235 D7. General Topics in Geochemistry and Metamorphic Petrology (Posters)

Wednesday, 12 October 2022
9:00 AM - 1:00 PM
Exhibit Hall F

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

Paper number: 235-23
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-380033

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