ORIGIN OF TONALITE-TRONDHJEMITE-GRANODIORITE (TTG) GNEISS IN THE BASEMENT COMPLEX OF SOUTHWESTERN NIGERIA: LU-HF ISOTOPIC VIEW

Abstract

This research investigates and report the origin of Tonalite-trondhjemite-granodiorite gneiss (TTG) in the basement complex of southwestern Nigeria. The unit forms the country rock and comprise of migmatite and assorted gneisses collectively referred to as migmatite-gneiss complex. Field investigation reveals the migmatite contains palaeosome and neosome while the gneisses contain melanosome, mesosome and leucosome. Petrographic examination reveals the unit comprise dominantly of quartz, orthoclase, plagioclase and biotite with hornblende forming supporting mineral aggregate. The mineral assemblage (quartz + biotite + K-feldspar + opaque) in the rock represents medium-grade metamorphic conditions, a temperature of about 550-600^oC and a pressure between 3 to 10 K bar. ¹⁷⁶Lu/¹⁷⁷Hf content with values between 0.00367 to 0.001669, ¹⁷⁶Hf/¹⁷⁷Hf ratio between 0.281955 to 0.282022, initial Hf isotope values between 0.281969 to 0.282005, εHf(t) values between -3.5 to -5.2 and a two-stage model age (tDM2) between 2080 and 2161 all indicate the inherited zircons in the rock separated from the ‘Depleted Mantle’ around 2.8 ~ 3.0 Ga. Even though, the Negative εHf(t) values signifies high crustal input, the tDM2 age implied the magma source originated from the Chondritic Universal Reservoir (CHUR) during Mesoarchean. The simultaneous occurrence of palaeosome and neosome in the migmatite, complex tortuous veins, porphyroblasts with augen structures and prevalence of veining (mainly quartzo-feldspartic) in the gneiss probably suggests anatexis is involved in the origin of the rock. Field relationships and textural evidences revealed the Tonalite-trondhjemite-granodiorite gneiss might have originated from partial melting of a poorly differentiated granitoid.

Keywords: Nigeria, TTG-gneiss, Chondritic Universal Reservoir, crustal input, partial melting

DOI: 10.7176/JEES/15-3-02

Publication date: May 30th 2025