Partial melting and strain localization in banded orthogneiss–quartzite assemblages of the Rogo, Northwestern Nigeria: geochemical and structural evidence for deep crustal evolution

Abstract

This study examines the tectono–metamorphic evolution of banded orthogneiss, inhomogeneous diatexites, mylonites, and quartzites within a high-grade metamorphic terrane. Focusing on the role of partial melting and strain localisation in the deep continental crust. Integrated field observations and whole-rock major and trace element geochemistry for 30 samples analyzed for XRF were used to constrain protolith composition, metamorphic processes, and deformation mechanisms. Banded orthogneiss displays geochemical characteristics of felsic to intermediate calc-alkaline granitoid protoliths, including large-ion lithophile element enrichment and Nb–Ta depletion, consistent with formation in an active continental margin setting. Partial melting of the orthogneiss resulted in migmatization and the development of inhomogeneous diatexites, characterised by elevated SiO₂ = 61.5 and K₂O = 10 contents, enrichment in Rb =782, depletion in Sr = 4.08 and pronounced negative Eu anomalies, reflecting plagioclase breakdown and residual phase control during anatexis. Mylonites derived from orthogneiss and migmatites largely preserve isochemical major and trace element signatures, indicating deformation dominated by solid-state processes, with limited melt- or fluid-assisted chemical modification along shear zones. Quartzites exhibit very high SiO₂ contents and extremely low trace abundances, confirming derivation from chemically mature quartz arenites and explaining their effectiveness in localising strain and preserving kinematic indicators. The combined data support a progressive deep-crustal evolutionary model involving granitoid crust formation, high-grade metamorphism, partial melting and rheological weakening, melt-assisted strain localisation, and ductile shear zone development during continental collision and subsequent exhumation.