UNIVERSIDADE DE BRASÍLIA / INSTITUTO DE GEOCIÊNCIAS

TESE DE DOUTORADO N^o 054

MARIA HELENA BEZERRA MAIA DE HOLLANDA

Palavras-chave:  Manto litosférico continental, geoquímica isotópica, Província Borborema

ÁREA DE TRABALHO DE TESE: Geologia Regional e Geoquímica Isotópica

DATA DA DEFESA: 21/06/2002
ÁREA DE CONCENTRAÇÃO: GEOLOGIA REGIONAL
ORIENTADOR: PROF. MARCIO MARTINS PIMENTEL (UnB)
EXAMINADORES: PROF. ELTON LUIZ DANTAS (UnB); PROF. JOSÉ AFFONSO BROD (UnB); PROF. ALCIDES NÓBREGA SIAL (UFPE); PROF. LEILA SOARES MARQUES (USP)

RESUMO
Esta tese reúne um conjunto de dados geoquímicos e isotópicos (Sr, Nd, Pb) de rochas máficas neoproterozóicas, mesozóicas e cenozóicas do Domínio Seridó (Província Borborema, NE do Brasil), como representantes de eventos magmáticos distintos em afinidade geoquímica e idade. Esses resultados refletem enriquecimento episódico em elementos incompatíveis no manto litosférico continental, sugerindo interação entre este reservatório e ambos astenosfera e plumas, durante eventos geológicos relacionados a processos de subducção, delaminação orogênica, rifteamento e evolução para margem passiva.
O magmatismo neoproterozóico (ca. 580 Ma) é representado por uma suíte gabro-diorítica alto-K, cujas características geoquímicas e isotópicas indicam envolvimento de um componente de subducção na fonte. Tal indicação é refletida pelo enriquecimento em elementos incompatíveis, anomalias negativas em Nb e Ti, razões isotópicas de Sr altamente radiogênicas (0,71202-0,7059) e valores bastante negativos de Nd(T) (-9,3 a –20,1). Idades modelo TDM sugerem que este evento de enriquecimento foi associado à orogenia Transamazônica, ca. 2,0 Ga, e esteve preservado no manto litosférico até o fim do Neoproterozóico, quando este foi fundido gerando os magmas parentais dos gabros e dioritos. Dados isotópicos de Pb sugerem que um componente astenosférico poderia ter participado como catalisador do processo de fusão parcial, demonstrado por razões dominantemente não-radiogênicas. 
O magmatismo básico mesozóico (ca. 180-110 Ma) é representado por um enxame de diques toleíticos - Magmatismo Rio Ceará Mirim, intrusivo durante os primeiros estágios de fragmentação do supercontinente Gondwana. A exemplo do magmatismo neoproterozóico, as características geoquímicas e isotópicas dos diques mesozóicos também indicam enriquecimento em elementos incompatíveis, combinados a altas razões iniciais de Sr (0,710 a 0,7047) e valores negativos de Nd(T) (-0,6 a –9), sugerindo semelhança química destes magmas com as composições de basaltos de ilhas oceânicas, tipo EM. Idades modelo TDM ca. 1,0 Ga indicam que o enriquecimento está relacionado a um componente antigo, paleoproterozóico, provavelmente manto litosférico delaminado. Esse componente foi provavelmente incorporado a uma pluma mesozóica, a qual foi responsável pela abertura do rifte Potiguar no Domínio Seridó. Tal assinatura geoquímica-isotópica enriquecida é também identificada em basaltos alcalinos terciários (ca. 30-20 Ma; Vulcanismo Macau). Tal particularidade constitui forte evidência de que os magmas parentais de ambos os grupos, basaltos toleíticos e alcalinos, devem ter compartilhado da mesma fonte litosférica enriquecida. Todavia, grande parte das composições isotópicas obtidas para os basaltos alcalinos são empobrecidas em termos de Sr (0,70463 a 0,70322) e Nd (Nd +0,6 a +4,7), indicando também a participação inconteste de um componente astenosférico na gênese destes magmas. A forte correlação observada entre todos os valores Nd (entre -8,42 e +4,7) e as idades modelo TDM (1,44-0,27 Ga) nesses basaltos pressupõe que o manto litosférico foi termalmente modificado por astenosfera durante a geração do magmatismo.
A partir dos dados geoquímicos e isotópicos, nós demonstramos que o manto litosférico abaixo do Domínio Seridó esteve submetido a eventos episódicos de enriquecimento químico ao longo de um período de ca. 2,0 Ga, entre o Paleoproterozóico e Mioceno. Estas heterogeneidades químicas estão intrinsecamente associadas a eventos tectônicos globais e regionais (caso do magmatismo terciário) que afetaram o sistema crosta-manto litosférico durante a evolução geodinâmica do Domínio Seridó.

     
UNIVERSITY OF BRASILIA / INSTITUTE OF GEOSCIENCES

PhD THESIS N^o 054

MARIA HELENA BEZERRA MAIA DE HOLLANDA

KeyWords: continental lithospheric mantle, isotopic geochemistry, Borborema Province

DATE OF ORAL PRESENTATION: 21/06/2002
TOPIC OF THE THESIS: REGIONAL GEOLOGY
SUPERVISOR: PROF. MARCIO MARTINS PIMENTEL (UnB)
COMMITTEE MEMBERS: PROF. ELTON LUIZ DANTAS (UnB); PROF. JOSÉ AFFONSO BROD (UnB); PROF. ALCIDES NÓBREGA SIAL (UFPE); PROF. LEILA SOARES MARQUES (USP)

ABSTRACT
The geochemical and isotopic compositions of mafic rocks ranging in age from Proterozoic to Tertiary in northeastern Brazil (Borborema Province) reflects episodic incompatible element enrichment in the continental lithospheric mantle, recording periodic interaction between this latter and the convective asthenosphere and mantle plumes. This interaction took place during major geological events such as subduction, orogenic delamination, rifting and development of Brazilian passive margin.
Late-Neoproterozoic (ca. 580 Ma) high-K gabbros and diorites are representative of the voluminous bimodal magmatism in the Borborema Province. These rocks show chemical signature that reflects derivation from a subduction-modified lithospheric mantle source: (i) enrichment in large-ion lithophile (Rb, Ba, K, Th) and light rare-earth elements (La/YbCN = 11 to 70), (ii) pronounced negative Nb anomaly and (iii) strongly radiogenic Sr (0.71202 to 0.7059) and unradiogenic Nd (Nd from –9.3 to –20.1) isotopic compositions. TDM model ages indicate a paleoproterozoic (mostly between 2.2 and 1.9 Ga) age for this metasomatic event, coincident with the Transamazonian/Eburnean tectonic processes in the region. REE modelling point out to 10-20% of partial melting of a metasomatised garnet lherzolite to produce these enriched compositions. Asthenosphere was the catalyst to promote partial melting of this mantle source in the Neoproterozoic, and we presume that lithospheric delamination was responsible for putting it in contact with lithospheric mantle at end of the Neoproterozoic. Beyond heat supply, asthenosphere probably contributed with some mass as suggested by the nonradiogenic Pb ratios (206Pb/204Pb = 16-17.3, 207Pb/204Pb = 15.1-15.6 and 208Pb/204Pb = 36-37.5), contrasting with the enriched Sr and Nd compositions and suggesting decoupling of Rb-Sr, Sm-Nd and U-Pb systems at time of intrusion of the basic magmas into the crust. This evidence shows that lithospheric mantle beneath the northeast Brazil was preserved of significant chemical modifications (except perhaps for Pb compositions) after the Transamazonian/Eburnean tectonics, until Neoproterozoic. 
Chemical signature of the continental lithospheric mantle at the Mesozoic was given by Rio Ceará Mirim magmatism. Its geological expression is a 400 km long juro-cretaceous dyke swarm in northeastern Brazil, formed in association with the opening of the Atlantic Ocean, during Gondwana break-up. The main dyke swarm is dominated by both high- and low-TiO2 tholeiitic basalts, which show chemical characteristics compatible with an enriched mantle source, such as: (i) strong enrichment in each large-ion lithophile and moderate enrichment in light rare-earth (La/Yb_CN = ca. 7 to 9) and high field strength elements, and (ii) strongly radiogenic initial Sr (0.710 to 0.7047) and unradiogenic Nd (Nd from –0.6 to –9) isotopic compositions. From a mantle end-member mixing modelling, the isotopic compositions can be explained by the strong involvement of both EM 1 and EM 2 end-members, and a depleted component. This one is especially identified in alkaline basalts that constitute a second order dyke swarm, to the south of the main swarm, which have 87Sr/86Sr as low as 0.703 and Nd as high as +5. 
The range of isotopic compositions of the Rio Ceará Mirim magmatism (including tholeiites and alkaline basalts) is compatible with interaction between a plume-related depleted source and an ancient enriched mantle source. The enriched component identified from the tholeiite isotopic compositions can be modelled to be lithospheric mantle, which had their Rb/Sr and Nd/Sm ratios increased due to subduction in Proterozoic times. Involvement of an old component is confirmed by the TDM model ages ca. 1.0 Ga. St. Helena plume, presently recognised as HIMU-like composition in oceanic island basalts, was probably the depleted component encountered in alkaline basalts.
A narrow genetic link is observed when the Sr-Nd isotopic compositions and incompatible element ratios of the mezosoic tholeiitic basalts are compared with those of some tertiary alkaline basalts. These latter, as well as the older (neoproterozoic and mesozoic) mafic rocks, equally exhibit typical enrichment in incompatible elements and strongly fractionated REE patterns. Two groups with distinct major element, LILE and LREE characteristics are identified: (i) low-SiO2 (= 40%) basalts, having mg# = 72, the highest LILE contents and (La/Yb_CN) ranging from 32 to 41, and (ii) high-SiO2 (= 43%) basalts, mg# between 62 to 70, lowest LILE contents and (La/Yb_CN) varying from 11 to 21. Initial 143Nd/144Nd ratios in the alkaline basalts as a whole exhibit variation from 0.512181 to 0.512583, values quite comparable to those from oceanic island basalts. The initial 87Sr/86Sr range between 0.70571 and 0.70322 and are anti-correlated with the Nd isotopic ratios.
REE modelling points that the alkaline basalts were produced by 5-20% partial melting of a metasomatically enriched garnet-bearing peridotite. The isotopic compositions exhibited essentially by the high-SiO2 basalts show that this enriched mantle source was initially similar to that of the mesozoic tholeiites. However, a noteworthy inverse correlation between 143Nd/144Nd and TDM model ages for the alkaline basalts (ranging from ca. 1.2 to 0.3 Ga) suggests that the mesozoic enriched lithospheric mantle was thermally eroded and progressively converted to asthenosphere, coeval with basaltic extraction at Tertiary.
From the geochemical and isotopic constraints, we presume that the upper lithospheric mantle beneath northeastern Brazil was submitted to three pervasive enrichment events in incompatible trace elements along of a period of ca. 2.0 Ga, between Paleoproterozoic to Miocene. These enriched heterogeneities were imprinted during tectonic processes that widely affected the crust, and had been each long-term preserved due the isolation of the lithospheric mantle from convective asthenospheric flow.