An overview of the geochemistry of Eoarchean to Mesoarchean ultramafic to mafic volcanic rocks, SW Greenland: Implications for mantle depletion and petrogenetic processes at subduction zones in the early Earth

This study reviews the geochemical characteristics of Eoarchean to Mesoarchean ultramafic to mafic volcanic rocks (now amphibolites) in SW Greenland and compares them with those of Cenozoic oceanic island arc basalts, to evaluate Archean subduction zone petrogenetic processes. Emphasis is placed on the Th–REE–HFSE (Zr, Ti, and Nb) systematics of the ca. 3800 and ca. 3700 Ma arc suites in the Isua greenstone belt, the ca. 3075 Ma Ivisaartoq–Ujarassuit greenstone belt, and amphibolites associated with the ca. 2970 Ma Fiskenæsset layered anorthosite complex.On N-MORB-normalized diagrams, the Isua, Ivisaartoq–Ujarassuit, and Fiskenæsset volcanic rocks are all characterized by depletion of Nb relative to Th and LREE, consistent with a supra-subduction (forearc–arc–backarc) geodynamic setting. Similarly, on the Th/Yb–Nb/Yb projection, these suites plot within the field of Cenozoic oceanic island arc basalts. On log-transformed immobile trace element ratio (Nb/Th, La/Th, Sm/Th, and Yb/Th) diagrams, they display a trend projecting from MORB (Mid-Ocean Ridge Basalt) to IAB (Island Arc Basalt) on the IAB–CRB (Continental Rift Basalt)–OIB (Ocean Island Basalt)–MORB diagram, as for Cenozoic oceanic island arc basalts. Accordingly, these trace element compositions are interpreted as reflecting the enrichment of Archean depleted upper mantle (MORB-source) by subduction-derived melts and fluids following the initiation of intra-oceanic subduction and arc migration.Concentrations of MgO and Ni in SW Greenland Archean basalts overlap with, but extend to 2 to 4 times higher than, those in Cenozoic oceanic island arc counterparts. In contrast, the majority of Archean basalts have REE and HFSE concentrations 2 to 4 times lower than Cenozoic oceanic island arc basalts, consistent with more depleted sub-arc mantle wedge peridotites in the Archean than in Cenozoic counterparts. Such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter Archean mantle. We infer that less refractory compositions of the Present-day depleted upper mantle, the source of MORB and conservative HFSE in arc basalts, have resulted from re-mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post-Archean times.Archean basalts in SW Greenland share the negative Nb and Ti anomalies of Cenozoic oceanic island arc basalts. On average, Archean basalts, however, have lower Th/Nb, La/Nb, Sm/TiO2 and Gd/TiO2 ratios than Cenozoic oceanic island arc counterparts. Given that rutile controls the Nb and Ti budgets in arc magmas, the lower Th/Nb, La/Nb, Sm/TiO2 and Gd/TiO2 ratios in SW Greenland basalts are attributed to the mobility of Nb and Ti in slab-derived melts involving rutile fusion in Archean subduction zones. These elements are less mobile in fluids originating from Cenozoic subducting oceanic crust where rutile appears to be generally stable. Accordingly, it is suggested that higher geothermal gradients in the Archean may have provided optimized conditions for slab melting and metasomatism of the sub-arc mantle wedge by slab-derived melts.

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► This study deals with the geochemistry of Archean volcanic rocks in SW Greenland.
► Trace element characteristics suggest a supra-subduction geodynamic setting.
► On tectonomagmatic discrimination diagrams, they display a trend from MORB to IAB.
► Their mantle sources were metasomatized by subduction-derived melts and fluids.
► The source of Archean arc basalts was more depleted than their Cenozoic counterparts.