Download Metamorphism of Ultramafic (UM) Rocks

Geol 285 - Dr. Helen Lang, West Virginia University, Spring 2003
Metamorphism of Ultramafic (UM) Rocks
Most ultramafic rocks come from Earth’s mantle
by definition UM rocks contain >90% mafic minerals
Original Mineralogy of UM Rocks:
Major Minerals
• Olivine
• Orthopyroxene
• Clinopyroxene
Ol
Opx
Cpx
Rock Names
• Dunite
• Harzburgite
• Lherzolite
>90% Olivine
Olivine + Opx
Olivine + Opx + Cpx
(Mg,Fe)2SiO4
(Mg,Fe)SiO3
Ca(Mg,Fe)Si2O6
Many UM rocks are part of Ophiolites
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Special sequence of rocks thought to represent a cross section of oceanic crust and
upper mantle formed near mid-ocean ridges
The Ophiolite Sequence
Deep sea pelagic sediments
Pillow basalts
Basaltic sheeted dike complex
Layered gabbros
Deformed ultramafic rocks (upper mantle)
The Ophiolite Sequence
How do Ultramafic Rocks get incorporated in the Crust?
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Obduction = overthrusting of big blocks, uncommon
Oceanic crust and upper mantle (ophiolites) get caught between colliding continental
blocks; Alps, Himalayas, Appalachians
Oceanic crust and upper mantle get scraped off in the subduction zone;
“dismembered” ophiolites
How do Ultramafic Rocks get incorporated in the Crust?
Most UM Rocks get Serpentinized as they are incorporated into continental
crust
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Serpentinization
Olivine + H2O = Serpentine
Mg2SiO4 + H2O = Mg3Si2O5(OH)4
Mantle UM rocks are high grade metamorphic rocks; serpentinization is a retrograde
(changing from higher grade to lower grade) metamorphic process
Retrogression usually involves hydration
Serpentine in Thin Section
We will look at changes in a serpentinite caused by increasing metamorphic
Temperature
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A serpentinite is a rock composed mostly of serpentine
Serpentine is a sheet silicate (2 main kinds)
– chrysotile: What do you know about this?
– antigorite: The main metamorphic serpentine
Metamorphism caused by increasing T is called prograde metamorphism
Metamorphosed UM Rocks are not especially common; why discuss them
now?
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They can be represented by a simple chemical system: CaO + MgO + SiO 2 + H2O
Changes mostly reflect increasing Temperature
Metamorphic chemical reactions in UM rocks are mostly dehydrations
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Dehydration (loss of H2O) is the main effect of most prograde metamorphism!!
We need a new Triangle!
CaO, SiO2, MgO triangle for UM Rocks
Di (Cpx) CaMgSi2O6
En (Opx) MgSiO3
Fo (Ol) Mg2SiO4
Other Minerals that can occur in UM Rocks
Antigorite (Atg) ~Mg3Si2O6(OH)4
Tremolite (Tr) Ca2Mg5Si8O22(OH)2
Talc (Tlc) Mg3Si4O10(OH)2
Quartz (Qtz) SiO2
Anthophyllite (Ath)
Mg7Si8O22(OH)2
Appalachian UM bodies = suture zone
Minerals of a low-grade serpentinite
P-T Phase Diagram for UM Rocks
from B to C (rxn Atg+Brc=Fo+H2O)
from C to D (rxn Atg+Di=Fo+Tr+H2O)
from D to E (rxn Atg=Fo+Tlc+H2O)
Notice limited range of Talc+Fo(Ol)
Olivine-Talc Rock with “jackstraw” texture
Almost all reactions are Dehydrations: H2O always on hi T side; curved as
shown
Dehydration is one of the Major effects of metamorphism!
Above about 900oC all UM rocks have completely anhydrous mantle
assemblage