Streckeisen diagram

Question 1

Granite group

Answer 1

-Usually 80-100 % felsic minerals
-Usually 0-20 % mafic minerals (most commonly biotite)
-20-60 % quartz of the felsic minerals
-40-80 % feldspar of the felsic minerals (of which 35 to 100 % alkalifeldspar)
-Often dotted appearance due to black biotite
-The feldspar may be red and / or white
-Quartz is colourless, it looks grey
-Medium to coarse-crystalline(crystals in mm to cm size)
-Often occurs along subduction zones
-Forms after partial melting of water-bearing crustal rocks:
I-type granite: formed after melting of igneous rocks
S-type granite: formed after melting of sedimentary rocks

Question 2

Syenite group

Answer 2

• Usually 80-100 % felsic minerals
• Usually 0-20 % mafic minerals (most commonly hornblende)
• 0-20 % quartz of the felsic minerals
• 80-100 % feldspar of the felsic minerals (of which 65-100 % alkalifeldspar)
• Often red due to high K feldspar content
• Usually very coarse-crystalline(crystals in cm size)
• Often occurs in thick continental crust
• Forms after (only little) partial melting of felsic, crustal material

Question 3

Diorite group

Answer 3

• Gabbro: An>50= more than 50 % of the plagioclase is anorthite (Ca plagioclase)
  Diorite: An<50
  -Usually 50-85 % felsic minerals
• Usually 15-50 % mafic minerals (most commonly hornblende)
  -0-5 % quartz of the felsic minerals
  -95-100 % feldspar of the felsic minerals (of which 90-100 % plagioclase)
  -Often black-white (or grey-green), dotted appearance due to white (or greyish) plagioclase and black (or green) hornblende
• Fine-crystalline(crystals in mm size)
  -Often occur along continental volcanic arcs (such as the Andes)
  z-Forms after partial melting of mafic rocks along subduction zones

Question 4

Gabbro group

Answer 4

Gabbro

• Usually 35-80 % felsic minerals
• Usually 20-65 % mafic minerals (most commonly augite and / or olivine)
• < 5 % quartz (very rare) of the felsic minerals (NEVER together with olivine)
• 95-100 % feldspar of the felsic minerals (of which 90-100 % plagioclase)
• Often black due to the high content of mafic components
• Medium to coarse-crystalline(crystals in mm to cm size)
• Occurs in the oceanic crust(underneath the pillow basalt & the sills)
• Forms along mid-ocean ridges

Question 5

Monzonite group

Answer 5

• Compositionally between syenite & gabbro / diorite
• Usually < 80 % felsic minerals
• Usually > 20 % mafic minerals (most commonly hornblende or biotite)
• 0-20 % quartz of the felsic minerals
• 80-100 % feldspar of the felsic minerals (of which 35-65 % alkalifeldspar)
• Often dotted but darker than both granite and syenite
• Medium to coarse-crystalline(crystals in mm to cm size)

Question 6

Larvikite - Monzonite group

Answer 6

Larvikite: variety of monzonite:

• cm-size feldspar
• alternating alkalifeldspar and plagioclase lamellae(in the same crystal)
• Schiller effect (blue twinkling of light on polished surfaces)
• Occurs ONLY in the Oslo rift(in the whole world)
• the national rock of Norway-Associated with the formation of the Permian Oslo rift
• Crystallisation in the lower crust

Question 7

Granodiorite

Answer 7

Question 8

Tonalite

Answer 8

Question 9

Rhyolite group

Answer 9

-Composition (& occurrence) like granite:
>80 % felsic minerals of which quartz & alkalifeldspar dominate
- Varieties (different fabric): Quartz porphyry - with quartz phenocrysts
Pumice - glass + macroscopic vesicles
Obsidian - glass + microscopic vesicles

Question 10

Andesite and basalt group

Answer 10

-Composition (& occurrence) like diorite & gabbro:
plagioclase + mafic components (hornblende [in andesite], augite, olivine [in basalt])
- They may (or may not) contain phenocrysts
-Variety: scoria -glass with vesicles

Question 11

Latite group

Answer 11

-Composition and occurrence like monzonite / larvikite:
between trachyte / syenite & basalt / gabbro
alkalifeldspar & plagioclase + some mafic components
-Variety: rhomb porphyry (rombeporfyr) -with rhombic feldspar phenocrysts

Question 12

Phonolite

Answer 12

• Clinkstone
• Dominated by foids(also called feldspathoids) and alkalifeldspar
• Mafic components: usually biotite, amphibole, pyroxene, or olivine
• NO quartz
• Often greyish
• May contain foid phenocrysts (nepheline, leucite)
• Phonolite never contains glass
• Occurs in many plate-tectonic settings
• Forms after partial melting of mantle rocks or continental rocks