly metamorphosed and deformed during the Brasiliano Orogeny. They show a dominantly Ediacaran (Brasiliano) flat lying foliation, with kinematic criteria indicating a top-to-the-westnorthwest sense of tectonic transport. The studied metabasites and meta ultramafic rocks show three petrographic – textural facies (Almeida et al., 2009): amphibolite, garnet amphibolites and garnet pyroxenites, gradating one to another. They are intruded into orthogneisses of granodioritic to granitic composition, along the sinistral sense Congo shear zone, which coincide with a highmagnetic in the Aeromagnetometric image. Due to intense migmatization associated to the Brasiliano orogeny, they are dispersing as pods within the enclosing orthogneissesmigmatites country rocks. However, some amphibolite lenses were mapped (Fig. 1). Corona textures constituted by symplectites of amphibole + plagioclase and orthopyroxene + plagioclase in garnet and clinopyroxene recorded in the studied rocks are evidences of retrograded metamorphism from eclogite facies to granulitic facies (Almeida et al., 2009), evolving to amphibolite facies. Messiga et al. (1991) interpreted the corona texture described above as the result of retrograded metamorphism from an eclogitic facies due to decompression, leading to the disappearance of omphacitic pyroxenes and pyrope – rich garnet. The “retroeclogites” are medium to fine grained, massif or slightly banded. They occur as nodules or pods within the amphibolites and orthogneisses. They can eventually enclose orthogneisses/migmatite fragments, and are composed of garnet, clinopyroxene, amphibole, opaque minerals and quartz. The last metamorphic stage occurs under greenschist facies metamorphic conditions, with clinopyroxene and amphibole replaced by chlorite, calcite and talc.
suggesting that their magma protholith evolution involved fractionation of plagioclase, apatite and biotite. The metabasites occur associated to lenses of Fe-Ti minerals. They have composition of tholeiitic basalt. Their REE patterns, normalized to the chondrite values, are horizontal to sub horizontal with absent to positive Eu anomalies. In general, the total REE contents are ten to twelve times higher than the REE chondrite contents, except one sample which have lower contents of total REE, about five times higher than the REE chondrite contents. These patterns differ from their country orthogneisses, which shows fractionated pattern with Eu negative anomalies (Fig. 2b). On the other hand, the REE patterns of the studied mafic rocks are similar to those recorded in komatiite which underwent distinct degrees of olivine fractionation (Rollinson, 1993). They have geochemical signature (spidergram patterns) similar to the MORB, except the amphibolite sample which shows a very disturbed spidergram pattern (Fig. 3b).
3. Geochemistry The Fazenda Salvador orthogneisses are slightly peraluminous, ferroan and show many geochemical signatures of alkaline granites. Their REE patterns (Fig. 2a) are fractionated with (Ce/Yb)N ratios ranging from 111.84 to 8.65 and are characterized by negative Eu anomalies (Eu\Eu* = 0.38 – 0.81). The samples free from migmatization show REE patterns less fractionated and deeper negative Eu anomalies (Fig. 2a). The spidergram patterns (Fig. 3a) normalized to the chondrite values (Thompson, 1982) are characterized by trough at Sr, Nb, P and Ti. These patterns are similar to those recorded in A2-type granites (Eby, 1992), which are extension related granites, derived from continental crust or underplated crust, that has been through a cycle of continent-continent collision or island arc magmatism. They are characterized by having high-Y and low-Nb contents. In the Harker diagrams, the Fazenda Salvador orthogneiss samples show negative trends for TiO2, Al2O3, Fe2O3, MnO, MgO, CaO and P2O5, and positives to Na2O, Al2O3 e P2O5,
Fig. 1. Simplified Geologic map of the studied area. 1- Quaternary cover, 2 - Ediacaran granitoids, 3 - Neoproterozoic metasedimentary sequences, 4 - Fazenda Salvador orthogneisses, 5 - Paleoproterozoic ganodiorite to granite orthogneisses, 6 - area with occurrences of metabasite and metaultramafic rocks, 7 - shear zone with kinematic sense, 8 - Thrust faults and CSZ - Congo shear zone. Fig. 1. Mapa geologico simplificado da area estudada. 1 - Coberturas quaternárias, 2 - Granitoides Ediacaranos, 3 - Sequencias metasedimentares neoproterozoicas, 4 - ortognaisses Fazenda Salvador, 5 - Ortognaisses granodioriticos a graniticos paleoproterozoicos, 6 - area de ocorrência dos metabasitos e rochas metaultramáficas, 7 - zonas de cisalhamento mostrando a cinemática, 8 - Falhas de empurrão e CSZZona de cisalhamento Congo.
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and 2094 ± 8 Ma respectively. These ages were interpreted as the ages of the protholith crystallization. An age of 600 ± 12 Ma was recorded in the lower intercept of the Concordia diagram for the garnet amphibolite and tt was interpreted as the age of the amphibolite metamorphic facies. An age of 1870 ± 27 Ma was defined by Sm-Nd method, using garnet and total rock (Almeida et al., 2005). This age was interpreted as the age of the high pressure metamorphism (eclogite facies). However, we must say that this probably high pressure metamorphic age, was not recorded in the analyzed zircon grains. 5. Conclusions
Fig. 2. Chondrite normalized REE plots for a) the Fazenda Salvador orthogneisses and b) metabasite and meta-ultramafic rocks. Chondrite values from Sun (1980). Fig. 2. Padrões de ETR normalizados em relação aos valores do condrito para a) ortognaisses Fazenda Salvador e b) metabasitos e rocha metaultraáficas. Valores do condrito de Sun (1980).
Our data suggest that during the Rhyacian, crustal reworked and crust formation occurred associated to extension in Central Domain of the Borborema Province. This event could be responsible by opening an ocean, which was closed during the Oriasian, (~1.87 Ga). In this way, the Congo shear zone could represent an Oriasian suture. This hypothesis is supported by the presence of a high- magnetic in the Aeromagnetometric image, coincident with the Congo shear zone. However, in our days, is difficult or even impossible to characterize this suture, due to the intense Brasiliano reworking. We must say that the closure age needs to be better constraint. Acknowledgments CAPES for a post-doctoral scholarship (BEX 4127-0702). A grant from the PRONEX/FACEPE program (APQ-04791.07/06) made possible SHRIMP analyses at the ANU, Canberra, Australia. We also thanks CPRM-Serviço Geológico do Brasil for financial support to the field work. References
Fig. 3. Spidergram patterns normalized to the values suggested by Thompson (1982) for the Fazenda Salvador orthogneisses (a) and to the MORB values (Sun, 1980) to metabasite and meta-ultramafic rocks (b). Fig. 3. Padrões spidergrams normalizados em relação aos valores sugeridos por Thompson (1982) para os ortognaisses Fazenda Salvador (a) e para os valores do MORB (Sun, 1980) para os metabasitos e rocha metaultramáficas (b).
4. Geocronology U-Pb zircon SHRIMP data for samples of augen gneiss and garnet amphibolite show similar ages i.e. 2095 ± 4 Ma
Almeida, C.N., Guimarães, I.P., Beurlen, H., Topitsch, W., 2005. Uma zona de sutura Transamazônica na Província Borborema, Nordeste do Brasil: os retroeclogitos da Faixa de Dobramentos Pajeú-Paraíba. In: XXI Simpósio de Geologia do Nordeste, Recife. Atas do XXI Simpósio de Geologia do Nordeste. Almeida, C.N., Guimarães, I.P., Beurlen, H., Topitsch, W., Ferrer, D.M.M., 2009. Evidências de metamorfismo de alta pressão na faixa de dobramentos Pajeú Paraíba, Província Borborema, nordeste do Brasil: petrografia e química mineral de rochas metamáficas. Revista Brasileira de Geociências, 39, 421-434. Eby, G.N., 1992. Chemical subdivision of the A-type granitoids: Petrogenetic and tectonic implications. Geology, 20, 641-644. Messiga, B., Tribuzio, R., Vanucci, R., 1991. Mafic and ultramafic pods with eclogitic relics from the Proterozoic Nagssugtoqidian mobile belt of East Greenland. Lithos, 25, 101-118. Rollinson, H., 1993. Using Geochemical data: evaluation, presentation, interpretation. Longman, 352 p. Santos, E.J., Gusmão, R., Paiva, I.P., 1997. Terrenos no Domínio Transversal da Provincia Borborema: controles sobre acresção e retrabalhamento crustais ao sul do lineamento Patos. Abstract vol. XVII Simpósio de Geologia do Nordeste, SBG-NE, Fortaleza, 15, 141-144. Sun, S.S., 1980. Lead isotopic study of younger volcanic rocks from mid-ocean ridges, ocean island and island arcs. Philosophical Transactions of the Royal Society, A297, 409-445. Thompson, R.N., 1982. Magmatism of the British Tertiary volcanic Province. Scottish Journal of Geology. 18, 50-107.
