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.
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