OpCh08_Lecture_Earth3

OpCh08_Lecture_Earth3 - Chapter 8 Metamorphism: A Process...

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Unformatted text preview: Chapter 8 Metamorphism: A Process of Change LECTURE OUTLINE Portrait of a Planet earth Third Edition ©2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 3rd 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.” Metamorphic Meta = Change. Morph = Form or shape. Parent rocks are called “protoliths.” Metamorphism can Metamorphism occur to any protolith. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Introduction Protoliths undergo pronounced changes in… Texture. Texture. Mineralogy. Burial. Tectonic stresses. Heating by magma. Fluid alteration. Due to change in physical or chemical conditions. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphism Metamorphism occurs in the solid state. Metamorphism solid It doesn’t include weathering, diagenesis, melting. Metamorphics often look totally unlike protoliths. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Character Metamorphic rocks have distinctive properties. Metamorphic Unique texture – Intergrown and interlocking grains. Unique minerals – Some that are only metamorphic. Unique only Staurolite, Kyanite, Sillimanite, etc. Unique foliation – A planar fabric from aligned minerals. These transformations can change the rock utterly. These Fossiliferous limestone Red mudstone Garnet gneiss Marble Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Processes Metamorphic change is slow and in the solid state. Several processes are at work. Recrystallization – Minerals change size and shape. Phase change – New minerals form with… Same chemical formula. Different crystal structure. Kyanite Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Processes Several processes may operate at the same time. Neocrystallization – New minerals with P-T changes. Neocrystallization Initial minerals become unstable; change to new minerals. h Decomposition of the original minerals in the protolith. h Chemical reaction of the elements to form new minerals. In this way, a shale can transform into a garnet mica schist. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Processes Several processes may operate at the same time. Pressure solution – Mineral grains partially dissolve. Plastic deformation – Mineral grains soften and deform. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Causes of Metamorphism The agents of metamorphism are… The Heat (Temperature – T). Pressure (P). Differential stress. Hydrothermal fluids. Not all agents are required; they often do co-occur. Not Rocks may be overprinted by multiple events. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Heat (Temperature) Metamorphism occurs as the result of heat. Temperature (T) ranges between 200oC and 850oC. The upper T limit is…melting. It varies based upon The rock mineral composition and water content. rock Heat energy breaks and reforms atomic bonds. Heat Sources of heat. The geothermal gradient. The Magmatic intrusions. Compression. Earth: Portrait of a Planet, 3rd 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. Metamorphism T and P both change with depth. and both Mineral stability is highly dependent upon T and P. Mineral This stability can be graphed on a “phase diagram.” Changes in T and P lead Changes to changes in minerals. to Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Differential Stress Pressure that is greater in one orientation. Pressure A commonplace result of tectonic forces. Two kinds of differential stress: Normal and shear. Normal stress – Operates perpendicular to a surface. Normal Tension – Pull-apart normal stress. Compression – Push-together normal stress. Earth: Portrait of a Planet, 3rd 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. Causes material to be “smeared out.” Earth: Portrait of a Planet, 3rd 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, 3rd 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. Inequant Platy (pancake-like) – 1 dimension shorter. Elongate (cigar-shaped) – 1 dimension longer. Differential stress causes these minerals to align. Alignment fabric records stress trajectory. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Differential Stress Differential Preferred platy mineral alignment is called foliation. Preferred Foliation imparts a layered or banded appearance. Rocks commonly break parallel to foliation planes. Minerals flatten, recrystallize, and rotate. Foliation develops perpendicular to compression. Inequant grains align by rotation and new growth. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Hydrothermal Fluids Hot water with dissolved ions and volatiles. Hydrothermal fluids facilitate metamorphism. Hydrothermal Accelerate chemical reactions. Alter rocks by adding or subtracting elements. Alter Hydrothermal alteration is called metasomatism. Earth: Portrait of a Planet, 3rd 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. Subjected to differential stress. Has a significant component of platy minerals. Has Classified by composition, grain size, and foliation type. Earth: Portrait of a Planet, 3rd 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. Crystallized without differential stress. Comprised of equant minerals only. Classified by mineral composition. Earth: Portrait of a Planet, 3rd 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. Has Develops by parallel alignment of platy clay minerals. Slaty cleavage oriented perpendicular to compression. Slate breaks along this foliation creating flat sheets. Slate Earth: Portrait of a Planet, 3rd 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. Formed Clay minerals neocrystallize into tiny micas. Clay Micas reflect a satiny luster. Phyllite is between slate and schist. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Rocks Metamorphic Schist – Fine - coarse rock with larger micas. Medium-to-high-grade metamorphism. Has a distinct foliation called schistosity. Has Parallel alignment of large mica crystals. Parallel Micas are visible because they have grown at higher T. Micas Schist often has other minerals due to neocrystallization. Quartz. Feldspars. Kyanite. Garnet. Staurolite. Sillimanite. Sillimanite. Large non-mica minerals are called porphyroblasts. Large Chapter 8: Metamorphism: A Process of Change Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak 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, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Rocks Compositional banding develops in several ways. Original layering in the protolith. Original Extensive high T shearing. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Rocks Compositional banding Compositional Solid-state chemical differentiation. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Migmatite Migmatite is a partially melted gneiss. Migmatite It has features of igneous and metamorphic rocks. It and Mineralogy controls behavior. Mineralogy Light-colored (felsic) minerals melt at lower T. Light-colored Dark-colored (mafic) minerals melt a higher T. Dark-colored Felsics melt first; mafics remain metamorphic. Felsics Earth: Portrait of a Planet, 3rd 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. Rock not subjected to differential stress. Dominance of equant minerals. Absence of platy minerals. Earth: Portrait of a Planet, 3rd 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. Usually Associated with plutonic intrusions. Finely crystalline. Finely Hornfels – Alteration by heating. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Rocks Quartzite – Almost pure quartz in composition. Quartzite Forms by alteration of quartz sandstone. Forms Sand grains in the protolith recrystallize and fuse. Sand Like quartz, it is hard, glassy, and resistant. Like Metamorphic Alteration Earth: Portrait of a Planet, 3rd 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. Forms Extensive recrystallization completely changes the rock. Original textures and fossils in the parent are obliterated. Original Used as a decorative and monument stone. Exhibits a variety of colors. Metamorphic Alteration Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Rocks Type depends on protolith. Type Minerals contribute elements. Some protoliths yield specific rocks. Pelitic. Basic (or Mafic). Calcareous. Quartzo-feldspathic. Broad compositional classes. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Classes Pelitic – Shale protoliths. Al-rich clay minerals yield micas. Al-rich Rock type depends on grade. Slate. Phyllite. Schist. Gneiss. Earth: Portrait of a Planet, 3rd 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, 3rd 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). Rich Recrystallize into calcite and dolomite marbles. Earth: Portrait of a Planet, 3rd 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, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Intensity Different minerals are stable as T and P changes. Different Grade is a measure of metamorphic intensity. Grade Low grade – Slight. High grade – Intense. High Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Intensity Specific minerals typify particular grades. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Grade Prograde – Metamorphism via increasing T and P. Prograde Common in rocks that are buried in orogenic belts. Progressive changes. Recrystallization causes mineral growth. Neocrystallization results in new mineral assemblages. Mineral changes release water. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Grade Example: Prograde metamorphism of a pelitic rock. Low grade – Shale protolith. Clays recrystallize into larger, aligned clays to yield a slate. Clays neocrystallize into tiny, aligned micas in a phyllite. Clays Intermediate grade – Intermediate Micas recrystallize and grow large to form a schist. New minerals grow in the schist. High grade – High Micas decompose; elements recombine into new minerals. Neocrystallization yields quartz and feldspars in a gneiss. Neocrystallization Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Grade Retrograde – Metamorphism via decreasing T and Retrograde P Common in rocks that are brought from depth by erosion. Accompanied by addition of H2O by hydrothermal fluids. Rocks at the surface can preserve prograde conditions. Many prograde rocks aren’t “retrograded.” Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Contact Metamorphism Certain minerals have a limited P-T range. Certain These “index minerals” record metamorphic grade. These Index mineral maps. Define metamorphic zones. Define Grade boundaries called Grade isograds. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Metamorphic Facies Metamorphic facies – Mineral assemblage from a Metamorphic specific protolith at specific P-T conditions. The same minerals result from the same… The Protoliths. Protoliths. T and P conditions. and Named for dominant Named mineral. mineral. Earth: Portrait of a Planet, 3rd 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, 3rd 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. Dynamic Regional – P and T alteration due to orogenesis. Regional Hydrothermal – Alteration by hot water leaching. Subduction – High P–Low T alteration. Shock – Extreme high P attending a bolide impact. Shock Mantle – Extreme high mantle P causes phase changes. Earth: Portrait of a Planet, 3rd 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. The Zoned from high (near pluton) to low grade (far from pluton). Earth: Portrait of a Planet, 3rd 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 size of the plutonic intrusion. The degree of metasomatism. The width of each aureole zone is due to… The dominant rock is hornfels. Earth: Portrait of a Planet, 3rd 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. increases This is ~ 8–15 km depending on the geothermal gradient. Requires burial below diagenetic effects. Requires Earth: Portrait of a Planet, 3rd 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. Fault Shallow crust – Upper 10-15 km. Rocks behave in a brittle fashion. Mineral grains crush and pulverize forming fault breccia. Deeper crust – Below 10-15 km. Rocks behave in a ductile manner. Rocks Minerals smear like taffy to form mylonite. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Regional Metamorphism Tectonic collisions deform huge “mobile belts.” Tectonic Directed compression thickens mountains. Rocks caught up in mountain building are… Heated via the geothermal gradient and plutonic intrusions. Squeezed and heated by deep burial. Squeezed Smashed and sheared by differential stresses. Earth: Portrait of a Planet, 3rd 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 This important in terms of the amount of rock altered. important Collisional belts are often… Collisional 1000s of km long. 1000s 100s of km wide. Earth: Portrait of a Planet, 3rd 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, 3rd 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 High P – Low T favor High glaucophane, a blue amphibole mineral. amphibole Earth: Portrait of a Planet, 3rd 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. Coesite. Stishovite. These conditions generate high-pressure minerals. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Exhumation How do metamorphic rocks return to the surface? Exhumation is due to... Exhumation Uplift – Compression squeezes deep rocks upward. Extensional collapse – Uplifted range spreads outward. Erosional unroofing – Weathering and erosion removes Erosional vast amounts of rock. vast Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change Finding Metamorphics Large regions of ancient high-grade rocks – called Large 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, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change This concludes the Chapter 8 Metamorphism: A Process of Change LECTURE OUTLINE Portrait of a Planet earth Third Edition ©2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 8: Metamorphism: A Process of Change ...
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