Magmatism and Tectonics

Basaltic Lava Fountain and Flows – (Hawaii)

Andesitic Explosive Eruption – (Mt. St. Helens)

Magma Types Based on Chemistry

gabbro/basalt diorite/andesite granite/rhyolite

50% SiO2 60% SiO2 70% SiO2

Physical Properties of Different Magma Types

Extrusive and Intrusive Igneous Rocks

Granite

Basalt

IUGS classification of phaneritic igneous rocks with more than 10% (quartz + feldspar + feldspathoids).

Quartz-richGranitoid

9090

6060

2020Alkali Fs.Quartz Syenite Quartz

SyeniteQuartz

MonzoniteQuartz

Monzodiorite

Syenite Monzonite Monzodiorite

(Foid)-bearingSyenite

5

10 35 65

(Foid)-bearingMonzonite

(Foid)-bearingMonzodiorite

90

Alkali Fs.Syenite

(Foid)-bearingAlkali Fs. Syenite

10

(Foid)Monzosyenite

(Foid) Syenite

(Foid)Monzodiorite

(Foi

d) G

abbr

o

Qtz. Diorite/Qtz. Gabbro

5

10

Diorite/Gabbro/Anorthosite

(Foid)-bearingDiorite/Gabbro

60

(Foid)olites

Quartzolite

Granite Grano-diorite

Tonalite

Alka

li Feld

spar

Gra

nite

Q

A P

F

60

Classification of Igneous Rocks

Quartz-richGranitoid

9090

6060

2020Alkali Fs.Quartz Syenite

QuartzSyenite

QuartzMonzonite

QuartzMonzodiorite

Syenite Monzonite Monzodiorite5

10 35 65 90

Alkali Fs.Syenite

Qtz. Diorite/Qtz. Gabbro

5Diorite/Gabbro/Anorthosite

Quartzolite

Granite Grano-diorite

TonaliteAl

kali F

eldsp

ar G

ranit

e

Q

A P

QAP diagram

IUGS classification of the phaneritic igneous rocks.Plagioclase

OlivinePyroxene

Olivine gabbro

Plagioclase-bearing ultramafic rocks

90

(b)

Anorthosite

OlivineOlivine

ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene

LherzoliteLherzoliteH

arzb

urgi

te

Wehrlite

Websterite

OrthopyroxeniteOrthopyroxenite

ClinopyroxeniteClinopyroxenite

Olivine Websterite

PeridotitesPeridotites

PyroxenitesPyroxenites

90

40

10

10

DuniteDunite

(c)

Gabbroic Rocks

Ultramafic Rocks

• Major elements: principle components of rocks mineralogy

expressed as wt% of oxide; usually greater than 0.1%– SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O

P2O5 H2O

• Trace elements: everything else in ppm or ppb; not principle

components in minerals

• Isotopes:– Special isotopes of certain elements (ie. Sr, Nd, Pb, O) that are

particularly important for understanding igneous processes

Bulk (or Whole Rock) Chemistry of Igneous Rocks

Table 8-3. Chemical analyses of some

representative igneous rocks

Peridotite Basalt Andesite Rhyolite Phonolite

SiO2 42.26 49.20 57.94 72.82 56.19

TiO2 0.63 1.84 0.87 0.28 0.62

Al2O3 4.23 15.74 17.02 13.27 19.04

Fe2O3 3.61 3.79 3.27 1.48 2.79

FeO 6.58 7.13 4.04 1.11 2.03

MnO 0.41 0.20 0.14 0.06 0.17

MgO 31.24 6.73 3.33 0.39 1.07

CaO 5.05 9.47 6.79 1.14 2.72

Na2O 0.49 2.91 3.48 3.55 7.79

K2O 0.34 1.10 1.62 4.30 5.24

H2O+ 3.91 0.95 0.83 1.10 1.57

Total 98.75 99.06 99.3 99.50 99.23

Major Elements

Early on it was recognized that some major element chemical parameters were very useful in regard to distinguishing magmatic groups (classification) and for showing important petrogenetic trends

– Total Alkalis (Na2O + K2O) vs (SiO2)

– AFM triangular diagram (Na2O + K2O, FeO, MgO)

– Harker diagrams (Oxides vs. SiO2)

Alkali vs. Silica diagram for Hawaiian volcanics

Classification of Volcanic Rocks

Alkaline

Subalkaline

AFM diagram can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline rocks

A = Na2O + K2O

F = FeO + Fe2O3

M = MgO

F

A M

Calc-alkaline

Th o l e iitic

Red circles = tholeiitic rocks from Iceland, the Mid-Atlantic Ridge, the Columbia River Basalts, and Hawaii

Pink circles = calc-alkaline rocks of the Cascade Range, Washington

CharacteristicSeries Convergent Divergent Oceanic ContinentalAlkaline yes yes yesTholeiitic yes yes yes yesCalc-alkaline yes

Plate Margin Within Plate

A world-wide survey suggests that there may be some important differences between the three series