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| Titel |
Geochemical Evidence of a Near Surface History for the Source Rocks of the Central Coast Mountains Batholith, British Columbia |
| VerfasserIn |
C. C. Stremtan, P. H. Wetmore, M. N. Ducea |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250019736
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| Zusammenfassung |
| Major and trace elemental concentrations as well as Sr, Pb and O isotopic data, completed on
41 plutonic samples from the Coast Mountains Batholith (CMB) ranging in age
from ~108 Ma to ~50 Ma indicate that the source regions for these rocks were
relatively uniform and typical for island arcs around the Pacific. The studied rocks are
mineralogically and chemically metaluminous to weakly peraluminous and are
mainly calc-alkaline with a few samples (mostly from the eastern part of the Coastal
Shear Zone (CSZ) and three samples from the western part) showing a high-K
calc-alkaline feature. Trace elements, especially REE, suggest a mafic source, probably
oceanic plateau or island arc in origin, buried to different depths in the crust and
that underwent various degrees of partial melting. Initial whole-rock 87Sr/86Sr
range from 0.7035 up to 0.7053, whereas lead isotopic data range from 18.586 to
19.078 for 206Pb/204Pb, 15.545 to 15.634 for 207Pb/204Pb, and 37.115 to 38.661 for
208Pb/204Pb. In contrast to these fairly primitive isotopic data, δ18O values for
quartz separates determined for 19 of the samples range from 6.8 up to 10.0 . Such
δ18O values exclude the possibility that these melts were solely generated from the
Mesozoic mantle wedge of this continental arc, just as the Sr and Pb data preclude
significant involvement of an old (Precambrian) crustal/mantle lithospheric source. We
interpret the high δ18O component to represent materials that had a multi-stage
crustal evolution. They were originally mafic rocks derived from a circum-Pacific
juvenile mantle wedge that experienced a period of near surface residence after initial
crystallization. During this interval these primitive rocks interacted with meteoric waters
at low temperatures, as indicated by the high δ18O values. Subsequently, these
materials were buried to lower crustal depths where they re-melted to form the
high δ18O component of the CMB. We estimate that, based on REE ratios and
the presence of magmatic garnet in the samples from Eocene intrusives, that the
source rocks for the samples on the east side of the CSZ were, at least partially,
mafic rocks with a strong basaltic component buried to a depth of more than 30 km.
This component makes up at least 45% (mass) of the Cretaceous through Eocene
batholith in the studied area. The remainder of the source materials making up
the CMB had to be new additions from the mantle wedge. A prolonged period of
contractional deformation beginning with the Early Cretaceous collisional accretion of the
Insular superterrane is inferred to have been responsible for underthrusting the high
δ18O into the lower crust. We suggest that rocks of the Insular superterrane (e.g.
Alexander-Wrangellia) are of ideal composition, and were accreted to and overthrust by
what would become the CMB just prior to initiation of magmatism in that region. |
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