Abstract

Fragmental rocks of the V10 units of the Vipond Formation of the Tisdale assemblage previously have been identified as pillow basalts, but many samples are shown to be intermediate-to-felsic in character, likely tholeiitic dacite in composition. Specifically, the V10b unit is mapped as a pillow-lobe dacite. Aside from being more geochemically evolved in terms of their “immobile” trace elements, these rocks differ from typical pillow basalts in that they have more abundant primary breccia and hyaloclastite. The pillow lobes are contorted, having been folded in a plastic state and are zoned, typically having a spherulite-rich core. Moreover, the flows are aphyric, interpreted to mean that they were erupted in a superheated state. This along with their pillow-lobe nature demonstrates that they were erupted as relatively low-viscosity melts for such silicic compositions. Interaction with water quenched the outer pillow lobe and contributed to the formation of the abundant breccia. The fact that the melt was crystal and microlite free inhibited crystal growth, such that the bulk of the lobes were quenched to crystal-free glass. Nucleation occurred only in the cores, where cooling rates were lower in comparison to the medial and exterior areas of the pillow lobes, although in the cores crystal growth rates were high so that abundant spherulite formation took place. The flows are exposed over a strike length of more than 10 km and are interpreted to be continuous and to have resulted from fissure eruptions. The resulting porous, permeable, high Fe/Mg, high-surface area glassy rocks may have been ideal for channeling and reacting with gold-bearing hydrothermal solutions. Descriptions of most other Archean subaqueous felsic lobes document that the lavas were phyric and that spherulites formed near the external lobe margin. The distinctly different zoning of the V10b pillow lobes suggests that they constitute a new Archean facies formed from superheated melts.

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