O and H isotopic evidence for a mantle source of water in appinite magma: An example from the late Neoproterozoic Greendale Complex, Nova Scotia

Abstract

Appinite suite rocks occur as small plutonic bodies, ranging from ultramafic to felsic in composition, that are characterized by abundant idiomorphic amphibole suggesting they are the products of water-rich mafic magmas. Appinites are also understood to record tectono-magmatic processes during the waning stages of subduction in convergent to collisional tectonic settings. Measuring D/H and 18O/16O ratios of hornblende in appinitic rocks offers opportunities to constrain the sources of water (mantle, crustal, sea water and or meteoric) in the magma during its crystallization, and to shed light on the role of water as arc magmatism shuts down. The ca. 607 Ma Greendale Complex in the Avalon terrane of Nova Scotia, Canada, is characterized by spectacular exposures of appinite suite rocks ranging from ultramafic to felsic in composition. Two populations of amphiboles are identified: one is characterized by δ18O values between 4.7 and 6.8‰ and anomalously low δD values (ca. < −90‰). These isotopic signatures are interpreted to represent incorporation of mantle-derived fluids into the crystal structure, possibly associated with mixing between a region of mantle upwelling and a subducting slab. This mixing process allows for hydration of the magma during its ascent into the overlying lithospheric mantle wedge. Some amphiboles retain mantle-like δ18O and δD values, but others may have partially re-equilibrated during low T assimilation of supracrustal units (e.g. organic-rich sediments) and or fluids derived from such bodies. A second generation of hornblende yields δ18O varying from 0.9 to 4.6‰ and δD from ca. −106 to −64‰, which supports equilibration with fluids from crustal sources. Assimilation of volatiles sourced from previously hydrothermally altered intruded sheets and or country rock is interpreted to produce these second-generation isotopic signatures. Regional syntheses indicate that arc magmatism in the Avalon terrane ended with the generation of a transform system, implying the potential generation of a slab window behind that system. In that context, the Appinites in the Avalon terrane have previously been interpreted to have been emplaced after the cessation of subduction in a slab window or slab failure setting behind the transform system, in which asthenospheric upwelling generated juvenile magmas and/or facilitated melting of the overlying continental lithospheric mantle. Our δD and δ18O data support this model and together with available Sm-Nd data, suggest asthenospheric upwelling triggered melting of the continental lithospheric mantle that was previously hydrated by subduction and this mantle-derived water contributed to the origin of appinites.