Metamorphism - Lecture 7

Metabasites

Metamorphic rocks of basaltic composition

Protoliths:

Mineralogy of protolith: ol, opx, cpx, plag, hbl, minor biot, minor qtz.

Chemistry: 8 components

Mineralogy of metabasites:

Graphical representation:

 

Project from Na2O - use ACF diagram

 

Difficulties with metabasites:

1. many chemical components, but few minerals

-Amphiboles, Chlorite - assemblage doesn't change, but composition does

2. Fluid is CO2 - H2O mixture - this causes complications in that must worry about XH2O in dehydration reactions


Mafic protoliths - diagnostic assemblages for different facies

I. Regional metamorphism (lots of continuous reactions)

1. Greenschist - (chlorite/biotite zone in pelites)

Diagnostic: first appearance of actinolite

actinolite + albite + epidote + chlorite quartz

2. Albite-epidote amphibolite facies (gt. zone in pelites)

hornblende + albite + epidote + chlorite quartz

(Diagnostic: hornblende is Na-rich)

3. Amphibolite facies (staurolite, kyanite, sillimanite zones in pelites)

hornblende + plagioclase quartz garnet cpx (plagioclase is An25-40)

Diagnostic:

4. Granulite (sillimanite + kspar zone)

Plagioclase + opx + cpx hornblende quartz

Diagnostic: appearance of opx, coexistence of plag with cpx and opx

II. Subduction zone metamorphism

5. Blueschist facies = glaucophane schist

glaucophane + lawsonite jadeite aragonite

6. Eclogite

omphacite + pyrope-rich garnet quartz glaucophane

III. Other

7. Zeolite facies

zeolites (wairakiite, laumontite)

Characteristic of ocean floor metamorphism, or shallow burial.

Diagnostic: presence of a zeolite mineral, but can also have prehnite + pumpellyite

8. Prehnite-pumpellyite facies

Distinguished by lack of zeolites and lack of actinolite

 


Ocean Floor Metamorphism

Large-scale infiltration metamorphism by hydrothermal fluids

Cartoon of hydrothermal metamorphism

If ocean is at 0C and magma chamber is at 1200C, there is a very strong thermal gradient which forms a pressure gradient. Get very strong circulation currents. Cold water infiltrates into cracks, migrates down and is heated up by magma chamber, then rises back up to feed "black smokers".

Ophiolites as analogues

Pillows: zeolite facies (laumontite, smectite, chlorite, albite)

Sheeted dikes: greenschist facies (albite, epidote, actinolite, chlorite)

Gabbros: amphibolite facies to fresh magmatic rock (plag, hornblende)

 


Subduction Zone Metamorphism

How is a blueschist assemblage preserved?

Typically must be attached or underplated in an accretionary wedge

If it stays too long in the accretionary wedge, it may heat up and change to blueschist facies (see P-T path)

Recent models suggest that the best way to get blueschists up to the surface quickly is via normal faults