Igneous Petrol. EPSC-423; Francis, 2013
Igneous Petrology LAB 9: Sifton Range Volcanic Complex
Today’s lab is the first of a two parts examining the relationship between
continental calc-alkaline volcanism and granitoid plutonism. We will use
the Eocene Magmatic Province of the northern Canadian Cordillera as a
case study. Today’s lab will focus on the volcanic rocks of the Sifton
Volcanic Complex, while next week’s lab will examine granitoids along two
transects of the associated Eocene plutons of the Coast Plutonic Belt.
The Eocene magmatism of the northern Cordillera is particularly well
suited to such an integrated study because uplift has exposed the deep core
of the Coast Plutonic Belt (CPB), but shallow plutons and thick volcanic
sequences are preserved along its eastern margin, and the SNORCLE
Lithoprobe transect provides 3-D geophysical constraints.
The final phase of extensive plutonism in the Coast Plutonic Belt occurred
during the Early Tertiary, between 48 and 62 Ma, followed by uplift and
erosion that has exposed the plutons to increasing depths from east to west
across the belt. Volcanic activity was pervasive during the Eocene and
remnant volcanic successions outcrop along the length of the western
Intermontane Belt. In the southern Yukon, major Eocene volcanic centres
occur at the Sifton Range, Mount Skukum, and Bennett Lake, and are all
assigned to the Skukum Group; while in northern British Columbia coeval
more effusive Eocene volcanics occur south of Atlin Lake and are referred to
as the Sloko Group. The intensity of activity during the Eocene was
probably linked to a change in the relative motion of the North American
plate with respect to the Kula-Farallon plate, from dominantly orthogonal
to oblique. The oblique northward motion of the Kula plate relative to
North America imposed a clockwise torque on western North America,
producing extension within the Coast and Omineca belts.
The magmatism of the CPB is characterized by the effusion of basalt to
andesite lavas, followed by the development of central volcanic complexes
characterized by the explosive eruption of fragmental rocks from dacite to
rhyolite in composition, along with minor felsic flows. These volcanic
complexes are then intruded by coeval intrusive rocks ranging from
granodiorites to granites, which are compositionally complimentary to the
basalts and andesites.
Crystal fraction models indicate that this
relationship can be understood in terms of the interplay between increasing
viscosity of a crystal-charged magma and the build up of water, which leads
to stagnation and water saturation at the transition from andesite to dacite
(Miskovic et al, 2003).
1
Igneous Petrol. EPSC-423; Francis, 2013
Sifton Range Geology:
The rocks below come from a section through the
Sifton Range Volcanic Complex, an erosion remnant
of an Eocene central volcanic complex that occurs as a
series of sub-horizontal sheets, underlain by a
younger Eocene granitoid batholith. Six stratigraphc
units can be defined on the basis of field occurrence
and characteristics.
The specimens within each stratigraphic unit are not
identical, but are from different cooling units sampled
stratigraphically across the stratigraphic unit. The
total stratigraphic thickness is ~ 1600 meters.
TASKS:
1. Plot the compositions of the Sifton volcanic rocks
that
you
will
study
in
thin
section
(PetLab9Sifton.xls) along with the more extensive Eocene volcanic
databases (PetLab9Solo.xls and PetLab9VolcBSS.xls)) in a total alkalis
versus SiO2 diagram. Examine both the hand specimen and thin sections,
making note of volcanic textures that will enable you to characterize the
type of cooling units found in each map sheet. Give the rocks of each map
unit a 2 part name, the first based on the rock’s composition (fields in
alkalis versus SiO2 diagram) and the second based on the probable type of
cooling unit (eg. basalt flow, rhyolite tuff, etc.). Make note of the phenocryst
phases and the range of SiO2 contents found in each map unit.
2
Igneous Petrol. EPSC-423; Francis, 2013
3
Igneous Petrol. EPSC-423; Francis, 2013
2. Plot both the compositions of the Sifton Volcanic Rocks in the Lab and those
of the Eocene volcanic databases in a plot of K2O versus SiO2 to characterize
the potassium content of the Eocene lavas. How does the variation of K2O
with SiO2 in the Eocene suite compared to the dividing lines of the
diagram?
3. Briefly describe the evolution of the Sifton Range Volcanic complex in terms
of the variations in both composition and eruption style with stratigraphic
position.
4
Igneous Petrol. EPSC-423; Francis, 2013
4. Plot the CIPW Norms in oxygen units for the Sifton volcanic rocks in this
lab (assume XFe3+ = 0.20) and those of the Eocene databases
(EoceneVolcNORM.xls) in a Quartz - Orthoclase - Plagioclase liquidus
projection (Petrogeny’s Residua). Where do they fall with respect to the
granite cotectic curve at 1 atm, and/or the granite eutectic and cotectic
curves at 5 Kbs H2O, and comment on possible implications for the depth of
fractionation of these felsic lavas.
Calculate the diagram end-members as follows:
Quartz
=
normative Quartz
Orthoclase
=
1.25 × normative Orthoclase
Plag
=
normative An + (normative Ab – 0.25 × Or)
5
Igneous Petrol. EPSC-423; Francis, 2013
6
Igneous Petrol. EPSC-423; Francis, 2013
5. PA-3 is the most primitive lava in the Eocene Volcanic suite
(PetLab10Sifton.xls). Assume that it is the parental magma to all the
Eocene lavas. Model the closed system, liquid line of descent of PA-3 by
crystal fractionation using AlphaMelts. Begin with dry conditions at 1 atm
buffered on FMQ, and compare the liquid line of descent to the spectrum of
Eocene volcanic compositions in a plot of Al versus Si. Adjust the pressure
and water content in successive models in order to best reproduce the
observed behaviour of Al with Si. How good is the best fit, and what does it
tell us about the conditions of crystal fractionation for this volcanic suite
(water content, pressure)?
Compare the rise in K, Zr, Ba, and Rb with increasing SiO2 in the best
AlphaMelts model to that observed in the Eocene lavas by plotting each
element versus Si and superimposing the calculated liquid line of descent.
Comment of the ability of closed-system crystal fractionation to reproduce
the observed variations.
Reference:
Miskovic, A., and Francis, D.; 2006: Interaction between mantlederived and crustal calc-alkaline magmas in the petrogenesis of the
Paleocene Sifton Range Volcanic Complex, Yukon, Canada. Lithos 87,
104-134.
7
Igneous Petrol. EPSC-423; Francis, 2013
Field Classification of Volcanic Rocks
8
Igneous Petrol. EPSC-423; Francis, 2013
9