2013 Igneous Petrology Field Trip – Francis
Earth and Planetary Sciences
Igneous Petrology Trip
Oct. 3, 2013
Don Francis
Don F. Cell Phone: 514-923-6759
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2013 Igneous Petrology Field Trip – Francis
The Monteregian Hills
Mont Brome Complex
Stops B-1a, b, c: Slate to Gabbro Contact
Begin by examining the outcrop of siltstone/slate just westof the intersection withn Chemin du
Centre (a). This is the country rock surrounding the intrusion. Drive towards the East to the
black outcrop on the North side of the road (b). What has changed? Drive further to the East,
past the driveway to the beginning of large outcrop in the woods on the North side of the road
(c). Begin looking for the contact with the gabbro of the intrusion by moving westward along
this outcrop scarp (be careful of the garbage). What lines the contact between the gabbro and
the country rock?
Stop B-2: Gabbro
Head North on Chemin Rogers. Examine the outcrops in the woods on the East side of the
road past the crest of the ridge. Walk along the outcrops from North to South. What is the
dominant lithology? Is there layering, and if so what defines it? Which way does the layering
dip with respect to the Brome intrusion?
Stop B-3: Syenite (pulaskite)
Head North via Chemins Sanborn and then Miltimore to Chemin de Gaspé, continue North to
Chemin de Missisquoi, take a right and then turn left at the top and drive to the large outcrop.
The dominant outcrop is called pulaskite, what would you call it? Distinguish between the
dark hornfels enclaves and the fine-grained mafic dyke in this outcrop. What is the honeybrown mineral?
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2013 Igneous Petrology Field Trip – Francis
Mont Brome Complex – cont.
Stop B-4: Nepheline Syenite: Chemin Missisquoi:
Enclaves of feldspathic hornblendite in nepheline syenite.
Stops B-4, B-5, and B-6 are structurally higher than the syenite of stop B-3, note the greater
abundance of dark enclaves.
Stop B-5: Nepheline Syenite Chemin Irelandais / Shannon
Stop B-6: Nepheline Syenite Chemin Irelandais / O’Connor
Stop B-7: Nepheline Syenite (tinguiate)
From the corner of Chemins de Gaspé and de Missisquoi, walk along the trail to the low
outcrop ridge. This outcrop represents a small late-stage body of aegirine-bearing nepheline
syenite (tinguaite). Examine the rock face in the hollow on the East side of the trail. How
does this unit compare texturally to the lithologies of the last three stops? What is the
significance of this difference?
Stop B-8: Syenite (pulaskite) with dykes
Head North on Chemin de Gaspé, take a right on Chemin de Chateauguay, and go to Chemin
Champlain and the Ski Centre parking lot. Walk up the ditch along the road above the
parking lot. This is another outcrop is the dominate felsic lithology of the Brome complex. Be
on the alert for small cross-cutting dykes and enclaves (xenoliths). How many different dyke
lithologies appear to be present?
Stop B-9: Siltstone Hornfels
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2013 Igneous Petrology Field Trip – Francis
Mont Brome Complex
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2013 Igneous Petrology Field Trip – Francis
Take Chemin Champlain to return to autoroute #10 and head towards Montreal, exit route
#227 for Mont. St. Gregoire. Drive ~ 5 km south and turn right towards the Mountain, follow
the signs for the CIME centre at Mont St. Gregoire.
Mont St. Gregoire
Climb the Panorama trail to the summit of Mont St. Gregoire. Identify the mapped units
along the way. In addition to basic lithology, characterize each map unit in terms of grainsize, presence or absence of layering, etc. Gather at the top for summary discussion.
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2013 Igneous Petrology Field Trip – Francis
Return to Autoroute #10 towards the West (Montreal), take route #134 North to the Jacques
Cartier Bridge, take the exit to Ile St. Hélène on the Bridge, and stop at Museum parking lot
under south access ramp.
Ile St. Hélène Diatreme Breccia
This stop is an exposure of the Ile St Hélène diatreme breccia pipe. A breccias pipe is a
fragmental intrusive body that is approximately carrot or funnel shaped, opening as a cone
towards the surface. It is thought to have been emplaced explosively as a solid–gas mixture
and contains many accidental fragments of the surrounding host rocks. The characteristic
feature that distinguishes diatreme breccias from other types of igneous breccias is that their
matrix is clastic rather than magmatic. At depths, such pipes commonly grade into dykes of
CO2-rich
magmas,
such
as
kimberlites,
aillikites,
or
carbonatites.
The present breccias are hosted by
Ordovician Trenton Limestone and
Utica Shale, but approximately 1km
of overlying Paleozoic strata (~ 1km)
has been lost through erosion since
the
diatreme’s
emplacement,
estimated by the recognition of
Devonian brachiopods in some
limestone fragments. Note the many
different
lithologies
that
are
represented in the fragments at this
location?
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