LAUSR

Layered intrusions are important for a number of reasons. Primarily, they present a record of how mafic magmas crystallize and, as a con­ sequence, change their compositions by the process of magmatic dif­ ferentiation. However, it is increasingly recognized that deciphering the record preserved by layered intrusions is not as easy as once thought. The classic models envisioned that magma differentiation was the result of crystals settling out of the magma. Alternatively, it has been sug­ gested that crystals precipitate ‘in situ’ on the floor and the walls of the magma chamber and that it is the evolved liquid that moves away from the crystals. Yet others have suggested that it is descending plumes of crystal­rich magma that accumulate at the floor of the magma chamber and then separate solid from liquid by compaction of the crystal pile. Layered intrusions also host important ore reserves of Ni, Cu, Cr, Ti, V, and unrivalled platinum­group element deposits; however, the mecha­ nisms by which these elements have been concentrated are still hotly debated. Finally, the larger layered intrusions with sill­like geometries (e.g. the Bushveld Complex of South Africa) that intruded into previ­ ously unheated sediments can have basal metamorphic aureoles several kilometers thick. The geometry of dehydrating country rock overlain by hot ultramafic rock is similar to that occurring in subduction zones, and, hence, these large ultramafic/mafic sill systems are potentially excellent analogs for understanding fluid migration from descending slabs into the overlying mantle wedge. If correct, layered intrusions will allow us to better understand how fluids, which come off a dehy­ drating ocean crust slab, can maintain their isotopic signatures and induce melting by lowering the melting temperatures as they move into the mantle.