Low-temperature hydrothermal alteration of intra-caldera tuffs, Miocene Tejeda caldera, Gran Canaria, Canary Islands

Abstract

ARTICLE I NFO The Miocene Tejeda caldera on Gran Canaria erupted ~20 rhyolite-trachyte ignimbrites (Mogan Group 14- 13.3 Ma), followed by ~20 phonolitic lava flows and ignimbrites (Fataga Group 13-8.5 Ma). Upper-Mogan tuffs have been severelyaltered immediately within the caldera margin, whereas extra-caldera Mogan ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid-rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar,plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid-rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The δ 18 O values of the altered intra-caldera tuffs are significantly higher than in unaltered extra- caldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extra- caldera ignimbrites have δD values between −110 ‰ and −173‰, which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The δD values of the altered intra-caldera tuffs range from −52‰ to −131‰, with ambient meteoric water at the alteration site estimated at ca. −15‰. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock δD values of around −45‰, given that ΔDclay-water is ca. −30‰ at 100 °C, and decreases in