DEUTERIC ALTERATION OF IRAGAWA DOLERITE SHEET, ATUMI, YAMAGATA PREFECTURE

Abstract

Drusy cavities of the Iragawa high-alumina dolerite sill, northeastern Japan are filled with gyrolite, analcime, thomsonite, heulandite and natrolite crystallized from residual solution at the deuteric stage of magmatic differentiation. The dolerite had been suffered intense alteration from cavity wall toward the inner part of the body. The width of alteration zone is about 1 to 10cm. Five samples of the altered zone have been analyzed chemically with a fresh dolerite. The results are given in Table 2. It shows that the most altered part increases strongly in SiO₂, CaO and K₂O and decreases in Al₂O₃, total MgO and Na₂O.