PETROGENESIS OF PRECAMBRIAN GRANITIC SUITES AROUND THE GINDI AKWATI AREA, WEST-EASTERN NIGERIA TERRANE: CONSTRAINTS FROM ZIRCON U-PB-LU-HF ISOTOPES, MINERAL CHEMISTRY, AND WHOLE-ROCK GEOCHEMISTRY

The Precambrian granite plutons along the western margin of the anorogenic Ropp complex at Gindi Akwati are preliminarily thought to represent a classic site with imprints of multiple orogenies (Eburnean, Kibaran, and Pan African events) that could define the geodynamic regime during the formation of the I- type granitic rocks in the west-eastern Nigeria terrane (ENT). Zircon LA-ICP-MS U-Pb dating of the granitic gneiss yield ages of 2157, 1427, and 631 Ma, while the Older Granite (OG) yield the age of 620 Ma. This suggests that metamorphism occurred at 1.43 Ga, between two magmatic episodes: 2.16 Ga (Eburnean orogeny) and 0.63-0.62 Ga (Pan-African orogeny). The OG is moderately evolved peraluminous calc-alkaline granites with high-K characteristics. The weak negative Eu anomaly, high Ba, Sr, and K/Rb, low Nb, Ta, HREE of the OG resemble those of typical high Ba-Sr granitoid. The low Mg# (<40), and the span of the ratios of some trace elements, including low Nb/La (average = 0.33), Nb/U (average = 10.60) is typical of magma sourced from the crust. The zircon ε_Hf(t) values (Granitic gneiss = −16.25 to +6.09 and OG = −3.96 to +3.86) for these plutons indicate that partial melting of older crust with mantle input formed the parent magma. Also, the low Y/Nb (average = 1.02), Yb/Ta (average = 1.34) in the OG also indicates mantle input signatures. The magma's physicochemical conditions from biotite and whole-rock analyses suggest the OG are ilmenite series granites formed under low ƒO₂ conditions (logƒO₂= ‒15.63 to ‒17.18). The OG's crystallization temperature and pressure are moderate (T = 706‒770ᴼC and P = 100 to 295 MPa), and the depth of emplacement was determined as 5.2 to 8.6 km using the newly improved Qtz-geobarometer. Metamorphism (the syn-collisional 631 Ma granitic gneiss) and magmatism (the post-collisional 620 Ma OG) in the area are contemporaneous with other granitoids in the ENT. The slab-melt from sinking oceanic crust subduction convects and underplates the lower crust to generate the granitic gneiss magma. Slab break-off during the waning stage of Pan-African orogeny induced the upwelling of the asthenosphere that partially melted the sub-continental lithospheric mantle (SCLM) and the lower crust to generate the parent magma, whose ascent follows a vertical tectonic extensional fault to the shallow crust to form the OG.