Geochemistry of alkaline basalts and associated high-Mg basalts from the 2.7 Ga Penakacherla Terrane, Dharwar craton, India: An Archean depleted mantle-OIB array

Abstract

Well-preserved alkaline basalts, bearing relict aegirine, leucite and nepheline mineralogy, are stratigraphically associated with high-Mg basalts in the Neoarchean Penakacherla greenstone belt, eastern Dharwar craton, India. Alkaline basalts (Mg #∼0.70–0.58) are enriched in alkalies (K 2O + Na 2O ∼ 7 wt.%), and TiO 2 (2.3–2.1 wt.%), and exhibit fractionated REE patterns with (La/Yb) N ranging from 23 to 29. On primitive mantle normalized diagrams they record a downturn from Ce to Th, small negative Nb anomalies relative to La, and Zr/Hf ratios higher than the primitive mantle value, in common with compositional characteristics of Phanerozoic alkaline ocean island basalts (OIB). Some interelement ratios are intermediate between EM1- and HIMU–OIB. Associated high-Mg basalts (MgO 17.2–9.2 wt.%) have comparatively lower TiO 2 (1.2–0.50 wt.%), and flat HREE patterns with slight depletion in LREE, and small positive Nb anomalies. These basalts are compositionally similar to tholeiitic basalts associated with komatiites in many Neoarchean greenstone terranes. Two samples have the conjunction of Nb/Th < 8 and fractionated LREE [(La/Yb) N 2.60–2.63] consistent with crustal contamination. On the global array of Phanerozoic to Recent ocean island basalts, in SiO 2 versus Nb/Y coordinates, alkaline basalts plot with counterparts from Aitutaki and Heard, whereas high-Mg basalts plot near Iceland tholeiites. Alkaline basalts plot with OIB, and high-Mg basalts near N-MORB, on the Th/Yb versus Nb/Yb MORB-OIB array of Phanerozoic intraplate basalts; accordingly the mantle components of that array were established in the 2.7 Ga mantle asthenosphere. Alkaline basalts are rare in Archean volcanic sequences. This occurrence of alkaline basalts indicates subduction, recycling, and incubation of Mesoarchaean oceanic and continental crust in the mantle, and generation of high-Mg and alkaline basalts from a mantle plume at 2.7 Ga, possibly analogous to counterparts of Iceland. The mantle plume likely erupted at a thin craton margin, given flat HREE of most high-Mg basalts.