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THE GEOCHEMICAL EVOLUTION OF GREATER THAN 100 MILLION YEARS OF SUBDUCTION-RELATED MAGMATISM, COAST PLUTONIC COMPLEX, WEST-CENTRAL BRITISH COLUMBIA. Goals of the Geochemical Component. Estimate bulk composition of the CPC for depths between 5 and 25 km. - PowerPoint PPT Presentation
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THE GEOCHEMICAL EVOLUTION OF GREATER THAN 100 MILLION
YEARS OF SUBDUCTION-RELATED MAGMATISM, COAST
PLUTONIC COMPLEX, WEST-CENTRAL 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.
Dean-Burke Channel Transect
Coast Shear Zone (CSZ)Western Late Jurassic
Eastern Late Jurassic
Western middle Cretaceous
Eastern Late Cretaceous
EoceneEcstall (Late K) CSZ Intrusive
(Paleocene)
Douglas Channel Transect
Dean-Burke Channel Transect
Coast Shear Zone (CSZ)Western Late JurassicWestern middle Cretaceous
EoceneEcstall (Late K)CSZ Intrusive (Paleocene)
Magmatic Flux
0
100
200
300
400
500
600
700
800
900
40 60 80 100 120 140 160 180
Time (Ma)
Appare
nt
intr
usiv
e fl
ux (
km
2/M
y)
Ecstall/western middle K
western & easternmiddle Jr
eastern Late K
Paleocene (CSZ) & Eocene
040 80 120 160
Time (Ma)
1000
2000
3000
DD
RS84
a.
Sierra Nevada fluxes
0.6
0.7
0.8
0.9
1.0
1.1
1.2
45 50 55 60 65 70 75 80 85SiO2 (wt.%)
A/C
NK
EoceneCSZ IntrusivesEastern Late KEastern Late JrEcstallEastern middle KWestern Late Jr
Peraluminous
Metaluminous
Coast Shear Zone Intrusives
1
10
100
1000
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Ro
ck/C
ho
nd
rite
Heavy RareEarth Elementdepletion dueto garnet in the residuum
1000
100
10
1La Ce Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Ro
ck/C
ho
ndri
te
0
50
100
150
200
250
-100 -50 0 50 100
Distance from CSZ (Km)
La
/Yb
EoceneCSZ IntrusivesEastern Late KEastern Late JrEcstallEastern middle KWestern Late Jr
>10 kbars
Crustal pressure correlations after Hildreth and Moorbath, 198810 kbars= 30-35 km depth
EastWest
0
50
100
150
200
250
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6Eu/Eu*
La
/Yb
EoceneCSZ IntrusivesEastern Late KEastern Late JrEcstallEastern middle KWestern Late Jr
0.0
5.0
10.0
15.0
20.0
25.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4Eu/Eu*
La
/Yb
Post Kinematic aABF
Pre-Kinematic SSPM
Post Kinematic SSPM
-4
-2
0
2
4
6
8
0.702 0.703 0.704 0.705 0.706 0.70787Sr/86Sri
Nd
EoceneCSZ IntrusivesEastern Late KEastern Late JrEcstallEastern middle KWestern Late Jr
Nd
Mantle
Array
BulkEarth
Assimilation?
Fields from Doe and Zartman, 1981
6
7
8
9
10
11
12
0 50 100 150 200
Age (Ma)
Maximum Mantle Signature
18 O
How about the stable isotopes?
Detailed petrography was completed on all samples to ensure that those withobvious evidence of alteration (e.g. sericite) were not analyzed!
6
7
8
9
10
11
12
45 50 55 60 65 70 75 80 85
SiO2 (wt%)
18O
~Eocene
CSZ Intrusives
Eastern Late K
Eastern Late Jr
Ecstall
Western middle K
Western Late Jr
18 O
Mantle
PRb trend from Taylor & Silver, 1978Klamath trend from Barns et al., 1990
How can we explain primitive radiogenicsignatures 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/ pre-Jurassic 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.
