LAVAS ULTRABÁSICAS DA UNIDADE BASAL DO GREENSTONE BELT DE CRIXÁS (Go): UMA NOVA CLASSE DE ROCHAS ULTRABÁSICAS NO ESTADO DE GOIÁS

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Abstract
The basal unit of the Crixás Greenstone belt is constituted of peridotitic and pyroxenitic flows intercalated with iron formation, chert, maficand acid tuffs and intrusive bodies of piroxenite and babbro. At the south end of the Crixás belt, the ultrabasic unite becomes thicker and the flows are best exposed, particulary along the Alagadinho Creek where detailed mapping was carried out. Three types of ultrabasic flows were recognized: flows with spinifex texture (olivine) in the medium zone; flows with polyedral jointing and spinifex texture (clinopyroxene) in the upper zone; flows with polyhedral jointing. The ultrabasic flows are composed of olivine, clinopyroxene, glass and opaques. The metamorphism of greenschist facies, together with deformation, have destroyed most of the primary volcanic textures and structures and changed partially olivine to serpentine and chlorite, clinopyroxene to tremolite-actinolite and glass to very fine chlorite. A preliminary chemical work sugests that these ultrabasic rocks belong to the komatiitic suite, characterized by the ratio CaO/Al 2 O 3 ≥ 1 and low TiO 2 , Na 2 O and K 2 O. The identification in the greenstone belt of Crixás, of komatiitic peridotite, which represents a new type of ultrabasic rock in the Central Western Part of Brazil, implies a reevaluation of the "serpentine belt" and ·"alpine type" ultramafic rocks of that region. The basal unit of the Crixás Greenstone belt is constituted of peridotitic and pyroxenitic flows intercalated with iron formation, chert, maficand acid tuffs and intrusive bodies of piroxenite and babbro. At the south end of the Crixás belt, the ultrabasic unite becomes thicker and the flows are best exposed, particulary along the Alagadinho Creek where detailed mapping was carried out. Three types of ultrabasic flows were recognized: flows with spinifex texture (olivine) in the medium zone; flows with polyedral jointing and spinifex texture (clinopyroxene) in the upper zone; flows with polyhedral jointing. The ultrabasic flows are composed of olivine, clinopyroxene, glass and opaques. The metamorphism of greenschist facies, together with deformation, have destroyed most of the primary volcanic textures and structures and changed partially olivine to serpentine and chlorite, clinopyroxene to tremolite-actinolite and glass to very fine chlorite. A preliminary chemical work sugests that these ultrabasic rocks belong to the komatiitic suite, characterized by the ratio CaO/Al 2 O 3 ≥ 1 and low TiO 2 , Na 2 O and K 2 O. The identification in the greenstone belt of Crixás, of komatiitic peridotite, which represents a new type of ultrabasic rock in the Central Western Part of Brazil, implies a reevaluation of the "serpentine belt" and ·"alpine type" ultramafic rocks of that region.

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