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Previous issue date: 2000-03-24 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The area studied is located on the north-easternmost portion of the Borborema Province, on the so-called S?o Jos? de Campestre Massif, States of RN and PB, Northeast Brazil. Field relations and petrographic, geochemical and isotope data permitted the separation of five suites of plutonic rocks: alkali-feldspar granite (Caxexa Pluton), which constitutes the main subject of this dissertation, amphibole-biotite granite (Cabe?udo Pluton), biotite microgranite, gabbronorite to monzonite (Basic to Intermediate Suite) and aluminous granitoid.
The Caxexa Pluton is laterally associated to the Rem?gio Pocinhos Shear Zone, with its emplacement along the mylonitic contact between the gneissic basement and the micashists. This pluton corresponds to a syntectonic intrusion elongated in the N-S direction, with about 50 km2 of outcropping surface. It is composed exclusively of alkali-feldspar granites, having clinopyroxene (aegirine-augite and hedenbergite), andradite-rich garnet, sphene and magnetite. It is classified geochemically as high silica rocks (>70 % wt), metaluminous to slightly peraluminous (normative corindon < 1%), with high total alkalis (>10% wt), Sr, iron number (#Fe=90-98) and agpaitic index (0.86-1.00), and positive europium anomaly.
The Cabe?udo Pluton is composed of porphyritic rocks, commonly containing basic to intermediate magmatic enclaves often with mingling and mixing textures. Petrographically, it presents k-feldspar and plagioclase phenocrysts as the essential minerals, besides the accessories amphibole, biotite, sphene and magnetite. It is metaluminous and shows characteristics transitional between the calc-alkaline and alkaline series (or monzonitic subalkaline). Its REE content is greater than those ones of the Caxexa Pluton and biotite microgranite, and all spectra have negative europium anomalies.
The biotite microgranites occur mainly on the central and eastern portion of the mapped area, as dykes and sheets with decimetric thickness, hosted principally in orthogneisses and micashists. Their field relationships as regards the Caxexa and Cabe?udo plutons suggested that they are late-tectonic intrusions. They are typically biotite granites, having also sphene, amphibole, allanite, opaques and zircon in the accessory assemblage. Geochemically they can be distinguished from the porphyritic types because the biotite microgranites are more evolved, peraluminous, and have more fractionated REE spectra.
The Basic to Intermediate rocks form a volumetrically expressive elliptical, kilometric scale body on the Southeast, as well as sheets in micashists. They are classified as gabbronorites to monzonites, with the two pyroxenes and biotite, besides subordinated amounts of amphibole, sphene, ilmenite and allanite. These rocks do not show a well-defined geochemical trend, however they may possibly represent a monzonitic (shoshonitic) series. Their REE spectra have negative europium anomalies and REE contents greater than the other suites. The aluminous granitoids are volumetrically restricted, and have been observed in close association with migmatised micashists bordering the gabbronorite pluton. They are composed of almandine-rich garnet, andalusite, biotite and muscovite, and are akin to the peraluminous suites.
Rb-Sr (whole rock) and Sm-Nd (whole-rock and mineral) isotopes furnished a minimum estimate of the crystallization (578?14 Ma) and the final resetting age of the Rb-Sr system (536?4 Ma) in the Caxexa Pluton. The aluminous granitoid has a Sm-Nd garnet age similar to that one of the Caxexa Pluton, that is 574?67 Ma. The strong interaction of shear bands and pegmatite dykes favoured the opening of the Rb-Sr system for the Caxexa Pluton and biotite microgranite.
The amphibole-plagioclase geothermometer and the Al-in amphibole geobarometer indicate minimum conditions of 560?C and 7 kbar for the Cabe?udo Pluton, 730?C and 6 kbar for the microgranite and 743?C and 5 kbar for the basic to intermediate suite. The Zr saturation geothermometer reveals temperatures of respectively 855?C, 812?C and 957?C for those suites, whereas the Caxexa Pluton shows temperatures of around 757?C. The Caxexa, Cabe?udo and microgranites suites crystallized under high fO2 (presence of magnetite). On the other hand, the occurrence of ilmenite suggests less oxidant conditions in the basic to intermediate suite.
Field relations demonstrate the intrusive character of the granitoids into a tectonically relatively stable continental crust. This is corroborated by petrographic and geochemical data, which suggest a late- or post-collisional tectonic context. It follows that the generation and emplacement of those granitoid suites is related to the latest events of the Brasiliano orogeny. Finally, the relationships between eNd (600 Ma), TDM (Nd) and initial Sr isotope ratio (ISr) do not permit to define the precise sources of the granitoids. Nevertheless, trace element modelling and isotopic comparisons suggest the participation of the metasomatised mantle in the generation of these suites, probably modified by different degrees of crustal contamination. In this way, a metasomatised mantle would not be a particular characteristic of the Neoproterozoic lithosphere, but a remarkable feature of this portion of the Borborema Province since Archaean and Paleoproterozoic times. / A ?rea estudada localiza-se na extremidade nordeste da Prov?ncia Borborema, no denominado Maci?o S?o Jos? de Campestre (RN e PB). Rela??es de campo e dados petrogr?ficos, geoqu?micos e isot?picos permitem individualizar cinco su?tes distintas de rochas plut?nicas representados por: ?lcali-feldspato granito (Pl?ton Caxexa), que constitui o principal alvo desta disserta??o, anfib?lio-biotita granito (Pl?ton Cabe?udo), biotita microgranito, gabronorito a monzonito (Su?te B?sica a Intermedi?ria) e granit?ide aluminoso.
O Pl?ton Caxexa est? lateralmente associado a Zona de Cisalhamento Rem?gio-Pocinhos, alojado ao longo da interface milon?tica entre o substrato gn?issico e os micaxistos. Este pl?ton corresponde a uma intrus?o sintect?nica alongada na dire??o N-S, com cerca de 50 km2 de superf?cie aflorante. Ele ? formado exclusivamente por ?lcali-feldspato granitos, tendo como minerais acess?rios clinopirox?nio (aegirina-augita e hedenbergita), granada (andradita), titanita e magnetita. Quimicamente, classificam-se como rochas alcalinas de alta s?lica (>70% em peso), metaluminosas a fracamente peraluminosas (cor?ndon normativo <1%), com altos valores de Na2O+K2O (>10%), Sr, raz?es de #Fe (90-98) e ?ndice agpa?tico (0,86-1,00), e anomalia positiva de Eu.
O Pl?ton Cabe?udo comp?e-se de rochas com textura porfir?tica, comumente contendo enclaves magm?ticos de composi??o b?sica a intermedi?ria, mostrando fei??es do tipo mingling e mixing. Petrograficamente, ? constitu?do por fenocristais de k-feldspato e plagiocl?sio como minerais essenciais, al?m de anfib?lio, biotita, titanita e magnetita como acess?rios. Quimicamente, mostra caracter?sticas metaluminosas e afinidade com rochas transicionais c?lcio-alcalina e alcalina (subalcalina monzon?tica). Apresentam espectros de terras raras com anomalia negativa de Eu e conte?dos de terras raras leves e pesadas mais elevados do que as rochas do Pl?ton Caxexa e do microgranito.
Os microgranitos ocorrem predominantemente na por??o centro-leste, sob a forma de diques e soleiras, com espessura decim?trica, alojados principalmente nas ortoderivadas e com menor freq??ncia nos micaxistos. Seu posicionamento tardio com rela??o ?s demais plut?nicas ? evidenciado atrav?s de diques encaixados em rochas dos pl?tons Caxexa e Cabe?udo. Petrograficamente, s?o biotita granitos, contendo tamb?m titanita, anfib?lio, allanita, opacos e zirc?o como minerais acess?rios. Quimicamente, diferem das rochas porfir?ticas, por serem peraluminosas, mais evolu?das, e terem espectros de terras raras mais fracionados.
As rochas b?sicas a intermedi?rias ocorrem como um grande corpo el?ptico de dire??o NE-SW, na parte SE da ?rea, bem como soleiras em micaxistos. Modalmente, s?o gabronoritos a monzonitos, contendo os dois pirox?nios e biotita como minerais m?ficos mais freq?entes, al?m de anfib?lio, titanita, ilmenita e allanita. Essas rochas mostram um comportamento qu?mico que n?o se adequa nem ?s s?ries c?lcio-alcalinas t?picas, nem ?s alcalinas, podendo representar possivelmente uma s?rie monzon?tica (shoshon?tica). Os espectros de terras raras possuem anomalia negativa de Eu menos pronunciada e conte?dos de terras raras maiores do que nas outras su?tes. Os granit?ides aluminosos s?o volumetricamente restritos, sendo identificados atrav?s da forte migmatiza??o em micaxistos que bordejam a su?te b?sica a intermedi?ria, destacando-se alguns corpos na por??o sul da ?rea. Mineralogicamente, s?o identificados granada, andaluzita, biotita e muscovita, sendo a su?te de caracter?stica geoqu?mica peraluminosa.
Dados isot?picos de Rb-Sr [rocha total (RT)] e Sm-Nd (RT + mineral) permitem estimar a idade m?nima de cristaliza??o (578?14 Ma) e a idade de fechamento final do sistema Rb-Sr (536?4 Ma) para o Pl?ton Caxexa. Os granit?ides aluminosos possuem idade Sm-Nd (rocha total + mineral) semelhante a do Pl?ton Caxexa, com valor de 574?67 Ma. A forte intera??o de bandas de cisalhamento e diques pegmat?ticos, facilitou a abertura do sistema Rb-Sr, impossibilitando a obten??o de idades geocronol?gicas para o Pl?ton Cabe?udo e o microgranito.
Dados termobarom?tricos utilizando o geoterm?metro anfib?lio-plagiocl?sio e geobar?metro do Al em anfib?lio indicam condi??es m?nimas de 560?C e 7 kbar para o Pl?ton Cabe?udo, 730?C e 6 kbar para o microgranito e 743?C e 5 kbar para a su?te b?sica a intermedi?ria. O geoterm?metro de Zr mostra temperaturas mais elevadas, de 855?C, 812?C e 957?C, respectivamente, para aquelas su?tes, enquanto o Pl?ton Caxexa apresenta temperaturas da ordem de 757?C. Os pl?tons Caxexa, Cabe?udo e microgranito cristalizaram-se sob condi??es de alta fugacidade de oxig?nio (presen?a de magnetita). Por outro lado, a ocorr?ncia de ilmenita na su?te b?sica a intermedi?ria indica condi??es menos oxidantes para a sua evolu??o.
Rela??es de campo demonstram o car?ter intrusivo dos granit?ides em uma crosta continental j? relativamente estabilizada. Isto ? comprovado por dados petrogr?ficos e geoqu?micos, que sugerem um contexto tect?nico tardi- ou p?s-colisional. Interpreta-se, da?, a gera??o e posicionamento das su?tes granit?ides durante os eventos tardios da orog?nese brasiliana. Finalmente, o confronte de eNd (600 Ma), TDM e raz?es isot?picas iniciais de estr?ncio (ISr) n?o permitem definir a(s) fonte(s) adequada(s) dentre as unidades crustais do substrato gn?issico atualmente aflorante no MSJC. Ensaios preliminares levando em conta a rela??o Rb/Sr vs. Sr deixam em aberto a possibilidade do manto metassomatisado (enriquecido em TRL, Ba, Sr, Zr) ter sido uma das fontes principais, contaminada em diferentes propor??es (menor na b?sica a intermedi?ria) por material da crosta continental. Desta forma, um manto enriquecido n?o seria uma particularidade da litosfera neoproteroz?ica, mas uma caracter?stica marcante da por??o nordeste da Prov?ncia Borborema desde o Arqueano e o Paleoproteroz?ico.
Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufrn.br:123456789/18752 |
Date | 24 March 2000 |
Creators | Nascimento, Marcos Antonio Leite do |
Contributors | CPF:15603180404, http://lattes.cnpq.br/1259445348649589, Sa, Jaziel Martins, CPF:04595769415, http://lattes.cnpq.br/2772507532078148, Galindo, Antonio Carlos, Souza, Zorano Sergio de |
Publisher | Universidade Federal do Rio Grande do Norte, Programa de P?s-Gradua??o em Geodin?mica e Geof?sica, UFRN, BR, Geodin?mica; Geof?sica |
Source Sets | IBICT Brazilian ETDs |
Language | Portuguese |
Detected Language | English |
Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/masterThesis |
Format | application/pdf |
Source | reponame:Repositório Institucional da UFRN, instname:Universidade Federal do Rio Grande do Norte, instacron:UFRN |
Rights | info:eu-repo/semantics/openAccess |
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