Silurian mafic magmatism related to post-collisional extension, Appalachian orogen, western Newfoundland

Gabbroic intrusions provide direct evidence of magmatic processes operating within the lithosphere and at the lithosphere–asthenosphere boundary, offering insights into magmatic differentiation, crustal growth, mantle–crust interaction, and tectonic evolution. The Taylor Brook Gabbro Suite (TBGS) is the northwesternmost expression of middle Silurian magmatism in the northern Appalachians. The petrogenesis and geochemical characteristics of the TBGS and crosscutting silicic magmatism are explored through U–Pb geochronology, mineral chemistry, lithogeochemistry, and Sr–Nd isotopic analyses, illustrating a complex history of mantle-derived mafic magmatism. The U–Pb zircon sensitive high-resolution ion microprobe ages indicate that, although the intrusive history is complex, the TBGS represents a magmatic event at 431.0 ± 2.7/4.3 Ma that was subsequently intruded by silicic magmatism at 419.7 ± 3.0/4.5 Ma. The mineral chemistry and lithogeochemistry of the TBGS suggest fractional crystallization of a magmatic system as the primary differentiation mechanism. The TBGS is tholeiitic and transitional to calc-alkaline and consists of non-arc-like, continental magmas derived from an enriched mid-ocean ridge basalt-like source. The silicic samples are granitic, magnesian, and alkali calcic in composition. The isotopic signatures of the TBGS are mostly juvenile with εNd (430 Ma) ranging from −1.1 to +6.4 and 87 Sr/ 86 Sr (i) values ranging from 0.703373 to 0.708250, with one sample having an εNd (430 Ma) of −8.6, indicating a minor role of crustal contamination by Mesoproterozoic to Neoproterozoic basement rocks. Magma ascent may have utilized inherited deep crustal structures, such as the precursor to the Doucers Valley Fault/Long Range-Cabot Fault systems. Lithospheric extension likely caused by slab rollback created a window that allowed for asthenospheric upwelling, inducing partial melting and generation of the TBGS magmas.