Abstract
Field, petrographic, geochemical and stable isotopic evidence for
timing of serpentinization in the 162 Ma Josephine ophiolite of NW
California and SW Oregon are presented in this study. Detailed
studies of brittle and ductile serpentinized shear zones have
revealed a complex serpentinization history, beginning at the
oceanic stage. Dikes intruded into serpentinized shear zones
provide time markers for serpentinization and deformation and are
of two types: (1) Fe-Ti enriched dikes with N-MORB magmatic
affinity, geochemically similar to the uppermost lavas and a late
Fe-Ti dike within the crustal sequence, and (2)
hornblende-bearing, calc-alkaline dikes intruded during ophiolite
emplacement. Cross-cutting relationships between dikes and
serpentinites indicate serpentinization of upper mantle
peridotites took place prior to the latest magmatic pulse, during
periods of amagmatic extension, at temperatures <300ºC.
The ultramafic cumulate sequence was completely serpentinized
prior to ophiolite emplacement and the paleomoho in the Josephine
ophiolite may be a serpentinization boundary. The occurrence of
oceanic serpentinites intruded by Fe-Ti basalts may indicate a
minor propagating ridge-tip setting, possibly associated with
overlapping spreading centers.
Lizardite-chrysotile serpentinites, interpreted as oceanic based
on field and geochemical evidence, exhibit a wide range of dD
values (-79 to -126), outside the range of modern oceanic
serpentinites. Comparison of field and isotope data strongly
suggests hydrogen isotopes in lizardite and chrysotile have
partially to completely equilibrated with modern meteoric waters,
by low-temperature, diffusive, hydrogen isotope exchange. dD
values for antigorite (-24 to -47) are compatible with formation
by interaction with oceanic or regional metamorphic fluids, and
indicate that antigorite is resistant to post-crystallization
hydrogen isotope exchange. d18O values for lizardite-chrysotile
serpentinites are within the range of oceanic serpentinites and
calculated d18Ofluid values indicate they could have formed by
interaction with modified oceanic hydrothermal fluids at w/r
ratios <0.5. Field, petrographic and stable isotope data are
compatible with formation of antigorite during ophiolite
emplacement and regional metamorphism. The findings of this study
necessitate re-evaluation of timing of serpentinization in
ophiolites, and caution against the use of hydrogen isotope data
alone as an indicator of timing of serpentinization.
Coulton, A.J., 1995. Fault-related oceanic and emplacement-age
serpentinization in the Josephine ophiolite of NW California and
SW Oregon.
Unpublished PhD dissertation, State University of New York at
Albany. 240pp., +xi;
University at Albany Science Library call number: SCIENCE
MIC Film QE 40 Z899 1995 C68
Copies of this PhD dissertation can be ordered
from Proquest UMI
Front matter (title,
table of contents, abstract, acknowledgements) - 0.45MB pdf
file
Photo pages in dissertation
(colour
and greyscale photos with captions): - 15.7MB pdf file
Appendix 2 - Josephine
Ophiolite shear zone and sample location maps 7.1MB pdf file
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