An Explosion-Breccia Complex at Back Settlement, Near Kentallen, Argyll

Abstract

Very coarse breccia, composed of quartzite, phyllite and limestone blocks with little interstitial material and no igneous matrix, occurs within the out-crop of the Appin Quartzite near Back Settlement on Ardsheal Peninsula. Some of the blocks are well rounded and many have rounded edges and corners. Several small masses of appinite, chilled to lamprophyre at the margins, intrude the breccia and in some places contain many quartzite blocks. Carbonation of the lamprophyre, and to some extent of the appinite, is a common feature. These phenomena are interpreted as the result of gas streaming ahead of a rising volatile-rich igneous magma, with the formation of an explosion-breccia approximately in situ followed by the intrusion of the magma which crystallised in a volatile-rich environment. Continued gas streaming altered the crystallised igneous rock, particularly by carbonation.