Chapter 8 Metamorphism A Process of Change LECTURE

Chapter 8 Metamorphism: A Process of Change LECTURE OUTLINE earth Portrait of a Planet Third Edition © 2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphism: A Process of Change Prepared by Ron Parker Earlham College Department of Geosciences Richmond, Indiana

Introduction < Metamorphic – Changed from an original “parent. ” = Meta = Change. = Morph = Form or shape. Parent rocks are called “protoliths. ” < Metamorphism can occur to any protolith. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Introduction < Protoliths undergo pronounced changes in… = Texture. = Mineralogy. < Due to change in physical or chemical conditions. = Burial. = Tectonic stresses. = Heating by magma. = Fluid alteration. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphism occurs in the solid state. < It doesn’t include weathering, diagenesis, melting. < Metamorphics often look totally unlike protoliths. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Character < Metamorphic rocks have distinctive properties. = Unique texture – Intergrown and interlocking grains. = Unique minerals – Some that are only metamorphic. 4 Staurolite, Kyanite, Sillimanite, etc. = Unique < foliation – A planar fabric from aligned minerals. These transformations can change the rock utterly. Fossiliferous limestone Red mudstone Garnet gneiss Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Marble Chapter 8: Metamorphism: A Process of Change

Metamorphic Processes Metamorphic change is slow and in the solid state. < Several processes are at work. < = Recrystallization = Phase – Minerals change size and shape. change – New minerals form with… 4 Same chemical formula. 4 Different crystal structure. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Kyanite Chapter 8: Metamorphism: A Process of Change

Metamorphic Processes < Several processes may operate at the same time. = Neocrystallization – New minerals with P-T changes. 4 Initial minerals become unstable; change to new minerals. a. Decomposition of the original minerals in the protolith. a. Chemical reaction of the elements to form new minerals. 4 In this way, a shale can transform into a garnet mica schist. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Processes < Several processes may operate at the same time. = Pressure = Plastic solution – Mineral grains partially dissolve. deformation – Mineral grains soften and deform. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Causes of Metamorphism < The agents of metamorphism are… = Heat (Temperature – T). = Pressure (P). = Differential stress. = Hydrothermal fluids. Not all agents are required; they often do co-occur. < Rocks may be overprinted by multiple events. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Heat (Temperature) < Metamorphism occurs as the result of heat. = Temperature (T) ranges between 200 o. C and 850 o. C. The upper T limit is…melting. It varies based upon rock mineral composition and water content. < Heat energy breaks and reforms atomic bonds. < Sources of heat. < = The geothermal gradient. = Magmatic intrusions. = Compression. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Pressure (P) < P increases with depth in the crust. = 270 to 300 bars per km (1 bar is almost 1 atm = 14. 7 psi). = Metamorphism occurs mostly in 2 to 12 kbar range. T and P both change with depth. < Mineral stability is highly dependent upon T and P. < = This stability can be graphed on a “phase diagram. ” = Changes in T and P lead to changes in minerals. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Differential Stress Pressure that is greater in one orientation. < A commonplace result of tectonic forces. < Two kinds of differential stress: Normal and shear. < = Normal stress – Operates perpendicular to a surface. 4 Tension – Pull-apart normal stress. 4 Compression – Push-together normal stress. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Differential Stress < Two kinds of differential stress: Normal and shear. = Shear stress – Operates sideways across a surface. 4 Causes material to be “smeared out. ” Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Differential Stress < At higher T and P, differential stress deforms rock. = Rocks change shape slowly without breaking. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Differential Stress < Deformation acts on minerals with specific shapes. = Equant – Roughly equal in all dimensions. = Inequant – Dimensions not the same. 4 Platy (pancake-like) – 1 dimension shorter. 4 Elongate (cigar-shaped) – 1 dimension longer. Differential stress causes these minerals to align. < Alignment fabric records stress trajectory. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Differential Stress < Preferred platy mineral alignment is called foliation. = Foliation imparts a layered or banded appearance. = Rocks commonly break parallel to foliation planes. < Foliation develops perpendicular to compression. = Minerals < flatten, recrystallize, and rotate. Inequant grains align by rotation and new growth. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Hydrothermal Fluids Hot water with dissolved ions and volatiles. < Hydrothermal fluids facilitate metamorphism. < = Accelerate chemical reactions. = Alter rocks by adding or subtracting elements. < Hydrothermal alteration is called metasomatism. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rock Types < Two major subdivisions of metamorphic rocks. = Foliated – Has a through-going planar fabric. 4 Subjected to differential stress. 4 Has a significant component of platy minerals. 4 Classified by composition, grain size, and foliation type. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rock Types < Two major subdivisions of metamorphic rocks. = Non-foliated – No planar fabric evident. 4 Crystallized without differential stress. 4 Comprised of equant minerals only. 4 Classified by mineral composition. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Slate – Fine clay, low-grade metamorphic shale. = Has a distinct foliation called slaty cleavage. 4 Develops by parallel alignment of platy clay minerals. 4 Slaty cleavage oriented perpendicular to compression. 4 Slate breaks along this foliation creating flat sheets. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Phyllite - Fine mica-rich rock. = Formed by low – medium grade alteration of slate. = Clay minerals neocrystallize into tiny micas. = Micas reflect a satiny luster. = Phyllite is between slate and schist. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Schist – Fine - coarse rock with larger micas. = Medium-to-high-grade metamorphism. = Has a distinct foliation called schistosity. 4 Parallel alignment of large mica crystals. 4 Micas are visible because they have grown at higher T. = Schist often has other minerals due to neocrystallization. 4 Quartz. 4 Feldspars. 4 Kyanite. 4 Garnet. 4 Staurolite. 4 Sillimanite. = Large non-mica minerals are called porphyroblasts. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Gneiss – Has a distinct banded foliation. = Light bands of felsic minerals (quartz and feldspars). = Dark bands of mafic minerals (biotite or amphibole). Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Compositional banding develops in several ways. = Original layering in the protolith. = Extensive high T shearing. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Compositional banding = Solid-state chemical differentiation. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Migmatite is a partially melted gneiss. < It has features of igneous and metamorphic rocks. < Mineralogy controls behavior. < = Light-colored (felsic) minerals melt at lower T. = Dark-colored (mafic) minerals melt a higher T. < Felsics melt first; mafics remain metamorphic. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Non-foliated rocks lack a planar fabric. = Absence of foliation possible for several reasons. 4 Rock not subjected to differential stress. 4 Dominance of equant minerals. 4 Absence of platy minerals. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Amphibolite – Dominated by amphibole minerals. = Basalt or gabbro protolith. = Usually not well foliated. < Hornfels – Alteration by heating. = Associated with plutonic intrusions. = Finely crystalline. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Quartzite – Almost pure quartz in composition. = Forms by alteration of quartz sandstone. = Sand grains in the protolith recrystallize and fuse. = Like quartz, it is hard, glassy, and resistant. Metamorphic Alteration Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Marble - Coarsely crystalline calcite or dolomite. = Forms from a limestone or dolostone protolith. = Extensive recrystallization completely changes the rock. = Original textures and fossils in the parent are obliterated. = Used as a decorative and monument stone. = Exhibits a variety of colors. Metamorphic Alteration Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Rocks < Type depends on protolith. = Minerals contribute elements. = Some protoliths yield specific rocks. < Broad compositional classes. = Pelitic. = Basic (or Mafic). = Calcareous. = Quartzo-feldspathic. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Classes < Pelitic – Shale protoliths. = Al-rich clay minerals yield micas. = Rock type depends on grade. 4 Slate. 4 Phyllite. 4 Schist. 4 Gneiss. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Classes < Mafic – Basalt or gabbro protolith. = Rich in Fe and Mg and poor in Si, Al, Na, and K. = Turn into biotite- and amphibole-dominated rocks. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Classes < Calcareous – Carbonate protolith. = Rich in Ca (limestones) and Ca and Mg (dolostones). = Recrystallize into calcite and dolomite marbles. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Classes < Quartzo-feldspathic – Granitic protolith. = Quartz + feldspar in granites stable under metamorphism. = Recrystallize and become foliated into granitic gneisses. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Intensity Different minerals are stable as T and P changes. < Grade is a measure of metamorphic intensity. < = Low grade – Slight. = High grade – Intense. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Intensity < Specific minerals typify particular grades. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Grade < Prograde – Metamorphism via increasing T and P. = Common in rocks that are buried in orogenic belts. = Progressive changes. 4 Recrystallization causes mineral growth. 4 Neocrystallization results in new mineral assemblages. 4 Mineral changes release water. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Grade < Example: Prograde metamorphism of a pelitic rock. = Low grade – Shale protolith. 4 Clays recrystallize into larger, aligned clays to yield a slate. 4 Clays neocrystallize into tiny, aligned micas in a phyllite. = Intermediate grade – 4 Micas recrystallize and grow large to form a schist. 4 New minerals grow in the schist. = High grade – 4 Micas decompose; elements recombine into new minerals. 4 Neocrystallization yields quartz and feldspars in a gneiss. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Grade < Retrograde – Metamorphism via decreasing T and P = Common in rocks that are brought from depth by erosion. = Accompanied by addition of H 2 O by hydrothermal fluids. < Many prograde rocks aren’t “retrograded. ” = Rocks at the surface can preserve prograde conditions. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Contact Metamorphism Certain minerals have a limited P-T range. < These “index minerals” record metamorphic grade. < Index mineral maps. < = Define metamorphic zones. = Grade boundaries called isograds. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Facies Metamorphic facies – Mineral assemblage from a specific protolith at specific P-T conditions. < The same minerals result from the same… < = Protoliths. =T < and P conditions. Named for dominant mineral. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Environments Metamorphism occurs in different settings. < Different settings yield different effects via… < = Geothermal gradient. = Differential stresses. = Hydrothermal fluids. < These characteristics are governed by tectonics. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Metamorphic Environments < The types (and settings) of metamorphism are. . . = Thermal – Heating by a plutonic intrusion. = Burial – Increases in P and T by deep burial in a basin. = Dynamic – Shearing in a fault zone. = Regional – P and T alteration due to orogenesis. = Hydrothermal – Alteration by hot water leaching. = Subduction – High P–Low T alteration. = Shock – Extreme high P attending a bolide impact. = Mantle – Extreme high mantle P causes phase changes. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Contact Metamorphism Due to heat from magma invading host rock. < Creates zoned bands of alteration in host rock. < = Called a contact (or metamorphic) aureole. = The aureole surrounds the plutonic intrusion. 4 Zoned from high (near pluton) to low grade (far from pluton). Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Contact Metamorphism < Grades of alteration form bands around the pluton. = Bands range from highly altered to slightly altered. = Analogous to changes in pottery with increased heating. < The width of each aureole zone is due to… = The size of the plutonic intrusion. = The degree of metasomatism. < The dominant rock is hornfels. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Burial Metamorphism < As sediments are buried in a sedimentary basin… =P increases because of the weight of the overburden. = T increases because of the geothermal gradient. < Requires burial below diagenetic effects. = This is ~ 8– 15 km depending on the geothermal gradient. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Dynamic Metamorphism Breakage of rock by shearing at a fault zone. < Fault location determines type of alteration. < = Shallow crust – Upper 10 -15 km. 4 Rocks behave in a brittle fashion. 4 Mineral grains crush and pulverize forming fault breccia. = Deeper crust – Below 10 -15 km. 4 Rocks behave in a ductile manner. 4 Minerals smear like taffy to form mylonite. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Regional Metamorphism Tectonic collisions deform huge “mobile belts. ” < Directed compression thickens mountains. < = Rocks caught up in mountain building are… 4 Heated via the geothermal gradient and plutonic intrusions. 4 Squeezed and heated by deep burial. 4 Smashed and sheared by differential stresses. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Regional Metamorphism Regional metamorphism creates foliated rocks. < This type of metamorphism is, by far, the most important in terms of the amount of rock altered. < = Collisional belts are often… 41000 s of km long. 4100 s of km wide. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Hydrothermal Metamorphism Alteration by hot, chemically aggressive water. < A dominant process near mid-ocean ridge magma. < = Cold ocean water seeps into fractured crust. = Heated by magma, this water then reacts with mafic rock. = The hot water rises and is ejected via black smokers. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Subduction Metamorphism Subduction creates the unique blueschist facies. < Trenches and accretionary prisms have… < =A low geothermal gradient – low temperature. = High pressures. < High P – Low T favor glaucophane, a blue amphibole mineral. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Shock Metamorphism Rarely, Earth is struck by a comet or asteroid. < Impacts generate a compressional shock wave. < = Extremely high pressure. = Heat that vaporizes or melts large masses of rock. < These conditions generate high-pressure minerals. = Coesite. = Stishovite. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Exhumation How do metamorphic rocks return to the surface? < Exhumation is due to. . . < = Uplift – Compression squeezes deep rocks upward. = Extensional collapse – Uplifted range spreads outward. = Erosional unroofing – Weathering and erosion removes vast amounts of rock. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

Finding Metamorphics Large regions of ancient high-grade rocks – called shields – are exposed in continental interiors. < Shields are eroded remnants of orogenic belts. < = Shield rocks form the basement under sedimentary cover. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change

This concludes the Chapter 8 Metamorphism: A Process of Change LECTURE OUTLINE earth Portrait of a Planet Third Edition © 2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change