Abstract
Large layered intrusions of the Porto Nacional area include two distinctively different groups. Mafic intrusions located west of the Tocantins River (Group 1) follow a tholeiitic (Fenner-trend) fractionation trend characterized by olivine compositions from moderately primitive (Fo 68) to highly fractionated (Fo 07). Zircon fractions of an olivine gabbronorite from a Group 1 intrusion yield a U–Pb concordia age of 526 ± 5 Ma. Highly variable T DM ages and negative ε Nd(530 Ma) values of samples from this Group 1 intrusion indicate crystallization in ca. 530 Ma from a magma highly contaminated with older crustal rocks. The layered intrusions located east of the Tocantins River (Group 2) have a large proportion of ultramafics (mainly wehrlite) and follow an Opx-free crystallization sequence. The compositional variation of olivine (Fo 85-84) and Cpx (En 49-45) of cumulates from Group 2 intrusions indicates moderately primitive compositions and very limited fractionation. Sm–Nd isotopic data for Group 2 intrusions are highly scattered and do not yield a crystallization age. Highly variable T DM ages and negative ε Nd(530 Ma) values suggest that the parental magmas of Group 2 intrusions were heterogeneously contaminated with old crustal material. These two petrologically distinct groups of layered intrusions open new opportunities for exploration for Ni–PGE deposits in central Brazil.
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