The apperance of almond-shaped augen in metamorphic rocks indicate which type of metamorphism?

Question

The apperance of almond-shaped augen in metamorphic rocks indicate which type of metamorphism?

Answer 1

Regional metamorphism

Augen

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A Gneiss with large eye-shaped feldspars Augen are large, lenticular eye-shaped mineral grains or mineral aggregates visible in some foliated metamorphic rocks. In cross section they have the shape of an eye.

Feldspar, quartz, and garnet are common minerals which form augen.

Augen form in rocks which have undergone metamorphism and shearing. The core of the augen is a porphyroblast or porphyroclast of a hard, resilient mineral such as garnet. The augen grows by crystallisation of a mantle of new mineral around the porphyroblast. The mantle is formed contiguous with the foliation which is imparted upon the rock, and forms a blanket which tapers off from either side of the porphyroblast within the strain shadows.

During shearing, the poprhyroblast may rotate, to form a characteristic augen texture of asymmetric shearing. In this case, the position of the tails is unequal across the foliation, with some augen showing clear drag folding of the mantle into the strain shadow. This derives a form of shear direction information.

A metamorphic rock which is clotted with augen is often called an augen gneiss.

See also

Foliation

Gneiss

Rock microstructure

Schist

Shear (geology)

Gneiss

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Gneiss Gneiss (pronounced /ˈnaɪs/) is a common and widely distributed type of rock formed by high-grade REGIONAL METAMORPHIC processes from preexisting formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse foliated and largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. However, depending on their composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Gneissose is used to describe rocks with properties similar to gneiss.