Evidence for subduction at 3.8 Ga: Geochemistry of arc-like metabasalts from the southern edge of the Isua Supracrustal Belt F.E. Jenner a , V.C. Bennett a, ⁎, A.P. Nutman a,b , C.R.L. Friend c , M.D. Norman a , G. Yaxley a a Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200, Australia b Beijing SHRIMP Centre at the Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037, P.R. China c 45 Stanway Rd. Headington, Oxford, OX3 OBP, U.K. abstract article info Article history: Accepted 22 September 2008 Keywords: Isua Supracrustal Belt Eoarchean Subduction Island arc basalts Greenland We present new major and trace element data for rare examples of preserved pillow basalts from a locality of little studied N 3800 Ma rocks at the southwestern edge of the Isua Supracrustal Belt (ISB), West Greenland. The 20 samples have 47.3–59.0 wt.% SiO 2 , 4.9–12.2 wt.% MgO, 6.5–11.2 wt.% CaO, 60–340 ppm Ni and 63– 1094 ppm Cr, consistent with pre-metamorphic basaltic compositions. Nb/La (n) and Ti/Gd (n) are lower than primitive mantle compositions (0.1–0.6 and 0.3–0.8, respectively), with these ratios interpreted to reflect the composition of their source. Correlations of Nb/La with La/Sm, Ti/Gd and Zr/Sm and between Zr/Nb and Nb/Th are comparable to those of modern subduction related magmas, whereby slab melts enriched the overlying mantle wedge and HFSE were retained by Ti-rich phases. Trends in Ba/La vs. Ba/Nb are comparable to modern day arc related basalts, which reflect a sediment influence and may indicate that, at least early in the evolution of Eoarchean subduction zones, the subducted Eoarchean oceanic crust retained a pelagic sediment cover, with this cover influencing the chemistry of the slab-derived fluid. Three of the ISB pillow lavas show extreme enrichments in LREE (La/Sm (n) = 1.9–2.6), large depletions in Nb, Ta, Zr and Hf relative to REE that are similar to rare high-K arc basalts from the Clark volcano of the presently active Tongan– Kermadec–New Zealand arc (Gamble, J.A., Christie, R.H.K., Wright, I.C., Wysoczanski, R.J.,1997. Primitive K- rich magmas from Clark volcano, southern Kermadec arc: a paradox in the K–depth relationship. The Canadian Mineralogist, 35, 275–290). The compositions of these highly enriched basalts indicate that the mantle source region of the Isua basalts was occasionally overprinted by small-volume sediment dominated melts rather than hydrous fluids. The compositional affinities of these 3.8 Ga pillow basalts with modern island-arc basalts provide strong evidence for the role of slab fluids and melts in basalt genesis in the Eoarchean, further demonstrating arc-like signatures in the oldest rock sequences. The compositions of Archean mafic rocks including samples from the ISB that are interpreted to have a petrogenesis similar to that of modern island-arc basalts, are compared with Archean ‘non-arc’ rocks (i.e. plume-related komatiites, komatiitic basalts and associated tholeiitic basalts). The two groups are distinguished by differences in La/Sm, Gd/Yb, Nb/La, Ti/Gd, Ba/La, Ba/Nb and Al 2 O 3 /TiO 2 . Thus clear compositional distinctions between different basaltic types, attributed to arc and non-arc origins, are a feature of the rock record for at least the last 3.8 Ga. © 2008 Elsevier B.V. All rights reserved. 1. Introduction At modern convergent plate boundaries a range of observations including those from structural geology, geomagnetic surveys, petrology, geochemistry and seismology can be used to place constraints on their structure and magma generation mechanisms. However, ancient terranes are incompletely preserved, and the tectonic and magmatic systems that formed them have been fragmented and modified by later tectonothermal events. Therefore, questions regarding the sources and processes responsible for the formation of these ancient crustal fragments are strongly biased towards interpreting the geochemistry of surviving mafic and ultramafic mantle-derived rocks, tonalites and granites and the detrital zircons derived from them (e.g. Campbell and Taylor, 1983; Mojzsis et al., 2001; Wilde et al., 2001; Polat et al., 2002; Harrison et al., 2005; Watson and Harrison, 2005; Bédard, 2006; Trail et al., 2007). On the modern Earth, mafic and ultramafic rocks produced at mid-ocean ridges, continental rifts, oceanic-islands, oceanic-plateaux and convergent margins carry distinctive geochemical signatures reflecting source variability and petrogenesis. Knowledge gained from studies of modern mafic and ultramafic rocks can provide a comparative context for the petrogenesis of Archean mafic and Chemical Geology 261 (2009) 83–98 ⁎ Corresponding author. Research School of Earth Sciences, Mills Rd. Bldg. 61, Canberra, ACT 0200 Australia. Tel.: +61 2 6125 5509; fax: +616125 8345. E-mail address: vickie.bennett@anu.edu.au (V.C. Bennett). 0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2008.09.016 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo