Metamorphism of oceanic crust Flashcards
What is the original rock type in oceanic crust?
Usually basalt
How are hydrous minerals formed?
Hydration of basalt, occur at any time, no need for deformation
Describe hydrothermal metamorphism
Cold seawater penetrates depths, replaced by hot fluid which returns to surface, creates convective scheme with large fluid volumes, significant retrogression occurs as crust moves away from heat
Name two common mineral alterations
Chlorite rich (Mg gain, Ca less) Epidote rich (Ca gain, Mg less)
Where does subduction occur and how does it work?
Collision zones with old cold dense ocean lithosphere being taken to depth. Cold water taken to depth to creating large geothermal gradient
What two facies does subduction create?
Blueschist - (glaucophane)
Eclogite - (garnet and pyroxene)
How do blueschists form?
At depth with low geothermal gradient
How do blueschists reach surface?
Underplating of accretionary prism causes uplift of blueschist while keeping low geothermal gradient
What happens to blueschist during dehydration?
Transition to amphibolites and eclogites
What are eclogites normally made of?
Metabasic (garnet and pyroxene) or Pelite (garnet, kyanite, Si rich white mica)
When do the three types of eclogites occur?
Xenoliths, high temp
blueschist dehydration, low temps
granulite or amphibolite facies, med temp, linked to crustal thickkening
What were crustal eclogites, what special mineral do they possess and how are they formed?
Originally basic intrusions, contain coesite which is a polymorph of quartz formed in deep subduction
What happened to eclogites on uplift?
Retrogressed to plag bearing gneisses
Explain the big density transitions that eclogites undergo
Rapid burial creates dense crustal root, delays uplift, slow heating, converts root to less dense rock, triggers uplift
