Geochemical systematics of tholeiitic basalts from the 2.7 Ga Ramagiri-Hungund composite greenstone belt, Dharwar craton

The 2.7 Ga Ramagiri-Hungund composite greenstone belt, or tectonostratigraphic superterrane (RHST), is one of a series of volcanic dominated supracrustal belts in the eastern Dharwar craton. Extensive outcrops of well preserved pillow basalt occur in the Penakacherla Terrane (PT); these are amphibolite facies, and relatively undeformed. Basalts are tholeiitic, and compositionally uniform, where Mg# spans 64 to 60. REE patterns are coherent, with no Eu anomalies, near flat HREE and systematically depleted LREE (La/Smcn = 0.68–1.08; Gd/Ybcn = 0.91–1.08). Chromium (284–671 ppm), Co, and Ni contents are high relative to Phanerozoic arc tholeiites. Dominantly negative Nb and Ti anomalies relative to REE, and Zr/Nb = 15–25 compared to 32 in average N-MORB, signify a convergent margin setting with a depleted mantle wedge; they plot with the low Ce–Yb trend of intraoceanic arcs. High Mg#, and Cr, Co, Ni contents may be indicative of a plume-influenced mantle wedge. The basalts are distinct from most Phanerozoic arc basalts in having a spectrum of negative to positive Nb and Zr–Hf anomalies, where the most positive anomalies are in conjunction with lowest La/Smcn. The former can be accounted for by addition of LREE from the slab to subarc mantle wedge in fluids, with conservative behaviour of Nb and Zr–Hf. The latter reflect a two stage melting history; first stage depletion of incompatible elements in the wedge, leaving positive anomalies of Nb and Zr–Hf, that are inherited by second stage melts. Cesium, Ba, and Rb show coherent enrichments relative to Th, Nb, and LREE, consistent with a frontal versus rear arc setting based on modern arcs.