ABSTRACT
The suite of gabbroic rocks collected by the DSRV ALVIN in 1976
and 1977 from the walls of the Mid-Cayman Rise spreading center
were studied in detail to provide the best available data on
plutonic rocks sampled directly from the ocean floor. The rock
types studied include variably deformed and altered gabbros,
orthopyroxene gabbros, olivine gabbros and troctolites, and a few
amphibolites. Mineral chemical analyses suggest that the various
rock types are representative of a fractionation trend from
magnesian troctolites through olivine and clinopyroxene gabbros to
iron-enriched orthopyroxene gabbros. Within many individual
samples, the primary mineral phases are apparently chemically
homogeneous despite sometimes considerable alteration, which
suggests reequilibration after original crystallization. Variation
in mineral-chemistry across the suite is larger than previously
reported for ocean-floor gabbros; this may be due to the larger
population of this study. The primary or secondary nature of
plagioclase and amphibole must be distinguished in discussions of
the igneous processes involved in the genesis of these rocks. In
this suite, textural evidence is often inconclusive, and although
sodic chemistry is assumed to indicate a secondary origin for
plagioclase, no chemical indicator was found to apply to
amphiboles. Many different deformation textures are observed in
the samples, indicating considerable variation in the conditions
of deformation within this slowly accreting ridge environment.
Ductile features range from mechanical twins and bent crystals
(low strain) to complete recrystallization with a well-developed
foliation (high strain). Textures suggestive of cataclasis include
zones .1-10 mm wide containing very fine, irregular grains; kinked
and cracked grains, usually very undulose; and crosscutting
fractures. These textures may occur separately, or next to or
overprinting each other. Of the variables controlling the
formation of deformation features, temperature and water pressure
are most easily estimated. Minimum temperature during deformation
is suggested by minimum temperature of formation of mineral
assemblages unaffected by this deformation. This temperature is
greater than 550°C (epidote-amphibolite facies) for the
majority of features observed, although sampling may have
introduced a bias away from lower temperature features.
Observations suggest enhanced recrystallization where the primary
mineralogy has been hydrated to a greater extent. Theoretically,
confining pressure for the gabbroic rocks in this suite may have
varied from about .3-.9kb; fluid pressure should have been within
these limits. Based on mineralogical evidence, cataclastic and
ductile textures developed at both high and moderate temperatures.
This suggests that strain rates and/or pressure vary considerably
within regions where rocks are hot as well as where rocks are
cooler, and that after high-temperature deformation some rocks
cooled quickly enough to prevent significant recrystallization
while others cooled more slowly. These interpretations indicate
that the Cayman plutonics were raised to their present position by
motions which varied from place to place and time to time, and
suggests that the structural history of plutonic.rocks
formed at slowly accreting plate boundaries is highly variable and
complex.
Malcolm, F.L., 1979. Petrography, mineral chemistry and
microstructures of gabbros from the Mid-Cayman Rise Spreading
Center. Unpublished MSc. thesis, State University of New York at
Albany. 418pp., +ix.
University at Albany Science Library call number: SCIENCE
Oversize (*) QE 431 M35X
thesis (scanned
text) - 9.5MB pdf file [p. 1-89, +vii]
Appendix 1 - descriptions
of rock samples recovered during ALVIN dives - 0.6MB pdf
file [p. 90-106]
Appendix 2 - thin
section descriptions - 6.3MB pdf file [p. 107-312]
Appendix 3 - microprobe
analyses of plagioclases in Cayman Trough plutonic rocks -
1.3MB pdf file [p. 313-342]
Appendix 4 - microprobe
analyses of olivines in Cayman Trough plutonic rocks - 0.5MB
pdf file [p. 343-353]
Appendix 5 - microprobe
analyses of pyroxenes in Cayman Trough plutonic rocks -
1.3MB pdf file [p. 354-382]
Appendix 6 - microprobe
analyses of amphiboles in Cayman Trough plutonic rocks -
1.8MB pdf file [p. 383-418]
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