Metamorphic Rocks

Goals for mastering metamorphic rocks Understand what it means to “metamorphose” a rock Explain the differences between various types of metamorphism processes – types of metamorphism Describe the various common metamorphic textures Associate the common textures with c0mmon metamorphic rocks Describe an index mineral – and how it relates to metamorphism Explain the association of metamorphic rocks with various types of plate tectonic settings

What is “metamorphism” ? meta (change) morphism (form) transformation of pre-existing rock from one to another through pressure and temperature relationships all changes take place through solid state transformation

takes place around 150oC (temperature) Types of Metamorphism takes place around 150oC (temperature) typically at depths of 5 km below the earth’s surface (Pressure) At this temperature and pressure new minerals are formed – changed from one mineral to another

Low-grade Metamorphism between 5-15 km below the earth’s surface temperatures up to 400oC common low-grade minerals quartz, chlorite, muscovite Chlorite Muscovite Quartz

High-grade metamorphism 15 km below the surface to the point melting (when melting occurs) Temperatures greater than 400oC Common high-grade minerals garnet, kyanite, sillimanite kyanite Garnet sillimanite

Migmatite How do fluids enhance metamorphism? minerals dissolve – re-precipitated in the rock fluids speed up chemical reactions – reduces mineral melting points creating “migmatites” Igneous rock material Migmatite Metamorphic rock Melting occurs from lowered melting points creating small pockets of igneous rocks surrounded by metamorphic rocks

When does metamorphism take place? solid rock coming in contact with hot magma solid rock contacts hot fluids (hot water) during episodes of intense mountain building along earthquake fault zones

What are the various types of metamorphism processes? contact metamorphism hydrothermal metamorphism regional metamorphism Metasomatism

What is contact metamorphism magma “contacts” solid rock – heat is transferred into overlying rocks through conduction – changing the pre-existing rock it intruded high heat / low pressure localized metamorphism “fast acting” like touching a hot frying pan

Country rock (surrounding) is baked by conduction

What is regional metamorphism? pre-existing rock is subjected to intense stresses and strains (deformation) usually from the forces of mountain building high heat / high pressure very wide spread geographically like experiencing a slow sun burn

Mountain building Wide spread belts

What is hydrothermal metamorphism the chemical alteration of pre-existing rock from the action of hot water – usually hot water from a magma source that is ion rich low pressures / high heat hot water alteration

Black smokers on the ocean floor hydrothermal alteration – “spewing” hot water concentrated in sulfide metals

Metasomatism fluids pass in / out of the rock changing the overall chemical composition fluids take away or add chemical compounds to the rock Chemical alteration from fluids --- moving and out of the rock

Heat: What does each metamorphic process have in common? provides the energy to cause recrystallization of pre-existing minerals into “new” type of minerals Heat sources: contact with hot magma – heat penetrates overlying rocks (conduction) geothermal gradient – heat increases with depth – rocks are hotter when buried

Temperature increases as depth increases Large thermal gradient Geothermal Gradient Temperature increases as depth increases Large thermal gradient at the surface gradient decreases after 1000 km depth Steep gradient 1000 2000 3000 4000

Pressure – increases with depth various types of pressure rearrange minerals – compacts or elongates Confining pressure Differential pressure

Chemically active fluids commonly hot water carries ions in solution and ppt various solids (metals) hot water dissolves minerals and deposits minerals to other sites – concentrated gold, silver, other earth metals Gold in Quartz vein Native Copper (Cu)

How does one know how much a rock has been metamorphosed? the degree of metamorphism is reflected in the rocks texture and composition Regional Metamorphism (HP/HT) – high grade rock exhibits a foliation texture rocks show a “preferred” mineral orientation parallel mineral alignments

How does one know how much a rock has been metamorphosed? Contact Metamorphism (HT / LP) low-grade rock textures decrease in foliated textures – “sub” parallel type textures rocks composed of one type of mineral (same chemistry) are called non-foliated

How does one know the amount of temperature / pressure the rock has been subjected? The Metamorphic facies the set of metamorphic mineral assemblages that form under various temperature/pressure relationships Associated with different plate boundaries Blueschist / Eclogite facies occur in the high pressure environment --- subduction Greenschist, amphibolite, granulite facies occur along convergent boundaries

Metamorphic facies – mineral assemblages that represent various P / T relationships

What plate tectonic settings do metamorphic rocks form? Divergent boundaries hydrothermal metamorphism with black smokers Convergent plate boundary metamorphism along plate boundary regional and contact metamorphism Contact Regional

Metamorphism and Metamorphic Rocks Define the following metamorphism terms: contact metamorphism nonfoliated foliation parent rock hydrothermal regional metamorphism metamorphism confining pressure differential pressure index mineral