lopolith

A lopolith is a large igneous intrusion that is concordant with the surrounding country rock. It is typically lenticular or saucer-shaped, with a relatively flat or gently curved top and bottom surfaces and outward-dipping margins. The body often spreads laterally between layers of sedimentary or volcanic rocks, yielding a disk-like or lens-shaped cross-section when viewed geologically.

Magma that forms a lopolith intrudes at shallow crustal levels, pools and spreads laterally, and cools to produce a layered cumulate sequence. Because the intrusion is largely flat-lying and laterally extensive, the contacts with host rocks are generally concordant rather than cross-cutting. The interior may show mineralogical zoning, with variations from the margins to the core.

Lopoliths are typically mafic to ultramafic in composition, with rocks such as basaltic to gabbroic compositions making up the bulk. They range from tens to hundreds of kilometers across and several kilometers thick, and they may preserve characteristic cumulate textures and internal layering. They can host economically important metal deposits, especially nickel, copper, and platinum-group elements, related to their cumulate mineralogy.

Geological significance: Lopoliths record shallow-level magmatic emplacement and prolonged crustal residence; their geometry helps geologists reconstruct crustal structure and magmatic evolution. They are distinguished from discordant intrusions by their concordant contacts and characteristic saucer-like shape.