W8L2 Metamorphic Facies Flashcards
Metamorphic Facies definition
A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. … The boundaries between facies (and corresponding areas on the temperature v. pressure graph) are wide because they are gradational and approximate
Metamorphic Grade ablauf
Lithification
low temp, low pressure
Low Grade Metamorphism → intermediate → metamorphic grade
Pete Envelope
bordered by partial melting in wet rock and partial melting in dry rock
water has big effect on melting in metamorphic rocks!
in this area very high grade dry rock or partial melting of wet rock
in presence of water can melt at much lower temps!
Prograde
prograde: increase burial path, into high grade
most metamorphic reactions are prograde
Retrograde
retrograde: return path, back to low grade bcs slow uplift of the crust
minerals can grow anywhere on this path
but time as key component, the faster the less “memory” so doesnt show record if this stage
Peak metamorphism
Peak metamorphism: maximum metamorphic condition which the system/ rock experiences
Zeolite . Facies on graph
upper left corner
lithification area
low temp low pressure
PP on graph
just below Zeolite and above Blueschist
left bordeer is “notpossible in nature”
right border is greenschist
greenschist on graph
further right so about 200-200 T zone left border: blueschist and PP right bordeer: Amphibolite bottom border: Eclogite goes down to like 8 Pa
Blueschist on graph
below PP and greenschist on graph
bottom border: Eclogite
stays by “not possible in nature” border on left
goes to about 8 Pa then eclogite, starts . at about 6Pa
Eclogite on graph
has highest pressure . at cold temp
so runs along bottom of graph till Granulite
top border amphibolite and greenschist
Amphibolite on graph
kinda the middle of the graph under hornfels above eclogite left broder greenschist right border granulite
Granulite
furthest right of the graph
almost takes up whole right bottom sectrion
kinda borders Hornfels at top
startsin hotter temp, all Pressure fields and depth
Hornfels on Graph
stretched along top of the graph as this like think layer till 900 Temo then Granulit etakes over
never lower than 2 Pa
Pathway 1 on graph
Conditions of thermal metamorphism
runs thru hornfels , stays at top till 10km at granulite
Pathway 2 on graoh
Condition beneath mountain belts
could almost say it goes straigh thru diagram from top left to bottom right
with lsight ciurve
pathway 3 on graph
conditions beneath stable continental interior
starts top left, then pretty stable fall
ends shortly after wet melting line
pathway 4 on graph
Condition in an accretionary plane
goes from top left steep down, kinda stays along left side, just off “not possible” zone
Moderate to high temp and high pressure . Facies
Regional Metamorphic Rocks • produced during mountain building episodes • result of burial & deformation • affect large portions of the Earth’s cr With increasing T & P … • greenschist facies • amphibolite facies • granulite facies metamorphosed rocks in mountain belts
Metamoprhic Index Materials
how to tell which grade a mineral is in
different minerals are stable at different T & P
index minerals are common minerals with well understood P & T stabilities
Barrovian Zones
By mapping the occurrences of index minerals in regionally metamorphosed rock zones of equal grade can be determined, separated by isograds
Areas of maximum grade
Areas of maximum grade indicate areas which have been most deeply buried
can reconstruct how crust ect have formed
High pressure - low temp facies
Subduction Zone Metamorphism • associated with subducting slabs • preserved in subduction trench environments • require rapid uplift for preservation With increasing P … • blueschist facies • eclogite facies (very HP, HT) low temperature facies
Low presure to high temp facies
Thermal (Contact) Metamorphic Rocks
• produced as a result of magmatic heating
• affect rocks local to the intrusion only
textures in thermally metamorphosed rocks
In lower grade parts of the aureole slatey rocks are formed
These often contain porphyroblasts of cordierite and/or andalusite
“spotted slates”
