Mesoarchaean aluminous rocks at Storø, southern West Greenland: New age data and evidence of premetamorphic seafloor weathering of basalts

Abstract

Metamorphosed Meso- to Neoarchaean supracrustal rocks in the central part of the island of Storø (Nuuk region, southern West Greenland), show field- and geochemical evidence of premetamorphic chemical alteration. This alteration changed basaltic precursors into aluminous lithologies, and following amphibolite grade metamorphism and penetrative ductile deformation, these garnet–biotite schists now resemble adjacent metapelitic schists of sedimentary origin. Mass balance calculations (isocon method), suggests that most major elements (Si, Fe, Mg, Ca, Na and P) were leached during alteration. The calculated overall net mass changes are between −18% and −45%, consistent with breakdown of olivine, pyroxenes, plagioclase and apatite in the basaltic precursor rocks. Major and trace elements such as, K, Cs, Rb, Ba, Pb, Zn, La and Ce were added during this alteration process, whereas high field strength elements (Ti, Al, Zr, Hf and Nb) remained essentially immobile and were thus residually enriched. Interestingly, Th which is generally assumed to be immobile in fluids, was also added during this process. These chemical changes reflect interaction between a basaltic protolith and hydrous fluids that established a new equilibrium and thus a different mineral assemblage. It is proposed that the premetamorphic alteration at Storø was due to low-temperature interaction between seawater and oceanic crust, and thus essentially represents in situ submarine seafloor weathering. This interpretation is consistent with the mass balances reported from well-documented examples in younger settings.New U–Pb zircon geochronology from the arc-related mafic sequences at Storø shows that they comprise at least two distinct age groups: an older anorthosite complex dated at 3051.3±2.6Ma and a younger supracrustal sequence with age brackets between 2840 and 2710Ma. The allochthonous nature of these two mafic igneous to sedimentary stacks is consistent with accretionary processes in island arc complexes and a compressional Archaean tectonic setting.