Veins and Porphyroblasts

Question 1

How do strain fringes form?

Answer 1

Growth of crystals next to a rigid object. Area where growth takes place is known as the fringe.

Question 2

How do veins form?

Answer 2

Rock breaks and crystals start to fill the space. Crystals grow continuously while the veins opens from centre outwards.

Question 3

Describe how blocky crystals form in veins

Answer 3

Veins open faster than the crystals can grow. Crystals develop facets, blocky growth.

Question 4

Describe how stretched crystals grow

Answer 4

Veins open faster and in large steps but some crystals reach the wall rock. Crystals cannot grow completely free, they become stretched.

Question 5

Describe how fibrous crystals grow

Answer 5

Veins open slower than crystals grow, crystals reach wall rock. If the wall rock is rough the grain boundaries track the opening history.

Question 6

What is antitaxial growth?

Answer 6

Vein is made up of material that is different as host-rock, growth from the centre outwards

Question 7

What is syntaxial growth?

Answer 7

Vein contains same material as host rock, growth is from host-rock inwards, vein opens always in the centre

Question 8

What is ataxial growth?

Answer 8

Vein contains same host-rock but fracture surface where the vein opens varies. Random fracturing leading to stretched crystals.

Question 9

What is meant by antitaxial and syntaxial growth in fringes?

Answer 9

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

Question 10

What happens during irrotational strain?

Answer 10

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.

Question 11

How do inclusion bands grow and what do these represent?

Answer 11

They mirror the vein fracture surface and give the opening direction. Each band represents a crack event.

Question 12

How do veins form during rotational strain?

Answer 12

Form tension gash geometries. Will open in the extensional ISA direction (like an extension crack)

Question 13

How do veins form during progressive deformation?

Answer 13

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.

Question 14

Where are tension gashes often found?

Answer 14

Next to large faults

Question 15

How do styolites form and how do they appear?

Answer 15

They develop during localized dissolution of a rock.

They appear as dark and rough seams.

Question 16

What direction is the styolite plane orientated?

Answer 16

Perpindicular to sigma 1

Question 17

What direction do styolite teeth grow?

Answer 17

Parallel to sigma 1

Question 18

How does styolite roughness develop?

Answer 18

Small particles that do not dissolve pin to the surface.

Question 19

What are porphyroblasts?

Answer 19

Minerals that grow during metamorphic reactions. During their growth they can capture the tectonic fabric of the rock.

Question 20

What is a strain cap in a porphyroblast?

Answer 20

Form at top of porphyroblast perpendicular to compressional ISA.

Question 21

Describe pretectonic porphyroblast growth

Answer 21

Porphyroblast grows before deformation occurs. Foliations then form around the rigid object. One metamorphic event (growth of porphyroblast) is followed by tectonic events (foliations)

Question 22

Describe what happens during intertectonic porphyroblast growth

Answer 22

This means we have a tectonic event followed by a metamorphic event. During this metamorphic event there is no further tectonic deformation.

Question 23

What is meant by syntectonic growth?

Answer 23

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

Question 24

What is meant by a post tectonic porpphyroblast?

Answer 24

Grows when the deformation is over.