Emplacement of the Monteregian Hills of Quebec; Geophysical evidence

Tecfonophysics,

Elsevier Scientific

Publishing

EMPLACEMENT GEOPHYSICAL

MAURICE

305

86 ( 1982) 305-3 17 Company,

Amsterdam-Printed

in The Netherlands

OF THE MONTEREGIAN

HILLS OF QUEBEC;

EVIDENCE

K. SEGLJIN

Department of Geology, UniversitP Lava& Quebec, GIK 7P4 (Canada) (Received

October

12, 1981; revised version accepted

January

26, 1982)

ABSTRACT

Seguin,

M.K.,

1982.

Emplacement

Teetonophysics, 86: 305-3 Constraints emplacement

are placed

pattern

saucers

nourished

dipping

normal

overlying

Integrated

information

indicate

by thin planar faults

motion

a continental

Monteregian

Hills

of

of intrusions

Quebec;

of the Monteregian

data (magnetic,

that the Monteregian

gravimetric

geophysical

evidence.

basement

Field geological

of the cooling

magma

and

intermediate

evidence,

and seismic reflection)

magma

tectonically

controlled thrust

is a more acceptable

rather

and geophysical

by steeply faults

injection

data, partial

origin for the Monteregian

in the

data indicate

than sub-horizontal

and paleomagnetic

and

having the form of

to low-angle

geochemical

and a sub-vertical

cold state of the intruded

melting of an olivine basalt magma

Hills and a model of their

Hills are shallow intrusives

feeders. These feeders are apparently

sequence.

In view of the relatively

progressive

the

geophysical

in the crystalline

Lower Paleozoic

a slow ascending mode.

on the shape

is proposed.

drainage

of

17.

and

Hills than

hot spot theory.

INTRODUCTION

Hilly igneous bodies which intrude the Lower Paleozoic sedimentary sequence of the St. Lawrence Lowlands and the western border of the Appa~ac~ans of southern Quebec were first described by F.D. Adams (1903). He recognized that the Montere~an Hills (MN) were genetically related products of regional magmatic activity and assigned them to a distinct consanguinous group that he called the “Monteregian Hills Petrographic Province”. As originally defined by Adams, the province included from west to east the following Monteregian Hills: Mt. Royal, Mt. St. Bruno, Mt. Johnson (St. Gregoire), Mt. St. ‘Hilaire, Mt. Rougemont, Mt. Yamaska, Mt. Shefford, and Mt. Brome and their satellitic dykes, sills, plugs and related breccia phenomena. Stansfield (1923) described the carbonatite complex near Oka as the ninth and most westerly of the Monteregians Hills. Osborne (1935; in Graham, 1944) suggested the inclusion of Mount Megantic. This addition was confirmed by subsequent 0040-i 95 1/~2/~-~/$02.75

0 1982 Elsevier Scientific

Publishing

Company

geological

and geophysical

1981). Subsequently, Kumarapeli

on Mount body

Megantic

(the

Grand

et al.. 1968) and the Boucherville

M.R.N.Q.,

1978) were shown to belong

Different

origins

field evidence, eroded

studies

the Iberville

were proposed

O’Neiil

volcanic

neck.

Royal was originally

For similar

1960, 1976: Seguin.

intrusive

by

et A.. 1976:

province.

the MH. On the basis of five pieces of

that St. Hilaire

reasons.

A decade

described

mass (Telford

to the same petrographic

to explain

(1914) concluded

a volcano.

(Reid,

Bois anomaly

Buchan

(1901)

later, an eruptive

mountain

represents

suggested origin

that

an

Mount

for the MH was

abandoned for a long period of time and few geologists disputed the intrusive origin of the Monteregians (e.g. Clark, 1955. 1972). The problem of the form of the intrusion of the MH is still being debated. At an early date, it was held that most of the Monteregians were a laccolith. Later on. after 1930 or so, the igneous bodies were visualized as long sub-vertical to conical intensive conduits. Quite recently, an origin of the MH based on the continental intraplate “hot spot’” has been proposed (Foster

and Symons,

1979).

Much can be learned about their three-dimensional shapes. constraints

the mode of emplacement of igneous bodies from In this paper, I first use geophysical data to place

on the shape of the MH intrusive

bodies and then re-open

the discussion

of their origin. GEOLOGiCAL

SETTING

AND

GEOGRAPHICAL

DISTRIBUTION

OF THE

MONTEREGIAN

HILLS

The Monteregian Hills are spread at varying 300 km along a somewhat curved belt extending

Fig. 1. Map showing 4= St. H&ire, II = Mount

the distribution

5 = Iberville.

M(%antic.

intervals over a distance of about from Carillon at the northwest to

of Monteregian intrusives. 7=Rougemont,

6 = St. Grkgoire,

I = Oka, 2 = Mount Royal, Z= St. Bruno. Il=Yamaska. 9=Brome. lO=Shefford,

307

Mount

Megantic

the St. Lawrence

at the southeast; Lowlands.

they rise abruptly

The geographic

limits of the Monteregian

Province is not fully known (Fig. 1). The igneous rocks of the Monteregian Pouliot,

1969) as large plug-like

genetically

related

(- 350 m) above the plain

Hills were described

masses and as numerous

to the plugs. The group of Monteregian

Petrographic

(e.g. Clark,

scattered

in

1955;

dykes and sills

Hills is characterized

by

the relatively rare alkaline suite of intrusive rock types and the concentric arrangement of the various rock types is explained by deep-seated differentiation and successive intrusions along the axial portion of the plugs with the more acidic rock-type toward the wall of the pipe-like body and the most basic in the central core (Adams, hornfels and

1913). The intrusive igneous rocks are surrounded by an aureole the sedimentary beds were not much disturbed by the process

intrusions. An outstanding feature of the Monteregian of the rocks of this petrographic province. AIRBORNE

MAGNETIC

(AM) AND

PALEOMAGNETIC

Hills is the alkaline

of of

character

DATA

Few geophysical studies of the MH and their surroundings have been published. However, AM, gravity and seismic reflection data are available. We shall first proceed to a semi-quantitative interpretation of the AM anomalies caused by the MH and thereafter discuss the relative importance of the remanent component. The magnetic 1978); it consists

signature

for the whole of the MH is quite

of a simple or multiple

positive

peak followed

similar

(Fig. 2). Generally, the two peaks are oriented along a N-S to NNW-SSE positive peak being located to the south and the negative to the north. MONT

r

I

RELATIM

Fig. 2. Airborne SSE-NNW

magnetic

profile

profile is characteristic

peak

axis, the

YAMASKA

I

I

I

SSE

(M.R.N.Q.,

by a negative

2 DISTANCE

on Mount

1

I

3

4

(Km)

Yamaska.

of the Monteregian

The magnetic Hills.

NNW signature

represented

by this main

The multiplicity explanations:

of the positive

(1) Heterogeneity magnetic

segment

of the lithology:

of the AM anomaly

each lithology

has two possible

being characterized

by its own

susceptibility.

(2) Presence

of natural

nent may either increase

remanent

magnetization

or decrease

the intensity

(NRM);

thus, the NRM compo-

of the measured

AM anomaly

and

change the orientation (D and I) of the magnetic polarity with respect to the present Earth’s field north pole (D = 344. I = 74). In the case of the MH. both normal (N) and

reverse

during

(R) polarities

Lower Cretaceous

are listed

in TableI;

may

have occurred

time. The directions

the data

are obtained

in different and polarity

subsequent

intrusions

of the NRM component

from Larochelle,

(1962.

1968, 1969).

Foster and Symons ( 1979) and Seguin (198 1). The magnetic signature as described in Table1 is similar for all the MH except for Brome and St. Bruno. In the case of Brome, this difference is easily explained by the proximity to the north edge of the mount

of a positive

anomaly

caused

by Mount

Shefford.

This positive

anomaly

masks the small negative anomaly to be expected on the north edge of Brome. The case of St. Bruno is more complex and an explanation of the reversal in the magnetic signature

TABLE

may be found

in a combination

of two factors:

I

Magnetic

characteristics

of the Monteregian

Intrusive

Hills

Sign of anomaly

Remanent

north

direction

south

D 339

Oka Mount

+

Royal

146

component polarity I

60

N R

-65

_

Boucherville

O--

f

St. Bruno

+ _.

153

-55

R

+

142

-- 54

R

+

327

St. Hilaire Rougemont Mount

Johnson

(St. Grkgoire)

_

Iberville Yamaska

+

Brome Shefford Mtgantic

* N = normal,

R = reverse, M = mixed

+ + + + + +

I 169 333

22(?) -54 62

MC) N

321

47

156

~~62

152

-56

R

165

~62

R

151

- 53

350

54

M

M

*

309

(1) Reverse

NRM

than the induced

component

the intensity

component.

(2) A basically

different

which is responsible

shape

of this intrusive

for an important

of St. Bruno and to a lesser extent other MH (Larochelle, In general, magnetization

of which is of the order of or larger with respect

demagnetization

Brome are higher (5 A m-‘)

1968) and consequently

to the other

effect. The NRM

ones

intensities

than the ones of the

factor (1) is probably

for most of the lithologies of the Monteregian Hills, is predominant over the NRM component as ascertained

preponderant. the induced by the signs

of the anomalies in Table I. In addition, some intrusives related to the MH can be detected by AM surveys; this is the case of the Boucherville subsurface elliptical plug which has an estimated strike length of 8 km, width of 4.2 km, dip of 40°SE and which is buried at about 800 m below ground level with a depth extent of some 3 km susceptibility contrast of 4.8 - 10-l the residual and regional magnetic

along the dip and an apparent The interpretation of both

A ml’. anomalies

corre-

sponding to the position of the MH yielded complementary information. For instance, in the case of the Iberville residual anomaly, the interpretation indicates an intrusive body having a circular diameter of about 1.1 km in plan; the vertical depth extent of this circular body is estimated at I km and its plunge approximately 75%. The depth of burial of the upper pole is calculated apparent susceptibility contrast at 1.06 A m-‘.

at 60m

below surface

and its

The model which fitted best the residual magnetic component of St. Bruno is an inverted tetragonal pyramid with its base at the surface and its apex at a vertical

4200

ST.

-

I

f

‘SE

I OBSERVED PRINCIPAL

I

I

2

4

SRUNO I REGIONAL TRANSVERSAL

transverse

profile

regional

magnetic

anomaly

intrusive

coincides

with the normal

DISTANCE

of the regional

was processed

I 1

NN’

I

I

I

8

6 R!%AllVE

Fig. 3. Principal

I ANOMALY PROFILE

IO

(Km)

aeromagnetic

in the wavenumber

fault in the crystalline

anomaly domain.

basement.

over Mount The position

St. Bruno.

The

of the St. Bruno

depth

extent

of 700m;

which is slanted The

regional

Figure3

the apparent

magnetic

component

shows the principal

intensity

step (180~)

faulted

susceptibility

to the SSE is estimated

of this anomaly

transverse

profile

in the 2.3 km central

model with the Precambrian

contrast

10 ’ A m

at 4.5

is also

of this regional

interval

basement

of thus magnetic

body

‘_

of great

interest.

anomaly.

The large

of the figure is characteristic

block to the SSE stepped

of a

down with

respect to the NNW block. It is instructing to note that the St. Bruno instrusive is located slightly to NNW of the mid-slope on the central part of the observed regional AM anomaly. This example demonstrates clearly the space relationship between

deeper

crustal

dislocations

and the location

of the intrusives.

The older

faults appear to have been reactivated locally at Lower Cretaceous time and served as planar conducts for the injection of magma which created the MH. GRAVIMETRIC

DATA

Gravity surveys constitute another source of information mode of emplacement of the MH. The information obtained two types:

TABLE

useful to determine the from gravitv data is of

II

Gravimetric

anomalies

of Monteregian

Name

Hills *

Location

Radius.

R

(m)

Agn,#*

An

Thickness,

(mgal)

(gem +)

(m)

long. w

lat. N

74001’

45”30

Mt. Royal

73”36’

45030

1100

4.7

0.23

+so+?

Mt. St. Bruno

73’20

45033’

1300

6.0

0.24

i 1000

Mt. St. Hilaire

730 10’

45O34’

1650

4.2

0.23

+600

Mt Rougemont

73”03’

45”29’

10.4

0.26

-z_12.50

73009

45”21’

Oka

z

_

Mt. Johnson (St. Gregoire)

0.21

500

Iberville

I

73” 12’

4.5’22’

450

1.7

0.2

Mt. Yamaska

72”52’

45”28’

1350

7.8

0.26

_

0.22

-

0.22 _

(Grand

Bois)

Mt. Brome

72”38’

45017’

4150

Mt. Shefford

72’36’

45’22’

2050

Mt. Megantic

71014’

45”26’

4250

* R =mean gravity

radius for an equivalent

anomaly

intrusive equivalent

body

at the centre and

circular

the sedimentary disk.

circular

area of a thin vertical cylinder;

of the vertical country

cylinder; rock;

be =estimated

z =calculated

depth

4 300 1100

4

Ag max=maximum density extent

contrast

residual

between

or thickness

the

of the

(1) regional, crystalline

i.e. gravity

basement

(2) local, because

under

surveys

and

the St. Lawrence

the density

the MH and the sediments

can

contrast

do detect

and

locate

faults

in the

Lowlands;

between

from the St. Lawrence

the basic and ultrabasic lowlands

rocks of

is large, even the small

intrusive bodies can be outlined by the gravity method. Thus, in view of the existence of a sufficient density

contrast

between

the

Precambrian crystalline basement and the lower Paleozoic sediments on the one hand and the Monteregian intrusives and the same Paleozoic sediments on the other hand, the positioning and the vertical displacement along gravity faults in the basement

as well as the dimension

from the gravity

data.

and the general shape of the MH can be obtained

Unfortunately,

the density

of the gravimetric

data (E.P.B.,

1978) is often insufficient to achieve these goals and to find out whether there exists a space relationship between the interpretation of the basement gravity faults and the occurrence of the MH using the gravity data. However, even if the density of gravity data satisfactory

modelling

of the individual

is often

Monteregian

insufficient

instrusive

bodies,

to allow

a

a reasonably

good estimate of their thickness (depth extent) may be obtained from the approximate maximum residual intensity observed over the intrusives (Table 11). A nearly vertical SEISMIC

circular

cylindrical

REFLECTION

model was used to carry out these calculations.

EVIDENCE

Detailed seismic reflection profiles were first run in 1969- 1970 by Shell Oil and later (1977-1978) by SOQUIP for oil and gas exploration in the St. Lawrence Lowlands. Some of these seismic reflection profiles are located as close as 0.3 km of certain MH (e.g. St. Bruno, St. Hilaire, Rougemont, Yamaska and Iberville). At all these locations, nearly

the seismic records do not show any break in the reflections

horizontal

observation

Lower Paleozoic

leads to two possible

(1) The walls of the intrusives that no diffractions (2) The intrusives

sediments

in the vicinity

from the

of the MH bodies.

This

conclusions: are perfectly

vertical

and extend

to great depth so

are present. are shallow and have no roots (only a thin dyke-like

umbilical

plane). These two possible geological situations have an important probable mechanism of emplacement of the MH.

bearing

on the most

The seismic reflection data do show low-angle thrusts (+ 30°-45”) with little lateral displacement (Q 1 km) in the lower Paleozoic sedimentary sequence of the St. Lawrence Lowlands. This is often terminated by zone of dragfolding along weakness zones in the shaly Groups (Utica, Lorraine or Richmond). Theses zone coincide often with the location of the MH. Such weakness zones are most likely filled with dykes and sills which served as feeders to the Monteregian Hills; this appears to be the case for St. Bruno and St.

Hilaire, for instance. fault in the basement In the overlying deformed the

Yamaska is located almost at the emplacement of a normal (vertical dispIacement + 1 km, dip of fault plane 4 65”SSE).

sediments.

shaly material;

velocity

of

the

tochthonous

zone

1 km-thick

crystalline

overthrusting a parautochthonous

sediments

is higher, basement

basement

is not involved

(apparent

doming)

is characterized zone overlies

(limestones

no velocity

by an overriding

and

pull-up

the autochthonous.

sandstones) created

from

in the thrust

sheet(s).

Over Rougemont.

is detected.

the

by the presence

to the NNW of the fault is observed:

caused by the intrusive

pile of

At Ibervitle,

As parauof the

the crystalline a small pull-up the normal

fault

(vertical displacement + 0.3 km, dip of the fault plane + 7VSSE) takes place mainly at the Potsdam level. The trace of this fault is located almost directly under the Iberville intrusive. The upper part of the sedimentary sequence is thrusted {small lateral displacement of + 0.4 km). This thrust represents the northernmost manifestation of the Taconic orogeny in the St. Lawrence Lowlands. As the roof of the platform is situated at approximately limited to this value.

700 m, the depth

extent

of this intrusive

is

Thus, seismic reflection evidences strongly suggest that the geological situation (2) is more acceptable and that the MH have the shape of an inverted saucer or a flat balloon. The feeders of this bubble of magma consists of umbilical chords located along curved fault planes, i.e. subvertical normal faults in the crystalline basement and the basal Lower Paleozoic sedimentary levels and along intermediateto low-angle thrust faults in the upper levels of the Lower Paleozoic sequence. These thrust

faults are characterized

of the typical Appalachian

by their small displacements

high-angle

and are the forerunners

overthrusts.

Deep drill holes which intersected sills and dykes at different levels in the sedimentary sequence also favour geological situation (2). The general NE-trending orientation normal

of the dykes and sills suggest

a structural

control

by both paralleling

and thrust faults.

DRAINAGE

PATTERN

AND STRUCTURAL

CONTROL

If the position of the MH is structurally controlled, the drainage pattern may still show the trace of the old thrust faults. As the empiacement of the MH is younger than the occurrence of faulting, either a disruption or a bifurcation should be observed in the drainage pattern. The data on drainage patterns were taken from D.N.D.C. topographic map (1972); Table III shows the results of the compilation. In order to eliminate as much as possible the problems related to differential erosion caused by important variations in lithological composition, only the MH intruding the Lower Paleozoic sedimentary sequences of the St. Lawrence Lowlands were retained in this compilation. Thus, Oka, Brome, Shefford and Megantic are not included in this compilation. Mt. Royal was also excluded because of man-made operations which obliterated the drainage pattern.

313

TABLE

III

Characteristics Name

of drainage

of MH

pattern Disruption

Correlation

NE sector

SW sector

Creek (c)

Creek (c)

or bifurcation

River (r)

River (r)

(b) *

St. Bruno

Premier

to Masse (c)

NE-SW

in straight

line

(b) to NW

St. Hilaire

Salvail (r)

Hurons

(r)

NE-SW

in straight

line

(b) to SE

St. Grtgoire

St. Louis (c)

Hurons

(r)

N-S

Rougemont

none

none

none

Yrmaska

(c)

(c)

NE-SW

Grand

(c)

* “d” means that the streams

(river or creek) are disrupted

are displaced

of the presence

or bent because

in straight

line

in straight

(d)

(b) to E _ line

(d)

by the MH body; “b” means that the streams

of a MH body.

Except for Rougemont, the compilation of Table III demonstrates clearly that in the St. Lawrence Lowlands, the MH are situated directly on the drainage lineament the trend of which is parallel to the strike axis of the faults. Thus, the drainage pattern also provides some evidence of structural control on the emplacement of the MH. PROPOSED

MODEL

Establishing

OF EMPLACEMENT

a model

OF THE MONTEREGIAN

of emplacement

HILLS

of the MH is important.

In addition

to

being a problem of its own, it is related to another problem which is just as important, i.e. the origin of the magma(s) responsible for the rock units observed. If the structurally controlled sill and dyke injection model is accepted as the mechanism of emplacement of the MH two injection modes are possible: (1) sub-vertical, (2) sub-lateral. First, direction

it is observed that the number of dykes and sills increase in the NNW (few at Megantic, Brome and Shefford, many around St-Hilaire, St-Bruno,

Mt. Royal and Oka). Second, the degree of alkalinity increases in the WNW direction, i.e. the chemical composition of the MH becomes more silicic to the east (Gold, 1967). Third intrusion dominates

explosive intrusion at the ESE end

occurs in the WNW directions and passive (Philpotts, 1970). Fourth, the size of the

intrusions increase in the ESE direction (Brome, Shefford, Megantic). Since the largest masses of MH rocks are situated to the ESE, one would be tempted to invoke a lateral movement of the original magma towards the WNW along weakness zones, i.e. close to the interface of the normal Appalachian thrust faults (a phenomenon According sedimentary

faults in the crystalline basement and the analogous to guided waves in seismology).

to Clark (1972), the contact of the intrusives with the surrounding rocks is an evidence of slow cooling. In addition, the flow structures

observed

in the intrusive

process.

A mainly

magma

in relatively

km). This appears reason, conduits

rocks indicate

lateral injection small planar impossible

a predominantly is more attractive

a slow motion

mode would require and tubular

conduits

on the basis of thermal

vertical

movement

of magma

(Fig. 4). i.e. smaller

of the magma

in the cooling

a fairly rapid circulation

f 150-200

for tong distances conduction

alone.

along structurally

lateral than vertical

of

For this controlled

fluid motion

and

thus a more or less static mode of emplacement of the intrusives in :I normal and possibly underheated coo&g environment with the fluid feeder(s) almost directly underneath the intrusive bodies. ‘The increase in alkalinity to the WNW is probably caused by the greater amount (thickness) of crystalline continental crust digested by the ascending magma. Even though the initial fluids did not have the necessary heat of fusion to melt large amounts

of crustal

rocks, at least the margins

of the intrusives

are the product

of

contamination through assimilation. Laurent and Pierson ( 1973) favour a partial and progressive melting rather than the differentiation of an alkaline oiivine basalt as the origin of the magma. The increase in the number

f

PC

CRYSTALLINE

a

PALEOZOIC

@

ORtGiNAL

Fig. 4. Schematic occurring

reflects the

BASEMENT.

SEDIYENTARY MAWA

UONTEREGIAN PLANAR 7 \EMPLACEMENT

of dykes and sills in the WNW direction

COVER.

POCKETS. PLUYONS.

AN0

CYLINDRICAL OF

MAIN

representation

CONDUITS. THRUST

FAULTS.

of the mode

of emplacement

at the base of the crust or deeper is mainly

attributed

of the Monteregian to stress-release.

Hills. The melting

315

greater

rigidity

sediments

of the crystalline

(shallow)

ments in particular).

basement

than the more plastic Actually,

and of the lower sequence Appalachian

all the MH located

of Paleozoic

rock sequences

in the St. Lawrence

(deep sediLowlands

are

surrounded by shaly materials which acted as an impermeable host rock favouring a dome-shape envelope for the cooling

and easily deformable magma mass. In this

respect,

area1 extent

the MH may be considered

into which the magma

intruded

as thick sills of limited

in a relatively

cool state. Therefore,

cannot be considered as “continental hot spots or thermal Morgan (197 1, 1972) Duncan et al. (1972) and Wilson

(pockets)

such intrusions

plumes” as described (1973) as advocated

by by

Laurent and Pierson (1973) or Foster and Symons (1979). Foster and Symons have shown that the MH plutons were emplaced progressively from WNW to ESE during two normal and two reversed polarity intervals lasting + 2 Ma at 120 2 4 Ma. Some 250 km south, in central Vermont, (1973)

described

WNW

to SSE in an interval

plutons

analogous

to the MH

lasting

and

which

+ 13 Ma (NNW

+ 98 2 2 Ma). Both Laurent and Pierson invoke a westward motion of the North

Laurent

and Pierson

were emplaced

+ 111 * 2 Ma

from

and

SSE

(1973) and Foster and Symons (1979) American plate. Assuming a stationary

Mid-Atlantic Ridge, the calculated westward drift is Fox, 1971). With a stationary thermal plume, a rate calculated for the MH of Quebec. In view of this Symons (1979) assumed a hot spot moving eastward

+ 1.4 cm/year (Le Pichon and of motion of + 6 cm/year is large discrepancy. Foster and at a rate of + 4.5 cm/year. In

New England, Laurent and Pierson (1973) related the progressive of the melts in Mesozoic time (180 Ma) from New Hampshire

change in the age (White Mountain

magma series) and in Cretaceous time (105 Ma) from central Vermont (MH analogs) to westward continental drift. If this is so, an average rate of motion of $0.25 cm/year

only is calculated

the one of 6cm/year. Wander

took place in this time interval.

fails to explain central

for this time interval;

This would then mean

the abrupt

Vermont

(W + E for emplacement same conduit necessitates the scarcity

MH)

change

(+ 150 km apart) vs NNW + SSE

this value is at a large variance that rapid changes

in Apparent

with Path

In any event, a single mobile hot spot (line)

of position and

of its track

the change

for central

between

of orientation

Vermont.

Finally,

of the intrusives (different magmas alternatively at differing period of time) observed by Laurent multiple hot spots and this is statistically of this phenomenon.

unlikely

the MH and in this track the

order

of

intruding upon the and Pierson (1973) taking

into account

Another important point to take into account is the relation between the change in the nature of the melts and the change in depth of magma generation along a specific trend. This idea was stressed by Laurent and Pierson (1973) to relate the New Hampshire Mesozoic intrusions to the Vermont plutons. Applied to the MH, a systematic increase in depth of magma generation is expected from SE to WNW (i.e. from more acidic to more alkaline and undersaturated magmas).

DISCUSSION

AND CONCLIJSION

Information

from magnetic,

gravimetric,

sion allows a coarse definition geometry

of a characteristic

saucer shaped

seismic data and topographical

of the typical geometric MH is associated

mass somewhat

analogous

expres-

model of the MH. The overall

to a rootless

to an oil bubble

or very deeply

suspended

rooted

in water. The

geometry of the MH restricts the numbers of mode of emplacement. On the basis of the geophysical data, a structurally controlled model of injection is proposed for the MH. The feeders (multiple dykes and sills. large and small) are concentrated zones of normal faulting in the crystalline basement and in thrust faults within

in the

Paleozoic sedimentary sequence which acted as bottlenecks to the fluid motion. A transfer of largely vertical to largely lateral position for the conduits took place in the vicinity of the interface between these two fault systems. This interpretation provides an alternative to the commonly accepted opinion relative to the emplacement of the MH. The mode of emplacement postulated puts magma(s) for the MH. Different explanations included intrusive

some constraints on the origin of relative to the origin of the MH

processes such as normal volcanic activity, intrusive magma differentiation, partial melting, and hot spot activity in a continental intra-plate regime.

Combining the geophysical information with structural, petrological, mineralogical and geochemical data, the origin of a relatively cold intrusion of normal magma characterized by partial crustal melting is preferred to the other alternatives and in particular to a thermal plume origin. In the Quebec MH and Vermont plutons. it appears that two magmas of contrasted composition are often alternatively intruding upon the same conduit. Under such circumstances, the thermal plume mechanism is inadequate unless the hot spot is converted to a hot line. Based on petrological grounds, a differentiation process of MH intrusive magma is also inadequate. ACKNOWLEDGEMENTS

The author Physics Branch,

thanks

A. Tr&panier

Ottawa)

(SOQUIP,

and P.S. Kumarapeli

Quebec

City), A. Goodacre

(Concordia

University,

(Earth

Montreal)

for

the time spent on discussion of various geological and geophysical features related to the Monteregian Hills. The writer is endebted to the two anonymous referees who improved tions.

the quality

of this paper

through

general

comments

and specific

sugges-

REFERENCES

Adams,

F.D.,

1903. The Monteregian

Hills-A

Canadian

Petrographical

Province.

J. Geol..

I I (3):

239-282. Adams,

F.D..

1913. The Monteregian

Hills. Geol. Sure. Can., Int. Geol. Cong..

12th. Guideb..

3: 29-80.

317

Buchan,

J.S., 1901. Was Mount

Royal an active volcano?

Clark.

T.H.,

1955. St. Jean-Beloeil

Clark,

T.H.,

1972. Region

Department Duncan,

de Montreal

of National

4, Scale:

Defense,

R.A., Petersen, Physics

Branch,

Townships. Foster,

(D.N.D.C.),

Geol. Rep., 66: 37-47.

Geol. Rep., 152: 115-154.

1972. Topographic

Map, Montreal

Area, Edition

Nature,

R.B., 1973. Mantle

plumes,

movement

of the European

plate,

239: 82-86.

Gravity

Division,

EMR,

Canada,

1978. Bouguer

anomaly

map of the Eastern

Open file No. 7% 12 (A). Map at a scale of I : 500.000. D.T.A.,

1979. Defining

a paleomagnetic

polarity

pattern

in the Monteregian

Can. J. Earth Sci., 16: 1716-1725.

D.P.,

1967. Alkaline

Ultramafic Graham,

Canada,

N. and Hargraves,

J. and Symons,

intrusives. Gold,

of Quebec,

area. Min. Rich. Nat. Quebec.

I: 250,000.

and polar wandering. Earth

Can. Rec. Sci., 8: 321-328.

area. Dep. Mines. Province

R.P.D.,

Geology.

P.S., Coates,

of another

rocks

in the Montreal

area,

Quebec.

In: P.J. Wyllie

(Editor),

Rocks. Wiley, New York, pp. 288-302.

1944. Montere~an

II. Descriptive Kumarapeli,

ultrabasic

and Related

Hills. In: J.A. Dresser

and T.C. Denis, The Geology

of Quebec,

Vol.

Que. Dep. Mines, Geol. Rep., 20: 455-482. M.E. and Gray, N.H.,

Monteregian

pluton.

1968. The Grand

Bois anomaly:

the magnetic

expression

Can. J. Earth Sci., 5: 550-553.

Larochelle,

A., 1962. Paleomagnetism

of the Monteregian

43 PP. Larochelle,

A., 1968. Paleomagnetism

of the Monteregian

Hills, S.E. Quebec.

Geol. Surv. Can. Bull., 79:

Hills: new results. J. Geophys.

Res.. 73 (10):

3329-3346. Larochelle,

A., 1969. Pal~ma~etism

of the Monteregian

Hills: further

new results. J. Geophys.

Res., 74

(10): 2570-2575. Laurent,

R. and Pierson,

Peninsula, Le Pichon,

T.C.,

1973. Petrology

J. Geophys. des Richesses

tiques residuelle,

of alkaline

rocks

from Cuttingsville

and the Shelburne

Can. J. Earth Sci., IO (8): 1244-1256.

X. and Fox, P.J., 1971. Marginal

Atlantic. Ministere

Vermont.

offsets

fractures

zones, and the early opening

of the North

Res., 76: 6294-6308. Naturelles

regionale

Quebec-Sherbrooke,

de la Province

et extension

Montreal,

Morgan,

W.J., 1971. Convection

Morgan,

W.J., 1972. Deep mantle

de Quebec

(M.R.N.Q.),

vers le bas de la composante

Trois-Rivieres

et Mont-Laurier.

plumes in the lower mantle. convection

1978. Composantes

magnetique Rappt.

Nature,

regionale;

magnb regions de

DP 709.

230: 42-43.

plumes and plate motions.

Am. Assoc. Pet. Geol. Bull., 56:

203-213. O’Neill, J.J., 1914. St. Hilaire Philpotts,

A.R.,

(Beloeil) and Rougemont

1970. Mechanism

of emplacement

Mountains,

Quebec.

of the Monteregian

Geol. Surv. Can., Mem., 43.

intrusions.

Can.

Mineral.,

IO:

395-410. Pouliot,

G. (Editor),

of Canada, Reid, A.M.,

1969. Guidebook

Montreal, 1960. Rapport

Frontenac. Reid, A.M.,

for the Geology

pr~li~najre

Min. Mines Quebec,

Hills. The Geological

Association

1976. Geologic

fur la geologic

de Mont Megantic.

J., 1923. Extensions

du Mont Megantic,

Corn&

de Compton

et de

R.P. 433: 6 pp.

Seguin, M.K., 1981. Etude paleomagnttique In press. Stansfield,

of Monteregian

169 pp.

Min. Rich. Nat. Quebec,

des formations

of the Monteregian

rocheuses

Petrological

ES-25: 59 pp.

du Mont M&antic

Province

et des environs.

to the West and Northwest.

Geol. Mag., 60: 433-453. Telford,

W.M.,

University

Geldart,

L.P.,

Press, Cambridge,

Wilson, J.T., 1973. Mantle

Sheriff,

R.E. and

Keys,

D.A.,

1976. Applied

Geophysics.

pp. l95- 198.

plumes and plate motions.

Tectonophysics,

19: 149-164.

Cambridge