The Horse Cove Complex (HCC) is a recently recognized late Ediacaran swarm of mafic-to-felsic dykes that occurs in the Newfoundland Avalon Zone. It is focused along the extrapolated trace of the Topsail Fault along the east coast of Conception Bay and is hosted by mafic submarine volcanic rocks, diorite, and, locally, granodiorite. The HCC has been divided into rock units that represent dykes of mafic-to-felsic composition, hosted by feldspar porphyry, gabbro, and diorite. The age of magmatism in the complex has been bracketed by U–Pb zircon ages, determined by chemical abrasion – thermal ionization mass spectrometry (CA–TIMS), of feldspar porphyry (581 ± 2.0 Ma) and an andesitic dyke (578 ± 2.3 Ma). Magmatism occurred over a period of 6.5 Ma (or less) and overlaps in age with felsic and mafic volcanic centres and related intrusions on the northeastern Avalon Peninsula. Feldspar porphyry and rhyolitic dykes show arc-related chemistry and may be co-magmatic. The mafic-to-intermediate rocks in the HCC are calc-alkaline to tholeiitic and exhibit a range of compositions, from those that have the characteristics of enriched mid-ocean ridge basalt (E-MORB-like) to those that show subduction-related light rare-earth element (LREE) enrichment and negative Nb anomalies. This range of compositions has εNd values (at 580 Ma) of +6.4 to +4.1. The HCC likely formed in a back-arc basin environment, in which rocks with LREE-enriched mantle sources and subduction-contaminated sources were emplaced side by side and closely in time. The complex may represent the last phase of subduction-related magmatism in the eastern Avalon Zone in Newfoundland prior to deep marine, deltaic, and alluvial fan sedimentation during the late Neoproterozoic.