Alteration and geochemical patterns in the 3.7–3.8 Ga Isua greenstone belt, West Greenland

Abstract

Volcanic rocks of the 3.7–3.8 Ga Isua greenstone belt have undergone heterogeneous, poly-phase post-magmatic alteration, resulting in mobilisation of many elements including LREE and Th. In contrast, Nb, Ta, Ti, Zr, Y and HREE were relatively immobile during alteration processes. The degree of element mobility appears to have been controlled by chemical composition, primary volcanic structures, carbonate and silica enrichment, and proximity to ultramafic units and shear zones. On the basis of the geochemical characteristics of the least altered samples, two distinct associations have been recognised in structurally separated lithotectonic sequences. These magmatic associations are: a low-HFSE association (TiO 2 =0.20–0.40 wt.%; Zr=12–30 ppm; Nb=0.13–0.80 ppm; Y=6–14 ppm) in the central arc sequence, resembling Tertiary boninites; and a high-HFSE association (TiO 2 =0.50–1.14 wt.%; Zr=34–77 ppm; Nb=1.2–2.7 ppm; Y=11–28 ppm) in the outer and inner arc sequences, comparable to Phanerozoic island arc basaltic to picritic volcanic rocks. These two geochemically distinct sequences were likely to have been juxtaposed as a consequence of Phanerozoic-style plate tectonic processes operating in the early Earth. The Isua volcanic rocks are characterised by sub-chondritic Nb/Ta (10–16) ratios. Post-magmatic alteration can be ruled out as the cause of these low ratios. Rather, these ratios reflect a compositional characteristic of the Paleoarchean mantle source(s). The sub-chondritic Nb/Ta ratios may have resulted from either preferential sink of Nb into the core as a weakly siderophile element , or preferential retention of Nb in the subducted oceanic crust during subduction zone petrogenetic processes. Alternatively, the building blocks of the Earth were not composed of CI-like chondritic material.