Metasomatic process in the formation of rodingite in Boso Peninsula, Chiba, Japan.

Abstract

Many masses and veins of rodingite derived from hornblende gabbro occur in serpentinized harzburgite of the Mineoka tectonic belt, Chiba. The original gabbro seems to have been composed mainly of plagioclase (Ane80Ab20) and actinolitic hornblende with a small amount of orthopyroxene (En75Fs23Wo2). Plagioclase changes progressively to thomsonite, prehnite and hydrogrossular, hornblende to cummingtonite and chlorite, and orthopyroxene to talc. Four metasomatic zones are recognized from gabbro to serpentinite as follows : 1) the least altered zone consisting of plagioclase, hornblende and cummingtonite ; 2) thomsonite zone of thomsonite, prehnite, hornblende, cummingtonite and chlorite ; 3) prehnite-hydrogrossular zone of prehnite, hydrogrossular, cummingtonite and chlorite ; and 4) hydrogrossular zone of hydrogrossular and chlorite. A small amount of orthopyroxene are distributed in all zones with talc. Plagioclase is altered together with hornblende, although the calcium contents of alteration products derived from both minerals show a negative correlation. The chemistry shows a characteristic variation in the altered zones as follows : 1) Al and O contents are nearly constant ; 2) Ca and H tend to increase toward the highly altered zone ; 3) Si, Fe and alkalis show the reverse trend ; 4) Mg abounds at both of fresh and highly altered zones. The water content of hydrogrossular increases with the metasomatism: i.e. The 2X value of Ca3Al2(SiO4)3-x(O4H4)x increases to 1.2-2.1 in the prehnite-hydrogrossular zone, and 1.9-2.4 in the hydrogrossular zone. The necessary amount of calcium for the formation of rodingite is supplied from both of hornblende in the orignal gabbro and surrounding peridotite. In rodingite occur many veinlets consisting of tobermorite, xonotlite, prehnite and hydrogrossular. In the least altered zone, the same metasomatic zonation of rodingite is found along these veinlets. The process of rodingite formation, therefore, is shown by a sequential reaction between Ca-rich solutions and hornblende gabbro.