Silicic Segregations of the Ferrar Dolerite Sills, Antarctica

The upper parts of the 300 m thick Basement Sill, one of the Ferrar Dolerite Sills in Antarctica, contain sharply defined, coarse-grained, leucocratic segregations that are temporally, spatially and chemically related to the host dolerite. The morphologies of the segregations vary gradationally with distance from the upper contact of the Basement Sill from thin (1 cm) streaks, pods and stringers close to the upper contact, (0·03–0·2 m) pipes and veins about 15 m below the contact, and (0·5–2 m) sub-horizontal, anastomosing lenses about 30 m beneath the contact. Their compositions range from diorite to granodiorite; they are enriched in P2O5, TiO2, FeOT, and to a lesser extent in alkalis compared with the host dolerite, and they display linear trends in Harker variation diagrams that are compatible with an origin as fractionated liquids formed after simultaneous crystallization of pyroxene and plagioclase from the host sill magma. The segregations show systematic variations in mineralogy and texture with composition, with the lowest SiO2 segregations having a similar mineralogy to the host and ophitic to sub-ophitic textures; the highest SiO2 segregations are granodioritic and exhibit a granophyric texture. In segregations with highly siliceous compositions, plagioclase crystals have resorbed edges and pyroxene forms blebby intergrowths with plagioclase and Fe–Ti oxides. Most segregations are interpreted to have formed when gravitational instability caused the upper solidification front of the sill to tear, and mixtures of crystals and residual liquid entered the tears by porous flow from the surrounding crystal mush and by channelling from lower levels in the sill. Modeling using the thermodynamic program MELTS shows that the bulk compositions of the segregations are consistent with wet (0·5 wt % H2O) fractional crystallization of the host dolerite magma at 100 MPa, although the data are not well matched by a single parent magma composition. We present a physical model of segregation formation involving multiple episodes of tearing triggered by fresh influxes of magma into the sill.