Lecture 15 intro to metamoprhic rocks +16 +17

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

1. Rounded grains can become elongated

2. 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