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 Eocene Ecstall (Late K) CSZ Intrusive (Paleocene)

Douglas Channel Transect Dean- Burke Channel Transect

Coast Shear Zone (CSZ) Western Late Jurassic Western middle Cretaceous EoceneEcstall (Late K) CSZ Intrusive (Paleocene)

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

Sierra Nevada fluxes

Peraluminous Metaluminous

Heavy Rare Earth Element depletion due to garnet in the residuum

>10 kbars Crustal pressure correlations after Hildreth and Moorbath, kbars= km depth EastWest

 Nd Mantle Array Bulk Earth Assimilation?

Fields from Doe and Zartman, 1981

Maximum Mantle Signature  18 O How about the stable isotopes? Detailed petrography was completed on all samples to ensure that those with obvious evidence of alteration (e.g. sericite) were not analyzed!

 18 O Mantle

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/ 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.