Question

For each of the following metamorphic rocks, indicate the likely parent rock and the grade and/or type of metamorphism: chlorite schist, slate, mica-garnet schist, amphibolite, marble.

Short answer

Chlorite Schist: Basalt or Shale (low-medium grade), Slate: Shale (low-grade), Mica-Garnet Schist: Shale (medium-high grade), Amphibolite: Basalt or Gabbro (medium-high grade), Marble: Limestone (medium-high grade).

Step-by-step solution

Identify parent rock of Chlorite Schist

Chlorite schist commonly originates from a parent rock called basalt or shale. The type of metamorphism involved is low to medium-grade. Chlorite is a mineral that forms under these conditions.

Identify parent rock of Slate

Slate is usually formed from shale. It undergoes low-grade metamorphism to become slate, characterized by fine grains and a foliated structure.

Identify parent rock of Mica-Garnet Schist

Mica-garnet schist typically forms from shale or mudstone. This process involves medium to high-grade metamorphism resulting in the presence of mica and garnet minerals.

Identify parent rock of Amphibolite

Amphibolite originates from basalt or gabbro and sometimes shale. It undergoes medium to high-grade metamorphism, identified by the presence of amphibole minerals.

Identify parent rock of Marble

Marble is derived from limestone or dolostone. The metamorphism is generally contact or regional type, and the grade is medium to high, resulting in crystalline structure.

Key concepts

Metamorphic Grades

Metamorphic grades refer to the intensity of heat and pressure that a rock experiences during metamorphism. These grades can be categorized into low, medium, and high grades.
Low-grade metamorphic rocks, like slate, have been subjected to relatively low temperatures and pressures. Medium-grade rocks, such as schist, have endured higher temperatures and pressures, while high-grade rocks, like gneiss, have experienced the most extreme conditions.
As the metamorphic grade increases, the minerals within the rock change into more stable forms, resulting in a transformation of the rock's texture and mineral make-up.

Types of Metamorphism

Metamorphism can occur in several different ways, primarily through contact and regional metamorphism.
Contact metamorphism happens when rocks are heated by nearby magma or lava, leading to thermal alteration. This type typically affects rocks on a smaller scale and generally results in the formation of non-foliated rocks like marble.
Regional metamorphism, on the other hand, occurs over large areas due to tectonic forces that cause widespread changes in pressure and temperature. This type results in the formation of foliated rocks like schist and gneiss.

Parent Rock Identification

Identifying the parent rock is crucial in understanding the metamorphic process. Each metamorphic rock originates from a specific parent rock, which has undergone transformation due to changes in temperature, pressure, or chemically active fluids.
For example, slate forms from shale under low-grade conditions, while marble originates from limestone or dolostone through contact or regional metamorphism. Similarly, schist can develop from shale or mudstone under medium to high-grade conditions.
Understanding the parent rock helps geologists determine the conditions that led to the formation of the metamorphic rock and provides insight into the geological history of the area.

Mineral Formation

Minerals within metamorphic rocks form as a result of various chemical processes under specific temperatures and pressures. Each mineral has a stability range which dictates the conditions under which it will form or transform.
In low-grade metamorphic rocks, minerals like chlorite typically form, while in medium-grade rocks, minerals such as mica and garnet appear. High-grade rocks often exhibit minerals like amphibole or kyanite.
These minerals not only give clues about the metamorphic conditions but also play a significant role in determining the properties and appearance of the resulting rock.