In Northern Araguaia Belt, in the region known as Serra do Tapa, in the municipalities of Sapucaia and Xinguara, southeast of Pará near the boundary with the state of Tocantins, are exposed one of the largest bodies elongated on N-S direction, and the best preserved lithosphere oceanic fragments of the Neoproterozoic in the Araguaia Belt. The Serra do Tapa Association is consist of a set of rocks that include metabasalts with remarkable pillow structure associated with banded iron formations, metacherts, silexites rich in hematite and / or magnetite plus metaperidotites/serpentinized dunites, tectonically setted in metasedimentary rocks of low-grade metamorphism of the Tocantins Group. The association of these rocks is interpreted as fragments of ancient ocean floors and exposed portions of the upper mantle, called ophiolitic suits. On this view, the study approached four main bodies of pillow metabasalts and chemical metasedimentary associated rocks. The metabasalts exhibit pillow structures and also hyaloclastites, locally. These rocks consists of a series of strokes and lava front overflow on the seafloor environment, on which were subsequently deformed and metamorphosed, but still preserve their magmatic characteristics. These rocks present an internal zoning, in which the core is defined by a green massive basalt, aphanitic, intersertal texture consisting essentially of strip form plagioclases. hypovitreos Basalts of the edge zone of yellowish green color, aphanitic and with ultrafast cooling textures like spherulites, acicular and radial crystals of plagioclase, and also textures like shallow-tail and hollow sections of these crystals. The hyaloclastites represent surface breccias of basaltic flow. They occur in hypovitreo basalts of the dark green interpillow. These rocks define elongated bodies preferentially in the NS direction with gentle dips to ESE. The basalts are weakly metamorphosed and the mineral association stabilized in green schist facies (actinolite, chlorite, epidote, and albite stilpnomelane). Additional studies of X-ray diffraction and Scanning Electron Microscopy (SEM) were performed to better characterize the mineral constituents of these rocks. The banded iron formations, characterized as jaspilites and cherts correspond to the marine chemical sediments of deep ocean marine environment, constituting the upper portion of the ophiolitic sequences present strongly crystallized portions indicating that they were also metamorphosed. These rocks still preserve their sedimentary structures such as banding and plane-parallel laminations composed of jasper or chert and hematite. Litochemical analyzes, based on geochemical classification diagrams, reveal the nature of the metabasalts with subalkaline-tholeiitic signature of oceanic magmatism N-type MORB. Considering the field, petrographic, litochemical data and accumulated knowledge in the
literature about these bodies it was concluded that these basalts, jaspilites and cherts are fractions of an ancient oceanic lithosphere which represent an environment where seafloor volcanism developed a submarine exhalative volcanism and tholeiitic basaltic volcanism extrusions during the oceanization stage of the Araguaia basin, during the Neoproterozoic, which was later obducted and weakly metamorfisated during inversion processes of the Araguaia Belt, along overthrust belts with transport toward the Amazonian Craton.
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