| Abstract: | Dyke swarms are well-known geological features but they occur in different environments. Intruded into the Coldbrook Supergroup of principally acid volcanic rocks is a swarm of closely associated amphibolitized diabases. Helmsteadt (1967) recognized that these rocks were not lavas, but discrete intrusive bodies.Work, now in progress, shows that the intrusions are dykes in several stages of deformation, so that the earliest are strongly lineated and the latest have an almost perfectly preserved diabasic texture. The early amphibolites have strings of boudined minor granitic veins. Separating the dykes are frequent septae, more or less deformed, of granitic, granophyric, rhyolitic and pyroclastic rocks, all in the upper greenschist fades of metamorphism. The total width of the zone in which the dykes form over 40% of the rock is as much as 1.5 km. It is proposed that the zone represents an initial stage in the opening of the Iapetus ocean in the late Precambrian times. |
