magmatism and tectonics. basaltic lava fountain and flows – (hawaii)
TRANSCRIPT
Magma Types Based on Chemistry
gabbro/basalt diorite/andesite granite/rhyolite
50% SiO2 60% SiO2 70% SiO2
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)
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