Barrovian

Definition: The term "Barrovian" refers to a specific type of regional metamorphism characterized by a sequence of metamorphic facies associated with increasing temperature and pressure. This sequence, often observed in orogenic belts, provides a record of the metamorphic conditions experienced by rocks during mountain building.

Characteristics: Barrovian metamorphism is typically associated with the regional metamorphism of pelitic (clay-rich) sediments. The characteristic sequence of metamorphic facies, with increasing grade, is:

• Chlorite Zone: Lowest grade, characterized by the presence of chlorite.
• Biotite Zone: Biotite becomes stable.
• Garnet Zone: Garnet appears.
• Staurolite Zone: Staurolite appears, often with garnet.
• Kyanite Zone: Kyanite becomes stable.
• Sillimanite Zone: Highest grade, characterized by sillimanite.

The presence and sequence of these index minerals (chlorite, biotite, garnet, staurolite, kyanite, and sillimanite) allow geologists to map metamorphic zones and determine the relative metamorphic grade of rocks within a region. The specific minerals present are also dependent on the bulk composition of the rock.

Origin of the Term: The term "Barrovian" is derived from the work of George Barrow, a British geologist who studied the metamorphic rocks of the Scottish Highlands in the late 19th and early 20th centuries. He was the first to systematically map the metamorphic zones and to recognize the progressive nature of metamorphism in the region.

Significance: Barrovian metamorphism is a significant indicator of tectonic processes, particularly those associated with convergent plate boundaries and orogenesis. The study of Barrovian metamorphic terranes provides valuable insights into the thermal and pressure history of mountain belts and the processes that shape the Earth's crust. The Barrovian sequence provides a standard framework for understanding and comparing metamorphic terranes worldwide.