UNIVERSIDADE DE BRASÍLIA /INSTITUTO DE GEOCIÊNCIAS

TESE DE DOUTORADO N^o 3

EMANUEL FERRAZ JARDIM DE SÁ

DATA DA DEFESA: 06/10/94
ÁREA DE CONCENTRAÇÃO: GEOLOGIA REGIONAL
ORIENTADOR: PROF. REINHARDT ADOLFO FUCK (UNB)
EXAMINADORES: PROF. UMBERTO GUISEPPE CORDANI (USP)
PROF. ALCIDES NOBREGA SIAL (UFPE)
PROF. LUIZ JOSÉ HOMEM D EL-REY SILVA (UNB)
PROF. ARIPLÍNIO ANTÔNIO NILSON (UNB)

RESUMO

Ciclo Brasiliano na Faixa Seridó é caracterizado por deformação dúctil transcorrente dextral (dobras D3 e zonas de cisalhamento contemporâneas), metamorfismo de baixa pressão/alta temperatura e volumoso plutonismo (granitóides G3) de origem mantélica (suites básico-intermediárias de afinidade shoshonítica ou cálcio-alcalina potássica) e crustal (suites monzoníticas e leucogranitos). Esse evento, cuja etapa principal é estimada em 580±30 Ma (datações Rb-Sr isocrônicas e U-Pb em zircões), constitui um processo de retrabalhamento atuando sobre metagranitóides (ortognaisses G2), metasupracrustais (o Grupo Seridó) e um embasamento gnáissico-migmatítico (o complexo Caicó), todos de idade paleoproterozóica.
Os ortognaisses do embasamento representam, na sua quase totalidade, magmas juvenis extraídos da cunha de manto metassomatizado acima de zonas de subducção. Dete modo, considera-se que o Complexo Caixó foi originado pela sucessiva acresção de arcos magmáticos, gerando uma protocrosta siálica, ao longo de um evento orogênico (D1) informalmente referido como o Paleotransamazônico (2,3-2,15 Ga). A porção inferior do Grupo Seridó é constituída por um pacote de sedimentos originalmente discordantes sobre o complexo Caicó, representando a evolução de ambientes desde um rift até uma margem continental passiva (formações Jucurutu e Equador). Os sedimentos basais são acompanhados por vulcanismo toleítico a transicional extraído de manto litosférico em terras raras leves (e valores EtNd negativos), o qual foi sucedico por tipos toleíticos associados a uma fonte normal. A inversão da bacia é marcada pela deposição de turbiditos flyschóides (a Formação Seridó), com restrito vulcanismo toleítico originado de uma fonte geoquimicamente empobrecida (valores positivos do parâmetro EtNd), possivelmente astenosférica. Nappes e dobras recumbentes perfazem um evento contracional de grande penetratividade (D2), em condições variando do fácies xisto verde a anfibolito (pressões intermediárias) e com transporte para S/SE. Suites granitóides sin a tardi-tectônicas (G2), de afinidades monzonítica, cálcio-alcalina, shoshonítica e alcalina, constituem uma feição marcante desse evento tectônico, o qual é atribuído a um processo colisional datado (isócronas Rb-Sr, datações U-Pb e Pb/Pb em zircões) em 1,9±0,1 Ga (ou 1,95±0,05 Ga), evento informalmente referido como o Neotransamazônico.
O estilo estrutural e a tipologia do magmatismo brasiliano se repetem, de forma semelhante embora guardando peculiaridades, ao longo da porção central da Província Borborema. A ocorrência de supracrustais e deformação tangencial de idades distintas (meso e paleoproterozóicas), separadas por zonas de cisalhamento e associadas a feições geológicas e geofísicas específicas, sugere que a Província Borborema é constituída de vários terrenos alóctones, aglutinados no curso da orogênese Brasiliana e, possivelmente, de um evento ca. 1,0 Ga. A acresção desses terrenos (arcos e microcontinentes) foi comandada por processos de colisão oblíqua e suturamento transcorrente/transformante. Numa etapa final, todo o bloco situado entre as suturas principais (também oblíquas) que margeiam os crátons do Oeste Africano/São Luís e São Francisco/Congo, foi submetido a retrabalhamento transcorrente com extrusão lateral de blocos, no estilo do platô Tibetano/sul da China.
No contexto geodinâmico Brasiliano/Pan-Africano, a Faixa Seridó e outros domínios da Província Borborema constituem microplacas continentais que preservam os registros de ciclos orogênicos antigos. A ocorrência de contextos semelhantes ao longo da Faixa Trans-Sahara, na África Ocidental, sugere que esses microcontinentes constituem fragmentos, tectonicamente desmembrados, de uma antiga faixa colisional (Transamazônica/Eburneana), bem como de segmentos precoces (ou referíveis a um evento prévio, Kibaran) da própria cadeia neoproterozóica.
Com base nos dados da literatura e nos resultados da presente tese, é feita uma discussão sobre a metodologia de estudo dos níveis crustais profundos das faixas orogênicas. É conferida ênfase à caracterização de sua anatomia em termos das unidades tectono-estratigráficas constituintes e de modelos cinemáticos, visualizando o desenvolvimento integrado e penecontemporâneo de dobras, zonas de cisalhamento e fases de deformação. Nesse contexto, são discutidos critérios e argumentos pertinentes aos conceitos de evolução monocíclica ou policíclica das faixas orogênicas.

  
 
UNIVERSITY OF BRASILIA / INSTITUTE OF GEOSCIENCES

PhD THESIS N^o 3

EMANUEL FERRAZ JARDIM DE SÁ

DATE OF ORAL PRESENTATION: 06/10/94
TOPIC OF THE THESIS: REGIONAL GEOLOGY
SUPERVISOR: PROF. REINHARDT ADOLFO FUCK(UnB)
COMMITTEE MEMBERS: PROF. UMBERTO GUISEPPE CORDANI (USP)
PROF. ALCIDES NOBREGA SIAL (UFPE)
PROF. LUIZ JOSÉ HOMEM D EL-REY SILVA (UNB)
PROF. ARIPLÍNIO ANTÔNIO NILSON (UNB) PROF. (UnB)

ABSTRACT

This thesis deals with the geodynamic evolution of the Seridó belt in the Borborema Province, NE Brazil, and its significance in the framework of the Brasiliano/Pan-African orogenic chain. Discussion of criteria concerning hypotheses of monocyclic vs. polycyclic evolution of the belt and adjacent domains is a major part of the investigation. The approach integrates field structural and stratigraphic date, age determinations and geochemical affinities concerning magma sources and tectonic setting, objectives and targets of this study were defined together with a review of the regional geology and crustal evolution of the Borborema Province.

In the Seridó belt, the Brasiliano orogeny is marked by deformation under a transcurrent kinematic regime (D₃) and low pressure, mostly amphibolite facies conditions (M₃), later evolving to a retrogressive stage during slow cooling of the region. Strain partitioning between domains of folding (with an important flattening component) and strike-slip or obliqueslip mylonitic belts (dominated by simple or general shear deformation) is an intrinsic characteristic of this event. This feature, together with early-D₃, low temperature, pressure solution veins and seams post-dating the high strain/high temperature D₂ fabrics, suggests that D₃ overprinting started in an already deformed crust undergoing progressive heating. Dextral transpression is dominant along the central (and probably over most of the western) part of the belt, including features like positive flower and variable crustal structures, contractional-transcurrent duplexes and variable crustal thickening. In the eastern part of the region the Brasiliano structures display a transtensional style with negative flower structures and extensional detachments.

Syn to late-tectonic plutona (G₃ granitoids) represent an outstanding kinematic marker of the Brasiliano event. They bear features like a continuum of the transcurrent fabrics and shear sense from the viscous to the plastic flow stages, sheets intruded along F₃ axial surfaces, characteristic foliation trajectories and dynamic metamorphic aureoles. Several plutons display an en cornue shape in the horizontal, XZ section, consistent with (mostly dextral) displacements along nearby shear zones; such an arrangement demands a forceful emplacement mechanism. The space problem is overcome by a combination of intrusion by sheeting or in transtensional P-bridges and hypothetical rotated, early sinistral tension fractures. Other plutons were emplaced in a more passive fashion, occupying transtensional bends, bridges and wedges, or extensional detachments of the shear zone network.

Dating of syntectonic Brasiliano granitoids by U-Pb zircon and Rb-Sr whole-rock isochron techniques point to the interval 580±30 Ma as the main period of pluton emplacement and coeval D3 ductile deformation. Late to post-tectonic plutonism, cooling and brittle deformation lasted from 540 to 500 Ma, as suggested by available Rb-Sr, K-Ar and 4OAr/39Ar. mostly mineral dates.

Besides subordinate calc-alkaline and alkaline plutons, the Brasiliano magmatism encompasses three kinds of suites: a) basic-to-intermediates shoshonitic or high-K calc-alkaline, metaluminous, transitional I-A types (K-díorites), coeval with; b) porphyritic granitoids of metaluminous, transitional I-A types with subalkaline/monzonitic affinities; c) younger, slightly peraluminous granites, a few of them being true two-mica, garnetbearing leucogranites.

Trace elements data confirm different parental magmas for the K-diorites and porphyritic granitoids, respectively ascribed to mantle and crustal sources on the basis of Sr isotopes and geochemical modelling. Even though some degree of mixing and crustal contamination is chemically detected and field supported in the K-diorites, fractional crystallization±combined assimilation (AFC) trends are still preservei in compatible vs. incompatible trace element variation diagrams. Mixing of mantellic and crustal components in the K-diorite suites produce anomalously old Rb-Sr pseudoisochrons (mixing lines). Sr and Nd isotopes, LREE-highly fractionated patterns and LILE-enriched, Nb-depleted spidergrams (even for the most primitive samples) support a lithospheric enriched mantle source for these rocks, with an old, arc-type component. The porphyritic granitoids are ascribed to lower crustal melting of an intermediate, I-type basementlike source combined with subordinate mantle contribution. The leucogranites derived by melting of dominantly I-type crustal, partly metasedimentary levels.

The Brasiliano orogeny in the Seridó belt overprints rock units and structures of Paleoproterozoic age. The Caicó Complex is a gneiss-migmatite basement encompassing older, subordinate supracrustals. Its dominant, plutonic (now gneissified) rock types represent juvenile magmas extracted from a metasomatized, enriched mantle wedge above subduction zones. Successive aglutination of arc-type sequences during the 2,3-2,15 Ga time interval (as defined by available Rb-Sr isochrons and U-Pb and Pb/Pb zircon its initial deformation (D₁, informally referred as the Paleotransamazonian event), preceding the Seridó Group deposition.

Inferred sedimentary protoliths and chemical affinities of interlayered volcanic rocks suggest that the lower unite of the Seridó Group (Jucurutu and Equador formations, originally unconformably overlying D₁-deformed Caicó gneisses) were deposited in a rift to passive margin setting. Continental, transitional to tholeiitic basalts and andesi-basalts at the lower levels of the Jucurutu Formation reflect an enriched lithospheric source (negative E^t_Nd values, high LREE contents) with an arc-type signature inherited from Caicó times. Stratigraphically higher volcanic levels record a more depleted or normal source. Deposition of the upper, turbidite-type unit (Seridó Formation micaschists) with penecontemporaneous contractional structures marks the onset of a convergent regime in the belt, corresponding thus to a flysch sequence. Subordinate tholeiitic volcanic layers at the base of this unit reflect a depleted (positive E^t_Nd values), probably astenospheric source which rose in response to the previous lithospheric stretching setting. In this context, the absence of calc-alkaline or other arc-type lavas suggests that this region was attached to the trailing edge of a subducting plate. A hypothetical suture zone to the north, plus the required active margin assemblages, should be looked for in the Nigerian shield.

This collisional scenario is antecipated mostly on the basis of D₂ deformation features, This is a highly penetrative and widely distributed, greenschist to amphibolite facies event. Unfolding of later (D₃) structures confirm a tangential geometry for its S+L fabric, with S/SE tectonic transport. Stratigraphic inversions caused by recumbent folds and nappe structures, coupled with medium pressure (kyanite) conditional during D₂, support its interpretation as a contractional, collision-related event. A later period of higher temperature, possibly extensional reworking of at least some of the tangential shear surfaces, seems to be related to the gravitational collapse of the thickened orogen. Considerations on kinematic incompatibility (two fabrics with different shear senses, recorded on markers of very different ages) confirm an older, pre-D₃ age for these tangential structures, allowing to discard their interpretation as flats or detachments of the D₃ strike-slip shear zones.

Syn to late-tectonic granitoid intrusions of monzonitic, calc-alkaline and shoshonitic affinities, ingluding several augen gneiss plutons, represent a conspicuous kinematic and chronological marker for the D₂ event. U-Pb, Pb/Pb zircon and Rb-Sr isochronic dates of these rocks, their D₂ mylonitic facies and pegmatite sheets injected and deformed along F₂ axial surfaces, as well as U-Pb dating of zircons from a high-grade Jucurutu paragneiss, point to a time interval of 1,9 O,1 Ga (or alternatively, 1,95±0,05 Ga) for the D₂ deformation and coeval granitoid emplacement (provisionally referred to as the Neotransamazonlan orogeny).

On the grounds above summarized, the Seridó belt is regarded as a polycyclic tectonic unit. At the present crustal level, the Brasiliano Cycle is essentially an intracontinental reworking event, Neoproterozoic cover rocks, eventually deposited in pull-apart basins, would have been eroded away in this region. This feature holds true over most of the central part of the Borborema Province, where the major supracrustal sequences are of Paleoproterozic (Seridó Group, Ceará Group, at least some of the highergrade metasedimente in the Transversal Zone domain) or Meeoproterozoic age (Orós-Jaguaribe sequences, Salgueiro-Cachoeirinha groups, the northern block of Riacho do Pontal belt). Although large areas remain poorly-dated both in NE Brazil and West Africa, Neoproterozoic sequences appear to be dominant only at the belts surrounding the large West Africa/São Luís and São Francisco/Congo cratonic masses - Atacora/NW Ceará and Riacho do Pontal/Sergipano/Oubanguides.

In a brief review, NW Ceará is regarded as a Brasiliano tectonic pile of Meso (?) and Neoproterozoic supracrustal units overlying and partly interleaved with Paleoproterozoic to Archean high-grade gneisses, as a result of oblique collision between the West Africa/São Luis Craton and the Hoggar/Nigeria/Borborema block. In both the Sergipe and Riacho do Pontal belts, the external and younger nappes were emplaced at high angles to the cratonic margin, while terrane collision at the inner regions is mostly oblique. This is the case of the northern part of the Riacho do Pontal belt, interpreted as a mesoproterozoic terrain accreted to the cratonic margin during an early Brasiliano stage.

Older tangential, thrust-related tectonics with NW transport directions is recorded over large areas of the Transversal Zone domain, south of the Patos shear zone. Ca. 950±20 Ma old (Rb-Sr isochrons, U-Pb and Pb/Pb zircon dates) augen gneisses, with similar setting and geochemical signature as compared to the Seridó G₂ granitoids, are correspondingly regarded as syn- to late-tectonic intrusions with respect to this tangential deformation. Penecontemporaneous arc-type sequences attest to the orogenic, probably collisional significance of this event, lending support to interpret the Cachoeirinha/Salgueiro groups as part of a polycycle, Meso to Neoproterozoic belt.

North of the Patos shear zone, both at the Seridó belt and Central Ceará domain, the tangential structures display southwards kinematic indicators and are ascribed to the older, 1,9tO,1 Ga orogeny, As the Paleoproterozoic and Mesoproterozoic supracrustals and plutonics have not been found in direct, original contacts but rather occur separated by major, tangential or strike-slip shear zones. Other reported features like basic, eclogitic rocks and positive gravity trends along these tectonic contacts have also to be considered. On these grounds, a terrane interpretation is posed for the central part of the Borborema Province, encompassing at least two suture zones between (and possibly partly coincident with) the Patos and Pernambuco shear zones. At least one of these sutures could be related to the 1,0 Ga old event. Alternatively, both structures are related to the Brasiliano orogeny, in this case performing a lateral, transform accretion style.

Geological interpretations in the Borborema Province are also constrained by correlations with the Trans-Sahara belt in West Africa. Integration of data from both continents highlights the following points: a) tangential structures in Nigeria and Hoggar are still poorly dated and although ascribed to the Brasiliano/ Pan-African event, some of them could also be of pre-Brasiliano age, just like in the Seridó belt and other polycyclic domains in NE Brazil; b) the predominance of dextral movements along the Pan-African shear zones demands an emphasis on oblique accretionary/collisional models at the scale of the belt, rather than on the frontal collision one traditionally held in western Hoggar,

Within the broader framework of the Brasiliano/Pan-African orogenic chain, the Seridó belt and similar domains in the Borborema Province and Africa should be regarded as discrete, allocthonous terranes preserving an old orogenic record. Amalgamation of these terranes is mostly the result of an early stage (700-600 Ma) of the Brasiliano/Pan-African orogeny. However, other accretionary/collisional events probably date back to ca, 1,0 Ga or even more, like in Seridó. During a later stage of the Brasiliano cycle (600-540 Ma), following the closure of oceanic domains surrounding the major cratonic landmasses, a larger plate encompassing most of the Hoggar, Nigeria-Cameroon and Borborema shields was subjected to reworking along crustal- and lithospheric-scale shear zones, in connection to a lateral extrusion process, Basic magmas (the K-diorite suites) were extracted from LILE- and LREE-enriched, sheared or delaminated lithospheric slabs. These melts, ascending through or trapped within the base of the crust, coupled with variable effects of crustal thickennig (along transpressional zones) and/or lithospheric thinning (in transtensional structures) related to the istrike-slip kinematics, triggered lower crustal anatexis resulting in the voluminous intermediate I-A type granitoids so characteristic of the belt.

Based on literature research and the studied examples, a discussion is presented concerning the anatomy and evolution of orogenic belts. The points emphasized are specially relevant to the inner, deeper parts of these belts and include the discrimination of their different kinds of tectonostratigraphic units, kinematic models explaining the development of folds and deformation phases coeval with shear zones, and the capability of the continental crust to preserve a polycyclic orogenic record.