DECODING THE EVOLUTION OF PYROCHLORE-BEARING ANOROGENIC ALKALI GRANITES BASED ON ZIRCON U-PB-HF ISOTOPES AND CHEMICAL FEATURES: A STUDY FROM THE YOUNGER GRANITE PROVINCE, NORTHCENTRAL NIGERIA

We combined reports of some petrological, whole-rock geochemical, and zircon trace elements systematics of A-type granites in Ririwai (~176 Ma), Dutsen Wai (~175 Ma), and Ropp complexes (~149 Ma) of the Nigerian Younger Granite province (NYG). They are typical within-plate granite that is quartz-saturated, highly differentiated ferroan, alkalic to alkalic-calcic. The peralkaline plutons are rich in F, Na-Fe silicates, acmite normative, and mainly comprise agpaite-like assemblages with accessory fluorite, and pyrochlore, and are enriched in HFSEs, LILEs and REEs. Zircon grains from Ropp peralkaline granite (Rp) largely have low silica (average = 13.4 wt%), high ZrO₂ (average = 83.7 wt%), and Na (up to = 3041 ppm) content suggesting that soda-rich fluids may have lowered silica activity and recrystallized monoclinic ‘baddeleyite’ (ZrO2) grains from stoichiometric tetragonal zirconium orthosilicate (ZrSiO₄). The grains separated from the aluminous granite (Dt and St4) contain higher abundances of Hf, Pb and U than those of Rp. The relatively high Y, Th, U and Zr contents in Rp suggest the bulk of the grains may have altered to other mineral phases like kobeite, polycrase, and yttrocrasite. Although most zircon grains exhibit chondrite-normalized REE patterns typical of magmatic zircons, few grains have fairly high LREE with somewhat flat chondrite-normalized patterns that are typical of hydrothermally-altered zircons. The 'altered' zircons largely had LREE-1 (Dy/Nd + Dy/Sm) values less than 30 and U/Yb vs Hf contents suggest zircons crystallized in the crust. Links between whole-rock Rb/Sr and zircon Eu/Eu* highlight that the latter can monitor magma fractionation in these systems. Zircon Ce/Ce* and Eu/Eu* might also define the conditions favourable for crystallization of rare metal-enriched accessory mineral phases. The Ti-in-zircon thermometer gives a relatively high mean temperature for the peralkaline granite (Rp = 840°C; probably due to higher saturation temperature of accessories) while the peraluminous granites are within wet solidus for granite (Dt = 671°C and St4 = 696°C). The redox condition of Rp, ΔFMQ = 0.4 to 1.5 and Dt, St4, ΔFMQ = -3.8 to -1.5 reflects higher oxygen fugacity in the peralkaline granite probably due to the nature of femic minerals; aegerine, arfvedsonite that contains ferric iron.