Download Magma Genesis in Orogenic Belts

MAGMA GENESIS IN OROGENIC BELTS
E1:K1:a)
Melting of rock material at depth to form magma occurs in a
number of different plate environments.
iii)`
In deeply buried lower continental crust during orogeny - melting
of crustal material generates granitic magma
Preview/Review:
Earth p380 Geoscience p58, 65-67
References:
Inaccessible Earth: Brown and Mussett
Websites:
http://www.ucl.ac.uk/~ucfbrxs/B164/Plates.htm
Eg Cornubian batholith, Cornwall, Himalayas
Collision of 2 continents complex process
Late stages of ocean closure:ANDESITIC magmas rise and join to form
DIAPIRS which rise into upper crust
As more and more diapirs rise, over-riding plate heats up
Heating leads to partial melting of early formed diorites, producing GRANITIC
magmas
Low density (2.4-2.6) viscous granitic magmas rise slowly through denser
crust (2.9)
Magma reaches equilibrium around 3-5km depth and cool to form large
coarse grained granite intrusions
Compression leads to thrust faulting and intense folding
Result is thickened crust - base deep enough for partial melting, generating
more granitic magma
PLUTONS/BATHOLITHS
Shallow FORCEFUL INTRUSIONS
sharp contact with country rocks
opens up joints + faults intruded by late stage aqueous magma rich in
dissolved ions forming veins with 'giant' crystals (several cm) PEGMATITES
and HYDROTHERMAL VEINS
Deep PERMITTED INTRUSIONS
diffuse contact with country rocks
assimilation of xenoliths by stoping of country rocks
GRANITIZATION; sediments changed to granite by invasion of volatile hot
gases and solutions from depth along microfractures ie METASOMATIC
REPLACEMENT
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