Regional and Contact Metamorphism

Metamorphism is grouped by setting and cause, from broad regional metamorphism in orogenic belts to localized contact metamorphism around intrusions.

Definition

The classification of metamorphism by geological setting and dominant cause, contrasting widespread regional metamorphism driven by tectonic burial and heating with localized contact metamorphism driven by igneous intrusion.

Scope

This topic covers the principal types of metamorphism distinguished by their geological setting: regional (orogenic) metamorphism over large areas during mountain building, contact (thermal) metamorphism in aureoles around intrusions, burial metamorphism, dynamic metamorphism in fault and shear zones, and hydrothermal and seafloor metamorphism. It relates each type to its driving heat source and pressure regime.

Core questions

• What distinguishes regional from contact metamorphism in scale and cause?
• How does a contact aureole form around an intrusion?
• What roles do burial, deformation, and fluids play in other metamorphic types?
• How do pressure and temperature regimes differ among settings?

Key theories

Setting-based classification of metamorphism

Metamorphic types are defined by their setting and heat source: regional metamorphism accompanies orogeny over broad areas at elevated pressure, whereas contact metamorphism is driven by the heat of nearby intrusions at low pressure in narrow aureoles.

Contact aureole zoning

Around an intrusion, declining temperature outward from the contact produces concentric zones of progressively lower-grade hornfels and spotted rocks, recording a thermal gradient at near-constant low pressure.

Clinical relevance

Recognizing the type of metamorphism constrains the tectonic and thermal history of a region, helps locate intrusions and associated mineralization through their aureoles, and frames the interpretation of facies and pressure-temperature paths.

History

Goldschmidt's early study of the Oslo contact aureoles and Barrow's mapping of regional zones in Scotland defined the two end-member types; Miyashiro's synthesis of metamorphic belts later placed both within the framework of plate tectonics.

Key figures

• Akiho Miyashiro
• George Barrow
• Victor Goldschmidt

Seminal works

• winter2013
• bucher2011
• miyashiro1973

Frequently asked questions

What is a metamorphic aureole?

The zone of contact-metamorphosed rock surrounding an igneous intrusion, where heat from the magma has altered the country rock, typically producing hornfels.

Why is regional metamorphism associated with mountain belts?

Mountain building buries and heats large volumes of rock under directed pressure over long periods, the conditions that drive widespread regional metamorphism.