MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
55
Paleomagnetism of the Avalonian Finn Hill sequence
of Eastern Newfoundland, Canada
fl.K. -Se.gu-in, TacvJtty o/ Sc-i&nce. and E.ngln&eyving
UnJ,vesu>j±e. Laval, QueJLe.c., Canada Q1K 7P4
As p a r t of a m a j o r study of the Avalon zone in eastern Newfoundland, the paleomagnetism of t h e Finn
Hill ignimbrite sequence located in the Colliers-Harbour Main area is described. Detailed a l t e r n a t i n g
field and thermal e x p e r i m e n t s performed on this Hadrynian ash-flow tuff sequence indicate t h a t it is
c h a r a c t e r i z e d by two significantly d i f f e r e n t mean directions of magnetization: I) A(S=l28, I=+25, a g s =7.1°,
K=171, N=4 sites) in-situ and (D=225, D + 59, 095 =13°, K=48) a f t e r tilt correction, 2) B (D=69, I=+52, N=
2 sites) in-situ and (D=303, I=+40) a f t e r tilt correction.
The A component is believed to be secondary and the paleopole corresponding to the in-situ direction of
m a g n e t i z a t i o n is 14°S, 1°W. The B tilt c o r r e c t e d component (D=303, I=+40) is i n t e r p r e t e d as a p r e - or
syn-folding direction of magnetization corresponding with the time of formation of t h e ignimbrite
sequence or slightly later; its corresponding paleopole position is 39°S, 29°E. It is worth noting t h a t t h e
paleopole (14°S, 1°W) of the secondary component found in the present study is not significantly d i f f e r e n t
from the paleopole (5°S, 8°W) obtained in t h e Cloud Mountain basalt overlying Grenville b a s e m e n t of
northwestern Newfoundland, the age of which is close to latest Precambrian (=620 Ma), this suggests
that in l a t e P r e c a m b r i a n t i m e , the eastern and western Newfoundland blocks w e r e not very d i s t a n t from
each o t h e r or else they were f a r apart but at the same paleolatitude. The secondary component is i n t e r p r e t e d as representing t h e m a g n e t i c imprint of the Avalonian (Cadomian) orogenic event. This last i n t e r pretation is reasonable since nearby Cambrian units (to the west) a r e located unconformably above the
P r e c a m b r i a n units and a r e almost flat lying. Consequently, the chances of this secondary c o m p o n e n t
being of Acadian age a r e relatively small. The new data set of this reasearch fills a d e f i n i t e gap in the
p a l e o m a g n e t i c record of the Avalon zone.
La description du paieomagnetisme de la sequence d'ignimbrite de Finn Hill, situfee dans la region de
Colliers - Harbour Main, s'insere dans le c a d r e d'une e t u d e d'envergure de la zone Avalon de l'est de
T e r r e - N e u v e . C e t t e sequence hadrynienne de tuf volcanique a <5t£ soumise a des experiences poussSes de
c h a m p a l t e r n a t i f e t d'anlyse thermique. Les r e s u l t a t s indiquent que la sequence est c a r a c t e r i s e e par deux
sens moyens de l'aimantation qui d i f f e r e n t de fapon significative: 1) A(S=128, I=+25, ogs =7.1°, K=I71, N=4
sites) in-situ e t (D=225, D+59, 095 =13°, K=48) apres correction de l'inclinaison, 2) B (D=69, I=+52, N=2
sites) in-situ e t (D=303, I=+40) apres correction de l'inclinaison.
La c o m p o s a n t e A serait secondaire e t le paleopole correspondant au sens in-situ de l ' a i m a n t a t i o n est
de 14°S, 1°0. On i n t e r p r e t e la composante d'inclinaison B corrigee (D=303, I=+40) c o m m e le resultat d'une
a i m a n t a t i o n qui a precede ou qui e t a i t c o n t e m p o r a i n e au plissement, e t correspondant done au t e m p s
d ' e m p l a c e m e n t de la sequence d'ignimbrite, ou survenant peu apres; la position du pal6p61e correspondant
est de 39°S, 29°E. II est interessant de noter que le palSpole (14°S, 1°0) de la composante secondaire,
r e l e v e dans la pr6sente etude, ne d i f f e r e pas de fagon significative du paiepSle (5°S, 8°0) obtenu a
p a r t i r du b a s a l t e de Cloud Mountain qui recouvre le socle grenvillien dans le nord-ouest de T e r r e - N e u v e
e t dont l'Sge est proche du Precambrien tardif (620 Ma). Ceci suggere que t a r d durant le P r e c a m b r i e n ,
les blocs est e t ouest de Terre-Neuve n ' e t a i e n t pas t r e s distants, ou bien qu'ils e t a i e n t t r e s 61oign6s 1'un
de l ' a u t r e mais situes a la mSme paleolatitude. On i n t e r p r e t e la composante secondaire c o m m e une
e m p r e i n t e magnetique repr6sentant l'orogenese avalonienne (Cadomien). C e t t e derniere i n t e r p r e t a t i o n est
raisonnable puisque les unites cambriennes voisines (a l'ouest) reposent subhorizontalement e t en discordance sur les unites pr6cambriennes. Consequemment, il est peu probable que c e t t e composante secondaire soit d'Sge Acadien. Ces nouvelles donnees comblent une lacune i m p o r t a n t e dans l'histoire paleom a g n e t i q u e de la zone Avalon.
[ T r a d u i t p a r le j o u r n a l ]
INTRODUCTION
The e a s t e r n s e g m e n t of t h e Appalachian
orogen in N e w f o u n d l a n d is underlain by
L a t e P r e c a m b r i a n (Hadrynian) rocks. The
P r e c a m b r i a n orogeny which a f f e c t e d t h e s e
rocks was n a m e d "Avalonian" a f t e r t h e
Avalon Peninsula of e a s t e r n N e w f o u n d l a n d
(Lilly 1966, Poole 1967) w h e r e they a r e
best exposed. S y s t e m a t i c mapping p r o g r a m s
of t h e Avalon Zone w e r e f i r s t u n d e r t a k e n
by Hutchinson (1953) and M c C a r t n e y (1956,
1957). D e t a i l e d s t u d i e s in t h e e a s t e r n AvaMARITIME SEDIMENTS A N D ATLANTIC GEOLOGY
21, 5 5 - 6 8 (1985)
Ion Peninsula (study area) have been c a r ried out by P a p e z i k (1969, 1970, 1972),
Nixon (1975), Nixon and P a p e z i k (1979) and
King (1979). The P r e c a m b r i a n s t r a t i g r a p h y
was
elucidated
subsequently
by
many
w o r k e r s including King e t al. (1974), King
(1979), Strong (1979) and Williams and King
(1979).
The Avalon Peninsula was a f f e c t e d by
two principal orogenies, a L a t e P r e c a m brian "Avalon orogeny" and t h e "Acadian"
orogeny of Middle to L a t e Devonian a g e
(Williams e t al. 1972, D a l l m e y e r e t al.
1983). D i f f e r e n t t e c t o n i c models have been
07U-1150/85/030055-14$3.10/0
56
M.K. Seguin
proposed for the origin of the Avaion
mini-continent: 1) volcanic islands (Hughes
and Bruckner 1972) and ensialic island arc
(Rast et al. 1976); 2) subduction and Basin
and Range - type rifting (Papezik 1970,
Schenk 1971, Strong et al. 1974, Strong
1979); 3) continental (Basin and Range)
extension and rifting (Rankin 1976). In
recent models the Avaion zone is regarded
as t e r r a n e exotic to North America and
a suspect t e r r a n e (Williams and Hatcher
1982, 1983). Precambrian rocks similar
to the ones in the Avaion Zone of Newfoundland occur along the length of the
Appalachian orogen in Nova Scotia, New
Brunswick, eastern Massachusetts, southeastern
United S t a t e s (Carolina
Slate,
Raleigh,
Eastern Slate,
Kiokee
Belts)
(Schenk 1971, Rankin 1976, Williams 1979),
and on the eastern side of the Atlantic,
i.e. Wales, Brittany, the Iberian Peninsula,
Czechoslovakia and Turkey as well as in
northeast A f r i c a (Choubert 1935, Schenk
1971, Cogne 1972, Rast et al. 1976, Strong
1979, Williams 1979, O'Brien et al. 1983).
GEOLOGICAL SETTING
In the Colliers-Harbour Main study area
(Fig. 1), the L a t e Precambrian rocks of
the Avaion Zone are subdivided into three
groups: 1) Harbour Main Group volcanic
rocks; 2) Conception Group composed of
volcaniclastics considered to be derived
from the Harbour Main Group; 3) an upper
sedimentary succession consisting of the
St. John's Group and Signal Hill Group.
North of Colliers (Fig. 1), these groups are
overlain locally by Lower Cambrian shales.
The Harbour Main Group is composed of
marine and terrestrial volcanics. Generally,
the lithological groups are weakly metamorphosed and contain prehnite-pumpellyite
sub-facies mineral assemblages. The Harbour Main Group is intruded to the south
by the Holyrood granite. Contact m e t a morphic e f f e c t s with some rock units of
the Harbour Main Group near and within
the Holyrood plutonic rocks have been
recognized, but not within the ignimbrites.
Small basic dikes (diabasic gabbro and diabase) cut the Harbour Main Group.
The Finn Hill ignimbrite sequence, part
of the Harbour Main Group, has a strike
length of 6.4 km (strike - 10°, dip - 80°W)
and comprises 20 recognizable lithological
units, including 12 ash-flow units intercalated with epiclastic breccia and finergrained sediments (Papezik 1969, 1972 see (Fig. 2). The largest volume of ignimbrites consists of reddish-gray to dark red
crystal t u f f s and vitric t u f f s with variable
amounts of albite, quartz, and biotite
crystals and lithic fragments in a recrystallized vitroclastic matrix cemented with
h e m a t i t e dust. Pyroclastic units include
crystal t u f f s with pumice fragments. The
mineralogy of the crystal fraction is characterized by small amounts of augite and
sparse chloritic pseudomorphs a f t e r olivine.
Basalts and porphyrite fragments are inclusions in the ash-flow sheets and pyroclastic
units. In summary, the ignimbrites include
an important proportion of felsic pyroclastics
(including
ash-flow tuffs) and
intercalated terrestrial sediments and a
smaller fraction of basaltic flows; the
environment is typically subaerial. The primary textures of these rocks are well preserved; a lack of penetrative deformation
is also evident.
AGE
Anderson (1972) suggested an age range
of about 800-600 Ma for the Conception
Group while Stukas (1977) provided 4 ( 5 Ar/
39
A r ages for Harbour Main Group volcanics as old as 1500 Ma (see also Stukas
and Reynolds 1976); this last age is anomalous. Rast et al. (1976) and Strong (1979)
correlated the Love Cove Group volcanics
from the Burin Peninsula,
southeastern
Newfoundland
with
the
Harbour
Main
Group. Rast et al. (1976) correlated the
largest part of the Coldbrook Group volcanic rocks with the Love Cove and Harbour Main Groups and they assigned an age
ranging from 750 to 650 Ma to the Coldbrook Group. Finally, on the basis of the
observed
field relationships,
McCartney
(1967) considers the Holyrood pluton as a
single intrusion post-dating the Harbour
Main Group volcanics; the granitic portion
of the pluton has been dated at 575±11
Ma using the Rb-Sr _ method (609+11 Ma
using &7Rb = 1.39 x 10 ^yr). Krogh et al.
(1983) obtained an age of 621+2 Ma.
In summary, the Harbour Main Group
volcanics are older than 620 Ma and prob-
57
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
ably younger than 800 Ma; an intermediate age of about 750 Ma is plausible
and on this basis King (1970) suggested
that the Harbour Main Group may be
related in t i m e to the Vendian of northern
Eurasia.
Field and laboratory sampling procedures
Some 35 oriented samples (99 specimens)
were collected on seven different sites;
the orientation was taken with a Brunton
compass. The number of samples per site
was fixed at 5 and the number of specimens per sample varied between 2 and 4.
The majority of the samples were collected
in red t u f f a c e o u s rocks and the others
(sites 8,9 as well as 4 out of 5 samples
from site 14) in grayish-green to dark green
t u f f a c e o u s rock types (Fig. 2). The oriented
hand samples were cored in the laboratory.
The sample localities were chosen with
r e f e r e n c e to detailed geological maps (Nixon
and Papezik 1979). Great c a r e was taken
to avoid excessively altered rock units.
EQUIPMENT USED
The direction and intensity of remanent
magnetization were measured with a digital
spinner m a g n e t o m e t e r (model DSM-1) manuf a c t u r e d by Schonstedt Inst. Co. coupled
53°30'
48°00'
47°45
47 3 0
Conception Group K
Harbour Main Group K&jjjl
Unseporated Units l-'wH
Including St. John's
and Signal Hill Croups
53 30
52°00'
Fig. 1 - General geology of the northern Avaion sector of Newfoundland and location of the study area.
58
M.K. Seguin
NIXON AND PAPEZIK
LEGEND
L
SILTSTONE, SANDSTONE (Conception Group)
DIABASE DYKES
BASALT FLOWS
RHYOLITE S I L L S
PORPHYRITE INTRUSIVES
VOLCANOGENIC SEDIMENTS
ASH-FLOW TUFFS, TUFF-BRECCIA
POND
FAULT
CONTACT (DefinedApproximate)
STRIKE AND DIP OF PLANAR STRUCTURES
PALEOMAGNETIC SAMPLING SITES
METRES
F i g . 2 - D e t a i l e d g e o l o g y of t h e s t u d y a r e a ( a f t e r N i x o n a n d P a p e z i k 1979) a n d d i s t r i b u t i o n of t h e
p a l e o m a g n e t i c s a m p l i n g s i t e s (HB 8 t o H B 14). F H S = F i n n Hill s e q u e n c e a n d WHS = W e a v e r ' s Hill
s e q u e n c e . T h e s i t e s a r e d i s t r i b u t e d o v e r s o m e 7 0 0 m e t e r s , t h a t is i t s e n t i r e t h i c k n e s s .
to PDP-1 and APPLE II minicomputers in
tandem. The volume susceptibility was measured at room temperature in the Earth's
field (H = 0.05 milliTesla (mT)), using a
Cenco MS-3B susceptibility bridge. Alternating field (AF) demagnetization was carried out using a demagnetizer built at Universite Laval having a maximum peak-topeak field of 90 mT, the performance of
which was largely improved by adding three
large concentric mu-metal cylinders around
the solenoid (Seguin 1975); a three-axis
tumbler was used with this unit. Other AF
demagnetizations were done using a Schonstedt GSD-1 apparatus (100 mT peak-to-peak
field). Stepwise thermal
demagnetization
tests were carried out using a Schonstedt
demagnetizer (TSD-I). The amount of residual ambient field is 5 to 10 nT in the AF
and thermal d e m a g n e t i z e s respectively.
REMANENT ORIENTATIONS AND
INTENSITIES
Sample-mean natural remanent magnetization (NRM) directions (Fig. 3), although
characterized
by
relatively
pronounced
smearing of positively inclined steep remanence, have a pronounced deviation away
from the present Earth's field (PEF) (D=
335°, 1=76°) at the site localities. Most of
the NRM directions are located in the
southeastern quadrant in a subvertical plane
oriented at 130°±20°. The extreme NRM
directions are roughly aligned with the PEF
and the A component respectively. Component A is obtained by both AF and thermal demagnetizations and will be described
in the two next sections. This observation
indicates that different samples contain
varying proportions of the A component and
the PEF-directed component. The site-mean
direction of the remanence is D 0 = 128°,
I 0 = 67°, N=7, a9 5=25.7°, K=6.5 for the Finn
Hill sequence (FHS) in-situ and
= 267°,
I x = 29°, a 9 5 = 24.9°, K=6.9 when tilted
back to the horizontal. These directions
indicate that the FHS rocks do carry some
ancient remanence especially since the
directions of the remanence for the four
samples that lie close to the PEF were
found to move away from these initial
directions during subsequent AF or thermal
demagnetizations. The intensities of NRM
(J n ) v a r y f r o m 2 x 10 3 to 5 x 10"1 Am 1
with Koenigsberger's ratio (Q n = J n /KH)
between T).2 and 20 but with predominant
ratios between 0.4 and 5 (Fig. 3).
a) AF demagnetization
For the pilot AF study, at least 10 but
usually 12 specimens for each of the seven
sites of FHS were demagnetized in 13 steps
up to 100 mT wherever the intensities were
large enough to warrant it, and vector dia-
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
grams (Zijderveld 1967) were used in the
subsequent analysis. For 50% of the specimens the intensities of magnetization fall
off to less than 10% of the original NRM
intensity in the interval: 30-90 mT (average: 60 mT). In 85%, the median destructive field ( H J is found to be below 100 mT
in the range? 10-70 mT (average = 25 mT).
Samples from sites HB12 and HB13 have
a median destructive field larger than 100
mT. In almost 40% of the AF demagnetized
samples, the measurements indicated acquisition of spurious magnetizations at higher
demagnetizing
fields (70-100 mT). This
phenomenon is caused by the instability
of magnetite at higher field and so thermal
treatment was more efficient than AF demagnetization in the majority of the samples. Generally, vector trajectories are
linear until stable end-points are reached
at higher fields and a univectorial component (SE, shallow positive) is obtained once
the low coercivity (soft) component is removed in the 5-20 mT range (average: 10
mT). This soft component was observed in
about
35% of the samples; upon its
removal, the declination remains unchanged
but the inclination (intermediate or steep
positive) becomes shallow positive. Specimen
HB13-3.B (Fig. 4) is a
representative
example of in-situ component A (SE, shallow positive) while specimen HB 8-l.B
(Fig. 4) is a typical example of a PEF
component ( H i = 5-10 mT) superimposed on
in-situ component B (East, intermediate
positive) in the 70-100 mT range. AF demagnetization tests and identification of
the opaques with the aid of a reflecting
microscope have shown that the magnetic
memory carrier in samples of sites HB 10,
11, 12 and 13 is mainly hematite. On sites
HB 12 and 13 in particular, the relative
intensity of NRM is still at the 80% level
a f t e r demagnetization at 100 mT. On sites
HB 8 and HB 9, the memory carrier is
principally
magnetite
with
an
average
median destructive field of 15 mT. In the
case of site HB 14, the memory is located
principally in the hematite for one sample
while for the other four samples it is
mainly in magnetite. Consequently, AF demagnetization was concentrated in samples
of sites HB 8, 9 and 14 and to a lesser
extent to sites HB 10 and 11.
b) Thermal cleaning
Five (5) to 10 specimens for each of the
seven sites were thermally cleaned in a
minimum of 13 steps up to 660°C, and
vector diagrams were used in the subsequent analysis of the results. Most commonly, the unblocking temperature (T UB ) is
located in the 300-660°C range (usually
500-650°C) with two main modes; one at
temperatures smaller than 560°C and the
other around 600°C or more. The first
PEF
*
'
60°
59
.1
-
INDUCED MAGNETIZATION KH (Am1)
Fig. 3 - Individual sample NRM directions plotted on Wulff net for sites HB8 to HB14 (left). The
solid circles represent projections on lower hemisphere. The star represents the present E a r t h ' s field
(PEF) at the sampling locality. Plots of intensity of NRM (Jn in Am ) against the induced magnetization (KH in AM j with resulting slope lines indicating the values of Koenigsberger ratio, Q^ (right), for
all individual specimens.
60
M.K. Seguin
(mT)
Fig. 4 - Equal-area projections of directions of magnetization (left) of two specimens a f t e r stepwise AF
demagnetization. Solid (open) circles represent projections on horizontal (vertical) plane. The numbers
next to the circles represent the demagnetization steps in mT as indicated. The respective AF decay
curves are shown on the right side. Figure 4 illustrates an example for the isolation of the A and B
components.
T U B is close to the Curie point of magnetite and the second one close to the one
of hematite. In slightly over 50% of the
samples, the intensities of magnetization
fall off to less than 10% of the original
NRM intensity in the interval: 500-650°C
(with two modes at 540 and 635°C). Typical kinds of behaviour of a specimen during
thermal treatment are shown in Fig. 5. A
characteristic
univectorial
component
is
obtained as depicted by the linear trajectories on Figure 5b from 300° to 590°C.
In the case of sample HB 12-5, the linear
trajectory extends from 450° to 670°C (fig.
5a).
The
occasional
intensity
increase
is
caused partly by the increase in magnetite
content (through oxidation ?) as observed
with increasing susceptibility and/or by a
low temperature reverse component.
This component (A) is a SE directed magnetization with a shallow downward inclination. In 90% of the cases, the AF demagnetized and thermally cleaned component
on samples from sites HB 10, 11, 12 and 13
coincided. The situation is more complex
on sites HB 8, 9 and 14. For the majority
of the specimens from these three sites,
AF and thermally cleaned components in
pure magnetite do coincide. This is not so
when both hematite and magnetite are
present in the same specimen (e.g. HB
8-2C, HB 9-3A, B, C, HB 9-5A, B and the
majority of specimens from HB 13). Occasionally both normal and reverse directions
are found in the course of thermal demag-
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
61
by some 15°. Tests based on stability
criteria such as Graham's (1949) fold test
cannot be fully performed to infer the age
of magnetization of the ignimbrites since
at most of the sites the layers have relatively similar strikes and dips. However, a
threefold reduction of Fisher's precision
parameter for A-sites (Table 1) points to
a significantly negative fold test.
netization (e.g. H B 9 - 3 A and H B 14-1B,C).
On sites HB 10, 11, 12 and 13, the NRM intensities generally decrease in a uniform
manner with increasing temperature. On the
three other sites, 60% of the specimens
have shown an increase in the intensity of
the residual magnetization in the temperature range: 5 9 0 - 6 5 0 (average: 6 3 0 ° C ) suggesting
mineralogical
change(s)
in
the
original magnetic memory carrier. AF demagnetization was thus believed to be more
efficient than thermal cleaning on these
three sites. Intermediate coercivities, high
Tub
and microscopic observations (both
transmitted and reflected light) indicated
that fine- to medium-grained magnetite is
the main memory carrier.
STATISTICAL ANALYSIS OF SITE-MEAN
DIRECTION AND ORIGIN OF
MAGNETIZATION
Site-mean directions for combined AF
and thermally cleaned results are shown in
Table 1 along with informations on e f f e c tive ranges of AF and thermal t r e a t m e n t s
and how they were combined in the final
data. A present Earth's field (PEF) component is relatively common on sites: HB
8, 9, 14 and to a lesser extent on site HB
10. The SE + component from sites 10 to
14 (A on Table 1) is definitely the more
stable one. A f t e r demagnetization, the declinations are the same as for the initial
NRM values; the inclinations did not change
substantially but they became more shallow
SAMPLE ALTERATION
Petrographic and mineralogical
studies
were then undertaken in order to determine the primary or secondary nature of
the opaque minerals which are the magnetic memory carriers. The samples are
divided into two sets: 1) HB 10, 11, 12 and
13 in which there is little or no alteration,
and 2) HB 8, 9 and 14 in which there have
been varying amounts of alteration.
1 - Samples HB 10, 11, 12 and 13
Transmitted and reflected light microscopic observations have shown that the
main opaque mineral in the red ignimbrites
(HB 10, 11, 12 and 13) is specular hematite.
A few large corroded crystals of titanomagnetite are also present and the specularite is present within such skeletal crystals. Red fine-grained earthy hematite is
usually
observed
around
titanomagnetite
crystals, along fractures within such crystals and also in the fine-grained groundmass. Therefore, specularite appears to be
an early stage oxidation product of titanomagnetite. Kono and Larson (1980) have
Table I
FINN HILL SEQUENCE (FHS)
Site
No.
N*
n**
Polarity***
Strike
C)
8
4
12
R
007
81W
B
R
R(?)
010
82W
B
9
3
2
7
5
Dip
<°>
Component
Sj)
(")
D.I
n
060
58
300
33
075
272
45
50
306
274
47
-32
Do
C)
It.
C)
01
95
n
K
Q
AF/T
10.2
82
6/6
32.
15
4/8
T
CC)
AF
(mT)
550-625
500-630
50-90
30-90
10
5
12
R
015
85W
A
123
31
254
59
10.6
53
3/9
575-645
80-100
ll
5
15
R
012
81W
A
127
20
219
64
11.3
47
2/13
575-675
70-90
12
5
13
R
010
82W
A
128
23
222
59
9.9
61
1/12
575-670
25-40
13
5
II
R
004
80W
A
134
24
210
50
13.3
34
2/9
620-670
50-60
14
2
2
1
4
5
3
M
R
008
77W
185
191
272
II
55
-07
199
243
261
05
09
-69
3/12
570-650
30-40
25
225
59
303
40
mean:
4 sites
in-situ
tilted
A
A
128
2 sites
in-situ
tilted
B
B
069
*N = number of samples
***R = reverse
52
-
7.1
13.4
171
48
-
**n = number of specimens
M = mixed
Do, Io = demagnetized directions (in-situ)
D | , Ii = demagnetized directions (tilt-corrected)
K = estimate of Fisher's precision parameter
Q - ratio of AF demagnetized over thermally cleaned
specimens
M.K. Seguin
62
suggested that specularite is the slightly
metamorphosed
iron
oxide
product
of
titanomaghemites occurring in recent subaerial volcanic rocksTransmitted light microscopic studies show
that the red ignimbrites are relatively fresh
(HB 10, 11, 12, 13); these feldspars (both
phenocrysts and microliths) are not much
altered. In other places, Nixon (1975) mentions that the feldspars are metamorphic.
No primary or even secondary porosity is
observed; the fragments are very hard and
part of the glassy matrix (perlitic) resembles recent obsidians. These observations
indicate that hematite is not a late alteration product because if this was so the
whole rock would be uniformly reddish.
Thus, if the specularite is not primary, it
is definitely primitive, i.e. acquisition of
magnetization took place only a short time
a f t e r the formation and solidification of
these ignimbrites. Large blobby droplets of
iron oxides in association with the glassy
material are definitely primary, while laths
and rods of specularite concentrated in the
N,UP
25
(l00 o C
X l03Am'
S.DOWN
ii 2 0 ° C
Fig. 5 - Representative orthogonal thermal demagnetization diagrams (Zijderveld 1967) of successive
magnetization vectors for two specimens as indicated. The other symbols are as in Figure 4. The numbers next to open circles represent the demagnetization steps (°C). The unblocking t e m p e r a t u r e (Tug )
in °C is indicated below the orthogonal plots. In Figure 5(b), the two points on the J r / J n versus T
diagram are not plotted on the Zijderveld diagram 'because they coincide with the value at 590°C.
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
rest of the m atrix (ashes?) are probably
prim ary as well. Modern equivalents are
seen in L ate Cenozoic volcanism in the
Basin and Range province of w estern United
S tates (Utah, Nevada) (e.g. C recra ft e t al.
1981).
2. Samples HB 8, 9 and 14
T ransm itted and refle cted light m icro­
scopic observations on specim ens from sites
HB 8, 9 and 14 show very variable phases
of oxidation and alteratio n in co n trast to
the' red ignim brites. Site HB 14 is the most
altered; the feldspars are highly saussuritized and the largest fraction of the rock
is chloritized. Specimen HB 14.1 contains
a large quantity of small rod-like crystals
of specularite. A few relicts and som etim es
corroded skeletal cry stals of titan o m ag n etite
are rim med by an aureole of specularite
which is in turn surrounded by red earthy
h em atite. In a few instances, T i0 2 has
m igrated and ferrous iron was transform ed
to ferric iron in the ou ter rim of the iron
oxide crystals. S pecularite and red hem a­
tite are the ch a ra c te ristic opaque m inerals
of the m atrix. The cleaned direction of
m agnetization of the specim ens from sample
HB 14.1 is quite sim ilar to th a t of the ones
from the HB 10, 11, 12 and 13 sites but the
polarity is reversed. This is not so for the
four other samples of HB 14 site in which
a m ixture of elongated cry stals of titano­
m agnetite and large equant cry stals of
m agnetite are present. Both titan o m ag n etite
and m agnetite are fractured, corroded and
surrounded by concentric rims of specularite
and red earth y h em atite. In sm aller crys­
tals, h em atite appears to be a pseudomorph
of m agnetite. Some reddish angular frag­
m ents contain what appears to be prim ary
or prim itive specularite and hem atite. Two
ages of h em atite w ere observed; one com­
ponent is older while the other, filling
fractu res
and
percolation
channels,
is
definitely secondary. M agnetic m em ories of
varying com positions and ages com plicate
the analysis and the in terp retatio n of the
residual com ponents from this site.
Samples from HB 8 and 9 are less altered
than HB 14 but much m ore so than samples
from the other sites. C rystals of m agnetite
were observed within feldspar phenocrysts
and possibly altered pyroxenes. However,
most of the oxides were found in the m at­
rix composed of m icroliths of feldspars and
very fine-grained m aterial (glass?). Under
63
high m agnification, the opaque m inerals
within the m atrix are more or less equidimensional; both m agnetite and titanom ag­
n e tite are present. S pecularite and fine­
grained red h em atite are present in the
reddish fragm ents. R are large m ag n etite
cry stals are partly replaced by h em atite.
It is then difficult to tell w hether the mag­
n e tite is prim ary or secondary.
AGE OF ACQUISITION
OF MAGNETIZATION
A synthesis of all the observations men­
tioned previously (Fig. 6) indicates th a t the
in-situ secondary A com ponent (Table 1)
represents a direction of m agnetization
acquired most probably in late Hadrynian
time; the site-m ean direction (4 sites) of
this com ponent is: 128, +25( fflgg - 7°, K171). Indeed, this com ponent can be corre­
lated with one of the following two orogenic events: 1) Cadomian (Avalonian), and
2) Acadian (Late Devonian) orogenies. Aca­
dian overprinting is shown from radiom etric
N
Fig. 6 - Site-mean directions of magnetization
plotted on Lambert equal-area net after AF
and thermal (combined) cleaning. The in-situ A
component and the tilted B component are iden­
tified respectively with solid triangles and
circles. The mean value of each component is
represented by a solid square and the dashed
circle shows the cone of 95% condidence (Fisher
1953). The solid symbols denote downward (posi­
tive) inclinations. The star represents the pres­
ent Earth's field (PEF) at the sampling sites.
64
M.K. Seguin
evidence (e.g. Dallmeyer e t al. 1983) to be
important in the western Avalon Zone of
Newfoundland. The e f f e c t of Acadian orogeny on the eastern Avalon Zone of Newfoundland is probably milder as the Cambrian-Ordovician
sedimentary
rocks
are
practically
unmetamorphosed
and
undeformed. However, unpublished data by E.R.
Deutsch and K.V. Rao (personal communication) on the Signal Hill, Black Head Formations and Late Precambrian tilites show
a negative fold test possibly due to Acadian
overprinting. So overprinting is a possibility
in the above mentioned rock units.
I favour a Cadomian rather than Devonian
age of acquisition for the A component
mainly because: 1) the Cambro-Ordovician
sediments close to the sampling region are
undeformed and unmetamorphosed, and 2)
the in-situ pole position of the A component (1°W, 14°S) is very different from
Late Devonian poles (115°E, 40°N) and Permian
poles
(115°E,
50°N)
from
North
America.
Additional
paleomagnetic
data
from Hadrynian rocks of the Eastern Avalon zone are needed to prove or disprove
the above mentioned statement. Component
B may either be primary or secondary.
Based on the above mentioned observations,
this component is more probably primary
and correlated with the 700-750 Ma period
of formation of the ignimbrite sequence.
The tilt corrected site-mean direction of
this component is: 303, +40. The fold axes
are subhorizontal and the correction was
done by tilting about such axes.
PALEOPOLE POSITIONS AND THEIR
RELATIONSHIP OF HADRYNIAN POLES
FROM AVALON PENINSULA, THE NORTH
AMERICAN CRATON AND NORTHERN
EUROPE
An important gap in the available paleomagnetic data for North America exists in
the 950-580 Ma interval (i.e. from the
Helikian-Hadrynian boundary to the Early
Cambrian). This remark applies particularly
to the Avalon microcontinent. The paleopoles obtained for the Cloud Mountain
basalt (Deutsch and Rao 1977) and the
Black Head (BH) Formation and Signal Hill
(SH) Group (Nairn et al. 1959) are 5°S-8°W,
5°S-96°W and 16°S-38°E respectively (Fig.
7). Only the two last rock units belong to
the Avalon microcontinent and their pole
positions are at least some 60° apart. The
paleopole obtained on the Finn Hill se-
quence from the tilt-corrected B component
(303, +40) is 39°S, 29°E (antipole; pole =
39°N, 209°E).
According to King (1979), these rock
units in stratigraphic order from bottom to
top are: Finn Hill (FH) sequence, SH Group
and BH Formation (which is now recognized
as the highest stratigraphic level of the
Signal Hill Group); their poles are 39°S29°E, 16°S-38°E and 5°S-96°W. No specific
trend for an APW trajectory is apparent.
E.R.
Deutsch
(personal
communication)
thinks that the inclusion of the SH and BH
poles in an APW trajectory is unjustified
in view of their uncertain reliability and
an uncertainty about their geographic N or
S polarity. Thus, the two present Finn Hill
results seem to be the more reliable poles
for use in reconstructions for the Hadrynian of Avalon. If the Finn Hill components
are correctly interpreted, the APW trajectory moved SSE in Hadrynian time on the
Avalon Zone.
Additional
paleomagnetic
studies are needed to construct a reliable
Late Precambrian APW trajectory for the
Avalon microcontinent and to establish a
suitable relationship with the North American craton and western Eurasia.
COMPARISON WITH NORTH AMERICA
However, some geological constraints can
be added to the paleomagnetic results obtained in this study. The late Carboniferous
Hercynian Orogeny has only a f f e c t e d parts
of the Avalon terrane in Canada, and in
Newfoundland its e f f e c t is minimal (O'Brien
et al. 1983). As the components of magnetization of the Finn Hill sequence cannot be
attributed to the Hercynian Orogeny or
else with some difficulty to the Acadian
Orogeny, it appears worthwhile to compare
the late Precambrian paleopoles and paleolatitudes of Newfoundland Avalon with those
of similar ages on the western European
and Pan-African terranes.
The very late Precambrian paleopoles of
cratonic North America (NA), and in particular the. Cloud Mountain basalt pole from
western Newfoundland which compares well
with other mainland NA late Precambrian
poles, are within 10° to 15° of pole A of
the Finn Hill sequence (Fig. 7). What is the
meaning of this coincidence? Taking for
granted that component A is secondary, the
most plausible age of acquisition of magnetization is Late Devonian (Acadian orogeny) or late Precambrian (Vendian). The
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
65
Table 2
LATE HADRYNIAN PALEOPOLES OF THE NORTH AMERICAN CRATON A N D AVALON
Pole Position
Rock unit and location
Age
(Ma)
Blackhead Fm., Nfld.
Lat.°
Long."
dp"
dm"
(<»95°)
Pole
Reference
Avaion Zone
= 600
5°S
96SW
9.1
13.4
6.2
11.8
BH
Nairn e t al. (1959)
Avaion
SH
Nairn e t al. (1959)
= 650
I6°N
I42°W
Finn Hill Sequence, Nfld.
(A)
= 590
14°S
1°W
8
FH (A)
This study
Finn Hill Sequence, Nfld.
(B)
= 750
39°S
29°E
30
FH (B)
This study
= 630
2°N
16I°E
= 665
5°N
I65°E
Signal Hill Gp., Nfld.
Cratonic North America
Victoria Island
Franklin Intrusive 1
VI
-
Palmer e t al. (1983)
5
FI,
Irving & Hastie (1975)
Franklin Intrusive 2
= 665
6°N
I66°E
4
fi 2
Irving & Hastie (1975)
Franklin Intrusive 3
= 665
5°N
165°E
3
FI 3
Irving & Hastie (1975)
Cloud Mtn basalt, Nfld.
= 605
5°N
172°E
CM
Deutsch & Rao (1977)
Rapitan Group X
= 760
7°S
194°E
5
RX
Morris (1977)
Rapitan Group Z
= 640
5°N
I49°E
4
RZ
Morris (1977)
pole position of component A is very
remote from any known NA Devonian
paleopole and thus a late Precambrian age
of magnetization is more appropriate for
component A. This leads to the conclusion
that at that time NA and Avaion were
probably not very remote from each other;
they could be at least 500 km away given
the amount of inherent error in the
method.
Comparison with Europe and
Pan-African belts
Paleomagnetic studies in the Precambrian
of the Avalonian, Pan-African and northwestern European terranes are critical in
understanding Late Precambrian tectonics
of these areas. Unfortunately, such paleomagnetic studies are very limited in number
and contain relatively large circles of confidence; this does not allow easy geodynamic conclusions. In attempting to draw
comparisons between the Avalonian and
other
Atlantic-bordering
terranes,
the
Armorican data especially should be used
with caution since the case for APW relative to Armorica is still ill-defined probably because of e f f e c t of a major Hercynian overprint, the internal consistency of
the data (e.g. Perigo et al. 1983) is poor;
still, using their own assessment of age
control of magnetization and of the magnetic reliability, a mean direction for Ca.
610 Ma can be obtained by respective
interpolation from a) one of two clusters
with poles KMB, SP and SQ, all paleomag-
2
4
netically reliable and reasonably well dated,
but with an age spread of about 60 Ma between the first two and the last pole (mean
pole: 37°N, 120°E) and b) a second cluster
consisting of 7 poles (GK1, SQ2, KH1, JD,
PP1, PP2 and PP3), of which two have low
paleomagnetic reliability, the second to
fourth have no age control and the other
four (GK1, PP1-3) have a substantial age
uncertainty (85 Ma) with a mean pole of
ca. 30°N, 230°E. The two clusters are more
than 100° apart. Hagstrum et al. (1981)
pointed out that the Eocambrian Armorican
APW path is quite similar to that of Africa
and reconstructed Gondwana. In NW Africa,
the paleopole is close to 40°N, 230°E with
a paleolatitude of 8°N (Kent e t al. 1984);
it is comparable to the mean pole (30°N,
230°E) of the second cluster (paleolatitude:
15°N). The anti-paleopole and paleolatitude
of the in-situ A component is 179°E, 14°N
and 13°N. Using this data base for comparison, it is noted that the paleolatitudes of
the Armorican, Pan-African and Avalonian
terranes could be similar (8° to 15°N) and
situated close to the paleoequator. As this
comparison is supported by incomplete data
sets from both Armorica and Avaion, Late
Hadrynian paleomagnetic data base ought
to be enlarged with additional local and
regional studies from both sides of the
Atlantic to test accurately the above mentioned possibility. The following section is
more speculative.
A f t e r allowing for the restoration of the
present Atlantic Ocean, the Eocambrian A
M.K. Seguin
66
• FH (B)
3530• NORTH AMERICA
25-
• AVALON MINI-PLATE
"lli
o
20*
"5
# SH
15-
RH
•BH
EAST
^(A-r*
X
Fl2 10- itr
•
•
• 5- - zo
f i I i 3 cm
RZ
•
1
100
1
120
1
140
1
160
1
160
l{)0
5-
H
O
|
140 WEST
BRX
10Fig. 7 - P a l e o m a g n e t i c
(Table 2).
poles
(late
Hadrynian)
dei
pole of Avalon is situated at approximately
214°E and 20°N; this relocated paleopole
position is not significantly d i f f e r e n t from
that of Eocambrian Armorican and PanAfrican terranes. The present latitudinal
difference
between
North
Africa
(e.g.
Morocco) and Armorica is 10-15° and it was
not much g r e a t e r in Eocambrian time; the
same conclusion applies to Newfoundland.
In
the
Eocambrian-Cambrian
interval,
Armorica, Pan-Africa and Avalonia probably
moved as a single unit. Their convergence
by Late Precambrian time may be related
to the Cadomian orogeny. Earlier than 620
Ma (i.e. at 750 Ma), the B paleopole (209°E
39°N) and paleolatitude (22°N) of the Avalonian terrane are not similar, a f t e r closure
of the present Atlantic Ocean, to those of
the Armorican and Pan-African terranes.
The reason for their d i f f e r e n c e may be related to the small quantity and/or poor
quality of the existing paleomagnetic data,
the unreliable age control of the magnetizations or other causes. In northwest Europe
(northern Great Britain, Scandinavia) the
paleopole for this period could be close to
:d from
Avalon
zone
and c r a t o n i c
North
America
300°E, 50°S while for Africa, the paleopole
position is close to 300°E, 60°S (Brock 1981). Consequently, it seems that Pan-Africa,
Armorica
and
Avalonia
were
dispersed
before 620 Ma but located close to each
other and the paleo-equator in late Hadrynian time.
ACKNOWLEDGEMENTS
The author thanks Dr. E.R. Deutsch and
Mr.
T.
Vallis
(Department
of
Earth
Sciences, Memorial University of Newfoundland) for assistance in drilling and cutting
of oriented rock samples, Mr. R.V. Rao
(Universite Laval) for field assistance and
Mr. R. Ouellet (Engineering Physics Program, University Laval) who made a large
part of the measurements. This work was
supported
by EMR
research
agreement
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REVIEWERS:
H . G . Miller
S. C o l m a n - S a d d
A n o n y m o u s ( f r o m D. C o l m a n - S a d d )