MIOCENE RHYOLITIC WELDED TUFF INFILLING A FUNNEL-SHAPED ERUPTION CONDUIT SHIOTANI, SOUTHEAST OF MATSUE, SW JAPAN

At Shiotani, SW Japan, rhyolitic welded tuff forms a steep-sided funne l-shaped body, confined by Paleogene granitic rocks to an elliptical a rea 1-1.5 km across. The Shiotani welded tuff is pervasively welded an d foliated concordantly with the contact that dips inward at angles of 70-90 degrees. In contrast, nearby contemporary volcaniclastic deposi ts are non-welded and gently inclined. Near the contact with the grani te, the tuff is plastically deformed and shows lineations that plunge inward at angles of 40-65 degrees. Lithic and crystal clasts in the rh eomorphic outer part are rotated in a plane normal to the foliations a nd parallel to the lineations indicating downward flow of the welded t uff. The geometry and internal structures suggest that the Shiotani we lded tuff was emplaced and welded in a funnel-shaped eruption conduit. Upon collapse of a plinian or phreatoplinian eruption column, the maj ority of the conduit-filling pyroclasts probably fell back en masse in to the conduit. Heat and steam from underlying magma and diffusion of interstitial volatiles into the glass perhaps reduced the viscosity of juvenile pyroclasts and facilitated welding in the conduit, especiall y at deep levels. The hot welded pyroclasts then flowed down the condu it wall during welding compaction and retreat of the magma. These proc esses resulted in increased welding toward the contacts and welding fo liations concordant with the steep wall. Emplacement of nearby correla tive volcaniclastic mass-flow deposits in a shelf to upper bathyal env ironment suggests a possibility that, when active, the Shiotani condui t was under the sea. Welding compaction would occur even under the sea provided that the steam generated in the upper part of the conduit fi ll prevented water access.