Low-temperature Hydrothermal Alteration Of Intra-caldera Tuffs, Miocene Tejeda Caldera, Gran Canada, Canary Islands

摘要

The Miocene Tejeda caldera on Gran Canada erupted ～20 rhyolite-trachyte ignimbrites (Mogan Croup 14-13.3 Ma), followed by -20 phonolitic lava flows and ignimbrites (Fataga Group 13-8.5 Ma). Upper-Mogan tuffs have been severely altered 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, plagiodase, 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 SD 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 6D values of around -45‰, given that △A_(clay-water) is ca. -30‰ at 100℃, and decreases in magnitude at higher temperatures. All altered tuff samples have δD values that are substantially lower than -45‰, indicating interaction with a meteoric water source with a δD value more negative than -15‰, which may have been produced in low-temperature steam fumaroles. Supported by numerical modeling, our Gran Canaria data reflect the near-surface, epithermal part of a larger, fault-controlled hydrothermal system associated with the emplacement of the high-level Fataga magma chamber system. In this near-surface environment, fluid temperatures probably did not exceed 200-250℃.