A mantle source for water in the Greendale (appinite) Complex: Implications for genesis of granitoid batholiths and crustal growth in magmatic arcs

Appinite complexes are small (∼1–5 km wide) hornblende-rich, predominantly mafic plutons that crystallized from anomalously water-rich mafic magma. The Neoproterozoic Greendale Complex, Nova Scotia, exemplifies these complexes, occurring as a small pluton, emplaced at ∼10–15 km depth, adjacent to a major fault along the periphery of granitoid plutons intruded in the waning stages of subduction. Stable isotopic data from hornblende in the complex yield −61 to −72 ‰ δD and + 3.7 to +7.0 ‰ δ¹⁸O values, indicating water in the appinite magma was dominantly mantle-derived. These data suggest appinites may represent aliquots of hydrous basaltic magma that segregated from a crystallizing mafic underplate before emplacement at higher crustal levels. Some amphiboles crystallized in situ; others crystallized at ∼12–20 km depth, cooled from ∼950-870 °C, and were entrained as the magma ascended.

Transfer of mantle-derived fluids and heat into the lower crust during mafic underplating triggered partial melting and generation of coeval granitoid magmas. These granitoids were emplaced at shallow levels (∼3–5 kbar) and formed the upper part of a plumbing system that created rheological barriers, impeding the ascent of coeval mafic magmas except along the deep crustal Hollow Fault, which bounded the plutonic system and along which the Greendale Complex was emplaced. More generally, these relationships imply feedback between active faulting and emplacement of overpressured hydrous mafic magmas, facilitating rapid ascent and high degrees of undercooling, and disequilibrium textures characteristic of water saturation. Appinite magmas may provide a direct window into the fluid conditions of the crystallizing mafic underplate and Deep Crustal Hot Zone. Therefore, appinite complexes provide deep insights into the processes that generate granitoid batholiths and crustal growth in arc systems.