Metamorphic Petrology. Jen Parks ESC 310, x6999

Transcription

1 Metamorphic Petrology Jen Parks ESC 310, x6999

2 Definition of Metamorphism The IUGS-SCMR SCMR definition of metamorphism: Metamorphism is a subsolidus process leading to changes in mineralogy and/or texture (for example grain size) and often in chemical composition in a rock. These changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting.

3 Definition of Metamorphism What does that really mean? Changes in P, T, X than result in a mineralogical or textural change in a rock Solid state transformation (no melting) What sort of change happens? Phase change: new metamorphic minerals due to a chemical reaction Textural change: new textures such as alignment of platy minerals, or progressive coarsening or fining of pre-existing igneous/sed minerals What should we care?

4 Limits of Metamorphism Lower limit : Digenesis Upper limit : Migmatites

5 Agents of Metamorphism: Temperature Temperature can be most important change Where does the heat come from? Increasing pressure with depth Radioactive decay Deformation Migrating magmas

6 Agents of Metamorphism: Temperature What does increasing T do? Adds energy to system, that helps to over come kinetic barriers in the aim of driving reactions to reach equilibrium 1. Promotes recrystallization increased grain size 2. consumes unstable mineral(s) and produces new minerals that are stable

7 Agents of Metamorphism: Pressure Lithostatic pressure Uniform, equal in all directions P=ρgh Normal gradients (general increase with depth) can be disturbed in certain tectonic environments What does changing P do? High T/P paths (low P) favor the formation of low-density metamorphic minerals as temperature rises (rifting or plutonic activity) Low T/P paths (high P) favor denser minerals (subduction zone)

8 Agents of Metamorphism: Deformation Deviatoric stress: unequal in every direction Compression Extension Shear What does deformation do? provide energy to overcome kinetic barriers to drive reactions

9 Agents of Metamorphism: Fluids Evidence of fluids during metamorphism: Fluid inclusions in meta minerals Formation of meta phases that require presence of a fluid Meta reactions often involve dehydration / decarbonation reactions that produce a fluid What can having a fluid in a system do? Fluid contains phases that will try to gain equilibrium with the rock phase, which drives reactions and results in mineralogical/textural changes of rock phase

10 Agents of Metamorphism: P & T High grade/low grade rocks: relative term to indicate high or low metamorphic conditions

11 Nature of Metamorphism Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions Retrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous event Basically can increase and decrease in grade, and do this multiple time

12 Types of Metamorphism 1.Contact metamorphism Heating of country rocks during igneous intrusion or beneath thick flows, results in growth of new metamorphic minerals in random orientations 2.Regional metamorphism a) Orogenic: Orogenic: large scale metamorphism characteristic of mountain belts and shield areas as a result of tectonism Burial: postdiagenetic, progressive changes occurring to sedimentary rocks during burial b) Burial:

13 Types of Metamorphism 3. Hydrothermal metamorphism Chemical reactions as a result of circulating fluids a) Sea floor spreading centres: Interaction of basalt and seawater, seawater infiltrates fractures systems and leaches metals and silica, black smokers b) Porphyry systems: Hot fluids interaction and circulating around plutons above subduction zones 4. Dynamic (Fault-Zone) metamorphism In response to high strain, localized on along structures 5. Shock metamorphism Impact of high velocity meteorites, creates high P/T minerals at surface (diamonds!)

14 Metamorphic Classification/Rock Types Metamorphic rocks are classified on the basis of texture and composition (either mineralogical or chemical) Use prefix-type modifiers to attach to names to stress important or unusual textural or mineralogical aspects (ie. muscovite schist) Broad classification based on foliated vs. unfoliated

15 Slate: Metamorphic Rocks Foliated fine grained rock with parallel cleavage, defined by parallel growth of micas, due to regional pressure regimes, dull foliation surface Phyllite: Same as a slate, contains very fine grained phyllosilicates, has a silky sheen on foliation surface

16 Metamorphic Rocks Foliated Schist: preferred orientation of fine to medium sized minerals and mineral aggregates that have been produced by meta processes, usually has planar features and is mica rich

17 Metamorphic Rocks Foliated Gneiss: banded rock with alternating light and dark mineral layers, segregation into layers by meta processes Orthogneiss: Igneous in origin Paragneiss: Sedimentary in origin

18 Metamorphic Rocks - Unfoliated Rocks that are not produced by high-strain metamorphism Granofels: a comprehensive term for any isotropic rock (a rock with no preferred orientation) Hornfels is a type of granofels that is typically very fine-grained and compact, occurs in contact aureoles, tough, tend to splinter when broken.

19 Metamorphic Rocks High Strain Usually results in textural change of a rock as it reacts to accommodate stain Mylonite, cataclasite.

20 Metamorphic Rocks High Strain

21 Metamorphic Rocks High Strain

22 Common Metamorphic Rock Types Marble: composed predominantly of calcite or dolomite, protolith is typically limestone or dolostone Quartzite: composed predominantly of quartz, protolith is typically sandstone Greenschist/Greenstone: a low-grade metamorphic rock that contains chlorite, actinolite, epidote, and albite, protolith is a mafic igneous rock or graywacke. Greenschist (foliated), Greenstone (unfoliated) Amphibolite: dominated by hornblende + plagioclase, may be foliated or non-foliated, protolith is a mafic igneous rock or graywacke

23 Common Metamorphic Rock Types Serpentinite: an ultramafic rock metamorphosed at low grade, contains mostly serpentine Blueschist: a blue amphibole-bearing bearing metamorphosed mafic igneous rock or mafic graywacke Eclogite: a green and red metamorphic rock,contains clinopyroxene and garnet,protolith is typically basaltic Skarn: contact metamorphosed and metasomatized carbonate rock containing calc-silicate minerals Granulite: high grade rock, composed of OH-free minerals, no muscovite, plagioclase and orthopyroxene are common, protolith is pelitic, mafic, or quartzo-feldspathic