Subduction Zone and High Pressure Rocks

Subduction Zone and High Pressure Rocks

Granulite Facies

• High T, P

• Base of Crust

• T > 700 C

• Very anhydrous

• Dry conditions inhibit melting

About 600 C

About 680 C

About 700 C

About 750 C

Subduction Zone Rocks

Steinmann Trinity (1905)

• Serpentinite

• Pillow Basalt

• Radiolarian Cherts

• Characteristic of ophiolite settings

• Other Distinctive Rocks– Graywacke and Flysch– Blueschist

Bedded Chert

Serpentinite

Serpentinite

Magnesite Mines in Serpentinite

Pillow Lava

Pillow Lava

Pillow Lava

Blueschist

Graywacke

Flysch

Melange

Melange

Coesite in Quartz• Not very dramatic

• Rare

• Relict in inclusions, often in garnet

• Mostly reverts to quartz

• Distinctive “palisade” texture in quartz

• Radial fractures due to 10% volume increase

Coesite in Gneiss

Palisade Texture in Quartz

Garnets

• Pyralspite series (Mg-Fe-Mn)– Pyrope Mg3Al2Si3O12

– Almandine Fe3Al2Si3O12

– Spessartite Mn3Al2Si3O12

• Ugrandite Series (Ca + Fe-Al-Cr)– Gossularite Ca3Al2Si3O12

– Andradite Ca3(Fe, Ti)2Si3O12

– Uvarovite Ca3Cr2Si3O12

Eclogite

• Start with Basalt: Labradorite (An + Ab) + Diopside + Olivine

• Ab breaks down to Jadeite + Quartz

• Jadeite + Diopside = Omphacite

• Anorthite + Olivine = Garnet

• Final composition:Omphacite + Garnet + Quartz

• Plagioclase no longer stable

Types of Eclogite• Type A: kimberlites, dunites, volcanic

pipes (Py 70%, Alm 20% T>900 C)– Metamorphism of mantle rocks

• Type B: high-pressure gneiss terranes (Py 45%, Alm 40% T=500-900 C)– Crustal thickening and deep metamorphism

• Type C: blueschists (Py 10%, Alm 60% T<500 C)– Next stage after blueschist metamorphism

Eclogite