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

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


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
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5
Douglas Channel Transect
Dean-Burke Channel Transect
6
Coast Shear Zone (CSZ)
7
Magmatic Flux
Ecstall/ western middle K
western eastern middle Jr
Paleocene (CSZ) Eocene
eastern Late K
8
Sierra Nevada fluxes
9
Peraluminous
Metaluminous
10
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11
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12
East
West
Crustal pressure correlations after Hildreth and
Moorbath, 1988
10 kbars 30-35 km depth
13
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14
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15
eNd
Assimilation?
Bulk Earth
Mantle Array
16
Fields from Doe and Zartman, 1981
17
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!
18
d18O
Mantle
19
PRb trend from Taylor Silver, 1978 Klamath
trend from Barns et al., 1990
20
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.

21
Conclusions
  • 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.

22
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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