Department of Earth and Planetary Sciences
McGill University
Understanding Planet Earth (186-201)
http://www.eps.mcgill.ca/~venetia/201c/index.html
Geologic Field Trip 2
THE IGNEOUS, SEDIMENTARY AND METAMORPHIC
HISTORY OF MOUNT ROYAL
This document was originally developed by Venetia Bodycomb and David Dolejs. Their wisdom is much
appreciated.
(The following description was liberally modified from a previous 186-201 handout which was based on Clark, 1972
and Gelinas, 19721.)
The Geology of Mount Royal
Mount Royal is one of the Monteregian Hills and is the prominent topographic feature of
the Montreal area with a maximum elevation of 254 m above sea level (just a little higher than
the restaurant at the top of Place Ville Marie). Mount Royal was formed by magma intruding
upward through the sandstone-limestone-shale sequence of the St. Lawrence Lowlands. The
oldest rocks of the St. Lawrence Lowlands sedimentary sequence were deposited during Early
Cambrian time almost 600 million years ago. The youngest rocks of this sequence that we can
now see intruded by the Mount Royal intrusion are approximately 470 million years old. The
magma came considerably later, approximately 125 million years ago, and the resulting igneous
intrusion is called a stock (a small pluton). During the past 125 million years, erosion has
removed at least 1.5 kilometres of rock, so what we are seeing today is only part of the story.
Sedimentary rocks
The sedimentary sequence of the St. Lawrence Lowlands is up to 2.6 km thick today and
represents sediments laid down on the old North American passive continental margin beginning
more than 500 million years ago. At the time, the eastern north American continent was
bordered by the Iapetus Ocean, which has since closed and been re-opened as the Atlantic
Ocean.
Sedimentation in a shallow inland sea of the Iapetus Ocean began more than 500 million
years ago with the deposition of fluvial to shallow marine sandstones (Potsdam Group). These
rocks were subsequently overlain by shallow marine limestones (commencing with the Lower
Ordovician Beekmantown Group), and finally the Utica black shales. The sequence of these
rock types (sandstone -> limestone -> shale) indicate that marine conditions underwent a
prolonged period of subsidence (transgression) in which water levels were rising over the land.
1
Clark, T.H., 1972. Montreal Area. Ministere de l’Energie et des Ressources du Quebec, Geol. Report 152, 244p.
Gelinas, L., 1972. Geology of Mount Royal. 24th International Geological Congress, Montreal, Guidebook B-12, 40p.
Subsequent uplift and disappearance of the inland sea (regression) was caused by the formation
of the Appalachian mountains to the southeast during ocean closure. Clastic sediments from the
eroding new mountains filled the sedimentary basin, covering the shales, forming the Lorraine
and Queenston rock groups.
Erosion (by rivers, glaciers, etc.) has since removed most of these rocks from the
Montreal area, except for the limestones and a small vestige of the shale. The sandstone is
buried under all the limestone.
How much of these rocks were already eroded away at the time of the magma intrusion
125 million years ago? We cannot tell with certainty, but we do have evidence that 400 million
year old rocks were still present at the time of the Mount Royal intrusion. This evidence is in the
form of limestone fragments in a diatreme breccia on St. Helen’s Island. The diatreme breccia
is an explosion pipe related to the Monteregian intrusive event: after magma violently shot up
through the area that is now St. Helen’s Island, pieces of ~400 Ma rock fell back into the
explosion pipe where they have since been preserved.
Igneous rocks
The igneous rocks of Mount Royal comprise intrusive rocks of various compositions
emplaced as a series of magmatic pulses 125 million years ago. The first and volumetrically
most important event involved basaltic magma that cooled to form a gabbro stock that we call
“Mount Royal”. Successive magmatic pulses gave rise to swarms of thin dykes around the
margins of the stock. Some of the rocks are porphyritic (recall: big crystals in a fine-grained
matrix). Some other rocks are explosive breccias that imply a violent (rapid) rise of magma.
Both of these rock types provide evidence that the intrusive complex we now see was formed
under a 1.5 km sedimentary rock cover which has since been eroded (this depth corresponds to
a pressure of about 0.5 kilobars).
Metamorphism
The sedimentary rocks in the vicinity of the intrusion have been thermally
metamorphosed (recrystallized by heat) to form what we call a contact aureole some 800 m
wide. Contact metamorphism means that if a rock at temperature X is suddenly heated up to
temperature Y by a nearby magma, then the rock’s minerals that were stable at temperature X
will have to change (recrystallize) into other minerals that are stable at the new, elevated
temperature Y. At Mount Royal, we would therefore expect that the sedimentary rocks closest to
the intrusion will show the greatest change and contain the higher temperature mineral
assemblages, and that the metamorphic aureole will diminish in intensity moving away from the
intrusion. In fact, it is only within the innermost 100 m of the contact metamorphic aureole that
we can observe the effects of thermal change with the naked eye. The results are “bleaching” of
the dark grey limestone to form white marble, and metamorphism of the soft and fissile shale to
a slightly coarser-grained and harder hornfels.
The Big Picture
The rocks of the St. Lawrence Lowlands represent continental shelf sequences deposited
on the western passive continental margin of the Iapetus Ocean - the ocean that existed before
the Atlantic Ocean. Part of the Iapetus Ocean system included widespread inland seas that
covered much of what is now known as North America (Lake Champlain is a remnant feature of
those seas). The shelf deposits were uplifted during Middle Ordovician time, causing the inland
seas to retreat and sea levels to drop relative to the continent. This uplift was caused by the onset
of mountain building as the Iapetus began to close as the African and North America collided
around 350 million years ago. The mountains that formed during this closure are known as the
Appalachians, and as other continents collided around the world, the supercontinent Pangea
was created. The rocks comprising the Appalachians were intensely deformed, but the St.
Lawrence Lowlands rocks to the northwest of the mountain chain were too far away from the
convergent plate boundary to have suffered extensive deformation. Instead, they were only
slightly deformed into large broad open folds (km scale), the most prominent of which is the
Chambly-Fortierville Syncline (Mount Royal is located at the southern end of this structure).
Folds are useful in that they tilt the original horizontal layers allowing erosional processes to
effectively expose a cross section of the underlying sedimentary rocks.
Mount Royal is only one of the Monteregian Hills formed by similar intrusions. The
hills extend in an approximate west-east direction from Oka to Lac Megantic in the
Appalachians. They are all alkaline in nature, which means that they are enriched in some of the
alkali elements (like K and Na) with respect to other igneous rocks. Alkaline rocks tend to be
associated with continental rift zones, and it has been suggested that the Monteregian province is
similar to the East African Rift Zone (Kumarapeli & Saull, 1966).
Long after the emplacement of the Mount Royal intrusion, an estimated 1.5 to 2 km of
overlying sedimentary and igneous rocks were removed by running water and ice (during the last
glaciation) to form the present surface.
Vocabulary:
basement:
older rock that underlies younger rock and is separated from the younger rock by an unconformity
(erosional event); typically metamorphosed and deformed.
carbonates:
carbonate-rich rock (typically limestone) derived from chemically precipitated calcite (=calcareous)
mud. Limestone composition = calcite = CaCO3.
diatreme breccia:
a volcanic pipe formed by a gaseous explosion and filled with large angular fragments in an igneous
matrix.
dyke:
thin, tabular intrusive body that cross-cuts the rock layers it intrudes.
fluvial:
of or pertaining to rivers.
gabbro:
mafic intrusive rock composed of Ca-feldspar and pyroxene; typically dark coloured.
Iapetus Ocean:
the ocean that disappeared about 350 Ma when Africa collided with North America during the formation
of Pangea - later rifting during the breakup of Pangea produced the Atlantic Ocean in its place, leaving
remnants of Africa and other smaller continental masses accreted onto N. America.
marine:
of or pertaining to seas/oceans.
porphyritic:
igneous rock texture whereby bigger crystals are surrounded by a finer-grained matrix.
sandstone:
rock formed from clastic (detrital) sediments (typically quartz-bearing) with grain size averaging 1/16 mm
to 2 mm in diameter.
shale:
a clastic rock formed from clay (mud) with minor silt component.
(black shale: shale containing organic matter to give it a blackish colour)
stratigraphic section:
a column displaying a chronologic succession of sedimentary rock from older (bottom) to younger (top).
stock:
irregularly
shaped
intrusive
igneous
body
<100km2;
type
of
intrusive
igneous
body.
QUESTIONS RELATED TO SPECIFIC FIELD TRIP STOPS:
Stop 1: Meeting Place (corner Peel and Pine avenue)
Most of Montreal is underlain by limestone, however the rock used to build the flagstone steps leading
up the mountain is not limestone.
a) What rock type is it? What criteria did you use to determine this?
b) Using your stratigraphic section in the handout, make a guess as to the stratigraphic name for this rock
(i.e., Trenton, Utica, Potsdam, etc.).
c) On the top step of the first set of steps, there is a flagstone with a distinctive sedimentary structure
(somewhat worn down). What is that structure and what does it signify?
Notice that the wall to your right as we climb higher is made of grey limestone blocks. We are now
following the approximate path of Jacques Cartier in 1535 when he left the Indian village of Hochelaga
to ascend the mountain and erect a cross in the name of the King of France.
Stop 2: Horizontal Beds
This outcrop consists of near horizontal beds of limestone that have been heated to about 400 degrees by
the Mount Royal intrusion. Layering (bedding) is very obvious due to a colour change.
a) What causes the colour change? Make a close-up sketch to illustrate your answer (include a scale).
b) What kind of fossils can you find at this outcrop? (there are two: we saw them on the first field trip)
Note that the monument is a type of granite (felsic intrusive rock) and not from the Montreal area.
Stop 3: Dykes and Sills
There is a bedded grey rock and inclined to subhorizontal layers of darker rock.
a) Name the bedded rock and the darker rock? Is the darker rock coarse-grained or fine-grained? What
terms do we give to the layers of dark rock?
b) Which of the two rock types is younger and why?
c) What was the temperature of the grey rock before the Mount Royal magma came up?
d) What would be a reasonable initial temperature for the dark rock?
e) Note the white material at the margins of the darker rock. Name this rock type and explain why it is
white. Make a close-up sketch of this feature, including a scale.
Stop 4: Steps to Lookout
This rock used to be Utica shale. Because it was “cooked” near the Mount Royal intrusion, we now call
it a hornfels (it has been recrystallized during contact metamorphism). Temperatures probably reached
~500 C. Compare the hornfels to the unmetamorphosed shale sample we will bring along.
a) Name two of the ways in which the hornfels differs from the shale samples we bring.
b) Name the feature that was not obliterated by recrystallization.
Stop 5: Lookout
At the lookout, check out the ornamental granite under your feet and at the water fountain etc. This
granite is much coarser-grained than that for the monument at Stop 2. None of the granites are from the
immediate Montreal area. From the lookout you can see the expanse of the St. Lawrence Lowlands rocks
(low-lying and flat) as well as some of the other Monteregian Hills (distant isolated mountains). Also
note Ile Ste-Helene, where the diatreme breccia described in the text can be found. Although it is
genetically related to the Mount Royal intrusion, it is quite distant from the main gabbro body.
a)
Why are the Monteregian Hills in topographic relief compared to the rocks of the surrounding St.
Lawrence Lowlands?
Stop 6: Black Outcrop
a) Name the black rock at this outcrop. How can you tell what it is?
b) How is its texture different from similar rocks at Stop 3?
Stop 6a: Black and White Outcrop
If it is still visible, we will stop at a flat outcrop to the side of the pathway.
Identify the minerals and explain their strange appearance.
Stop 7: Camilien-Houde
a) Make a sketch of this outcrop. Reconstruct the order of geological events by numbering the rock from
1 (oldest) to 3 (youngest). Rocks are: gabbro intrusion, Utica shale, and Trenton limestone (marble).
Another feature that you must add is the fault zone. The sketch should be very simplified - do not try
to draw all the details, but do show a general indication of bedding, the contacts between different
rock types, and the approximate location of the fault plane. Indicate which direction you are facing
and include a scale.
ADDITIONAL QUESTIONS FOR THE REPORT:
1. (a) Draw a horizontal time line from 600 million years to the present showing the events that shaped
the Montreal area as we know it today. Make your time line to scale! [Events include:
sedimentation on the old passive continental margin, the closing of Iapetus Ocean as the
Appalachian mountains formed, the Mount Royal intrusion, and recent glaciation which eroded
the land to its present level.]
(b) Kumarapelli and Saull (1966) suggested that the Monteregian Hills were probably associated
with E-W rifting which failed to produce a new ocean. The Monteregian intrusions would
therefore mark an ancient failed rift arm at a triple junction. Rifting along the other two arms
and elsewhere was successful. What new feature formed during that rifting event? Add the
feature to your time line.
2. At what approximate depth did the Mount Royal gabbro stock form? Give three lines of evidence
that together suggest that the intrusion was emplaced at such a shallow depth.
3. How do you think we know the age of the sedimentary fragments in the Ile Ste. Helene diatreme
breccia if we have only found dislodged fragments?
4. Using the attached sheet stratigraphic sequence, complete the second and third cross sections through
the Montreal area showing how depositional environments change during transgression (sea level
rise) and how they explain the vertical sequence of rocks found under Montreal.
5. Why does the metamorphic aureole diminish in intensity away from the stock? What would it look
like seen from above? Include a plan-view sketch (with scale) to illustrate your answer (you can
trace the outline of the intrusion from the attached map and simply draw the metamorphic zone
around it). What are the highest temperatures attained and indicate where they are found in the
aureole.