Finding the Water for Partial Melting of the Lower Crust

Jim McLelland and his colleagues have documented the 1160-1150 Ma intrusion of the Adirondack-Frontenac region by anorthositic, mangeritic, charnockitic and gabbroic (AMCG) intrusions, coeval with the intrusion of four anorthosite massifs, and have suggested that ponding of mantle partial melts at the base of the crust generated partial crustal melts (McLelland et al., 1984, 1990, 1991, 2002). These partial melts ultimately became the AMCG suite and the anorthosite bodies. Production of large volumes of crustal melts probably requires fluid fluxing of the lower continental crust in addition to heating from an enormous supply of underplated basaltic magma. Exposures of the lower crust in the Athabasca mylonite zone of northern Saskatchewan, a setting similar to that of the Adirondacks, show that the lower crust is heterogeneous, with compositions ranging from gabbro to granite. Dehydration melting of hornblende-rich lower crust produced tonalitic melt in the Athabasca mylonite zone lower crust. Water needed to produce hornblende was probably subduction zone water, which may have been present in the lower Adirondack crust as well. In addition to dehydration melting, in the Athabasca mylonite zone, granitic melt was derived from partial melting of the 2.6 Ga Fehr granodiorite. We suggest that trace concentrations of structural water (to ~100 ppm) in feldspar and quartz, and fluid inclusions in the same minerals (to ~1000 ppm) in the Fehr granodiorite contributed significant water to flux partial melting of the source. Auband et al. (2004) showed that nominally anhydrous mantle minerals with 200 ppm water could lower the dry mantle solidus by approximately 50oC. Similar concentrations of water in quartz and feldspar in the lower crustal source rocks of intrusive suites in the Adirondack setting may have assisted in massive crustal partial melting.