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Kinds of metamorphism Lecture based on

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1 Kinds of metamorphism Lecture based on And PowerPoint lecture of Dr. J.D. Winter

2 The Types of Metamorphism Different approaches to classification 1. Based on principal process or agent –Dynamic Metamorphism –Thermal Metamorphism –Dynamo-thermal Metamorphism

3 2. Based on setting (P-T conditions/tectonic) –Contact Metamorphism PyrometamorphismPyrometamorphism –Regional Metamorphism Orogenic MetamorphismOrogenic Metamorphism Burial MetamorphismBurial Metamorphism Ocean Floor MetamorphismOcean Floor Metamorphism –Hydrothermal Metamorphism –Fault-Zone Metamorphism (cataclastic or mylonitic in your text) –Impact or Shock Metamorphism

4 Types of metamorphism based on pressure and temperature conditions

5 Blueschist metamorphism HIGH pressure/Low temperature Typically associated with subduction zones Example: Blueschist (from glaucophane) Geothermal gradient: Indicates the average increase of temperature with depth

6 1mm Glaucophane in a blueschist (ppl).

7 Eclogite metamorphism VERY HIGH pressure/moderate temperature Happens in the mantle Example: Eclogite (pyroxene/garnet)

8 Eclogite (ppl) Eclogite (xpl)

9 Hydrothermal metamorphism Low pressure/low temperature Hot, chemically active waters react with the surrounding rocks Example: Serpentinite, soapstone

10 Hydrothermal metamorphism Usually involves METASOMATISM (metamorphism plus exchange of ions from an external source) Difficult type to constrain: hydrothermal effects often play some role in most of the other types of metamorphism

11 Contact metamorphism Low pressure/HIGH temperature “Country” rocks baked by igneous intrusion (pluton) Example: Hornfels, skarn (baked carbonate rocks), quartzite, marble


13 Contact Metamorphism Most easily recognized where a pluton is introduced into shallow rocks in a static environment (Pluton=intrusive igneous body, like a batholith)  Hornfelses (granofelses) commonly with relict textures and structures

14 Spotted hornfels

15 Pyrometamorphism A minor type of contact metamorphism Very high temperatures at very low pressures, generated by a volcanic or subvolcanic body Also developed in xenoliths (pieces of solid rocks carried up by magma) Pyrometamorphism may be accompanied by various degrees of partial melting

16 Barrovian metamorphism Intermediate pressure/Low-high temperature Example: Slate-phyllite-schist-gneiss, also quartzite and marble Also referred to as “regional” metamorphism

17 Regional Metamorphism sensu lato: metamorphism that affects a large body of rock, and thus covers a great lateral extent Three principal types: –Orogenic metamorphism –Burial metamorphism –Ocean-floor metamorphism The term, “regional metamorphism” is often used synonymously with “orogenic metamorphism” (OROGENY=mountain building)

18 Orogenic Metamorphism is the type of metamorphism associated with convergent plate margins Dynamo-thermal: one or more episodes of orogeny with combined elevated geothermal gradients and deformation (differential stress) Foliated rocks are a characteristic product

19 Slate (ppl) Phyllite (xpl) Foliation Quartz-muscovite schist (xpl) Muscovite Quartz

20 Crenulation cleavage in a muscovite-biotite-garnet schist (xpl)

21 Gneiss in hand specimen Gneiss under a microscope (xpl) Quartz BiotiteFeldspar

22 Marble (xpl) Calcite (note cleavage)

23 Burial metamorphism = low-grade metamorphism in sedimentary basins –Metamorphic effects attributed to increased temperature and pressure due to burial –Occurs in areas that have not experienced significant deformation or orogeny –Mild deformation, no igneous intrusions discovered

24 Restricted to large, relatively undisturbed sedimentary piles away from active plate margins –Term coined by Coombs (1961) based on Southland Syncline in New Zealand: thick pile (> 10 km) of Mesozoic volcaniclastics Modern examples: –The Gulf of Mexico? –Bengal Fan?

25 Ocean-Floor Metamorphism affects the oceanic crust at ocean ridge spreading centers A wide range of temperatures at relatively low pressure Seawater penetrates down fracture systems, where it becomes heated, and leaches metals and silica from the hot basalts Considerable metasomatic alteration, notably loss of Ca and Si and gain of Mg and Na

26 Highly altered chlorite-quartz rocks- distinctive high-Mg, low-Ca composition Black smokers - Another example of hydrothermal metamorphism

27 Fault-Zone and Impact Metamorphism –High rates of deformation and strain with only minor recrystallization (a) Shallow fault zone with fault breccia (b) Slightly deeper fault zone (exposed by erosion) with some ductile flow and fault mylonite

28 Width of view=4mm

29 –Impact metamorphism at meteorite (or other bolide) impact craters –Both correlate with dynamic metamorphism, based on process Fault-Zone and Impact Metamorphism –High rates of deformation and strain with only minor recrystallization

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