Navegando por Autor "MIYAGAWA, Luciana de Jesus Penha Pamplona"

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Basaltos almofadados da Suíte Ofiolítica Morro do Agostinho: registros de fundo oceânico na porção centro-oeste do Cinturão Araguaia
(2013-12) MIYAGAWA, Luciana de Jesus Penha Pamplona; GORAYEB, Paulo Sérgio de Sousa
Along the domain of low metamorphic grade (Midwestern portion) of the Araguaia Belt, outcrop dozens mafic and/or ultramafic bodies of ophiolitic suites. An example is the Morro do Agostinho Ophiolite Suite, in the surroundings of the city of Araguacema (Tocantins) that configures a small isolated body that sustains the Morro do Agostinho and is tectonically emplaced in meta-sandstones, slates and phyllites of Couto Magalhães Formation (Tocantins Group). The Morro do Agostinho Ophiolite Suite consists of serpentinized peridotites, basalts and ferriferous cherts. The association of basalts is characterized by a significant layer of submarine pillow lavas overlying serpentinized peridotites. The basalts were classified into massive and hialobasalts types. Massive basalts are homogenous, with intersertal texture that is essentially composed of thin laths and acicular crystals of plagioclase and clinopyroxene and present rare crystals of olivine, chalcocite and chalcopyrite. Hyalobasalt shows ultra-fast cooling features (quenching) as spherulites, radial, acicular and skeletal crystals of plagioclase and clinopyroxene with forms of swallowtail. Geochemical data, including (La/Yb)n and (La/Sm)n ratios less than 1, indicate a subalkaline-tholeiitic MORB nature of the basalts. These rocks show that during the geological evolution of Araguaia Belt, there was an oceanization stage of Araguaia Basin, with litospheric mantle's exposure, followed by lava's extravasation and sedimentation of cherts and banded iron formation in deep ocean environment. After sedimentary filling of the Couto Magalhães Formation (Tocantins Group), the tectonic inversion, which led to the exhumation of the ophiolitic bodies, especially along thrust surface, with fragmenting and tectonic mixing with supracrustal rocks, accompanied by regional metamorphism under conditions of low green schist facies. The Morro do Agostinho Ophiolite Suite therefore represents a small allochthonous fragment of a well-preserved ocean mantle/crust lithosphere, similar to the others examples found in the Araguaia Belt who marks the Araguaia Basin's moment of oceanization during the Neoproterozoic.
Acesso aberto (Open Access)
Geologia, petrografia e geoquímica da Suíte Ofiolítica Araguacema - Cinturão Araguaia
(Universidade Federal do Pará, 2012-06-25) MIYAGAWA, Luciana de Jesus Penha Pamplona; GORAYEB, Paulo Sérgio de Sousa; http://lattes.cnpq.br/4309934026092502
Ophiolites suites were identified along Araguaia Belt’s domain of low metamorphic grade; they are distributed figuring dozens of disrupting bodies. Araguacema Ophiolitic Suite stands out among them and it is the porpouse of this research. Araguacema Ophiolitic Suite is located in the city of same name, northwest of the State of Tocantins and is tectonically emplaced in metasedimentaries rocks of low metamorphic grade of Couto Magalhães Formation (Tocantins Group). This suite consists of three main rock types: serpentinized peridotites, pillow basalts and ferrigeous metacherts. The serpentinized peridotites are the predominant type, in general, they are phaneritic thick rocks, fractured and display intersected by numerous veins of chrysotile. The peridotites were classified into two types: harzbugites and wehrlites. The harzbugites, considered mantle peridotites, present predominantly pseudomorphic texture given for mesh and the features bastite. The wehrlites represent cumulative peridotites, and have combined texture evidenced by large crystals of olivine, clinopyroxene and orthopyroxene, which are partially serpentinized. On the NE flank of Araguacema Ophiolitic Suite arises a significant layer of pillow basalts. We identified three types of basalts: massive, spherulitic glassy and hialoclastites. The massive basalts are encountered at the core of the pillow structure and they have a little interact with sea water that characterizes then as the most preserved type. Massive basalts are homogeneous, aphanitic, with intersertal texture that is essentially composed of plagioclase laths and clinopyroxene, whose are located upon glassy material. The spherulitic glassy basalt occupies the middle of the pillow structure and shows more ultra-fast cooling features (quenching) as spherulites, radial and acicular crystals of plagioclase, opaque minerals and clinopyroxene with skeletal forms of “swallow-tail” variety. The hialoclastites occur at the edge of the structure, therefore, represent the most contaminated by sea water lythotipes and the contamination caused mineralogical changes by replacing the crystals of augite and Caplagioclase by albite, epidote, chlorite and carbonate and chemical transformations by silica e alkalis depletion and enrichment of MgO and Al2O3 concentrations which led to the formation of these minerals. The ferrigeous metacherts are found in the NW flank of the suite and exhibit millimetric discontinuous banding of layers that are rich in quartz and magnetite. The scanning electron microscopy (SEM) identified in serpentinized peridotites magnesian olivine (forsterite), the augite type: clinopyroxene, magnetite, Fe-Mg chromite, sulfides and oxides of Ni. At basalts whose are analyzed we have found sodic plagioclase type, albite, augite, magnetite, and chalcopyrite chalcocite. The X-Ray Diffraction (XRD) was also utilized to help at the identification of cuprite ferrite magnesium in the harzbugites and clinochlore, nimite (mica that is rich in Ni) and polylithionite (mica rich in Li and K) inside internal structure of the spherulites. The geochemistry of harzbugites revealed that they were enriched in MgO, FeO and Ni rocks and depleted in CaO, which reflects the mineralogy composed mainly of olivine and orthopyroxene. While the wehrlites are rich in CaO, Al2O3, and Cr and they’re related to the presence of magnesium chromite and augite. Although we can find differences at some major element’s concentration, in general, peridotites are a little depleted at light rare earth elements (REE) than at heavy REE, with gochemistry signatures sugesting lherzolitic mantle derivation. The pillow basalts reveal subalcaline-tholeiitic nature of MORB type. Reasons La/YbN <1 confirm this nature and reasons La/SmN <1 show that they are N-MORB-type, resulting from processes of partial melting of a depleted mantled source. The negative anomaly of Sr indicated the retention of plagioclase during the stages of partial melting to the formation of basalts and confirms that the source is the depleted mantle of plagioclase lherzolite type, which is typical of N-MORB mid-ocean ridge’s basalts. The data analysis allowed characterizing the Araguacema Ophiolitic Suite as a small allochthonous fragment of a well preserved and low metamorphic ocean mantle/crust, similar to other examples found in the Araguaia Belt who mark the Araguaia Basin’s moment of oceanization during the Neoproterozoic.