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Serpentine (Ophiolite (= remnant of oceanic crust & upper mantle,…
Serpentine
Ophiolite
= remnant of oceanic crust & upper mantle
obducted
onto continental crust often
exposed
above sea level
often rich in serpentine minerals
n.b. obduction
= overthrust
Minerals
Lanzo Massif
Original minerals
may still be evident i.e. pyroxene
Greenish to yellow/orange
Mineral alteration
can be pervasive
i.e. chlorite = low grade metamorphic product
Antigorite
- two phases with cross cutting relations
antigorite veins cut fibrous antigorite
Fracture/veining/foliation
Evolution of
Oceanic Lith Fragment
P-T path of ophiolite reconstructed
from serpentine mineral relations
Serpentine minerals
-
only allow temperature reconstruction
Pressure reconstruction - requires stability of other minerals be studied
Vertical lines on stabilithy diagram
shows only temp constrained by serpentine phase stability
Serpentine Formation
via hydration of Mg silicates i.e. olivine & pyroxene
mantle rock + water ---> serpentinite
density reduction accompanies formation (3.3. - 2.7 g/cm^3)
Serpentine Stability
Serpentine mineral type present
= f ( temperature )
∴ Pressure has little effect
i.e. straight vertical lines separate mineral stability fields
Lanzo Massif,
W. Alps
Peridotite massif
= fragment of oceanic lithosphere
generated in oceanic-continental transition
eclogitised during alpine collision
Ophiolite
- obducted & deformed
Sedimentary cover
= preserved on massif during subduction history
Slightly serpentinized peridotites
surrounded by foliated serpentinite envelope
bordered by oceanic metabasalts & metasediments
(weathered mantle rock)
Serpentine Phases Present
- Lanzo Massif
Early lizardite
present in peridotites
∴ serpentinisation began during oceanisation
= confirmed by meta-ophino carbonates bordering it
Prograde
antigorite
partially replaced lizardite / relict primary peridotite minerals
indicates serpentinisation @ ridge / in subduction zone
Retrograde Antigorite
formed during exhumation
partially / fully obliterating previous textures in massive/foliated serpentinites
3 kbar = 10 km depth