Geochemistry of the NeoArchaean high-Mg basalts, boninites and adakites from the Kushtagi–Hungund greenstone belt of the Eastern Dharwar Craton (EDC); implications for the tectonic setting

Abstract

Volcanic rocks of the Kushtagi–Hungund greenstone belt from the Eastern Dharwar Craton (EDC) show extreme diversity due to the presence of high-Mg, high Fe-, high Mg–Fe basalts, high Mg-dacites and andesites (boninitic affinities), and sodic–plagioclase–quartz rich felsic rocks (adakites with Na 2O (∼4–6 wt.%). The adakites ( Drummond and Defant, 1990; Martin, 1999) and high-Mg-dacites–andesites are being reported for the first time from greenstone belts of the Dharwar craton. Trace elements in these different types of volcanic rocks have distinct characteristics. For example, the basalts have (1) Al 2O 3/TiO 2 ∼16 (PM value of 17), (2) (La/Yb) N ∼1–3, (3) low order negative anomalies of Nb/Nb*, Zr/Zr*, Hf/Hf* and Ti/Ti*, (4) Zr/Nb ∼4–10 and (5) Ce/Nb ∼3–4. Two samples of andesites having Mg-enrichment and Al 2O 3/TiO 2 ratios of 31–35 are interpreted to have boninitic affinities. Other dacites and andesites are characterized by (1) high-Mg (8–10 wt.%), (2) high Al 2O 3/TiO 2=22–27, (3) Zr/Nb=8–14, (4) (La/Yb) N ∼2–7, (5) negative Nb/Nb*, Zr/Zr*, Hf/Hf* and Ti/Ti* anomalies and (6) Sc/Lu=67–172. The adakites are characterized by (1) a narrow range of SiO 2 (67–70 wt.%), (2) enriched Na 2O (4–6 wt.%) with very high Na 2O/K 2O (2.5–7.7), (3) Low Mg# (0.38–0.48), (4) high (La/Yb) N (13–31) with fractionated patterns, no Eu anomaly and (Gd/Yb) N =2–4, (5) very high Al 2O 3/TiO 2 (50–80) and (6) enriched Ni, Cr and Co (total 50 ppm). These adakites resemble 3.0 Ga tonalite–trondhjemite gneisses of the Dharwar craton, which are interpreted as products of partial melting of the basaltic slab. The available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting.