Ticiano José Saraiva dos Santos

Evidence for Neoproterozoic Continental Arc Magmatism in the Santa Quitéria Batholith of Ceará State, NW Borborema Province, NE Brazil: Implications for the Assembly of West Gondwana

Abstract Recent field investigations and geochronological studies of Neoproterozoic rocks in the northwestern part of the Borborema Province, Ceara State, NE Brazil provide important clues pertaining to the nature of convergence between the Borborema Province and the West African-Sao Luis craton during the assembly of West Gondwana. U-Pb zircon data indicate that the earliest evidence of convergent magmatism along the northwest margin of the Borborema Province occurred around 777 Ma, and was followed by the development of a large continental arc batholith (Santa Quiteria batholith) between ca. 665 and 591 Ma within the central part of Ceara State. These findings, along with supporting geophysical data, suggest that convergence between the Borborema Province and the West African-Sao Luis craton involved closure of an oceanic realm with subduction polarity to the southeast beneath the northwestern part of the province. Consequently, it seems likely that the Pharusian Ocean was continuous from the Hoggar Province in West Africa into South America during the late Neoproterozoic and additional data suggests that it may have even been connected with the Goianides Ocean of the Brasilia Belt farther to the southwest.

Evidence for 2.35 to 2.30 Ga juvenile crustal growth in the northwest Borborema Province, NE Brazil

Abstract The c. 600 Ma Brasiliano Borborema Province of NE Brazil comprises a complex collage of Precambrian crustal blocks cut by a series of continental-scale shear zones. The predominant basement rocks in the province are 2.1–2.0 Ga Transamazonian gneisses of both juvenile and reworked nature. U–Pb zircon and Sm–Nd whole-rock studies of tonalite-trondhjemite-granodiorite basement gneisses in the NW Ceará or Médio Coreaú domain in the northwestern part of the Borborema Province indicate that this represents a continental fragment formed by 2.35–2.30 Ga juvenile crust. This block has no apparent genetic affinity with any other basement gneisses in the Borborema Province, and it does not represent the tectonized margin of the c. 2.1–2.0 Ga São Luis Craton to the NW. The petrological and geochemical characteristics, as well as the Nd-isotopic signatures of these gneisses, are consistent with their genesis in an island arc setting. This finding documents a period of crustal growth during a period of the Earths history which is known for its tectonic quiescence and paucity of crust formation.

Provenance and tectonic setting of neoproterozoic supracrustal rocks from the Ceará Central Domain, Borborema Province (NE Brazil): constraints from geochemistry and detrital zircon ages

Major- and trace-element and U–Pb analyses of detrital zircons were performed on metavolcano sedimentary sequences and igneous rocks from the Ceará Central Domain (CCD) in the Borborema Province of northeastern Brazil. No significant geochemical differences were found between these rocks, which were possibly initially deposited as parts of a very large metavolcano-sedimentary sequence. Weathering in the source area was moderate, and the sediments were deposited as both sands and clays. The sources of the sediments were likely mixtures of felsic and intermediate rocks deposited predominantly in an active-margin setting with minor contributions of both continental arc and passive margin components. Three main source ages were identified: Palaeoproterozoic (~2.2 Ga), for which potential sources include the Palaeoproterozoic Madalena-Algodões Suite; early Neoproterozoic (~850 Ma), related to felsic volcanic magmatism due to continental rifting, initial phases of the Santa Quitéria Magmatic Arc, or magmatic arc systems on the margins of the Palaeoproterozoic crust; and late Neoproterozoic (~650 Ma), associated with extensive granite generation and migmatization events accompanying Santa Quitéria Arc activity. Deposition of the CCD volcanosedimentary rocks occurred shortly before regional, collision-type metamorphism accompanying the amalgamation of the São Francisco-Congo Cratons (~ 620–630 Ma).Major- and trace-element and U–Pb analyses of detrital zircons were performed on metavolcano sedimentary sequences and igneous rocks from the Ceara Central Domain (CCD) in the Borborema Province of northeastern Brazil. No significant geochemical differences were found between these rocks, which were possibly initially deposited as parts of a very large metavolcano-sedimentary sequence. Weathering in the source area was moderate, and the sediments were deposited as both sands and clays. The sources of the sediments were likely mixtures of felsic and intermediate rocks deposited predominantly in an active-margin setting with minor contributions of both continental arc and passive margin components. Three main source ages were identified: Palaeoproterozoic (~2.2 Ga), for which potential sources include the Palaeoproterozoic Madalena-Algodoes Suite; early Neoproterozoic (~850 Ma), related to felsic volcanic magmatism due to continental rifting, initial phases of the Santa Quiteria Magmatic Arc, or magmatic ar...

Geologia da Faixa Eclogítica de Forquilha, Domínio Ceará Central, noroeste da Província Borborema

A partir de mapeamento geologico em escala de detalhe, 1:10.000, e estudo petrografico sistematico foram caracterizados o modo de ocorrencia, a distribuicao e as associacoes minerais do retroeclogito de Forquilha, que situa-se em uma faixa N-S ao longo de mais de 16 km na regiao NW da Provincia Borborema. Tais rochas ocorrem como boudins em gnaisses orto- e paraderivados e, raramente, associados a rochas calcio-silicaticas. Esta associacao compoe a Faixa Eclogitica de Forquilha, que estruturalmente esta em contato tectonico de empurrao com metapelitos e corpos subordinados de rochas metacarbonaticas e metamaficas metamorfizadas entre condicoes facies granulito de alta pressao e facies anfibolito superior pertencentes ao Grupo Ceara. O retroeclogito mostra tres principais estagios de retrometamorfismo representados pelas seguintes reacoes: 1o) [Grt + Cpx + Qtz + H 2 O → Pl + Amp] e [Grt + Cpx + Rt + Qtz + H 2 O → Amp + Ilm + Pl]; 2°) [Omp → Di + Pl]; 3o) [Grt + Di + Pl + H 2 O → Amp] e [Grt + Pl + Qtz + H 2 O → Grt 2 + Pl (An) + Amp], alem de incipiente formacao tardia de Fe-clorita e actinolita. O retrometamorfismo ocorreu de forma heterogenea nestes corpos, sendo mais intenso nas bordas devido a interacao com fluidos metamorficos canalizados ao longo de zonas de cisalhamento durante o processo de exumacao progressiva. A ocorrencia de retroeclogito em Forquilha e importante para a contextualizacao da evolucao tectonica da Provincia Borborema durante a formacao do Gondwana Ocidental, uma vez que este litotipo pode representar uma zona de sutura neoproterozoica.

Airborne geophysical and tectonics of the Ceará central domain, eastern region of the Santa Quitéria magmatic ARC, Borborema Province, NE Brazil

The airborne geophysical data gamma-spectrometric and magnetic from Itatira Project (1977) was carried out, in the central region of the State of the Ceara, Brazil. Their processing produced products useful to indirectly identify regional geologic features. The verification and applicability of this approach was tested in a pilot-area whose limits are within the Umirim and Caninde topographic sheets, at 1:100,000 scale. The integration between the airborne geophysical and the geologic field data supported to the geologic mapping at the same cited scale. Two main units were recognized: the Santa Quiteria magmatic arc and the supracrustal sequences that surround the arc. The rocks fram Santa Quiteria magmatic arc are characterized by high counting on the K-channel in its almost entire region and intermediate and low counting of eTh and eU. Generally, the supracrustal sequences are characterized by high values on the eU-and-eTh-channels and low on K-channel. The subunit metaultramafics and, mafics granulites, are remarkable by the occurrence retrograde eclogites bands and mafic granulites as lenses within gneisses and migmatites. These subunits present strong negative anomalies, with low counting on the three K, eTh and eU channels. That area also has positive magnetic anomalies, and may reflect high values of magnetic susceptibility.

Petrographical and geochemical signatures of the Granja paragneisses (Médio Coreaú Domain, NW Ceará, Brasil)

The Granja Granulite Complex (GGC) exposed in the Médio Coreaú Domain (NW Ceará, Brasil) consists mainly of garnet and sillimanite migmatitic paragneisses enclosing discontinuous lenses of mafic granulites and enderbites. According to the published geochronological data, this high-grade metamorphic belt represents a segment of the Paleoproterozoic basement intensely reworked during the Brasiliano / Pan-African Orogeny in the Neoproterozoic (600 Ma). The Granja paragneisses are strongly foliated rocks characterized by the alternance of dark garnet-biotitesillimanite-rich layers and millimeter-thick leucocratic quartz-feldspathic bands, interpreted as indicative of incipient melting. As melt contents increase, layer-parallel leucosomes become thicker and a well-developed stromatic layering is defined. Both the gneissic and stromatic fabrics are strongly overprinted by a penetrative mylonitic foliation correlated to the last reactivation of the dextral NE-SW trending Granja Shear Zone (GCZ) that cuts across the studied area. Mineral assemblages and microstructures indicate that these rocks were affected by granulite-facies metamorphism and anatexis followed by decompression and cooling. In order to constrain the protolith composition of the Granja paragneisses, twelve whole-rock samples from the parts of the migmatitic paragneisses that appear to have undergone little or no melt extraction were analysed for major and trace elements. In the classification diagram of Herron (1988), the samples plot in the transition between the greywacke and the pelite fields, suggesting that the pre-metamorphic sequence was dominantly composed by shales and immature clastic sediments (greywackes). Their chondrite normalized REE patterns show a moderate LREE enrichment (LaN/YbN = 9.46–15.50), flat HREE profiles and negative Eu anomalies (Eu/Eu* = 0.63–0.82), closely resembling those of PAAS (Post-Archean average Australian Shale) and Early Proterozoic Greywackes. Geochemical data also suggest that the precursor sediments of the Granja paragneisses derived from source areas of felsic to intermediate composition and were deposited in a tectonically active continental margin / continental island arc setting.