Paleoarchean variole-bearing metabasalts from the East Pilbara Terrane formed by hydrous fluid phase exsolution and implications for Archean greenstone belt magmatic processes

Abstract

Archean greenstone belts contain low grade metamorphic volcanic sequences with localised examples of preserved original volcanic textures and relict magmatic minerals, which are invaluable for inferring volcanic processes and by extension magma genesis. A feature observed in many Archean greenstone belts but rare in modern volcanic rocks is variolitic texture with millimetre- to centimetre-sized leucocratic globular structures (varioles), in mafic lavas. Models proposed to explain this texture include rapid undercooling, magma mingling and liquid immiscibility. Here, we investigate samples of Paleoarchean variole-bearing pillow lavas from the ca. 3469 Ma Mount Ada Basalt of the Pilbara Supergroup, Western Australia. The samples contain relict magmatic clinopyroxene and spinel and preserve sharp contacts between varioles and host. No plagioclase is observed in the samples. The clinopyroxene mineral composition is indistinguishable between in the varioles and host. This is despite the variole material having lower MgO and Fe2O3(total) and higher SiO2 and Al2O3 concentrations. However, the variole and host materials have indistinguishable TiO2, Cr2O3, NiO2 and Sm-Nd isotope systematics. These observations are consistent with an immiscibility reaction from a parental melt to a hydrous fluid phase and conjugate mafic liquid, where minerals that formed before the reaction become incorporated into both endmembers where they continued to grow by diffusive exchange between the hydrous fluid and silicate melt. Two lines of evidence indicate that hydrous flux exsolution occurred due to H2O saturation in the precursor magmas, including: (1) the lack of plagioclase, for which crystallisation is supressed at high H2O content; and (2) Nb/Th < 7, which indicates a hydrated mantle source. The variole-bearing pillow lavas are part of the 10-Myr-duration Coongan Subgroup volcanic cycle and occur interspersed with tholeiite basalts and minor komatiites of the Mount Ada Basalt and mafic to felsic volcanic rocks of the Duffer Formation. These distinctly different volcanic rock types, which have εNdi −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites, indicate that there was variation in magma genesis, which included hydrous fluxed melting likely genetically related to delamination, during the 10 Myr formation of the Coongan Subgroup volcanic cycle, as part of the Paleoarchean East Pilbara Terrane formation.