ABSTRACT
Little Sitkin island is an Aleutian calc-alkalic volcanic center
that
has erupted a suite of lavas ranging from andesite through
rhyodacite.
Whole-rock chemistry of these lavas indicates contrasting
evolutionary
processes; major-oxide silica variation diagrams exhibit linear
trends
that are suggestive of magma-mixing while trace-element trends are
largely
controlled by accessory-phase fractionation.
Plagioclase, the dominant phenocryst phase in all lavas, commonly
occurs
in two distinct populations with markedly different compositions
and textures.
Both normal and reverse zonation is noted in the plagioclase and
clinopyroxene
of several samples. In addition, clinopyroxene is found as rims on
orthopyroxene
grains and as cores with orthopyroxene rims in one sample. These
inhomogeneities
and indications of disequilibrium are supportive of mixing.
The phase chemistry of the Little Sitkin samples indicates that
several
andesites of intermediate composition formed as a result of
mixing. There
is considerable overlap of phase compositions in the intermediate
andesites
and other samples, however the most complete overlap occurs with a
rhyodacite.
This suggests that the intermediate andesites formed as a result
of mixing
between a silicic rhyodacite and a more primitive low-silica
andesite.
The andesitic endmember has not been sampled, although its
bulk-rock major-element
chemistry has been estimated.
Least-squares modelling of four whole-rock mixes shows close
agreement
between observed and calculated andesite compositions. The sum of
the squares
of the residuals for these calculations are all less than one. The
best
match is given by a mixing pair of rhyodacite and the estimated
andesite
composition, for which the sum of the squares of the residuals is
approximately
0.02.
Comparison of the petrography and whole-rock chemistry of a sample
previously described as a basalt suggests that addition of
approximately
35 to 45 weight percent clinopyroxene to an andesite resulted in
the observed
composition. Least-squares analysis of the Little Sitkin samples
indicates
that incorporation of a smaller proportion of clinopyroxene,
approximately
34 weight percent, plus lesser olivine, plagioclase and magnetite
by an
andesitic liquid formed the "basalt". These phases are possibly
cumulate
and were incorporated into the andesitic liquid when a fresh pulse
of parental
magma carried them up to a shallow crustal magma chamber from
lower crustal
depths. This indicates that andesite, not basalt, is the most
primitive
composition sampled on the island, and therefore the composition
of the
parental magma must be inferred. Amphibole compositions indicate
that the
most reasonable composition of the parent is that of high-alumina
basalt
(HAB).
A model is proposed in which partial melting of mantle peridotite
yields
an olivine tholeiite liquid. High-pressure fractionation of this
liquid
at the base of the crust produces the HAB parent magma. The HAB
magma then
undergoes low-pressure fractionation at upper crustal depths. This
process,
along with mixing between fresh HAB and more evolved compositions
yields
the suite of rocks found on the island. Late stage mixing of
magmas intermediate
between HAB and the silicic component is recorded by intermediate
andesite
compositions.
Wolf, D.A., 1987. Identification of endmembers for magma mixing
in Little
Sitkin Volcano, Alaska.
Unpublished MSc. thesis, State University of New York at Albany.
201
pp., +xiii
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1987 W65
thesis (scanned
text) - 16.8MB pdf file
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