The emplacement history of a remarkable heterogeneous, chemically zoned, rheomorphic and locally lava-like ignimbrite: ‘TL’ on Gran Canaria

Ignimbrite ‘TL’ on Gran Canaria is a complex, compositionally zoned rheomorphic tuff, that locally exhibits features previously considered to be diagnostic of lavas. It is made up of two locally overlapping lobes of ignimbrite that were emplaced during a single eruptive episode. The eastern lobe is high-grade, with rheomorphic zones and localised patches that are lava-like. The western lobe is extremely high-grade, more extensively lava-like, and welded to its top surface. Both parts are zoned, with a basal comendite-rich zone grading up, through a mixed zone, into an upper trachyte-rich zone. Lithic contents, and the relative proportions of comendite and trachyte pyroclasts vary with height. Each comendite-rich zone is vitroclastic, whereas each trachyte-rich zone is partly lava-like with local gradations into vitroclastic ignimbrite. Mixed zones are intermediate in character, and locally show compositional banding. Gradational zoning in massive ignimbrite, best seen in lower strain zones, and welding fabrics that are pervasively lineated and oblique to bedding, suggest that deposition was sustained, agglutination was rapid, and rheomorphic deformation began during the sustained deposition. The viscosity and porosity of the agglutinate varied with height because successively deposited pyroclast populations varied in grainsize, composition and temperature. The hot agglutinate continued to compact and shear downslope after the density currents had dissipated, causing further rheomorphic folding, thrusting, attenuation and autobrecciation. The western lobe locally overlies the partly welded top of the eastern lobe, in part because it advanced rheomorphically across it for at least 300 m. Hot-state loading and auto-intrusion occurred due to unstable density layering in the chemically zoned agglutinate. Deformation behaviour changed during cooling and degassing, and because of heat transfer between juxtaposed agglutinates, and localised retention of dissolved volatiles where there was an overlying impermeable cap.