Geology, petrology and geochronology of the layered mafic-ultramafic intrusions in the Porto Nacional area, central Brazil

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• 作者：Humberto Alcâ ; ntara Ferreira Lima ; Cesar Fonseca Ferreira Filho ; Má ; rcio Martins Pimentel ; Elton Luiz Dantas ; Sylvia Maria de Araú ; jo
• 关键词：Layered intrusion ; Mafic-ultramafic ; Geochronology ; Cambrian ; Porto Nacional ; Sm– ; Nd ; U– ; Pb
• 刊名：Journal of South American Earth Sciences
• 出版年：2008
• 出版时间：November 2008
• 年：2008
• 卷：26
• 期：3
• 页码：300-317
• 全文大小：2138 K

文摘

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 (Fo₆₈) to highly fractionated (Fo₀₇). 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.