Abstract
The mineralogical and chemical changes by alteration of the in-situ basalts from the Shimanto belt are studied using 16 weakly- to strongly-altered rocks from an outcrop of lava flows. The samples show variolitic to intersertal-intergranular texture, and are mainly composed of plagioclase, clinopyroxene, chlorite, sericite, hematite and titanite. No glass is preserved. Plagioclase is mostly albitized, as its anorthite component is less than 10%. Particularly in the samples cut by prehnite veins of high-temperature hydrothermal origin (ca. 250°C), plagioclase is severely albitized, clinopyroxene and sericite are much less, and the other secondary minerals are more abundant than in the remaining samples. These petrographical variations observed in a single flow suggest that the alteration occurred in at least two stages: an earlier one where sericite and the other secondary minerals appear at temperatures higher than 250°C, and a later one in which clinopyroxene and sericite disappear, accompanied by additional formation of the other secondary minerals, at ca. 250°C.
Remarkable compositional variations are observed in LILEs (K2O, Ba, Rb, and Sr), MnO, Ce, and Pb. Although MnO, Ce, and Pb show no consistent trends with modal proportions of any minerals, LILEs show positive correlations with the sericite abundance. In consideration of previous works on interactions between seawater and basaltic rocks (Thompson, 1991), that show LILEs to be lost from rocks at high temperature (>100°C), secondary enrichment of LILEs is unlikely to occur in such alteration stages. LILEs are preserved by sericite replacing feldspar and glass in the first stage of alteration, and are depleted by decomposition of sericite in the later stage. Original abundance of total alkalis (Na2O+K2O) is retained by formation of albite after the disappearance of sericite.
Except for a sample having the signature of intraplate type volcanics, the in-situ basalts originally have much higher abundance of total alkalis and LILE/HFSE ratios than N-MORB, and negative Nb anomaly on MORB-normalized spidergrams, which are characteristics of subduction-related volcanics.
