Studied by 8 people
How are metamoprhic rocks formed?
Transformation of prexisting rocks under
-pressure
-temperature
-fluids
During metamorphic process what form do rock stay in?
Solid. The mineralogy texture and very slight composition changes
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
Metasomatism
the alteration of rocks through fluids, such as water or hydro thermal comes incontact with the rock
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
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
Lithostatic stress:
Confining pressure equal in all directions
-forms a rock that doesn’t have a preferred mineral orientation (burial)
Deviatoric stress
Differential stress ( pressure is stronger in a certain direction)
(regional metamorphism)
If differential stress is present during metamorphism
can greatly affect texture of the rock
Rounded grains can become elongated
Minerals that crystallize have a preferred orientation
Regional metamorphism
occurs over a wide area and affects a large rock volume
Ocean floor spreading etc
Local metamorphism
Limited areal extent
rock directly touches lava
Cataclastic metamorphism
Metamorphism due to mechanical deformation when two bodies of rock slide past one another along a fault
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
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
Index mineral:
Metamorphic mineral that is used to characterize a certain zone of a metamorphic grade and is an indicator of progressive metamorphism
Isograd
Surface along a metamorphic rock sequence shown by a line or defined by appearance or disappearance of a certain mineral
What is the mineral assemblage of a rock indicative of?
Of the temperature and pressure the rock was subjected to
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
Foliated rocks
Grains platy or elongate and orientated parallel to each other to make a planar fabric
What are the regional metamorphism states achieved ?
Shale Slate schist Phyllite Gneiss Migmatite
Slate
Elongate platy mineral, breaks smoothly along planes
Granoblastic metamorphic texture
Grains are equidimensional with platy and elongate minerals so subordinate so foliation is not developed
Hornfelsis
granoblastic associated with thermally metamorphosed rocks
Cataclastic
Clastic textures resulting from breaking or grinding usually at faults
Quartzite
non-foliated recrystallized sandstone
Indicative of prior sedimentary structures
contact or regional metamorphism
Is SiO2
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)
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
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
What are peltic rocks more sensitive to over basic rocks?
changes in P-T conditions
Buchan series (lower pressure)
Characterized by presence of andalusite and cordierite, occurs at lower pressure and intermediate grade minerals
regional metamorphism associated
Contact metamoprhism series
commonly produces hornfels
found in aureoles (associated with plutons)
Low pressure high temperature (touching lava etc)
Fransiscan series:
High pressure low T, produces bluschist
Note 
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