Chapter 8: Metamorphism & Metamorphic Rocks

Introduction “Meta” = change & “morph” = form Metamorphic = to change form Transition of one rock into another by application of pressure and/or temperature unlike those from which it formed. Metamorphic rocks are produced from Sedimentary rocks Igneous rocks Other metamorphic rocks Parent rock = protolith Rock from which the metamorphic rock was formed

Types of Metamorphism Progresses incrementally Low-grade metamorphism (< 200oC) High-grade metamorphism (> 600oC) ** Rock must remain solid!! If melting occurs, then igneous rocks are formed Agents of Metamorphism Heat Pressure & differential stress Chemically active fluids

Settings of Metamorphism Three settings: Thermal metamorphism AKA contact metamorphism Intrusion of magma body Change driven by rise in temperature Hydrothermal metamorphism Chemical alterations that occur as hot ion-rich water circulates through rock Regional metamorphism AKA large-scale deformation Large quantities of rock are subject to P and high T

Heat as Metamorphic Agent Most important agent b/c provides energy for chemical reactions & recrystallization Sources: Geothermal gradient Contact metamorphism

Pressure & Differential Stress Confining pressure  compaction Equal pressure in all directions Differential stress = directed pressure Causes folds & faults

Brittle vs. Ductile Behavior At surface, rocks are brittle Tend to fracture & break into smaller pieces At high-T, rocks are ductile Grains tend to flatten and elongate

Chemically Active Fluids Mostly water (H2O) and carbon dioxide (CO2) Sources of fluids: Hydrated minerals e.g. Clays & amphiboles Movement: Pore spaces of sedimentary rocks Fractures in igneous rocks

Importance of Protolith Remember: protolith = parent rock Most metamorphic rock have same chemical composition of their parent rock Except for gain or loss of volatiles e.g., H2O & CO2

Metamorphic Textures Texture = size, shape & orientation (arrangement) of grains in rock Foliation = any planar arrangement of mineral grains or structural features within a rock e.g., parallel alignment of platy and/or elongated minerals e.g., parallel alignment of flattened mineral grains or elongated pebbles e.g., compositional banding e.g., slaty cleavage where rocks can be easily split into thin, tabular sheets

Foliation: Alignment of Platy Minerals

Foliation: Alignment of Elongated Pebbles

Foliation: Compositional Banding Granite protolith resultant Gneiss with compositional banding

Foliation: Slaty Cleavage

Foliated Textures Rock cleavage (AKA slaty cleavage) Schistosity Closely spaced planar surfaces along which rocks split e.g., slate (originally shale) Schistosity Platy minerals (e.g., micas) are discernible with unaided eye Exhibit planar or layered structure e.g., schist (originally slate) Gneissic banding During higher grades of metamorphism, ion migration results in separation of light and dark minerals Exhibit distinctive light & dark compositional banding

Formation of Slate

Garnet – Mica Schist

Gneissic Banding

Other Metamorphic Textures Nonfoliated Form in environments where deformation is minimal e.g., fine-grained limestone + heat = marble Porphyroblastic texture Large grains surrounded by smaller grains Porphyroblast = large grains Matrix = fine-grains around porphyroblast

Common Metamorphic Rocks Foliated Rocks: Slate Phyllite Schist Gneiss Nonfoliated Rocks: Marble Quartzite

Slate Very fine-grained Excellent rock cleavage Slaty cleavage Most often generated from low-grade metamorphism of shale, mudstone or siltstone Different colors: Black = carbonaceous Red = Fe-oxide Green = chlorite

Phyllite Degree of metamorphism between slate & schist Platy minerals not large enough to be identified with unaided eye Glossy sheen & wavy surfaces Has rock cleavage Composed of platy minerals such as micas & chlorite Phyllite Slate

Schist Medium- to coarse-grained Medium-grade metamorpism Platy minerals dominate Mainly micas Schistositic texture To indicate composition, mineral names are used e.g., mica schist 

Gneiss Medium- to coarse-grained Banded layered appearance High-grade metamorphism Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian-rich layers

Marble Coarse-grained Crystalline Protolith = limestone Composed of essentially calcite [CaCO3] and/or dolomite [CaMg (CO3)2] crystals Exhibits a variety of colors Used as decorative & monument stone

Quartzite Medium- to coarse-grained Crystalline Protolith = sandstone Composed dominately of quartz [SiO2]

Classification of Metamorphic Rocks

Metamorphic Environments Contact Metamorphism AKA Thermal Metamorphism Hydrothermal Metamorphism Regional Metamorphism Other Metamorphic Environments Burial Metamorphism Metamorphism along fault zones Impact metamorphism AKA Shock Metamorphism

Contact Metamorphism

Hydrothermal Metamorphism

Regional Metamorphism

Burial Metamorphism

Fault Zone Metamorphism

Impact Metamorphism

Metamorphic Zones Metamorphic Grade: Index Minerals: Systematic variation in mineralogy & textures are related to the variations in the degree of metamorphism Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism Index Minerals: Certain minerals which are good indicators of the degree of the metamorphic conditions under which they form e.g., chlorite ~200oC

Progressive Regional Metamorphism With increasing P & T, higher degree of metamorphism

Index Minerals Typical transition in mineralogy due to progressive metamorphism of shale

Metamorphic Zones in New England, USA Grades concentric with high-grade in center of mountain range

Migmatites Highest grade metamorphic rock Transitional to Igneous rock Partial melting of felsic minerals Contain light bands of igneous components along with dark metamorphic rock

Fault Zone Metamorphism Fault breccia at and near surface Mylonite (AKA migmatite) forms at depth

Metamorphism & Plate Tectonics Most metamorphism occurs along convergent boundaries Compressional stresses deform edges of plates Formation of Earth’s major mountain belts e.g. Alps, Himalayas, Appalachians Mountainous terrains along subduction zones contain linear belts of metamorphic rocks High P, low T rocks nearest the trench e.g. blueschist facies (glaucophane = index min) High T, low P zones further inland In region of igneous activity Large-scale metamorphism also occurs along subduction zone at convergent boundaries Several metamorphic environments exist here Important site for magma generation

Metamorphism & Plate Tectonics High P deformation Mountain Building Migmatites Blueschist facies Decompression melting

Ancient Metamorphism Canadian shield AKA craton

Homework Assignment #1 Investigate rocks in courtyard Determine if igneous, sedimentary, metamorphic Give five observations to support choice F-A F-C 2 3,4 5 F-B 6 1 7