Contrasting Neoproterozoic and Mesozoic granitoids in Zaranda complex (Nigeria): insights into the distinct origins, tectonic settings and mineralization potential

摘要

Combined whole-rock geochemistry, zircon U-Pb dating, and Sr-Nd-Pb isotopic analyses were carried out to constrain the petrogenesis and tectonic implications of the "Zaranda Older Granites" (ZOG) and "Younger Granites" (ZYG) in north-central Nigeria. Zircon U-Pb dating yielded 633 +/- 3 Ma for the ZOG and 203 +/- 1 Ma for the ZYG, corresponding to Late Neoproterozoic and Late Triassic, respectively. The ZOG consists dominantly of biotite granite, which displays a whole-rock eNd(t) values of - 3.48 to - 5.00 and Nd model ages of 1620-1720 Ma. Most ZOG samples exhibit elevated Sr/Y (107-128) and La/Yb ratios (36-40), with a positive Eu anomaly (Eu/Eu* = 2.2-2.58), interpreted to reflect magma derivation through partial melting of a thickened lower crust. The ZOG is geochemically and geochronologically similar to the Pan-African granites from Togo-Benin (West Africa) and Northeast Borborema, Sao Paulo (Brazil), suggesting that their emplacement probably marked the initial phase of the Pan-African orogeny during which the Gondwana was amalgamated. The ZYG consists of compositionally diverse alkaline granitoids including aegirine syenites, quartz syenite, and fayalite granite porphyry. These rocks are strongly ferroan, alkalic and show petrographic and geochemical features of classical A-type granites. Substantial depletion in Ba, Sr, P, Ti, and Eu signifies fractionation of felsic and mafic minerals such as feldspar, biotite, amphibole, and Ti-rich augite. The low Y/Nb and Yb/Ta ratios (= 2) indicate that the ZYG samples display geochemical features typical of OIB-derived A-type granites. The eNd(t) values in the ZYG range from + 4.16 to - 2.76 and the Nd model ages range from 0.6 to 1.2 Ga. Based on these results, we propose that the ZYG samples and by extension, other alkaline granitoids from the Nigerian Younger Granite (NYG) province with similar geochemical compositions were likely generated through extensive differentiation of OIB-type mantle-derived magma. The concentrations of Nb and other HFSEs are much higher in ZYG than ZOG, indicating that the former has higher rare metal mineralization potential.