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. 312pp., +ix.
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