Alkali feldspar syenites with shoshonitic affinities from Chhotaudepur area: implication for mantle metasomatism in the Deccan large igneous province

Abstract

Two petrologically distinct alkali feldspar syenite bodies (AFS-1 and AFS-2) from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidiomorphic texture and consists of feldspar (Or₅₅Ab₄₃ to Or₂₅Ab₇₁), ferro-pargasite/ferro-pargasite hornblende, hastingsite, pyroxene (Wo₄₇, En₅, Fs₄₆), magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene (Wo_47-50, En_22-39, Fs_12–31) set in a groundmass matrix of alkali feldspar (Or₉₉Ab_0.77 to Or_1.33Ab₉₈), titanite and magnetite. In comparison to AFS-1, higher elemental concentrations of Ba, Sr and ∑REE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is ∼1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.