Field and Geochemical Characteristics of the ca. 3.0-billion-years-old Ivisaartoq Greenstone Belt, West Greenland: Geodynamic Implications

The 3.0 Ga Ivisaartoq greenstone belt is the largest Mesoarchean (~3075 Ma) supracrustal assemblage in southern West Greenland. The belt is composed mainly of pillow basalts ultramafic flows, gabbros, and serpentinites. Pillow basalts display well-preserved core and rim structures. Many pillow cores have cavities at the centre, which are filled mainly with diopside-quartz-plagioclase-epidote assemblage, indicating Mesoarchean seafloor hydrothermal alteration processes. Pillow cores and gabbros have pre-alteration ocelli structures, composed predominantly of plagioclase-quartz assemblage. These ocelli structures are morphologically and mineralogically similar to those found in other Archean belts. Accordingly, they have been interpreted to represent immiscible liquids. The diopside-quartz-plagioclase-epidote metasomatic assemblage in the pillow cores is interpreted as metamorphosed epidosite formed under upper-greenschist to lower-amphibolite facies metamorphic conditions during hydrothermal alteration of the Mesoarchean oceanic crust. Early seafloor hydrothermal alteration was followed by amphibolite facies regional metamorphism, characterized by diopside-garnet-hornblende-plagioclase-quartz-vesuvianite calc-silicate metasomatic assemblage. Given the preservation of primary ocelli-matrix texture in the outer pillow cores, they are considered as the least altered sections of the pillow basalts. On chondrite- and primitive mantle-normalized diagrams, these pillow cores have the following trace element characteristics: (1) moderately enriched LREE patterns; (2) near-flat HREE patterns; and (3) variably negative Nb, Ti, and Zr anomalies. Ultramafic flows, with picritic geochemical characteristics, and gabbros also have similar trace element characteristics, consistent with a subduction zone geochemical signature. Given the observations that epidosites and subduction zone geochemical signatures have been identified in several Phanerozoic ophiolites, the Ivisaartoq belt is interpreted as the crustal section of a dismembered Mesoarchean supra-subduction zone ophiolite.