What major constituents are lost during prograde regional metamorphism and what

Question

What major constituents are lost during prograde regional metamorphism and what important roles do they play?
Also, what causes rocks to become coarser grained during metamorphism, and are there any exceptions? What major constituents are lost during prograde regional metamorphism and what important roles do they play?
Also, what causes rocks to become coarser grained during metamorphism, and are there any exceptions?
Also, what causes rocks to become coarser grained during metamorphism, and are there any exceptions?

Explanation / Answer

Metamorphism is the change of minerals or geologic texture (distinct arrangement of minerals) in pre-existing rocks (protoliths), without the protolith melting into liquid magma (a solid-state change). The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering and/or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis (max. 200°C), and melting (~850°C).

Three types of metamorphism exist: contact, dynamic, and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.

Metamorphism is further divided into prograde and retrograde metamorphism. Prograde metamorphism involves the change of mineral assemblages (paragenesis) with increasing temperature and (usually) pressure conditions. These are solid state dehydration reactions, and involve the loss of volatiles such as water or carbon dioxide. Prograde metamorphism results in rock characteristic of the maximum pressure and temperature experienced. Metamorphic rocks usually do not undergo further change when they are brought back to the surface.

Retrograde metamorphism involves the reconstitution of a rock via revolatisation under decreasing temperatures (and usually pressures), allowing the mineral assemblages formed in prograde metamorphism to revert to those more stable at less extreme conditions. This is a relatively uncommon process, because volatiles must be present.

Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges, particularly those associated with convergent tectonic plates or the roots of previously eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism. These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects, characterized by paired metamorphic belts. The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane.

When rocks are subjected to elevated temperatures and pressures, for example due to deep burial in orogenic (mountain building) zones when two continents collide, they may become metamorphosed (metamorphism is from the Greek, to change in form). They slowly recrystallize while remaining in the solid state. This may takes thousands or millions of years. Metamorphism is essentially an isochemical process, i.e. the bulk chemical composition of a rock body is more or less unchanged from the protolith, or original rock. But the minerals may be largely recrystallized into a new mineral assemblage. In addition, new structural features are frequently imparted to the rocks, such as slaty cleavage or schistosity.Metamorphic rocks form as a result of significant changes in temperature, pressure, and chemical composition in regions of the Earth which we cannot directly observe. So, like igneous rocks, they provide an important 'window' to the study of internal Earth processes. Many mountain ranges (such as the Blue Ridge) are characterized by metamorphic rocks, so they also provide important information about how mountain systems form. In certain cases, metamorphism results in the formation of minerals which have important economic value.Hornfels are the exception.

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