The Mugford Group Volcanics of Labrador: Age, Geochemistry, and Tectonic Setting

Abstract

The Mugford Group is a sequence of volcanic and sedimentary rocks exposed within the Khaumayät (Kaumajet) Mountains of Labrador. Separated from an intensely deformed and deeply eroded Archean basement complex by an angular unconformity, these rocks are nearly everywhere flat-lying and only locally altered. The volcanic rocks within the Mugford Group are of three types: tholeiitic basalts, komatiitic basalts and greenstones. A phosphorus fractionation diagram indicates that the tholeiitic and komatiitic basalts may be differentiates of a common magma. The greenstones, however, have undergone a separate crystallization history, but plot within the field of tholeiitic basalts on a FMA diagram, suggesting they were originally tholeiites. K–Ar whole-rock ages show that the Mugford volcanics are at least 1490 m.y. old. Rb–Sr whole-rock isotopic analyses of the tholeiitic and komatiitic basalts and the greenstones define an isochron of 2369 ± 55 m.y. with an initial 87Sr/86Sr ratio of 0.7033 ± 0.0002. This age is interpreted as approximating the time of extrusion of the Mugford volcanics. The low initial 87Sr/86Sr ratio indicates that the magmas giving rise to these rocks were not appreciably contaminated with older crustal material.The Mugford volcanics are presently the oldest recognized continental flood basalts. Their extrusion apparently occurred contemporaneously with the intrusion of the Okhakh granite at Okhakh (Okak) Harbour, 25 km to the south. This suggests that while no regional metamorphism accompanied extrusion of these volcanics, some local igneous activity did occur at that time. The Mugford volcanics may represent the extrusive equivalents of numerous basic dikes that were intruded during the final stages of stabilization of the North Atlantic craton.