THE BEN-NEVIS INTRUSIVE RING TUFF, SCOTLAND - REINTERPRETATION OF THEFLINTY CRUSH ROCK AS PART OF AN IGNIMBRITE CONDUIT IN THE ROOTS OF ANANCIENT CALDERA

Intermittently exposed around the margins of the Pen Nevis volcanic pi le is a thin (< 5m) rhyolite with a well developed magmatic flow folia tion that includes rock previously described as 'flinty crush rock'. P revious models for the development of this 'flinty crush rock' suggest ed that it developed as a result of intense mechanical shearing during subsidence of the volcanic pile into the underlying Inner Granite. Ne w field, petrographic and geochemical evidence presented here, indicat e that the 'flinty crush rock' is magmatic and part of a larger body o f flow-foliated rhyolite that represents the remains of an ignimbrite conduit formed during caldera collapse. Post-eruption closure of the c onduit imposed a well developed flow-foliation on the rhyolite destroy ing any primary pyroclastic textures. The rhyolite is compositionally identical to a small outcrop of amphibole-biotite Inner Granite, sugge sting an intimate and probably co-genetic relationship between flow-fo liated rhyolite and Inner Granite magmas. It is recommended that the t erm 'flinty crush rock' be abandoned and replaced by 'Ben Nevis Intrus ive Ring Tuff'. This unit affords a rare opportunity to examine a ring fracture conduit associated with a caldera-forming eruption.