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Pr Fabricio de Andrade Caxito a
présenté une conférence intitulée "Towards an
integrated model of geological evolution for NE
Brazil-NWAfrica: The Borborema Province and its
connections to Hoggar through the Benino-Nigerian
Shield and Cameroon "
à la bibliothèque du CRAAG le dimanche 06 octobre
2019 à 10h00.
Résumé:
Towards an integrated model of geological evolution for NE
Brazil-NW Africa: The Borborema Province and its connections
to Hoggar through the Benino-Nigerian Shield and Cameroon
Fabrício
de Andrade Caxito1
1
Centro de Pesquisas Manoel Teixeira da Costa, Instituto de
Geociências, Universidade Federal de Minas Gerais
(CPMTC-IGC-UFMG), Campus Pampulha, Av. Antônio Carlos 6627,
CEP 31270-901, Belo Horizonte, MG, Brazil; caxito@ufmg.br
The Borborema Province of NE Brazil is a region
characterized by great crustal complexity, with
superposition of various deformational, metamorphic and
magmatic events and main structuring during the Pan-African
/ Brasiliano Orogeny (ca. 625-510 Ma). The geological
framework of the province is composed of: (i) a major
Paleoproterozoic (2.2-2.0 Ga) basement strongly reworked
during the Brasiliano Orogeny, with a few Archean nuclei
including the oldest rocks in South America (ca. 3.5-3.2 Ga
orthogneisses of the São José do Campestre massif); (ii)
Paleo-mesoproterozoic sequences (starting at 1.8.-1.7 Ga,
and anorogenic granites at ca. 1.5 Ga) developed under a
continental rift setting, mainly in the Orós-Jaguaribeano
belt of the Ceará Central domain; (iii) conspicuous
orthogneisses of the Cariris Velhos Belt, dated at ca.
1000-960 Ma, which are characteristic of this province and
not found elsewhere in the Atlantic Shield of Brazil; and
(iv) the components of complete plate tectonic cycles at the
Neoproterozoic, including rift and passive margin sequences
(900-670 Ma) superseded by syn-orogenic sedimentary basins
(< 650 Ma) throughout the province. Those are intruded by
various generations of granitic and syenitic plutons,
including: (i) calc-alkaline suites interpreted as related
to continental magmatic arcs at ca. 640-625 Ma (Stage I,
Tamboril-Santa Quitéria, Conceição-type granites, and
continental arcs of the Riacho do Pontal and Sergipano
belts); (ii) two-mica syn-collisional granite sheets
following crustal anatexis and the peak of upper greenschist
to amphibolite facies metamorphism and tangential (thrust
and nappe) deformation at ca. 625-590 Ma (Stage II), marking
the peak of continental collision, as also demonstrated by
eclogites dated at ca. 625-610 Ma; followed by (iii)
extensive late- to post-collisional high-K granitic and
syenitic plutons emplaced in combined transpressional and
extensional fields at ca. 590-530 Ma, during a late phase of
lateral escape which produced the extensive network of shear
zones that crosscut the province and characterizes the
structural framework of NE Brazil. Comparisons with the
geological evolution of the provinces in NW Africa are drawn
and an evolutionary model for the whole Pan-African /
Brasiliano affected areas in this region is tentatively
proposed. While connection of the mountain chains that frame
the province, bordering the eastern margin of the West
African / São Luis craton (Médio Coreaú – Dahomeydes –
Gourma – Pharusian) and the northern margin of the São
Francisco / Congo craton (Rio Preto – Riacho do Pontal –
Sergipano – Oubanguides – Central Africa) seem progressively
better constrained, correlations within the interior, highly
reworked and sectioned portions of both the Borborema, the
Benino-Nigerian shield, Central and East Hoggar, and NW
Cameroon, are more complicated and tentative scenarios
demand novel geological, isotopic and geochronological data
in order to be refined. Some of the questions of prime
importance in this context are the continuation or not of
the 1000-960 Ma Cariris Velhos belt, now well defined in
Brazil, into NW Africa, where rocks of this age have not yet
been described; and if the basement-dominated North
Borborema / Benino-Nigerian and Central-southern Borborema /
East Nigeria / NW Cameroon (?) domains could represent major
metacratonic blocks such as LATEA. Discover and
characterization of key tectonic units, such as ophiolites,
eclogites, HP/UHP rocks, and both oceanic and continental
magmatic arcs are helping to clarify those issues and no
doubt will have an enormous impact in tectonic models for
the geological evolution of NE Brazil / NW Africa in the
near future.
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