ABSTRACT
During a geological and geophysical survey of the Oceanographer
Fracture Zone (35°N, 35°W), seventeen dredge hauls
containing a variety of rocks were obtained. Petrographic study
shows that these rock samples can be classified into six main rock
types: fresh basalt, weathered basalt, metabasalt, gabbro,
metagabbro, and serpentinite. Most of the dredge hauls were
positioned on the steep, southern wall of the fracture zone, and
an inference from the dredging results suggests that basalt is the
most abundant rock type which outcrops at the junction between the
rift valley and the fracture zone; however, as we move away from
the junction, metabasalts and metagabbros apparently become more
abundant in the outcrops on the wall of the fracture zone.
Fresh basalts from the Oceanographer Fracture Zone are
characterized by the petrography, mineralogy, and major element
chemistry typical of abyssal tholeiites, but are somewhat enriched
in large ion lithophile elements (K, La, etc.). The major
constituent minerals observed in these fresh tholeiitic basalts
are olivine, plagioclase, clinopyroxene, and opaque minerals.
Compositional variation of olivine within a given specimen is
small, and fayalite contents steadily increase as FeO*(total iron
as FeO)/MgO in the host basalt increases. Clinopyroxenes of these
tholeiitic basalts are mostly augites, and no Ca-poor pyroxene is
observed. Plagioclase including those that occur in the groundmass
show an overall compositional range from An86 to An44. There is a
positive correlation between albite content and Fe/(Fe+Mg) in
these plagioclases; i.e., Fe/(Fe+Mg) increases as plagioclase
becomes more sodic.
Major element analyses of tholeiitic basalts from two adjacent
dredge hauls (RD 7 and 8) show that these tholeiitic basalts are
closely related chemically and that they apparently represent
points on a continuous liquid line of descent. This compositional
variation can be ascribed to fractional crystallization of
phenocryst phases observed in these tholeiites.
Metamorphic rocks from the Oceanographer Fracture Zone range in
metamorphic grade from zeolite to greenschist facies. Metabasalts
are massive and show no evidence of deformation; metagabbros are
to some degree deformed. Petrographic study of these metagabbros
indicates that. plagioclase grains are commonly strained, bent,
and ruptured, whereas pyroxene and hornblende grains are replaced
by a metamorphic mineral assemblage made up mainly of actinolite
and chlorite. Apparently, plagioclase responded differently than
the mafic minerals to these tectonic and metamorphic conditions.
The Oceanographer rock collection is characterized by abundant
rocks with the hornblende plus plagioclase assemblage. It is
suggested that this rock type must be considered as one of the
potential candidates for the constitution of oceanic crust.
Shibata, T. 1976. Petrology of the Oceanographer Fracture Zone
(35ºN35ºW). Unpublished PhD dissertation, State
University of New York at Albany. 129pp., +x; Appendix 27pp.
University at Albany Science Library call number: SCIENCE
MIC Film QE 39 S56X
Copies of this PhD dissertation can be ordered
from Proquest UMI
Front matter (title,
table of contents, abstract, acknowledgements) - 0.25MB pdf
file
Photo pages in dissertation
(greyscale
photos
with captions) - 4.9MB pdf file
Appendix 2 - Electron
microprobe analyses of olivine, plagioclase and clinopyroxene
- 0.8MB pdf file
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