Pillow basalts of Morro do Agostinho Ophiolite Suite: records of seafloor in the midwestern portion of Araguaia Belt

Abstract

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.