Historiografia da região de dobramentos do médio Coreaú

Abstract

The regiao de Dobramentos do Medio Coreau, known since the beginning of the century, is the most spectacular geologic feature of Ceara and may be one of the most important of the northeastern Brazil. According to Brito Neves (1995), it corresponds to an Orogenic Belt, which includes a variety of lithotypes with different ages and origins within one single tectonic unit. This region, together with Macico de Granja, extends along the whole portion of the extreme northwest of Ceara, and is situated to the north of the Sobral-Pedro II lineament. In this paper we present a summary of the historical stratigraphic concepts of the seven main units described in the region, which comprise: 1) the Jaibaras Group, which represents a molassic unit with volcanic intercalations deposited in a graben and unconformably overlying the Ubajara.Group. A total rock Rb/Sr isochron yield and age of 535 ± 27 Ma (Novais et al. 1979). This unit probably underwent prehnita-pumpelleyte facies (Jardim de Sa et al 1979). 2) the Ubajaras Group includes pelites-psammites, sandstones, graywackes and limestones (Costa et al. 1973), and is cut by granite with a total rock Rb/Sr age of 550 ± 30 Ma (Sial et al. 1981). 3) the Chaval Granite, a sinorogenic and strongly sheared intrusion of total rock Rb/Sr age of 507 ± 27 Ma (Nogueira Neto et al. 1989). 4) the Martinopole Group, made up of phyllites, schists, limestones, metavulcanics, and mylonitic quartzite, all metamorphosed under green schist facies. U/Pb data of zircon of metavolcanic rocks intercalated in the metasedimentary rocks yield a Neoproterozoic age of 808 ± 7,8 Ma for the sedimentation (Fetter et al. 1995). 5) the Sao Joaquim Group, which includes 1.500 m thick quartzites and many intercalation of ortho and paragneisses (Torquato et al. 1987). 6) the Granja Granulitic Belt, represented by khondalites, charnockitic gneisses, enderbites and mafic granulites. These rocks yield a Pb/Pb zircon age of nearly 2.0 Ga (Gaudette et al. 1993), and a U/Pb age of zircon of 2.2 Ga (Fetter et al. 1995). 7) the Granja Complex Ortogneisses of tonalitic-granodioritic composition, which may represent reworked basement rocks. Pb/Pb ages of zircon from two gneisses of the vicinities of the city of Granja are of 2.028 ± 30 Ma and 2.253 ± 27 Ma (Gaudette et al. 1993), while U/Pb in zircon of the same region yield ages between 2.290 and 2.360 Ma (Fetter et al. 1995).