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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. 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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  1. THE GEOCHEMICAL EVOLUTION OF GREATER THAN 100 MILLION YEARS OF SUBDUCTION-RELATED MAGMATISM, COAST PLUTONIC COMPLEX, WEST-CENTRAL BRITISH COLUMBIA

  2. 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.

  3. Dean-Burke Channel Transect

  4. Eastern Late Jurassic CSZ Intrusive (Paleocene) Ecstall (Late K) Eocene Coast Shear Zone (CSZ) Western Late Jurassic Eastern Late Cretaceous Western middle Cretaceous

  5. Douglas Channel Transect Dean-Burke Channel Transect

  6. CSZ Intrusive (Paleocene) Eocene Ecstall (Late K) Western middle Cretaceous Western Late Jurassic Coast Shear Zone (CSZ)

  7. Magmatic Flux Paleocene (CSZ) & Eocene Ecstall/ western middle K western & eastern middle Jr eastern Late K

  8. Sierra Nevada fluxes

  9. Peraluminous Metaluminous

  10. Heavy Rare Earth Element depletion due to garnet in the residuum

  11. >10 kbars West East Crustal pressure correlations after Hildreth and Moorbath, 1988 10 kbars= 30-35 km depth

  12. eNd Assimilation? Bulk Earth Mantle Array

  13. Fields from Doe and Zartman, 1981

  14. 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!

  15. d18O Mantle

  16. PRb trend from Taylor & Silver, 1978 Klamath trend from Barns et al., 1990

  17. 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.

  18. 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.

  19. 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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