Ferroan charnockites in the Wanni Complex, Sri Lanka: Evidence for extensional setting magmatism and crustal evolution

Orthopyroxene-bearing granitic rocks, widely described as charnockites, display a wide range of geochemical and isotopic characteristics. Although several mechanisms have been proposed for the origin and evolution of charnockite, more comprehensive studies are needed to understand the elusive occurrence of its ferroan varieties. In this study, ferroan charnockites together with associated orthogneisses from the central region of the Wanni Complex, Sri Lanka were investigated in detail using integrated petrographic, whole-rock geochemical, geochronological and isotopic constraints to elucidate the processes governing their petrogenesis and evolution. Unlike previously studied charnockites from the Wanni Complex with calc-alkaline signatures typical of arc magmatism, these ferroan charnockites from the Wanni Complex are characterized by high FeO contents (up to 10.5%), extremely low Mg# (<28), and a trend towards tholeiitic field, with monzonitic to syenitic to granitic compositions marked by relatively high total alkali contents (Na 2 O + K 2 O), potassic affinities, and comparatively low CaO contents. Geochemical signatures reveal an extensional setting, with A-type granite affinities and within-plate granite (WPG) signatures. Geochronological data show several distinct age groups. Younger zircon ages (∼531–587 Ma) are explained to represent late Neoproterozoic to Cambrian metamorphic overprinting. The oldest ages (∼955 ± 22 Ma) are interpreted as dating igneous emplacement of these A-type magmatic rocks and zircon Hf isotope data supported by whole rock Nd-Sr isotopic results indicate this early magmatic stage was sourced predominantly from juvenile mantle-derived materials with variable degrees of crustal contamination whereas younger emplacement age of ∼705 Ma reflecting later reworking. These findings indicate that the ferroan charnockite originated from mixing of mantle-derived magmas and arc-derived crustal materials. Therefore, the Wanni Complex can be considered a collage of magmatic arc and accretionary orogen material developed through repeated magmatism of Grenville age, interspersed with extensional magmatism producing a hybridized geochemical signature in ferroan charnockites from its central region.