Igneous Petrology EPSC 423, Francis 13
Lab 4: Classification of Basalts
Basalts are mafic volcanic rocks composed predominately of:
Feldspar
Pyroxene
Olivine
45-55 %
30 - 40 %
0 - 15%
picritic basalts contain > 15% olivine phenocrysts
ankaramitic basalts contain >15% cpx and olivine phenocrysts
Because of their fine-grain size, basalts are typically informally classified on the basis
of their phenocrysts phases in the field, ie.; olivine-phyric basalt, plagioclaseclinopyroxene phric basalt, etc.
The formal classification of basalts, however, is based on their chemical composition
as expressed in their calculated normative mineralogy (NORM) with respect to the
plagioclase – olivine – clinopyroxene plane, the low pressure thermal divide.
Alkaline Basalts:
Alkaline Olivine Basalt – (alkali olivine basalt)
Olivine and minor nepheline (feldspathoids: 0-5%) in the NORM, typically
olivine and plagioclase phyric, with olivine, plagioclase, titan-augite, oxides,
olivine, and possibly K-spar in the groundmass.
Basanite
Olivine and significant nepheline (feldspathoids: 5-15%) in the NORM,
typically olivine and titan-augite-phyric, with clinopyroxene, plagioclase,
oxides, K-spar, and possibly nepheline, and apatite in the groundmass.
Nephelinite (Foidite)
Olivine and much nepheline in the NORM (feldspathoids: >15%), no normative
plagioclase. Olivine-phric with titan- to aegirine-augite, oxides, K-spar, and
nepheline or other feldspathoids in the groundmass. Plagioclase is absent, and
olivine is typically absent in the groundmass. Not strictly a basalt because of
the absence of plagioclase. Potassic varieties contain leucite rather than
nepheline and are termed leucitites. Calcic varieties contain melilite and are
termed melilitites.
Igneous Petrology EPSC 423, Francis 13
Sub-Alkaline Basalts:
Hyperstene-normative basalt
Olivine and orthopyroxene in the NORM, typically olivine-phyric, with subcalcic pigeonite or orthopyroxene in the groundmass along with plagioclase,
clinopyroxene, and oxides.
Quartz-normative basalt
Orthopyroxene and quartz in the NORM, typically clinopyroxene and
plagioclase-phyric with sub-calcic clinopyroxene, plagioclase, oxides, and
possible quartz in the groundmass. Olivine is absent in the groundmass.
Igneous Petrology EPSC 423, Francis 13
Fort Selkirk Volcanic Complex
The Fort Selkirk Volcanic Complex is one of a series of late Tertiary to Recent
alkaline volcanic complexes that have erupted at “pull-aparts” along the length of
the Canadian Cordillera. The Fort Selkirk Complex is located at the junction of the
Pelly and Yukon Rivers in the central Yukon and comprises three mapable volcanic
units (A, B, and C). Despite the relatively small size of the Fort Selkirk Complex,
the range of alkaline magmas that have erupted rivals that see at large Ocean
Island Basalt provinces such as Hawaii. The different samples of each map unit
have approximately the same composition, but vary in appearance because of the
type of cooling unit they sample, and where in the cooling unit they were positioned.
The samples from different map units, however, have very different chemical
compositions that lead to different crystallization sequences and liquid lines of
descent.
Igneous Petrology EPSC 423, Francis 13
Fort Selkirk Alkaline Volcanic Complex
Map Unit
A1
A2
A3
Map Sub-Unit
Samples
A1-Fort Selkirk
HF-45
A2-Wootton’s Cone
DF-13
A3-Volcano Mt.
VM-11 or VM-13
VM-20 or GR-15
DF-42,
W-5 or W-6
Wo-11
B
PY-1 or PY-19
PY-32, PY-35 or PY-36
HF-2
YK-16 or YK25
C
Map Unit
SiO2
A
41.82
B
45.87
C
48.05
PD-17
46.00
TiO2
3.16
2.90
2.54
0.89
Al2O3
11.38
13.25
13.74
11.50
MgO
11.25
8.16
8.31
16.80
FeO
13.15
12.24
10.06
10.50
MnO
0.20
0.17
0.15
0.18
CaO
9.33
8.90
9.67
10.20
Na2O
4.90
4.24
3.20
1.20
K2O
2.25
2.19
1.43
0.05
P2O5
1.36
0.90
0.51
0.08
LOI
1.00
0.71
2.22
2.20
Total
99.85
99.55
99.95
99.50
Igneous Petrology EPSC 423, Francis 13
Tasks :
Speculate on the type of volcanic cooling unit (eg. pillow lava, lava flow, pyroclastic,
etc.) that each basalt sample likely represents, as well as its approximate position
in its cooling unit (eg. flow top, middle, bottom, pillow margin, etc), using the
criteria you learned in Lab 3. For reference, compare the samples here to the
samples taken across the Baffin Is. or AOB flows in the Lab last week (Series A-1
and A-2, Station A), and/or the Baffin pillows.
Determine the order of crystallization of the minerals for each magmatic unit (map
unit) by examining thin sections of the finer-grained samples that best approximate
chilled margins. In each case, use AlphaMelts to predict the 1 atm crystallization
sequence (at FMQ-1) and compare the calculated results with your petrographic
determination.
Determine the late groundmass phases for each magmatic unit (map unit) by
examining thin sections of the coarser-grained samples that come from the interiors
of thicker cooling units. Calculate the CIPW normative mineralogy (See appendix
in Winter for recipe) of each magmatic unit, as well as Pd-17, and compare the
normative mineralogy to the modal mineralogy observed in the coarse-grained
samples.
Determine the basalt classification of each magmatic unit according to its
normative mineralogy, and plot them in the attached olivine liquidus projection.
Speculate how each magma type would evolve in this diagram if it underwent lowpressure crystal fractionation.
Plot graphs of P versus Mg and Si for the 4 basalt samples in cation units. With the
aid of vectors, show the expected effects of the fractionation of olivine and/or cpx
phenocrysts on the magma composition. Use AlphaMelts to calculate the liquid line
of descent for each magma type until the point of saturation in apatite. Plot these
liquid lines of descent in your P versus Mg and Si plots. Compare the amounts of
normative feldspathoid or quartz in each of the liquids at the appearance of apatite
in the AlphaMelts models. Comment on whether it is possible to relate the three
magmatic units at Fort Selkirk by low pressure crystal fractionation.
Francis, D., and Ludden, J.; 1995: The signature of amphibole in mafic alkaline
lavas, a study in the northern Canadian Cordillera. J. Petrology 36, 11711192.
Igneous Petrology EPSC 423, Francis 13
Igneous Petrology EPSC 423, Francis 13