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Oceanic Lithosphere Evolution (Lanzo Massif (Oceanic exhumation (of…
Oceanic Lithosphere Evolution
Lanzo Massif Ophiolites evolution
= inferred from studying mineral stability
Lanzo
Massif
Oceanic exhumation
of peridotite in ocean-continent transition i.e. rifting
mantle rocks brought to surface & cooled in process
Oceanic cooling
& partial hydration
Subduction
Re-exhumation
following subduction (during continental collision?)
Ophiolite obducted & deformed
Ultramafic rocks record 4 geodynamic stages
Deriving
P-T Path
Serpentine mineral stability
only allow constraint of temperature ∆
Other minerals' stability
required to constrain pressure ∆
e.g. chlorite, amphibole, talc
Parageneses
= set of minerals which formed together
Serpentinite Phase Evolution
Lizardite
Hydrothermal alteration
of hydrated ultramafic peridotites
during
ocean expansion
(rifting)
Antigorite
(atg1)
Lizardite recrystallises to form antigorite
during alpine
subduction
Antigorite
(atg2)
crystallised from previous minerals
i.e. 1° atg, 2° oliving
retrograde serpentinisation during
massif exhumation
n.b. secondary olivine
forms by dehydration of atg 1 during prograde metamorphism (increasing depth)
Prograde vs retrograde metamorphism
Mineral assemblage / composition ∆
Prograde - burial & heating
(z increasing)
Retrograde - uplift & cooling
Cross-cutting antigorite
Indicates rock underwent
oceanic exhumation / cooling / subduction to form antigorite 1
subduction past antigorite 1 field
--> to form crosscutting antigorite 2