Amphibolite is a medium-to-high grade metamorphic rock common in mountain belts and deep continental crust. It forms under intense heat and pressure, transforming its original minerals and structure. The texture of amphibolite—whether foliated or nonfoliated—is not a simple binary choice, as it depends heavily on the specific geological conditions during its formation.
Defining Foliated and Nonfoliated Textures
Metamorphic rocks are broadly categorized by their texture, which refers to the arrangement of their mineral grains. Foliation describes parallel layers or planes created by the realignment of mineral grains. This layered appearance results directly from differential stress, where pressure is applied more intensely from one direction than from others.
Platy or elongate minerals, like micas or hornblende, rotate and recrystallize perpendicular to the greatest applied pressure. This creates a visible alignment called schistosity or cleavage. Schist is a common example of a rock exhibiting this texture.
In contrast, nonfoliated rocks lack this parallel arrangement of mineral grains. This texture typically arises under conditions where pressure is uniform, known as confining pressure.
Nonfoliated rocks are also common when the rock is composed mainly of equidimensional minerals, such as quartz or calcite.
Minerals like quartz and calcite do not readily align because they are not platy or needle-like, preventing foliation even under differential stress. Quartzite, composed almost entirely of interlocking quartz grains, is a classic example of a nonfoliated metamorphic rock.
Mineral Composition and Formation of Amphibolite
Amphibolite is primarily defined by its mineral content, consisting predominantly of amphibole and plagioclase feldspar. The amphibole is usually hornblende, a dark, elongate mineral resulting from medium-to-high grade metamorphism. Plagioclase feldspar is the light-colored constituent, with little to no quartz typically present.
The parent rock, or protolith, of amphibolite is most often a mafic igneous rock like basalt or gabbro, which are rich in iron and magnesium. If the protolith is igneous, the rock is an ortho-amphibolite. Amphibolite can also form from the metamorphism of chemically appropriate sedimentary rocks, called a para-amphibolite.
Amphibolite forms under the specific conditions of the amphibolite facies of regional metamorphism. These conditions are characterized by moderate to high temperatures and high pressures. This environment is common in the deep crust during mountain-building events, where elevated temperature and pressure are sustained over large areas.
The formation process involves the recrystallization of original minerals into the stable assemblage of hornblende and plagioclase. The presence of hornblende, which naturally grows as an elongate, prism-shaped crystal, predisposes amphibolite to textural variability.
The Role of Stress in Amphibolite Classification
Amphibolite’s texture is resolved by considering the specific type and intensity of stress present during metamorphism. If the rock is subjected to strong differential stress, the elongate hornblende crystals rotate and grow into parallel planes. This alignment creates a clear foliation, often resulting in a schistose texture.
This foliated version is common in settings where tectonic plates are colliding, causing significant directional pressure and deformation across a wide region. The resulting rock has the characteristic layered look of a foliated metamorphic rock due to the aligned hornblende crystals.
Conversely, if the rock forms under high confining pressure with minimal differential stress, the hornblende crystals grow in random orientations. This lack of parallel alignment results in a nonfoliated texture, and the rock is then described as a massive amphibolite. Such massive textures can occur in areas where the metamorphic event was dominated by uniform pressure and high heat.
Amphibolite is fundamentally variable because the same mineral composition can exhibit two distinct textures. Therefore, amphibolite can be either foliated or nonfoliated, existing on a spectrum determined by the intensity of differential stress. Its potential for foliation is rooted in the elongate hornblende, but its realization depends entirely on directional pressure.