Fluid–Rock Interaction in the Miocene, Post-Caldera, Tejeda Intrusive Complex, Gran Canaria (Canary Islands): Insights from Mineralogy, and O- and H-Isotope Geochemistry

The intra-caldera volcaniclastic deposits of the Miocene Tejeda caldera on Gran Canaria host an ∼12 km diameter intrusive complex comprising a peralkaline, trachytic to phonolitic cone sheet swarm surrounding a central core of hypabyssal syenite stocks. Both intrusive rock types display textural and mineralogical features indicative of secondary fluid–rock interaction, including (1) deuteric mineral phases (e.g. aegirine, alkali-amphibole, analcime), (2) turbid alkali feldspars, and (3) hydrothermal mineral phases (phyllosilicates, Fe–Ti oxides, Mn-oxides, and quartz). Altered cone sheets have whole-rock δ18O values ranging from 0·1 to 10·0‰ (n = 22), and whole-rock δD values between −62 and −149‰ (n = 28). Three altered syenite samples have whole-rock δ18O values of 2·5, 1·5, and 0·9‰, and corresponding δD values of −91, −99, and −121‰. The H2O concentrations of the altered cone sheets range from 0·4 to 0·8 wt % (n = 28), and the altered syenites have H2O concentrations of 0·5, 0·5, and 0·6 wt %, respectively. The majority of altered samples are depleted in 18O relative to the typical δ18O range for unaltered trachytes and syenites (δ18O = 6–8‰), indicative of interaction with local meteoric water (δ18O c. −8‰) at temperatures ≥ 150°C. Only one cone sheet sample appears petrographically unaltered and has a typical ‘igneous’ isotopic composition (δ18O = 7·1‰, δD = –48‰) and a relatively high H2O concentration (2·2 wt %). A weak correlation (r = 0·55) between δD and H2O is observed in the cone sheets, reflecting the combined effects of magmatic H2O exsolution, and subsequent deuteric and hydrothermal alteration. No systematic variation in δ18O or δD was detected across the cone sheet swarm, most likely reflecting overprinting of isotopic compositions during successive intrusive events. However, the highest δ18O values (8·2–10·0‰) occur in clay-bearing cone sheets from the central part of the intrusive complex, suggesting enhanced infiltration of relatively cool meteoric water in this area. Overall, at least three phases of fluid–rock interaction can be distinguished: (1) deuteric alteration (c. 300–500°C) by late magmatic fluids expelled from a solidifying crystal mush; (2) hydrothermal alteration (≥150–300°C) by meteoric water during the final stages of crystallization and/or immediately following solidification of the intrusive complex; (3) retrograde alteration related to the influx of relatively cool (≤150°C) meteoric waters.