Lecture 15 intro to metamoprhic rocks +16 +17

Transformation of prexisting rocks under

-pressure

-temperature

-fluids

Solid. The mineralogy texture and very slight composition changes

Stability increases through the loss of volatiles, new minerals form at the expense of others

the alteration of rocks through fluids, such as water or hydro thermal comes incontact with the rock

If it was, all rocks would melt at a certain point. This is why we get polymorphs

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

Confining pressure equal in all directions

-forms a rock that doesn’t have a preferred mineral orientation (burial)

Differential stress ( pressure is stronger in a certain direction)

(regional metamorphism)

• can greatly affect texture of the rock

1. Rounded grains can become elongated

2. Minerals that crystallize have a preferred orientation

occurs over a wide area and affects a large rock volume

Ocean floor spreading etc

Limited areal extent

rock directly touches lava

Metamorphism due to mechanical deformation when two bodies of rock slide past one another along a fault

Increasing T gives rise to mienrals that are typically a higher grade, this occurs when the rock is increasing in depth and getting buried

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

Metamorphic mineral that is used to characterize a certain zone of a metamorphic grade and is an indicator of progressive metamorphism

Surface along a metamorphic rock sequence shown by a line or defined by appearance or disappearance of a certain mineral

Of the temperature and pressure the rock was subjected to

Formation and P/T undergone by the rock during the formation processes

Grains platy or elongate and orientated parallel to each other to make a planar fabric

Shale Slate schist Phyllite Gneiss Migmatite

Elongate platy mineral, breaks smoothly along planes

Grains are equidimensional with platy and elongate minerals so subordinate so foliation is not developed

granoblastic associated with thermally metamorphosed rocks

Clastic textures resulting from breaking or grinding usually at faults

non-foliated recrystallized sandstone

Indicative of prior sedimentary structures

contact or regional metamorphism

Is SiO2

Regional, folliated in peltic rocks

Chlorite is lowest grade

Sillimanite is highest grade

Uses index minerals

(Not the same as metamorphic facies)

It is present everywhere, it does not define a zone and does not disapear when entering the next zone

• It occurs at a particular zone

• It doesn’t disappear when u go to the next zone

• the texture of rocks changes

changes in P-T conditions

Characterized by presence of andalusite and cordierite, occurs at lower pressure and intermediate grade minerals

regional metamorphism associated

• commonly produces hornfels

• found in aureoles (associated with plutons)

  Low pressure high temperature (touching lava etc)

High pressure low T, produces bluschist