THE GEOCHEMICAL EVOLUTION OF GREATER THAN 100 MILLION YEARS OF SUBDUCTION-RELATED MAGMATISM, COAST P...
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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.

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Goals of the Geochemical Component

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Goals of the geochemical component

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

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.


Goals of the geochemical component

Dean-Burke Channel Transect


Goals of the geochemical component

Eastern Late Jurassic

CSZ Intrusive (Paleocene)

Ecstall (Late K)

Eocene

Coast Shear Zone (CSZ)

Western Late Jurassic

Eastern Late Cretaceous

Western middle Cretaceous


Goals of the geochemical component

Douglas Channel Transect

Dean-Burke Channel Transect


Goals of the geochemical component

CSZ Intrusive (Paleocene)

Eocene

Ecstall (Late K)

Western middle Cretaceous

Western Late Jurassic

Coast Shear Zone (CSZ)


Magmatic flux

Magmatic Flux

Paleocene (CSZ)

& Eocene

Ecstall/

western middle K

western & eastern

middle Jr

eastern

Late K


Goals of the geochemical component

Sierra Nevada fluxes


Goals of the geochemical component

Peraluminous

Metaluminous


Goals of the geochemical component

Heavy Rare

Earth Element

depletion due

to garnet in

the residuum


Goals of the geochemical component

>10 kbars

West

East

Crustal pressure correlations after Hildreth and Moorbath, 1988

10 kbars= 30-35 km depth


Goals of the geochemical component

eNd

Assimilation?

Bulk

Earth

Mantle

Array


Goals of the geochemical component

Fields from Doe and Zartman, 1981


Goals of the geochemical component

How about the stable isotopes?

d18O

Maximum Mantle Signature

Detailed petrography was completed on all samples to ensure that those with

obvious evidence of alteration (e.g. sericite) were not analyzed!


Goals of the geochemical component

d18O

Mantle


Goals of the geochemical component

PRb trend from Taylor & Silver, 1978

Klamath trend from Barns et al., 1990


Goals of the geochemical component

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…..

  • Alteration had to occur when the source rocks were near the surface with cool meteoric waters.

  • No interaction with ocean water as that would elevate Sr (even w/ pre-Jurassic seawater).

  • 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

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

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.


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