Geology, Petrology And Geochronology Of The Layered Mafic-ultramafic Intrusions In The Porto Nacional Area, Central Brazil

摘要

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.