Migmatite, Welcome Springs, Utah
Migmatites have undergone partial melting with one or more minerals in a liquid state (usually the lightly colored feldspar and quartz) and the rest (dark colored minerals) remaining in a solid to semi-solid state. These two components are mixed to form a migmatite. Here, both metamorphism and mixing are the result of subducting oceanic mud sediment from the passive margin of ancient North America into an oceanic trench, exposing the mud both to heat and pressure. This happened about 1.7 billion years ago.
This is a amphibolitic migmatite, about 100cm across
Migmatite, Welcome Springs, Utah
Migmatites have undergone partial melting with one or more minerals in a liquid state (usually the lightly colored feldspar and quartz) and the rest (dark colored minerals) remaining in a solid to semi-solid state. These two components are mixed to form a migmatite. Here, both metamorphism and mixing are the result of subducting oceanic mud sediment from the passive margin of ancient North America into an oceanic trench, exposing the mud both to heat and pressure. This happened about 1.7 billion years ago.
This is a amphibolitic migmatite, about 100cm across