科研之友
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摘要
The intra-caldera volcaniclastic deposits of the Miocene Tejeda caldera on Gran Canaria host an similar to 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 delta(18)O values ranging from 0 center dot 1 to 10 center dot 0 parts per thousand (n = 22), and whole-rock delta D values between -62 and -149 parts per thousand (n = 28). Three altered syenite samples have whole-rock delta(18)O values of 2 center dot 5, 1 center dot 5, and 0 center dot 9 parts per thousand, and corresponding delta D values of -91, -99, and -121 parts per thousand. The H(2)O concentrations of the altered cone sheets range from 0 center dot 4 to 0 center dot 8 wt % (n = 28), and the altered syenites have H(2)O concentrations of 0 center dot 5, 0 center dot 5, and 0 center dot 6 wt %, respectively. The majority of altered samples are depleted in (18)O relative to the typical delta(18)O range for unaltered trachytes and syenites (delta(18)O = 6-8 parts per thousand), indicative of interaction with local meteoric water (delta(18)O c. -8 parts per thousand) at temperatures epsilon 150 degrees C. Only one cone sheet sample appears petrographically unaltered and has a typical 'igneous' isotopic composition (delta(18)O = 7 center dot 1 parts per thousand, delta D = -48 parts per thousand) and a relatively high H(2)O concentration (2 center dot 2 wt %). A weak correlation (r = 0 center dot 55) between delta D and H(2)O is observed in the cone sheets, reflecting the combined effects of magmatic H(2)O exsolution, and subsequent deuteric and hydrothermal alteration. No systematic variation in delta(18)O or delta D was detected across the cone sheet swarm, most likely reflecting overprinting of isotopic compositions during successive intrusive events. However, the highest delta(18)O values (8 center dot 2-10 center dot 0 parts per thousand) 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 degrees C) by late magmatic fluids expelled from a solidifying crystal mush; (2) hydrothermal alteration (epsilon 150-300 degrees 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 degrees C) meteoric waters.
- 出版日期2010-10
