Rare-earth elements as source indicators of Pan-African granites from Obudu Plateau, Southeastern Nigeria

• Published in: Acta geochimica 2008-06, Vol.27 (2), p.130-134
• Main Authors: Ukaegbu, V. U., Beka, F. T.
• Format: Article
• Language: English
• Subjects: Earth and Environmental Science; Earth Sciences; Fractionation; Geochemistry; Granite; Rare earth elements; Rocks; Trace elements; 指示剂; 稀土元素; 花岗岩
• ISSN: 1000-9426; 2096-0956; 1993-0364; 2365-7499
• DOI: 10.1007/s11631-008-0130-2

The rare-earth element （REE） concentrations of representative granite samples from the southeast of the Obudu Plateau, Nigeria, were analyzed with an attempt to determine the signatures of their source, evolutionary history and tectonic setting. Results indicated that the granites have high absolute REE concentrations （190×10^-6-1191×10^-6; av.=549×10^-6） with the chondrite-normalized REE patterns characterized by steep negative slopes and prominent to slight or no negative Eu anomalies. All the samples are also characterized by high and variable concentrations of the LREE （151×10^-6-1169×10^-6; av.= 466×10^-6）, while the HREE show low abundance （4×10^-6-107×10^-6; av.=28×10^-6）. These are consistent with the variable levels of REE fractionation, and differentiation of the granites. This is further supported by the range of REE contents, the chondrite-normalized patterns and the ratios of LaN/YbN （2.30-343.37）, CeN/YbN （5.94-716.87）, LaN/SmN （3.14-11.68） and TbN/YbN （0.58-1.65）. The general parallelism of the REE patterns, suggest that all the granites were comagmatic in origin, while the high Eu/Eu＊ ratios （0.085-2.807; av.=0.9398） indicate high fo2 at the source. Similarly, irregular variations in LaN/YbN, CeN/YbN and Eu/Eu＊ ratios and REE abundances among the samples suggest behaviors that are related to mantle and crustal sources.