Lecture 15 intro to metamoprhic rocks +16 +17

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Geology

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32 Terms

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How are metamoprhic rocks formed?
Transformation of prexisting rocks under

\-pressure

\-temperature

\-fluids
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During metamorphic process what form do rock stay in?
Solid. The mineralogy texture and very slight composition changes
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As metamorphic grade increases, what typically happens to a rock
Stability increases through the loss of volatiles, new minerals form at the expense of others
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Metasomatism
the alteration of rocks through fluids, such as water or hydro thermal comes incontact with the rock
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Why is pressure not linearly related to temp?
If it was, all rocks would melt at a certain point. This is why we get polymorphs
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What are the limits of metamorphism
Temperature

* Low T limit (minerals that are not stable at earths surface 100C)
* High T limit ( when a rock reaches solidus, the first melt starts to form)

Pressure

* Pressure in the crust increases by 1 kb for 3.3 km of burial

How fast does T increase with depth

* Geothermal gradient (oceanic crust and continental crust has different geothermal gradient)

Continental crust is thinner than oceanic
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Lithostatic stress:
Confining pressure equal in all directions

\-forms a rock that doesn’t have a preferred mineral orientation (burial)
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Deviatoric stress
Differential stress ( pressure is stronger in a certain direction)

(regional metamorphism)
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If differential stress is present during metamorphism
* can greatly affect texture of the rock


1. Rounded grains can become elongated
2. Minerals that crystallize have a preferred orientation
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Regional metamorphism
occurs over a wide area and affects a large rock volume

Ocean floor spreading etc
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Local metamorphism
Limited areal extent

rock directly touches lava
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Cataclastic metamorphism
Metamorphism due to mechanical deformation when two bodies of rock slide past one another along a fault
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Prograde metamorphism= progressive
Increasing T gives rise to mienrals that are typically a higher grade, this occurs when the rock is increasing in depth and getting buried

\
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Retrograde metamorphism =retrogressive
Decreasing T forms minerals typical of a lower grade than the former mineral assemblage

* as P and T fall the rocks would follow a reverse path
* it is not common, if it was we wouldn’t see metamorphic rocks at the surface of the earth

Why? chemical processes occur slower at higher elevations, and there is more water so this also prevents retrograde metamorphism
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Index mineral:
Metamorphic mineral that is used to characterize a certain zone of a metamorphic grade and is an indicator of progressive metamorphism
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Isograd
Surface along a metamorphic rock sequence shown by a line or defined by appearance or disappearance of a certain mineral
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What is the mineral assemblage of a rock indicative of?
Of the temperature and pressure the rock was subjected to
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A metamoprhic facies is characterized by mineral associations which are indicative of the
Formation and P/T undergone by the rock during the formation processes
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Foliated rocks
Grains platy or elongate and orientated parallel to each other to make a planar fabric
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What are the regional metamorphism states achieved ?
Shale Slate schist Phyllite Gneiss Migmatite
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Slate
Elongate platy mineral, breaks smoothly along planes
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Granoblastic metamorphic texture
Grains are equidimensional with platy and elongate minerals so subordinate so foliation is not developed
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Hornfelsis
granoblastic associated with thermally metamorphosed rocks
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Cataclastic
Clastic textures resulting from breaking or grinding usually at faults
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Quartzite
non-foliated recrystallized sandstone

Indicative of prior sedimentary structures

contact or regional metamorphism

Is SiO2
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Barrovian series: (medium pressure)
Regional, folliated in peltic rocks

Chlorite is lowest grade

Sillimanite is highest grade

Uses index minerals

(Not the same as metamorphic facies)
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Why is quartz not an index mineral?
It is present everywhere, it does not define a zone and does not disapear when entering the next zone
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What occurs in the barrovian series to make a mineral an indicator mineral
* It occurs at a particular zone
* It doesn’t disappear when u go to the next zone
* the texture of rocks changes
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What are peltic rocks more sensitive to over basic rocks?
changes in P-T conditions
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Buchan series (lower pressure)
Characterized by presence of andalusite and cordierite, occurs at lower pressure and intermediate grade minerals

regional metamorphism associated
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Contact metamoprhism series
* commonly produces hornfels
* found in aureoles (associated with plutons)

Low pressure high temperature (touching lava etc)
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Fransiscan series:
High pressure low T, produces bluschist