ALTERATION PROCESSES OF THE ROQUE-NUBLO IGNIMBRITES (GRAN-CANARIA, CANARY-ISLANDS)

The non-welded, lithic-rich ignimbrites of Roque Nublo are the most characteristic deposits of the second magmatic cycle (Pliocene in age) on Gran Canaria. They are very heterogeneous, with 35-55% by volume of lithic clasts, 15-30% of moderately vesiculated pumice, 5-7% of crystals and 20-30% of ash matrix. The juvenile components (pumice fragments and ash matrix) are largely altered. The mineral assemblage that originated by the alteration of vitric components consists mainly of zeolites (chabazite, phillipsite and analcime) and subordinate smectities. The alteration does not show any lateral or vertical gradation in the deposits and does not change in intensity with respect to the distance from the emission center. These alteration processes affect only the Roque Nublo ignimbrites; their associated fall and surge deposits remain unaltered. A geo-autoclave mechanism is proposed to explain the alteration processes, which devitrified the juvenile components of the Roque Nublo ignimbrites and gave the rocks their characteristic strong induration. This implies that the eruptions which led to the formation of the Roque Nublo ignimbrites had an important phreatomagmatic component, and that the resulting pyroclastic flows were too dense and too poorly expanded to permit the water vapour to be separated from the vitric fragments during transport. The alteration took place in situ and immediately after the deposition of the deposits when they were still hot, by the reaction between the vitric components and the condensed vapour, and cannot be attributed to late diagenesis or weathering.