Porosity evolution and mineral paragenesis during low-grade metamorphism of basaltic lavas at Teigarhorn, eastern Iceland

Low-grade alteration of basaltic lavas at Teigarhorn, eastern Iceland, resu lted in three distinct stages of mineral paragenesis that correlate to even ts in the burial and intrusive history of Icelandic crust. Metasomatism and brittle deformation during the paragenetic stages dramatically affected th e paleohydrology of the lavas ana formed temporally distinct mineral assemb lages. The lavas initially contained up to 22 percent total porosity concen trated near the tops and bottoms of individual lava flows. Celadonite and s ilica (Stage I) precipitated along the walls of primary pores prior to deep burial of the lavas and occluded similar to 8 percent of the initial poros ity. During burial (Stage II), hydrolysis of olivine and basaltic glass led to the formation of mixed-layer chlorite/smectite clays in the matrix of t he lavas and as rims filling similar to 40 percent of the volume of primary pores. Chlorite contents in Stage II mixed-layer clay rims increased from similar to 20 to similar to 80 percent during the infilling of individual v esicles, reflecting increasing temperatures with time as the lavas were bur ied. The end of Stage II occurred after burial and is represented by fillin g of similar to 40 percent of total primary porosity by zeolites (scolecite or heulandite + stilbite + mordenite + epistilbite) and replacement of pla gioclase by zeolites and albite, The Stage II zeolite assemblages are indic ative of two regional metamorphic mineral zones in eastern Iceland, the mes olite + scolecite and heulandite + stilbite zones. The presence of the boun dary between the mesolite + scolecite + and heulandite + stilbite zones ind icates that the maximum temperature during burial metamorphism was 90 degre es +/- 10 degrees C, Localized areas of intense hydrothermal alteration ass ociated with intrusion of basaltic dikes (Stage III) overprint Stages I and II. Extensive fracturing and hydrothermal brecciation during Stage III add ed 3 to 11 percent total porosity in which mm- to cm-scale museum-grade cry stals of quartz, calcite (Iceland spar), stilbite, scolecite, heulandite, a nd laumontite precipitated. Estimates of the temperature during Stage III ( based on fluid inclusion homogenization temperatures in calcite, chlorite g eothermometry, and the zeolite assemblage) range from 120 degrees to 180 de grees C. Although the thermobarometric conditions during Stages II and III led to similar mineral assemblages, careful attention to textural and geolo gic relations permits seemingly complex, multi-stage parageneses in metabas alts to be interpreted in a petrotectonic context.