ARRESTED CO-CONTAMINATION OF DOLERITE AND GRANITIC MUSH, FERRAR IGNEOUS COMPLEX, ANTARCTICA

During the breakup of Gondwana at ≈180 Ma, the Ferrar Igneous Complex, a ≈10⁴ km³ system of interconnected sills and flood basalts, was emplaced in what would become the McMurdo Dry Valleys of Antarctica. The Basement Sill, the lowermost and latest arrival, was injected at a paleodepth of ≈3 km from a deeper source in central Bull Pass, which may have also fed the entire complex. The sheer volume of magma passing through the Basement Sill conduit was enough to partially melt the host granite on a massive scale (≈150 m x 15 km) proximal to the filling site. The system was shallow enough, however, for cooling to arrest and preserve key features of melting and interaction, that might have been lost if the system had been deeper. In and around the granitic partial melt zone we observe two means by which granitic melt and dolerite have comingled: 1) Contact Digestion and 2) Reverse-Injection. Contact Digestion occurs when prolonged flow of magma remelts the original quenched contact, eventually eating into the granite itself. This manifests itself as a bulging of the Basement Sill into overlying granite, with coarse grained dolerite occupying the contact. Mingling occurrs on a limited scale, within a sub-meter thick boundary layer along the contact, reflecting that this is an inefficient process. Also observed is a large lens of felsic rock (≈80 m x 1.5 km) located fully within the Basement Sill. This lens originated as granitic melt, and was Reverse-Injected into molten dolerite. Rounded, clustered, quartz grains, diagnostic of the partially melted granite zone, are extant within portions of the felsic lens, but elsewhere, the bulk chemistry departs markedly from the original granitic composition, owing to intimate mingling and diffusional exchange with dolerite. The ability of the dolerite to incorporate the silicic 'xenomagma' is much greater in this environment, owing to the increased surface area and the already molten granite migrating into a much hotter part of the doleritic sill. Some large ore deposits, as in the Nokomis deposit in northern Minnesota, required sulfur from granitic country rock to facilitate ore mineralization. The combination of Contact Digestion and Re-Injection may be processes instrumental to incorporating ore-forming nutrients from host rock to mafic magma.