The genetic relationship between coeval Ediacaran mafic and granitoid plutons in the Antigonish Highlands, Nova Scotia

Abstract

Abstract Granitic plutons dominated by felsic-intermediate compositions are commonly spatially and temporally associated with mafic intrusions; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is controversial. To better understand this relationship, we present new lithogeochemical and isotopic data from coeval late Neoproterozoic plutonic rocks in the Antigonish Highlands of Nova Scotia where the regional context is well documented. The predominantly mafic Greendale Complex contains lamprophyre, appinites and leucocratic dykes. The appinites are remarkably variable in their textures, and consist of hornblende pegmatites, hornblende cumulates, porphyritic hornblende gabbro and coarse-grained, equigranular hornblende gabbro. Geochemical data show enrichment in large-ion lithophile elements and depletion in the high field strength elements suggesting an arc setting. ε Nd (607) values from the Greendale Complex range from +3.2 to +5.0 and are on average slightly more juvenile than coeval granitic rocks which have petrological characteristics typical of continental arc magmas. Hydrous mafic magmas were likely contaminated by subducted sediments and their ascent was facilitated by lithospheric-scale faults. Felsic magmas were derived by anatexis of heterogeneous Avalonian crust that oscillated between fluid-saturated to fluid-deficient (dehydration) melting, consistent with the evolution from arc to intra-arc rift environment.