Magnmas

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Part II: Magmatic processes

Partial melting

Melting:
• Partial melting of crustal materials
gives GRANITES

• Partial melting of mantle materials
gives BASALTS/ANDESITES

Partiallymolten rocks

= migmatites

Differenciation
• Fractionnal crystallization • Other processes

Fractional crystallisation

22
10

Al2O3
17

MgO
5

Liquids composition

12

015

FeO* 10

10

CaO

5

5

Harker diagram for Crater Lake

0 6

0 4

Na2O
4

3 2

K2O

2

1
0 50 55 60 65 70 75 45 50 55 60 65 70 75

0 45

SiO2

SiO2 Differenciation
• Always towards high SiO2 contents • Depending on the initial composition, this can mean…
– Evolution towards saturated rocks (rhyolites), as in sub-alkaline series – Evolution towardsunder-saturated rocks (phonolites), as in alkaline series – Little or no evolution… if the initial magma was already SiO2-rich !

Figure 4-21. Kangâmiut dike swarm in the Søndre Strømfjord region of SEGreenland. From Escher et al. (1976), Geology of Greenland, © The Geological Survey of Denmark and Greenland. 77-95.

Ploumanac’h, France

Torres del Paine, S. Chile

Half-dome granite(s),Yosemite, USA

a

b
N

Structures and Field Relationships
Figure 4-32. Developmental sequence of intrusions composing the Tuolumne Intrusive Series (after Bateman and Chappell, 1979), Geol. Soc.Amer. Bull., 90, 465-482. a. Original intrusion and solidification of marginal quartz diorite. b. Surge of magma followed by solidification of Half Dome Granodiorite. c. Second surge of magmafollowed by solidification of porphyritic facies of Half Dome Granodiorite. d. Third surge of magma followed by solidification of Cathedral Peak Granodiorite and final emplacement of Johnson Granite Porphry.Magma Magma

Marginal quartz diorite Half Dome granodiorite (porphyritic)

Half Dome granodiorite (non-porph) Cathedral Peak granodiorite

Johnson granite porphry

10 kilometers

c...
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