Magmatic differentiation processes in saucer-shaped sills: Evidence from the Golden Valley Sill in the Karoo Basin, South Africa

Analysis of compositional variations along profiles from tholeiitic sills provides insights into syn- and post-emplacement magmatic differentiation processes. We present here 18 whole-rock compositional profiles sampled from a saucer-shaped sill emplaced in the Karoo Basin (South Africa), the Golden Valley Sill. We show that different compositional profile patterns previously described in basic-ultrabasic sills may be found in different parts of a single saucer-shaped sill. The detailed examination of the mineral grain assemblage and compositions suggests that processes taking place in hundred-meter-thick sills relate to early and late fractional crystallization. Our observations in the Golden Valley Sill suggest that a significant part of fractionation takes place at a late stage of cooling when a crystalline skeleton or mush zone is formed. We show that porous flow of interstitial melt driven by forces related to the particular geometry (saucer-shaped) of the sill may result in a post-emplacement compositional evolution. We propose that the process of post-emplacement melt flow regionally overprinted compositional patterns produced by earlier crystal segregation from the cooling magma at fluid-like stages during the emplacement.