Igneous Petrol. EPSC-423; Francis, 2013
Igneous Petrology Lab 10: Eocene Granitoids of the Northern Cordillera
Today’s lab is the second of a two part lab examining the relationship between continental
calc-alkaline volcanism and granitoid plutonism using as an example the Eocene Magmatic
Province of the northern Canadian Cordillera. Today we focus on the Eocene granitoid
rocks that underlie the Sifton Range volcanic rocks that you examined last week.
Most of the granitoid rocks (SiftonPlutons.xls) in today’s lab come from the vicinity of the
Sifton Range Volcanic Complex, but a few are from further a field within the Coast
Plutonic Belt. The samples from Groups A and B are Eocene in age, contemporaneous with
the Sifton Range volcanics that you studied last week , whereas the samples in Groups C, D,
and E, may be somewhat older. In addition you have been given two data bases of Eocene
granitic rocks, one for relatively high level or shallow granitoids
(EoceneShallowPluton.xls), and the other for deep-seated, possibly older, granitoids
(EoceneDeepPluton.xls).
Group
Unit
Sample
Sample
Sample
A
B
C
D
E
high-level dykes
sub-volcanic plutons
deeper pluton(s)
mafic enclaves in C
older granitoids
SF-8
SF-44
SF-33
SF-20
SG-36
SF-12
SG-35
SG-16
SG-41
PX-235
DI-1
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SF-54
Igneous Petrol. EPSC-423; Francis, 2013
Tasks:
1. Examine and classify each of the Sifton plutonic rock using the IUGS official
classification scheme (see below). To do this, you will have to distinguish and estimate
the proportions of K-spar and plagioclase, as well as quartz. Make a note of average
grain-size, percent and types of mafic minerals, presence or absence of phenocrysts,
types and textures of feldspars, etc., and interpret them in terms of cooling history.
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Igneous Petrol. EPSC-423; Francis, 2013
2. Calculate the CIPW Norms in oxygen units for the Shifton granitoid rocks
(SiftonPutons.xls) in this lab and plot them, along with the larger Eocene plutonic data
base, in the two Quartz – Orthoclase - Plagioclase liquidus projections (Petrogeny’s
Residua) from last week’s lab, one at 1 atm and the other at 5 Kbs H2O).
Calculate the diagram end-members as follows:
Quartz
=
norm Qtz
Orthoclase
=
1.25 × norm Or
Plag
=
norm An + (norm Ab – 0.25 × norm Or)
1 atm.
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Igneous Petrol. EPSC-423; Francis, 2013
3. Construct plots of the variation in the ratios of Zr/Y and Ba/Zr versus Si content in the
Eocene volcanic database from last week’s lab and the shallow Eocene granitoid suites
(EoceneShallowPluton.xls). In the same diagram, plot the best liquid line of descent
you obtained last week using AlphMelts for closed-system crystal fractionation of the
most primitive lava (PA-3.
In the same diagrams, plot mixing curves between the composition of the model parent
magma (PA-3) and rhyolite (SL-08). Then, run AlphaMelts AFC crystal fractionation
models using the optimal conditions arrived at last week, but now including the
assimilation of variable amounts of a rhyolite contaminant of SL-08’s composition. Plot
the results of the best model in terms of fitting the observed volcanic data in the
foregoing diagrams and comment on the ability of closed-system crystal fractionation to
explain the observed variation. Finally plot the compositions of the deep seated
granitoids -what is going on with these rocks in comparison to what is seen in the
volcanic suite and shallow granitoids?
4. Calculate the following chemical parameters in cation units for the average composition
of Eocene granites containing between 70 and 74 wt.% SiO2:
Alkalinity Index
= (K + Na) / Al
Mg No.
= Mg / (Fe + Mg)
Aluminum-Saturation index = Al / (Ca×2 + Na + K)
Using these chemical parameters, and your petrographic observations, classify the
northern Cordilleran granitoid suite in terms of A, S, or I types according to the
criteria in the attached sheet and comment on the implications for the source of these
granitoids.
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Igneous Petrol. EPSC-423; Francis, 2013
Characteristics of Granitoid Suites
Feature
Rock Types :
Mafic
Minerals:
A -Type
granite
I - Type
tonalite to granodiorite
to granite
Na-amphibole
Na-pyroxene
Accessories:
fluorite
topaz
tourmaline
Oxide:
ilmenite
hornblende
biotite
S - Type
granodiorite
to granite
muscovite
biotite
garnet
Al-silicates
cordierite
corundum
magnetite
ilmenite
Feldspar :
perthite
hypersolvus
K-spar & Plag
subsolvus
(Na+K) / Al:
>1
per-alkaline
<1
<< 1
<< 1
<1
meta-aluminous
>1
per-aluminous
very low
low
very low
HFSE elements:
(Zr, Nb, REE)
high
low
low
Restite/
Enclaves:
few
Al / (Na+K+Ca×2)
Mg No.
Source
Regions:
Tectonic
Setting:
granulitic
lower crust
with halogen
volatile flux
extensional
within-plate
amphibolite
biotite-cordierite
sillimanite-garnet
gneiss
mafic igneous
rxs or
mantle-derived
magmas (M)
sedimentary rxs
compressional
volcanic arcs
extensional
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K-spar & Plag
subsolvus
pelites
greywackes
compressional
continental
collisions