Rare-earth and trace element imprints on the origin and tectonic setting of gabbro-diorite complex in the Pan-African be
A gabbro-diorite plutonic complex from the Southeast Obudu Plateau, representing limited volumes of magma, was studied for its trace and rare-earth element characteristics, in an attempt to document its genetic and geodynamic history. Geochemical studies indicate that the gabbro samples are characterized by variable concentrations and low averages of such index elements as Cr (40×10-6-200×10-6; av. 80×10-6), Ni (40×10-6-170×10-6; 53.33×10-6) and Zr (110×10-6-240×10-6; 116.67×10-6); variable and high averages of Rb (3×10-6-270×10-6; 80.67×10-6), Sr (181×10-6-1610×10-6; 628.17×10-6) and U (0.14×10-6-3.46×10-6; 1.51×10-6), and fairly uniform Co (34×10-6-49×10-6; 36.33×10-6) and Sc (23×10-6-39×10-6; 34.5×10-6), while the diorite samples exhibit higher trace element compositions. The range of REE contents and distinctive chondrite-normalized patterns indicate moderate fractionation with slight positive Eu anomaly in the diorites to very low fractionation with flat patterns and slight positive Eu anomaly in the gabbros. However, the general element systematics of the samples, especially LILE (Ba, Rb, Sr, Cs and Pb), HFSE (Zr, Th, U, Hf, Mo, W, Nb and Sn), relatively immobile elements (Zr, Ni, Cr) and REE, suggests a differentiation model, involving fractional crystallization of olivine and clinopyroxene from a partial melt generated beneath an island arc complex. A possible model for the complex is therefore an island arc setting, the development of which was dominated by calc-alkaline magmatism across the Obudu Plateau.
作 者： Ukaegbu V.U. Beka F.T. 作者单位： Department of Geology, University of Port Harcourt, Port Harcourt, Nigeria 刊 名： 中国地球化学学报（英文版） 英文刊名： CHINESE JOURNAL OF GEOCHEMISTRY 年，卷(期)： 2009 28(3) 分类号： P5 关键词： rare-earth and trace element imprints origin and tectonic setting gabbro-diorite complex Pan-African belt Southeast Obudu Plateau Nigeria