Veins and Porphyroblasts Flashcards
How do strain fringes form?
Growth of crystals next to a rigid object. Area where growth takes place is known as the fringe.
How do veins form?
Rock breaks and crystals start to fill the space. Crystals grow continuously while the veins opens from centre outwards.
Describe how blocky crystals form in veins
Veins open faster than the crystals can grow. Crystals develop facets, blocky growth.
Describe how stretched crystals grow
Veins open faster and in large steps but some crystals reach the wall rock. Crystals cannot grow completely free, they become stretched.
Describe how fibrous crystals grow
Veins open slower than crystals grow, crystals reach wall rock. If the wall rock is rough the grain boundaries track the opening history.
What is antitaxial growth?
Vein is made up of material that is different as host-rock, growth from the centre outwards
What is syntaxial growth?
Vein contains same material as host rock, growth is from host-rock inwards, vein opens always in the centre
What is ataxial growth?
Vein contains same host-rock but fracture surface where the vein opens varies. Random fracturing leading to stretched crystals.
What is meant by antitaxial and syntaxial growth in fringes?
Antitaxial fringe growth - crystals in fringe different from the central object. Grows in the host rock towards the fringe, opening surface is at the object.
Syntaxial fringe growth - crystals in the fringe are the same as central object. Crystals grow on the object towards the host rock
What happens during irrotational strain?
Veins open as simple mode 1 extension fractures parallel to sigma 1 or they originate from the shear fractures that open later on. Fringes develop next to rigid objects where strain shadows form that lead to local extension sites. These strain shadows fill with material and become strain fringes that are being pulled away from the rigid object.
How do inclusion bands grow and what do these represent?
They mirror the vein fracture surface and give the opening direction. Each band represents a crack event.
How do veins form during rotational strain?
Form tension gash geometries. Will open in the extensional ISA direction (like an extension crack)
How do veins form during progressive deformation?
The vein will rotate (act ductile). At the same time the vein grows in length opening in the direction of the extensional ISA. The new parts of the vein are found iat the tips and these tips indicate the orientation of the ISA and thus the shear sense.
Where are tension gashes often found?
Next to large faults
How do styolites form and how do they appear?
They develop during localized dissolution of a rock.
They appear as dark and rough seams.
What direction is the styolite plane orientated?
Perpindicular to sigma 1
What direction do styolite teeth grow?
Parallel to sigma 1
How does styolite roughness develop?
Small particles that do not dissolve pin to the surface.
What are porphyroblasts?
Minerals that grow during metamorphic reactions. During their growth they can capture the tectonic fabric of the rock.
What is a strain cap in a porphyroblast?
Form at top of porphyroblast perpendicular to compressional ISA.
Describe pretectonic porphyroblast growth
Porphyroblast grows before deformation occurs. Foliations then form around the rigid object. One metamorphic event (growth of porphyroblast) is followed by tectonic events (foliations)
Describe what happens during intertectonic porphyroblast growth
This means we have a tectonic event followed by a metamorphic event. During this metamorphic event there is no further tectonic deformation.
What is meant by syntectonic growth?
Tectonic event starts and at the same time the porphyroblast starts to grow while the rock is still deforming. The centre of the porphyroblast will capture the older deformation whereas the outside will show more complex younger deformation
What is meant by a post tectonic porpphyroblast?
Grows when the deformation is over.
