Silicic frothy xenoliths (xeno-pumice) in recent volcanics from Gran Canaria, Canary Islands

Abstract

The Quaternary small-volume alkaline magmatic episode on Gran Canaria erupted dominantly basanite and nephelinite lavas and scoria deposits that contain a range of mantle and crustal xenoliths. These xenoliths comprise peridotite nodules, partially melted plutonic and volcanic rock fragments, and a group of light colored, felsic, and commonly frothy quartz-bearing rock fragments (xeno-pumice) that show evidence for intense interaction with their host magmas. Here we study a selection of these felsic and, in part, glassy and vesicular xenoliths from North and North-East Gran Canaria, with the aim to unravel their ultimate origin and learn more about magma storage and ascent within and below the island. Inspection of textures, mineral assemblages and glass compositions reveal one group of felsic xenoliths with fresh to partly altered igneous phenocryst assemblages and relict magmatic textures in addition to δ18O values of 3.6 to 6.6‰. This group is interpreted to be of igneous origin. A second group of frothy felsic xenoliths displays mineralogy and textural characteristics more similar to sedimentary rocks with frequent occurrence of quartz, a mineral usually not present as phenocrysts in magmatic rocks from the Canary Islands. This second group displays relatively high δ18O values (8.1 to 16.8‰), more typical for sedimentary lithologies, and is thus interpreted to represent material derived from the extensive pre-island sedimentary part of the ocean crust. The investigated xenoliths from North Gran Canaria thus provide a snapshot of pre-island sedimentary geology as well as the island's “magmatic” interior. These new data help constrain the available subsurface compositional variations within and below the Canary Islands and will hence be useful in interpreting magma evolution trends and magma storage levels.