A mantle source for water in Appinite complexes: implications for genesis of granitoid batholiths and crustal growth

Appinite plutonic rocks range from ultramafic to felsic in composition, are characterized by idiomorphic hornblende as the dominant mafic mineral in all lithologies, and by spectacularly diverse textures, including planar and linear magmatic fabrics, multiple comb layers, mafic pegmatites and widespread evidence of mingling between mafic and felsic compositions. These features suggest that they are anomalously water-rich mafic magmas.  

The ca. 607 Ma Greendale Complex in the Antigonish Highlands of Nova Scotia is typical of appinite complexes which commonly occur as small (~2 km diameter) plutons adjacent to major deep crustal faults along the periphery of voluminous granitoid plutons emplaced in the waning stages of regional arc activity. Isotopic data from hornblendes in the Greendale Complex yield δD values ranging from -61 to -72 and δO¹⁸ from 3.7 to 7.0, indicating the water in the appinite magma has a strong mantle component. These data suggest the appinites may represent aliquots of hydrous basaltic magma derived from mafic underplates originally emplaced along the base of the crust during protracted subduction. Transfer of heat and fluids to the base of the crust triggered generation of coeval (615-604 Ma) granitoid magmas by partial melting in the overlying MASH zone. The granitoid magmas were emplaced in the shallow crust when transient stresses activated favourably-oriented structures which became conduits for magma transport. The ascent of late mafic magmas within the Antigonish Highlands was impeded by the rheological barriers created by the structurally overlying granitoid magma bodies. Magmas that form the Greendale Complex evaded those rheological barriers because they preferentially exploited the deep crustal Hollow Fault that bounded the plutonic system.

Collectively, these mineralogical, textural and geochemical features suggest a complex magmatic history involving repeated water saturation episodes within the plumbing system as mafic, mantle-derived magmas ascended and differentiated at mid-to-upper crustal levels (ca. 3-5 kbar). More generally, the most mafic components of appinite complexes may provide a window into the composition of the mafic underplate and insights into processes that generate granitoid batholiths and crustal growth in arc systems.