contact metamorphism

Contact metamorphism typically manifests as heat-induced metamorphic aureoles in the country rock surrounding intrusive igneous rocks (plutons).

The size of the metamorphic aureole depends on the temperature differential between the intrusive magma and the country rock, and on the size of the intrusion. Small intrusions, such as dykes typically have small aureoles with minimal metamorphism, whereas large ultramafic intrusions often display thick aureoles with well-developed contact metamorphism. Contact metamorphic rocks (hornfels) are often fine-grained and show little or no evidence of strong deformation.

The metamorphic grade of an aureole is that of the peak metamorphic mineral which has formed within the aureole – andalusite hornfels, sillimanite hornfels, or pyroxene hornfels. These grades relate to the metamorphic temperatures of pelitic or alumonisilicate rocks and the minerals that they form.

Magmatic fluids derived from the intrusive rock can impact metamorphic reactions, and extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks (metasomatism). Intrusions rich in carbonates produce skarns, fluorine-rich magmatic solutions often form greisens within granites. Metasomatic altered aureoles may include metallic ore minerals and are of economic interest.

hornfels

Hornfels are hard, mostly fine-grained metamorphic rocks that results from contact metamorphism.

Banding from the country rock can be retained, but because original bedding planes have typically been lost from the country rock upon baking by intruding magma, hornfels tend to separate into cubical fragments rather than into thin plates. Hornfels display characteristic mosaic-like interlocking of minerals, sometimes with enclosed particles of the other minerals [3]. The minerals rarely show crystalline form and are typically of nearly equal dimensions. The interlocking, equidimensional texture has been called pfiaster or pavement structure [photomicrographs 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]


Hornfels types and mineral assemblages depend upon the country rock that was baked during contact metamorphism:
● biotite hornfelses (the commonest) derive from slates, shales and clays
● calcite-silicate-hornfelses from impure limestone (purer limestones recrystallize as marbles)
● hornfelses with eldspar with hornblende (generally of brown color) and pale pyroxene arise from diabases, basalts, and andesites

Whole specimens range are dense, dull (not shiny), may be spotted, and vary in color : biotite hornfelses from slates, shales and clays are dark-brown to black; lime hornfelses are often white, yellow, pale-green, or brown; andalusite may be pink and pleochroic.

Some hornfels do retain layering [1]
hand specimens : andalusite hornfels : graywacke : hornfels : hornfels (Løkken Verk) : hornfelse :
by location : St Sjöfallet : Triangle Lake : Svenådalen :
rock outcrops : Tonschiefer-Hornfels : New Zealand : Japan :
photomicrographs : andalusite hornfels : biotite hornfels : biotite hornfels 2 : cordierite hornfels : garnet hornfels : pyroxene hornfels : sillimanite hornfels : spotted hornfels