Isotopic evidence of a near surface history for the source rocks of the
central Coast Plutonic Complex, British Columbia, Canada
Paul H. Wetmore#*, Kelley N. Stair#, and Mihai Ducea#
UNIVERSITY OF
SOUTH FLORIDA
#Department of Geosciences, University of Arizona, Tucson, AZ 85721
*Department of Geology, University of South Florida, Tampa, FL 33620
Western Early K
18.4
Western Jurassics
GJP
Pb
204
Pb isotopic compositions all plot within the “mature arc primitive” field of Zartman
and Doe (1981). Ratios become more radiogenic and exhibit less scatter through time.
The intrusions with the most radiogenic Pb are located nearest the Coast Shear Zone
(CSZ).
17.8
17.0
17.5
18.0
18.5
206
Pb/204 Pb i
206
East
19.0
19.5
20.0
-100
20.5
-75
-50
-25
0
25
50
75
Distance from CSZ (km)
Pb/ 204Pb
19.2
15.68
19.0
15.66
15.64
18.8
15.62
18.6
15.60
18.4
15.58
18.2
15.56
18.0
1
-8
15.54
West
17.8
East
100
150
200
-100
-75
-50
-25
0
25
50
75
Distance from CSZ
O Isotopic data
80
PGJ
0.710
0.709
0.706
0.706
0.705
9
-7
7
PGJ
GJP
a
5
9. 7 M
87
5 1.
l
ana +
Sr/ 86 Sr i
0.707
8
l
ba
West
0.703
East
0.702
0.702
0
25
50
75
100
125
150
175
-100
200
-80
-60
-40
7
-7 Ma
GJP 0.7
+
.6
Coast Shear Zone Intrusive
Suite
-20
0
20
40
60
INTERPRETATIONS AND CONCLUSIONS
80
Distance from CSZ
Age (Ma)
55
Douglas Channel
Transect
Ecstall Intrusive Suite
latest Early to earliest Late
Cretaceous Intrusives
12
12
11
11
10
10
Kilometers
25
Paleozoic/Mesozoic
Metasediments and Volcanics
Coast Shear Zone
T
OR
AF
SE
7
7
6
6
-100
West
0
50
100
G
100.0 + 2.3 JP
-4
0
Key
Bella
Bella
-60
-40
-20
0
20
40
Distance from CSZ (Km)
18
G
JP
G
JP 36
-3
5
G
G JPJP 3
-3 3
4
Sr vs. d O
12
G
JP
-2
3
82.8 + 2.8
latest Paleocene and Eocene
intrusives.
Coast Shear Zone Intrusive
Suite
Eastern Late Cretaceous
intrusives.
182.4 + 4.7
61.5 + 1.7
G
JP
-2
Ecstall Intrusive Suite
DEA
G
JP
-3
G
JP
-3
1
N CH
AN N
G
JP
G
-2
JP
8
-2
78.8 + 1.8 9
EL
0
G
JP
-2
1
G
D
G JPG JP- 25
JP 2
-2 6
7
2
C
EAN
HA
11
EL
NN 157.1 + 3.2
10
L
18
G
JP
-1
9
G
JP
-2
BURK
E
9.3 + 0.4 G
JP
-
CHAN
17
G
JP
-1
4
0
NEL
G 52.7 + 1.5
JP
-1
6
G
JP
-1
ACKNOWLEDGMENTS
8
This research was funded through the Continental Dynamics project,
BATHOLITHS. The data and conclusions presented here were made possible through
the direct assistance of many, many colleagues. Specifically, we would like to thank
George Gehrels, P. Jonathan Patchett, and Mark Barton at the University of Arizona,
Jeffrey Ryan at the University of South Florida, Margaret E. Rusmore at Occidental
College, and Glenn J. Woodsworth with the Geological Survey of Canada.
5
G
JP GJ
-1 P98.4 + 1.8 3 12
7
58.4 + 1.9
SO
UN
H
UG
H
FI
TZ
G
JP
-1
1
H
RT 77.3 + 1.7
O
N
BE
G
N
G
JP
-1
JP
-0
9
0
TIN
G
JP
CK
-0
8
G
JP
-0
7
6
MORB
Bella
Coola
5
0.702
0.703
Mariana Arc
0.704
0.705
0.706
87
H
SOUT
G
JP
-0
3
G
G JPJ
G P- 0 4
JP 0
-0 5
6
144.5 + 2.9 GJ
P0
1
The isotopic data from CPC samples reported herein indicate that the composition of
the source rocks for these magmas are not simple mantle-derived diorites that ponded in
the lower crust. While Pb and Sr data are consistent with a relatively primitive source
composition (e.g. mantle-derived diorites) the oxygen data are far to elevated to
represent simple mantle derivation. The heavy oxygen compositions require that
source rocks interacted with meteoric waters at low temperatures. This implies that
prior to being melted to form the CPC magmas, the source rocks were at the surface in a
subaerial environment.
In addition to a period of residence in a subaerial environment interacting with
meteoric waters the rocks that comprise the source(s) of the CPC magmas must have
been geographically isolated from any continent or continental terrane. This follows
from the fact that Pb and Sr isotopic data are not sympathetically enriched with oxygen.
If such rocks were spatially associated with these source rocks it is expected that the
meteoric waters would have acquired the more evolved radiogenic isotopic
compositions that characterize continental rocks.
Japan Arcs
D
BU
E
K
R
A
H
C
E
NN
G
JP
-
Ch.
Coast Shear Zone.
9
n
a
R
s
e
g
18
Paleozoic/Mesozoic
Metasediments and Volcanics.
d O
E
ER
CH
Jurassic Intrusives.
r
a
l
u
ns lith
i
n o
e
P ath
B
G
JP
-2
4
U
LABO
G 92.1 + 2.1
JP
-3
2
Jurassic volcanics.
-80
Age (Ma)
King Island Syenite.
latest Early to earliest Late
Cretaceous Intrusives
Jurassic metasediments.
200
L
20
G107.5 + 1.5
G JPJP 3
-3 7
8
150
East
NE
10
Kilometers
107.6 + 3.0
G
JP
-3
9
Miocene volcanics.
BENTINCK
Powders of each sample were dissolved using traditional HF-HNO3 methods. Sr
and Pb were isolated using Sr-spec resins in disposable columns. No spikes were
added. Isotopic concentrations of both Sr and Pb were determined on a
multicollector Inductively Coupled Mass Spectrometer (GVI Isoprobe) at the Arizona
LaserChron Center, University of Ariznoa. Measured Sr ratios were normalized to a
87
Sr/86Sr = 0.7102 4 value for NBS 987. NBS 987 was analyzed 145 times during the
87
86
207
206
study with an average Sr/ Sr = 0.71027. Lead data were normalized to Pb/ Pb =
207
206
0.91475 for NBS 981. The average Pb/ Pb value for 112 analyses of NBS 981
during this study was 0.91460. Clean, hand picked quartz separates were
decomposed in Nickle bombs in the presence of BrF5 and converted to CO2 gas in the
of heated graphite in Stable Isotope lab of Dr. Mark Barton at the University of
Arizona. Oxygen isotopic values were normalized using in-house standards (Barton
Lab Brazilian Quartzite).
8
9
AN
CH
0
ANALYTICAL METHODS
8
H
Dean-Burk Channel
Transect
9
d 18 O
Paleozoic Intrusive
d 18 O
Central Gneiss Complex
Western Jurassic Intrusives
The d O values range from 6.9 to 11.3 %0 with an average value of 8.8 %0. Samples of
intrusions of all ages yield an ~2%0 range in compositions with the western Late Jurassic
18
yielding the heaviest oxygen values. However, for nearly all samples the d O is
significantly elevated over those of typical island arc or MORB derived lavas.
0.704
0.704
latest Paleocene and Eocene
intrusives.
S r/ 8 6 S r I
0.708
0.708
rC
dne
Gar
ss
ce
in l
Pr ya
Ro land
Is
a
M
60 .7
P- + 1
GJ 8.4
14
nal
50
0.710
Jurassic volcanics.
INTRODUCTION
The average Sr isotopic composition of all samples analyzed is 0.7042. With the
exception of the King Island Syenite, compositions overlap those of most island arc lavas
(e.g. Mariana Arc). The wester Late Jurassic plutons exhibit the largest scatter of initial
ratios (0.7035 to 0.706).
18
Key
N
Sr Isotopic Data
15.52
0
Age (Ma)
nel
Ursula Chan
18.0
87
Prin
Cha cipe
nne
l
Is.
pani
a
Pb Isotopic Data
Pb/204 Pb
7
West
The isotopic data from this study are displayed in the plots to the left. Here we
enumerate some of what we consider to be the significant insights the data provide.
18.2
207
-8
Fields derived from
Zartman and Doe (1981)
MORB
15.10
16.5
GJP
a
Gardner C
1
le
a
h
W
l
e
n
n
a
h
C
Pb/
15.50
18.6
206
Pb/204Pb
207
1
-9 a
GJP22M
3
88
PGJ
nne
l
Ecstall Equivalent
a
M
6
Cam
E Late Jr Intrusives
15.60
18.8
15.30
86 a
P- .1 M
GJ+ 1
.9
Cha
at
0
-9 a
GJP 52M
d
slan ks
I
c
i
c
Ro
ean
Oc canic
Vol
E Late K Intrusives
15.40
81
7
84 0 .
P- +
GJ 60.9
-6
10.23
v
as
n
tio
a
t
el
n
an
Ch
KEY ASPECTS OF THE DATA
19.0
CSZ Intrusives
15.20
0
Generation and evolution of continental arcs remains a fundamental problem in the
Earth sciences. Specifically, the dominant magmas from the mantle in all tectonic
settings are basaltic, yet the bulk composition of continental crust is andesitic. An
increasingly accepted view of Cordilleran arc magma generation is the anatexis of
mantle derived diorites that have ponded in the lower crust. In large part this model
stems from the observation that relatively silicic magmas possess juvenile or primitive
radiogenic isotopic compositions. While this model may, in fact, be generally applicable
to some portions of the North American Cordillera we argue here that at least some
portions of this extensive Mesozoic and early Cenozoic arc system do not necessarily
conform to this view of batholith generation.
The Coast Plutonic Complex (CPC) of western British Columbia, Canada is one of
the northern most segments of the Mesozoic and Tertiary batholiths that help to define
the North American Cordillera. As part of a Continental Dynamics Project,
BATHOLITHS, we have generated a regional isotopic database on the numerous coastparallel age-equivalent plutonic belts that comprise the CPC. Approximately 60
plutonic samples were collected along two transects of the CPC and Pb and Sr isotopic
compositions were determined each. When combined with major and trace elemental
data the CPC rocks are characterized as being both relatively silicic and isotopically
primitive suggesting possible derivation from the melting of mantle-derived diorites
ponded in the lower crust. However, oxygen isotopic data determined for 22 of the
samples suggest that at least some of the source rocks must have experienced near
surface residence prior to being melted.
Mature Arc
Primitive
~Eocene Intrusives
15.70
a
M
7 a
0.
+ .8 M
1
.5
49 .8 +
82 1
P- 3 8
GJ JP-8
G
Squ
ally
tim
Ki
15.80
65
P- a
GJ.4 M
1
+
38.154
9
0.
3
-6
15.567
62 a
P- .1 M
GJ + 1
.5
18.591
94
0.70474
-6
142.5 to 188
GJP
Western Late J Intrusives
De
10
GJP
8.65
64
P- Ma
GJ1.7
+
7
7.
38.239
8.54
10
15.568
l
Gi land
Is
18.745
38.353
8.00
9
0.70434
15.593
37.966
5
PGJ
94.5 to 107.7
18.902
15.559
a
M
Western Middle K Intrusives
0.70414
18.375
8
-5 Ma
GJP1.5
+
2
4.
78.8 to 92.1
0.70334
14
Ecstall Intrusives
144.5 to 182.4
GJP
4
Eastern Late J Intrusives
57 a
P- 7 M
GJ+ 1.
7
6.
9.31
55 .5
P- + 1
GJ7.8
14
38.259
6
-5 Ma
GJP 1.9
+
5
2.
15.573
8.10
14
14
18.735
38.209
3
0.70363
15.588
5
PGJ
61.5 to 81.9
18.857
y
ne
wd
De
Is
Eastern Late K Intrusives
0.70438
-5
58.4 to 60.9
GJP
CSZ Intrusives
D
a
M
8 9
1. P-8
GJ
9.54
Douglas Channel
5
-8
GJP a
M
59
38.425
7
15.597
6
PGJ
18.959
2
0.70437
5
PGJ
a
9M
49.5 to 52.7
+
a
M
1
1.
+ -68
.0
97 GJP
14
ch
ut
Tr land
Is
Paleocene-Eocene Intrusives
as
l
g
u
o
2
0.
7.35
69
P- a
GJ .2 M
1
+
38.468
.9
1
15.595
91
-5
19.034
7
2. Ma
15 1.9
+
0.70602
0
9.3
5
PGJ
King Is. Syenite
l
e
n
n
a
h
C
0
8. Ma
18 2.6
+ -7 6
GJP
 O
19.2
King Is. Syenite
13
18
74
P- 5
GJ -7
GJP
73
PGJ 2
7
PGJ
Pb/
204
Pbi
1
-7 a
M
GJP0.9
+
208
70
P- Ma
GJ110
94
Pb/
204
Pbi
.6
207
GJP
Pb/
204
Pbi
82
206
G
r
e
n
v
i
l
le Chan
ne l
Sr/86Sr
initial
tt
Pi land
Is
87
15.90
za
ta
is
Ar land
Is
Radiogenic isotopic data from intrusive suites have recently been the key to
interpretations that many of the Cordilleran batholiths are the direct products of
partial melting of mantle derived diorites that ponded within the lower crust. However,
results from a regional study of the Coast Plutonic Complex of British Columbia,
Canada, part of the BATHOLITHS Continental Dynamics project, conflict with this
interpretation and indicate that a substantial portion of the source rocks resided at near
surface levels at some time prior to burial and partial melting.
Radiogenic isotopic data, including Sr, and Pb, completed on more than 60 plutonic
samples ranging in age from 322 Ma to 9 Ma indicate that the source regions for these
melts were indeed very primitive. Initial Strontium 87/86 ratios range from 0.7032 up to
0.7062 with a mean of 0.7042. Similarly, lead isotopic data range from 19.675 to 19.520
206
204
207
204
208
204
for Pb/ Pb, 15.550 to 15.662 for Pb/ Pb, and 38.155 to 39.589 for Pb/ Pb. In
contrast to these relatively primitive isotopic data, oxygen data indicate a somewhat
more evolved component to or characteristic of the deep crustal source regions for these
melts. Delta 18O values for quartz separates determined for more than 30 of the
samples range from 7.1 %0 up to 9.8 %0. Such high oxygen values clear preclude the
possibility that these melts were generated from mantle derived diorites just as the Sr
and Pb data preclude significant contamination from an evolved crustal source.
We argue that these data indicate that the rocks that formed the source region for
these melts were originally derived from the mantle but experienced a period of near
surface residence after initial crystallization and before they were again melted to form
the igneous bodies of the Coast Plutonic Complex. During this interval these highly
primitive rocks would have interacted with meteoric waters at low temperatures in
order to change their oxygen isotopic composition away from their mantle signatures.
Such a process of depression and remelting of once epizonal rocks is also consistent with
the common Pb data which tend plot within the Mature Primitive Arc field of Zartman
and Doe (1981).
Pb/238U zircon
age (Ma)
s
nk
Ba land
Is
ABSTRACT
206
N
Average Isotopic Compositions
USF
0.707
0.708
0.709
0.710
86
Sr/ Sr
Modified from Taylor & Scharf, 1986