Geochemistry and geodynamic origin of the Mesoarchean Ujarassuit and Ivisaartoq greenstone belts, SW Greenland

The Ivisaartoq (ca. 3075 Ma) and Ujarassuit (ca. 3070 Ma) greenstone belts are the largest Mesoarchean supracrustal lithotectonic assemblages in the Nuuk region, SW Greenland. Both greenstone belts underwent polyphase deformation and amphibolite facies metamorphism, and were in due course variously dismembered. Pillow lavas, pillow breccia, magmatic layering, and relic sedimentary structures are well preserved in the Ivisaartoq belt. Volcanic rocks include basalts, and minor andesites and picrites. Boninite-like rocks occur in the Ujarassuit belt. Metasedimentary rocks in the Ujarassuit belt occur as thin layers (0.5–1.0 m) of biotite schists, and in the Ivisaartoq belt as ~ 500 m-thick biotite schists intercalated with minor quartzitic gneisses. There is no field evidence indicating that the Ivisaartoq and Ujarassuit supracrustal rocks were deposited on older continental basement, and their volcanic rocks do not exhibit any geochemical trends indicating contamination by Archean upper continental crust. Four groups of amphibolites (metavolcanic rocks) were recognized in the Ujarassuit greenstone belt on the basis of their REE and HFSE characteristics: (1) Group 1 is characterized by near-flat REE patterns (La/Smcn = 0.77–1.14; La/Ybcn = 0.84–1.24) and moderate negative Nb anomalies (Nb/Nb⁎ = 0.60–0.79); (2) Group 2 displays LREE-depleted patterns (La/Smcn = 0.53–1.02; La/Ybcn = 0.32–0.61) and pronounced negative Nb anomalies (Nb/Nb ⁎ = 0.32–0.67); (3) Group 3 consists of LREE depleted patterns (La/Smcn = 0.69–0.84; La/Ybcn = 0.55–0.91) and absence of significant Nb anomalies (Nb/Nb⁎ = 0.92–1.15); and (4) Group 4 has concave-upward REE patterns (La/Smcn = 1.64–2.42, Gd/Ybcn = 0.57–1.01) and large negative Nb anomalies (Nb/Nb ⁎ = 0.28–0.42). The depletion of LREE in Group 2 amphibolites appears to have resulted from mobility of these elements during metamorphism. Group 1, 3, and 4 amphibolites retain their near-primary geochemical signatures and their trace element patterns are comparable to those of Phanerozoic oceanic island arc tholeiites (IAT), normal-mid-ocean ridge basalts (N-MORB), and boninites, respectively. The trace element patterns of the least altered meta-ultramafic rocks (La/Ybcn = 2.48–1.35; Nb/Nb⁎ = 0.31–0.60) are comparable to those of modern subduction-related picrites. Amphibolites with an andesitic composition show large negative Nb anomalies (Nb/Nb⁎ = 0.21–0.55), enriched LREE patterns (La/Ybcn = 6.29–15.64), and fractionated HREE (Gd/Ybcn = 2.61–3.12) implying deep melting in equilibrium with residual garnet in the source. Biotite schists and quartzitic gneisses have low chemical indexes of alteration values (CIA = 46 to 62) and trace element characteristics indicating volcaniclastic sedimentary protoliths derived from poorly weathered felsic to mafic source rocks. Collectively, the geochemical features of metavolcanic and metasedimentary rocks suggest that the Ivisaartoq and Ujarassuit greenstone belts represent dismembered fragments of Mesoarchean supra-subduction zone oceanic crust formed in an arc-forearc-backarc tectonic setting.