THE GEOCHEMICAL EVOLUTION
OF GREATER THAN 100 MILLION
YEARS OF SUBDUCTIONRELATED MAGMATISM, COAST
PLUTONIC COMPLEX, WESTCENTRAL BRITISH COLUMBIA
Goals of the Geochemical Component
• Estimate bulk composition of the CPC for depths
between 5 and 25 km.
• Constrain the depth of melt generation through time.
• Characterize the source of granitoids and distinguish
between crustal and mantle contributions.
• Calculate the composition and size of residual
assemblages created during batholith formation.
• Use Tertiary dikes and volcanics to help identify potential
crustal delamination events.
DeanBurke
Channel
Transect
CSZ Intrusive
(Paleocene)
Ecstall (Late K)
Eastern Late Jurassic
Eocene
Coast Shear Zone (CSZ)
Western Late Jurassic
Western middle Cretaceous
Eastern Late Cretaceous
Douglas
Channel
Transect
DeanBurke
Channel
Transect
Ecstall (Late K)
Western middle Cretaceous
Western Late Jurassic
CSZ Intrusive
(Paleocene)
Coast Shear Zone (CSZ)
Eocene
Magmatic Flux
900
Paleocene (CSZ)
& Eocene
Apparent intrusive flux (km2/My)
800
Ecstall/
western middle K
western & eastern
middle Jr
700
600
500
eastern
Late K
400
300
200
100
0
40
60
80
100
Time (Ma)
120
140
160
180
Sierra Nevada fluxes
3000
a.
2000
DD
1000
0
40
RS84
80
120
Time (Ma)
160
1.2
Peraluminous
1.1
A/CNK
1.0
Metaluminous
0.9
Eocene
CSZ Intrusives
Eastern Late K
Eastern Late Jr
Ecstall
Eastern middle K
Western Late Jr
0.8
0.7
0.6
45
50
55
60
65
70
SiO2 (wt.%)
75
80
85
Coast Shear Zone Intrusives
Heavy Rare
Earth Element
depletion due
to garnet in
the residuum
Rock/Chondrite
1000
100
10
1
La
Ce
Pr
Nd Sm Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Rock/Chondrite
1000
100
10
1
La Ce Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
250
Eocene
CSZ Intrusives
Eastern Late K
Eastern Late Jr
Ecstall
Eastern middle K
Western Late Jr
La/Yb
200
150
100
50
>10 kbars
0
-100
West
-50
0
Distance from CSZ (Km)
50
100
East
Crustal pressure correlations after Hildreth and Moorbath, 1988
10 kbars= 30-35 km depth
250
Eocene
CSZ Intrusives
Eastern Late K
Eastern Late Jr
Ecstall
Eastern middle K
Western Late Jr
La/Yb
200
150
100
50
0
0.0
0.2
0.4
0.6
0.8
Eu/Eu*
1.0
1.2
1.4
1.6
25.0
Post Kinematic aABF
Pre-Kinematic SSPM
20.0
Post Kinematic SSPM
La/Yb
15.0
10.0
5.0
0.0
0.0
0.2
0.4
0.6
0.8
Eu/Eu*
1.0
1.2
1.4
8
Eocene
CSZ Intrusives
Eastern Late K
Eastern Late Jr
Ecstall
Eastern middle K
Western Late Jr
6
Nd

eNd
4
2
Assimilation?
0
Bulk
Earth
-2
-4
0.702
0.703
0.704
87
86
0.705
Sr/ Sr i
0.706
0.707
Fields from Doe and Zartman, 1981
How about the stable isotopes?
12
11
d18O
10
9
8
7
Maximum Mantle Signature
6
0
50
100
150
Age (Ma)
Detailed petrography was completed on all samples to ensure that those with
obvious evidence of alteration (e.g. sericite) were not analyzed!
200
12
~Eocene
CSZ Intrusives
11
Eastern Late K
Eastern Late Jr
18
d18O
O
10
Ecstall
Western middle K
Western Late Jr
9
8
7
Mantle
6
45
50
55
60
65
SiO2 (wt%)
70
75
80
85
PRb trend from Taylor & Silver, 1978
Klamath trend from Barns et al., 1990
How can we explain primitive radiogenic
signatures and heavy oxygen?
•Assimilation?
Metamorphic rocks found as screens within and between intrusions have
extremely evolved radiogenic isotopic signatures (e.g. Boghossian and Gehrels,
2000). Minor amounts of assimilation would dramatically increase Nd-Pb-Sr
isotopic signatures of the melts!
• Unique source composition? Okay, but…..
1. Alteration had to occur when the source rocks were near the surface with
cool meteoric waters.
2. No interaction with ocean water as that would elevate Sr (even w/ preJurassic seawater).
3. The interacting waters, and therefore the rocks that would become the source
to the CPC melts, must have been isolated from exposures of evolved
continental rocks.
Conclusions
• The CPC represents the roots to a very long
lived arc system (>150 m.y.).
• Magmatism within the CPC was very
episodic.
• Melt generation occurred, in most cases, at
depths in excess of ~35 kms (i.e. w/ garnet in
the residuum). A dramatic crustal thickening
event near the end of the Cretaceous.
Conclusions continued
• Radiogenic isotopes indicate that the CPC
was primitive but mature arc (a long lived
island arc?).
• Oxygen isotopes suggest that the source
rocks experienced some residence at near
surface levels where they interacted with
meteoric waters that had not previously
flowed over or through older, evolved
continental rocks.