Arc-nascent back-arc signature in metabasalts from the Neoarchaean Jonnagiri greenstone terrane, Eastern Dharwar Craton, India C. MANIKYAMBA 1 * , SOHINI GANGULY 2 , M. SANTOSH 3 , M. RAJANIKANTA SINGH 1 and ABHISHEK SAHA 1,2 1 National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad, India 2 Department of Geology, University of Calcutta, Kolkata, India 3 School of Earth Science and Resources, China University of Geosciences, Beijing, China The Neoarchaean Jonnagiri greenstone terrane (JGT) is located at the centre of the arcuate Hutti–Jonnagiri–Kadiri–Kolar composite greenstone belt in the eastern Dharwar Craton. High MgO (MgO = ~14 wt.%; Nb = 0.2 ppm), low Nb (LNB) (MgO = 7.8–12 wt.%; Nb = 0.1–5.1 ppm) and high Nb basalts (HNB) (MgO = 5.6–10.1 wt.%; Nb = 9.0–10.6 ppm) metamorphosed to lower amphibolite facies are identified based on their geochemical compositions. These metabasalts exhibit depleted HFSE (Nb–Ta, Zr–Hf), pronounced LREE and LILE enrichments suggesting contribution from subduction-related components during their genesis. Th and U enrichment over Nb–Ta indicates influx of fluids dehydrated from subducted oceanic lithosphere. The high MgO basalts with higher Mg# (51) than that of the associated LNB and HNB (Mg# = 34–47) represent early fractionated melts of subduction-modified mantle peridotite. The LNB were produced by partial melting of mantle wedge metasomatized by slab-dehydrated fluids, whereas the HNB represents melts of subducted oceanic crust and hybridized mantle wedge. Lower Dy/Yb and variable La/Yb ratios suggest their generation at shallower depth within spinel peridotite stability field. The low Ce–Yb trend of these metabasalts reflects intraoceanic type subduction which straddles the fields of arc and back-arc basin basalts, resembling the Mariana-type arc basalts. The Jonnagiri metabasalts were derived in a paired arc-back-arc setting marked by nascent back-arc rift system that developed in the proximity of an intraoceanic arc. Copyright © 2014 John Wiley & Sons, Ltd. Received 24 February 2014; accepted 26 May 2014 KEY WORDS Jonnagiri greenstone terrane; high Nb basalts; slab dehydration-wedge melting; paired arc-back-arc; nascent back-arc rift 1. INTRODUCTION Understanding the geological and tectonic processes involved in the generation of continental crust provides important constraints on global crustal evolution and continental growth (Condie and Aster, 2013; Condie and Kröner, 2013). Phaner- ozoic continental crust formed mainly along subduction zones by tectonic accretion and emplacement of juvenile magmatic rocks (Sengör et al., 1993; Polat, 2012; Xiao and Santosh, 2014; Zhai, 2014). However, the nature of petrogenetic and geodynamic processes that generated the Archaean continental crust still remains one of the most challenging problems in Earth science (Foley et al., 2002; Rapp et al., 2003; Hawkesworth et al., 2010; Santosh et al., 2013). Nd–Hf isotopic data from Archaean rocks in several cratons and high Nb/Th and Nb/U ratios in Neoarchaean komatiites and tholeiitic basalts suggest that by 2.5 Ga, the Earth’s upper mantle had been strongly depleted in incompatible elements (Sylvester et al., 1997; Kerrich and Xie, 2002; Bennett, 2003; Polat, 2012). Such depletion of the early mantle implies the extraction of a large volume of incompatible-element enriched mafic to ultramafic components from the mantle. It is widely accepted that ~60% of the continental crust formed by the end of the Archaean (Taylor and McLennan, 1995). This contention is also endorsed by recent models for Archaean continental evolution which indicate a peak of crustal growth through terrane accretion processes and amal- gamation of oceanic plateaus and island arcs at convergent plate margins during the Neoarchaean (Polat et al., 1998; Isley and Abbott, 1999; Kerrich and Polat, 2006; Manikyamba and Khanna, 2007). Therefore, subduction zone magmatism is considered to be the most viable mechanism for generation of new continental crust. However, the periodicity of crustal growth is also linked with the mantle plume activity, and it is important to identify the relative contributions of the *Correspondence to: C. Manikyamba, National Geophysical Research Insti- tute (Council of Scientific and Industrial Research), Uppal Road, Hyderabad 500 007, India. E-mail: cmaningri@gmail.com Copyright © 2014 John Wiley & Sons, Ltd. GEOLOGICAL JOURNAL Geol. J. 50: 651–669 (2015) Published online 10 July 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/gj.2581