The metabasites and garnet amphibolites of Glencolumbkille, Co. Donegal and the early mafic intrusions into the Dalradian rocks of Donegal, Connemara and Scotland

Abstract

The petrochemistry of the major elements (33 new rock analyses) of the Dalradian Glencolumbkille metadolerite sills, now amphibolite, shows them to have been quartz tholeiites like those of most of Donegal. Their compositions overlap with those of Connemara (Co. Galway) and Knapdale (Scotland), except the latter two also extend into the olivine tholeiite field. The syn-D2 (~470Ma) sheared schistose margins of the Glencolumbkille bodies are garnet amphibolites, unlike the generally garnet-free centres, and have suffered fluid-derived metasomatism and garnet growth. Garnets are postulated to have been nucleated by the activation energy from the shearing, but the main garnet growth and metamorphism was under static post-D2 pre-D3 conditions in certain favourable rock compositions. Garnet growth in the Glencolumbkille and Connemara amphibolites was generally restricted to rocks with low Mg/Fe, low Fe2O3/FeO and high MnO compositions, whether original or metasomatic. The long-standing puzzle as to why amphibolites that were originally dolerite sills in the Dalradian succession are clustered at certain horizons (at Glencolumbkille in and near the 654–635Ma Marinoan glaciation Portaskaig Tillite Formation) is examined. The clustering is suggested to be partly explained by recent structural research showing that the intrusion of widespread lateral sheets of basaltic magma is restricted to closely interbedded competent beds with thin incompetent pelites. The former act as lids, while the latter allow lateral expansion of the original sills. The whole magmatic suite studied intruded sediments deposited before 600Ma. It is thought to be part of the major ~600Ma magmatism and crustal extension that affected the Scottish Highlands and eastern North America as the supercontinent Rodinia split apart and the Iapetus Ocean opened. Of the metabasites, only the Knapdale ones have been U-Pb dated, yielding a magmatic 600Ma age and thus definitely shown to be pre-D1.