Title: THE GEOCHEMICAL EVOLUTION OF GREATER THAN 100 MILLION YEARS OF SUBDUCTION-RELATED MAGMATISM, COAST PLUTONIC COMPLEX, WEST-CENTRAL BRITISH COLUMBIA
1THE GEOCHEMICAL EVOLUTION OF GREATER THAN 100
MILLION YEARS OF SUBDUCTION-RELATED MAGMATISM,
COAST PLUTONIC COMPLEX, WEST-CENTRAL BRITISH
COLUMBIA
2Goals 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.
3Dean-Burke Channel Transect
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5Douglas Channel Transect
Dean-Burke Channel Transect
6Coast Shear Zone (CSZ)
7Magmatic Flux
Ecstall/ western middle K
western eastern middle Jr
Paleocene (CSZ) Eocene
eastern Late K
8Sierra Nevada fluxes
9Peraluminous
Metaluminous
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12East
West
Crustal pressure correlations after Hildreth and
Moorbath, 1988
10 kbars 30-35 km depth
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15eNd
Assimilation?
Bulk Earth
Mantle Array
16Fields from Doe and Zartman, 1981
17How 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!
18d18O
Mantle
19PRb trend from Taylor Silver, 1978 Klamath
trend from Barns et al., 1990
20How 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.
21Conclusions
- The CPC represents the roots to a very long lived
arc system (gt150 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.
22Conclusions 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.