ACADEMIA NACIONAL DE CIENCIAS Gondwana 12, Mendoza 2005- Abstracts

Geological fit between the -Alagoas terrane of NE and Central African Fold Belt in Camero on, based on Proterozoic structures and magmatism / I da SilvaJ~'i1h o " W.R. Van Schmus 2, RB. Brito Neves 3, I.P. Guimaracs \ A-.X.. - . 4 , S.F. Toteu and L.S. Osako

I Dept. Geologia . Univ. Federal de Pernambuco, -Pli, Brazil. 1 . Dept. Geology, Univ. Kansas, Lawrence, Kansas , 66045 USA. 3 Institute de Geociencias. Universidade de Sao Paulo, Sao Paulo-S}', Braz il. " C R.G.M., BP 333, Garoua, Cameroon.

The Congo craton (CC) ofAfrica and the Sao Francisco craton (SFC) ofBrazil were joined at about 2.05 Ga during the Eburnian () and Transamazonian (Brazil) orogenies, possibly during formation of a Paleoproterozoic sup ercontinent. Studies in NE Brazil and Central Africa suggest that prior to this event Paleoproterozoic crust extended north ofthe two cratons, and that after fusion the nell', larger continent included Paleopro terozoic and Archean nucleii. Mesoproterozoic and Neoproterozoic events have extensively altered the Precambrian basement north ofthe SFC and the CC, so that direct comparisons are more difficult . In NE Brazil, crust north ofthe SFC comprises the Sergipano Fold Belt (SFB), a region of metasedimentary sequences deposited on or adjacent to extended crustal blocks ofthe SFC These rocks are ofloll' metamorphic grade in the south, where platform deposits overlie the SFC, and ofmoderate metamorphic grade in the north, where turbiditic sediments fil l larg e basins with 110 appa rent older basement. Sni-Nd data for metasedimentary rocks 0/ the SFB indicate that much of the provenance came from late Mesoproterozoic to Neoproterozoic crust. Similar relationships occur in central Afric a, where rocks ofthe Central African Fold Belt (CAFB) overlie or are interspersed with extended Paleoproterozoic crust ofthe CC Sm-Nd datafor metasedimentary rocks ofthe Yaounde Series, which occur along the northern edge ofthe CC, also indicate derivation front sources containing abundant Mesop roterozoic to Neoproterozoic crust. The metamorphic grade ofthe Yaounde Series is much high er than that of the SFB, suggesting that exposed rocks in Africa may represent deeper-seatedparts ofthe orogen. The SFB is bounded to the north by the Pernambuco-Alagoas terrane (PEAL) , which is composed main ly of migmatitic para- and orthogneisses or Brasiliano (ca. 600 Ma) plutons. Based on Sm-Nd and U-Pb data , there are both Paleoproterozoic and Neoproterozoic protoliths /0 the gneisses, although exact relationships between older andyounger protoliths are obscured by the metam orphism and migmatization. PEAL -like domains may extend into central Cameroon, but exact correlations are not yet possible. The southern margin ofthe PEAL terrane is thrust southward over the SFB, whereas the north ern margin of the PEAL terrane is thrust northward over Paleoproterozoic to Neoproterozoic basement 0/ the "Zona Transversa l Domain" (DZT). The E-W striking Pernambuco shear zone (PeSZ) ofNE Brazil mostly occurs along the north ern part ofthe PEAL terrane, but it is not a major crustal boundary; since protoliths to the north and south are similar. The PeSZ continues eastward into Cameroon as the Adamaoua shear zone (IISZ), which is also not a major crustal boundary. The PeSZ-ASZ shea r zone instead represents a majorfault zone that developed during late stages ofthe assembly ofWest Gondwana and was not controlled by pre-exist ing crustal boundaries. The Garan huns shear zone (GSZ) within the PEAL terrane seems to divide dominantly Paleoproterozoic protolith to the north from Neoproterozo ic protolith to the south. The region between the PeSZ and the GSZ exposes high-grade supracrus tal rocks. possibly related to a foreland basin developed over a Paleoproterozoic to Archean basement. and high -K metaluminous and peraluminous granites with or without garnet, derived from Paleoproterozoic protoliths. The high-grade supracrustal rocks y ield Mesoproterozoic, Paleoproterozoic and Archean detrita l zircons. Orthogneisses associated with the PeSZ record a ca. 625 Ala. low-angle defo rma tion event. Magmatism associated with the PeSZ y ields ages ofca. 580 Ma. Dating ofgranites associated with the GSZ y ield ages ca. 580 Ma. similar those obtained/or the PeSZ, suggesting they acted as a duplex fault system. Paleoproterozoic migmatitic gneisses around Bafia in Cameroun and the older metasedim ents [rotn Meiganga may correspond to similar lithologies located between the PeSZ and the GSz. The NNE-SSW striking Sanaga shear zone (SSZ) located to the south ofthe AF. extends into the PEAL terrane, and SSZ-related gn eiss yielded an age 0/ ca. 625 Ma. The Ngo ndo and Tibati granitic complexes, located adja cent to the ASZ, are similar to granites in the PeSZ and y ield an age ca. 580-600 Ma, suggesting coevalfaulting.

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