Week 19 Flashcards
Importance of intrusion
Magmatic = abundant rock type in continental crust
Reflects process of thermally-driven differentiation of Earth’s crust
Derived from mantle/lower crust = info on nature of crust
Important mineral deposits
Reflect melting/transport/crystallisation processes associated with tectonic belts
What is most continental crust made up of?
Large no. of granite plutons
= space problem in emplacement zone/source region
‘Bottle of red ink’
Gabbro glacier model
All lower crust crystallises in high level melt lens
Subsides down and away from ridge axis
Multiple sill model
Lower crust forms by intrusion of multiple melt lenses throughout lower crust
Gabbro glacier or multiple sill?
Hybrid
Can form décollements = thin-skinned deformation = lower orogenic slope
Upper crustal magmatic system terms
Sill Dyke Lopolith Laccolith Stepped geometries/fingers
Lopolith
Space = erosion
Lacolith
Space = source volume withdrawal
Why does deformation occur near intrusions?
To accomodate intrusions = ‘instantaneous’ localised extension
At intrusion margins
Depends on strength of host rock
= stepped/faulted margin as complex builds up
Moving magma out of the source region
G-driven compaction too slow for viscous melts = fracturing is 1’ mechanism
Intrusion and driving forces in magmatic intrusions
- Melting = source region porous and dilates
- fluid-absent melting = up to 15% volume increase - = hydraulic overpressure in source
- = doming + therefore uplift of Earth’s surface
- = radial stress field with vertical tensile cracks/dykes/conduits
- Buoyancy forces drive magma up conduit
Ascent and emplacement in magmatic intrusions
Feedback loop set up:
- initial dyke feeds sill = releases excess mass volume (EMV) in source
- sill supports roof
= floor decoupled and subsides back into source
= expels magma - = increase sill size and dilates conduit
How do emplacement-accommodation space mechanisms vary with depth?
SHALLOW = doming
DEEPER = laterally
- overburden too much for doming
- tectonic forces required
DEEPER STILL = country rocks move down
- = diapirism/cauldron subsidence
Aphantic vs holocrystalline
Aphantic = fine grained, surface
Holocrystalline = below surface
Intrusion systems
Central piston/satellite bodies
Multilayer complex
Multi-feeder laccolith complex
Halle-type
Donnerberg type
Bysmalith
Saucer-shaped sills
How far can intrusions travel?
?! 1000s km horizontal transport
Dyke =
‘instantaneous’ localised extension
Vertical extent of intrusions
Stack of tabular shaped intrusions i.e. not ‘bottomless’
- competition between horizontal lengthening and vertical thickening
Mud volcanoes =
over-pressured shale = intrusion/extrusion
Salt intrusion =
salt = low viscosity/density
= diapirs/veneers/glaciers
“Allochthonous salt”
CAN OCCUR UNDER COMPRESSION AND EXTENSION
e.g. South Caspian Basin
Active salt intrusion
Salt pushes up
Reactive salt intrusion
Faults = buoyancy
Passive salt intrusion
Sedimentation and loading
Speed of sedimentation affects shape
Salt withdrawal minibasin
Forms above salt
Subsides as salt flows away from depocentre
Salt and seismic sections
Does not reflect well
Salt intrusions; sedimentary and petroleum implications
Sandstone (reservoir) adjacent to salt because deposited same time
= “controlled sedimentary architecture”
Salt diapirs in a contractional regime
Diapir pre-existing
= pedestal, salt weld and rotated flap
Subduction-related magmatism: Cordilleran batholiths
Large regions in W N/S America
e.g. Patagonian batholith
Compressional regime = sill favoured rather than dykes = no volcanism
Caledonian Late Orogenic granites
Emplacement controlled by crustal scale lineaments e.g. transtension
= tectonic-controlled
Intruded late in orogenic cycle
Examples in Ireland
Cornubian Batholith
Tectonic controls
Variscan convergence = S-S
Post Variscan extension = emplacement of basalt sills
Cornwall granites
Radioactive (K/U) decay = drill = heat
Ketilidian Orogen, South Greenland
3 different suites of intrusion:
- Cordilleran-style = arc-magmatism (mafic)
- Forearc partial melting of sedimentary rocks
- Post-orogenic; deep lithosphere alkaline
- Rapakibi granites - perialkaline
- >50km2 tabular shaped granite intrusions
- evidence for floor depression
