Igneous Petrol. EPSC-423; Francis, 2013
Igneous Petrology LAB 7: Layered Igneous Intrusions
Last week we looked at a relatively simple zoned dyke in which a flowing magma
crystallized inwards along the wall of the dyke. The crystal fractionation that produces the
spectrum of lava compositions observed in volcanics on the surface is probably produced in
more sizeable magma chambers within the deep crust and/or at the crust-mantle interface.
The solid crystal assemblages that crystallized from cooling magmas in these magma
chambers commonly form layered cumulate rocks with a stratigraphy that reflects the
sequence of cumulate minerals that crystallized from the parental magma as it cools below
its liquidus.
Some plutonic rocks are almost completely cumulate in nature, while others reflect a
mixture of accumulated crystals and frozen interstitial liquid. When examining cumulate
plutonic rocks, it is important to distinguish between minerals which are cumulus versus
those which are post-cumulus, crystallizing from the interstitial liquid between the primary
cumulus grains.
Today’s lab deals with the sequence of cumulus minerals in large layered intrusions, with a
focus on the Muskox Intrusion of the Northwest Territories. In addition to answering the
specific questions for each station, give each rock you examine two names. One that reflects
its total mineralogy according to the formal IUGA classification (see appendix in Lab 6) and
a second which indicates which minerals were interpreted to be cumulus phases, eq.
A dunite is also an olivine cumulate.
A feldspathic olivine clinopyroxenite might be an olivine clinopyroxene cumulate,
with the plagioclase being post-cumulus. It might even be an olivine cumulate, with
both the clinopyroxene and plagioclase being post-cumulus.
Some gabbros might be olivine-clinopyroxene-plagioclase cumulates, while others
might be olivine clinopyroxene cumulates, with the plagioclase being post cumulus
Station A – Crystallization Sequence
The three trays at this station contain cumulate rocks from three different layered intrusions
with different crystallization sequences. Determine the order of crystallization of olivine,
plagioclase, orthopyroxene, and clinopyroxene for each intrusion by examining the rocks
and thin sections that are available. Specify the textural criteria upon which you base your
determination.
A1 specimens: V-5, V8b, V13B
A2 specimens: BV-1, BV-2, BV-4
A3 specimens: VD-710, VD-711, VD-724 (no hand specimens)
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Igneous Petrol. EPSC-423; Francis, 2013
Station B : Changing Parental Magma Composition
This station contains a series of cumulate rocks from the upper part of the third megacycle
and lower portion of the fourth megacycle of the Muskox Intrusion (Francis, 1994).
Examine the rocks to determine their sequence of crystallization. Contrast this with the
sequence of crystallization in the specimens you observed at station A3, which come from
the lower part of the third megacycle of the Muskox Intrusion, and the underlying first
megacycle. Explain this difference in terms of a change in the composition of the magma
in the chamber with time, perhaps because of the effect of the assimilation of Si rich crust
using an olivine liquidus projection.
B specimens: V3-06 (no hand specimen), V3-05, VD-550, VD-553, VD-594
Station C - Chromite Reef
The specimens at this station are from “reefs” in the Muskox and Bushveld intrusion.
C-1
These samples straddle the chromitite reef in the Muskox intrusion, which occurs in
Megacycle 4, near the level at which plagioclase becomes a significant cumulus
phase for the first time. The two sets of samples represent two locations along the
reef. Examine at least 1 each of:
VD-847 and/or M10 are located about 1.5 meters below the reef.
VD-849, M9, and/or M11 are pieces of the reef itself, which is ~ 0.5 meters
in width.
VD-851 and/or M13 are located 1.5 meters above the reef.
By examining and contrasting the samples from just below the reef with those from
just above the reef speculate on the change that occurred in the magma chamber at
the interval the reef occupies. Describe the reef samples themselves and contrast
them with their surrounding rocks. Speculate on the implications of these
differences for the origin of the reef.
C-2
This station contains samples of chromitite reefs in the Bushveld Layered Intrusion
of South Africa.
BV-3 is a sample of the LG-6 chromitite reef.
crystallization.
Determine its sequence of
Merensky Reef Hand Samples: Try to determine which way is tops in the hand
samples by examining the edges of the chromitite seam. How does the
cumulate rock below the Merensky reef differ from that above it. Compare
this with your observations for the Muskox Reef in C-1.
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Igneous Petrol. EPSC-423; Francis, 2013
Station D – Gabbros to Granophyres
The rocks at this station are all located in the gabbroic top of the Muskox Intrusion.
Specimen 33 is located in the middle of the gabbroic unit, whereas specimens M-26 and M27 are close to the very top of the intrusion. Specimen M-29 is in between. Examine each
rock paying particular attention to the interstitial phases between the cumulus pyroxenes and
plagioclase. What changes occur as the roof of the intrusion in approached. What might
these changes reflect?
Skaergaard Stratigraphy
The compositional stratigraphy of igneous layered intrusions is characterized by abrupt
discontinuities caused by the appearance of successive cumulus phases on the liquidus, on
scales ranging from local cyclic cumulate layers to the intrusion as a whole; as the
composition of the magma evolves due to crystal fractionation.
The file
“SkaergaardRx.xls” (see Lab 7, C423 course web site) contains a whole rock data set for
samples taken across the stratigraphy of the Skaergaard Layered Intrusion in Greenland.
Plot graphs of Ti and P content versus stratigraphic position (“Strat” in data file) and
determine the points a Fe-Ti oxide and then apatite became cumulus phases by looking for
discontinuous increases in Ti, and P respectively. Plot V, Y, and Cu against stratigraphy.
What cumulus phases do you think might be controlling each of these elements, at what
point do they appear as cumulus phases? What are the Mg#s of the rocks and the fractions
of liquid left in the intrusion (F in the f) at the points of appearance of Fe-Ti oxide and
apatite as cumulus phases. (PS: You can use wt. units, no need to convert to cations)
References:
Francis, 1994: Chemical interaction between picritic magmas and upper crust along the
margins of the Muskox Intrusion, Northwest Territories. GSC Special Paper 92-10, 94 p.
Smith, C.H., Irvine, N.T., and Findlay, D.C., 1966; Muskox Intrusion, Geological Survey
maps 1213A and 1214A.
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