On coarse gabbroid diabase in Westfield, Massachusetts


Shannon, E.V.

The Journal of Geology 27(7): 579-581

1919


ON
COARSE
GABBROID
DIABASE
IN
WESTFIELD,
MASSACHUSETTS'
EARL
V.
SHANNON
United
States
National
Museum
In
the
western
part
of
the
town
of
Westfield,
Massachusetts,
a
number
of
quarries
have
been
opened
in
the
Holyoke
diabase
sheet.
These
quarries
are
the
same
which
have
furnished
the
beautiful
datolite
specimens
which
are
to
be
found
in
all
collections.
The
diabase
or
trap
in
which
these
quarries
are
located
is
the
middle
or
main extrusive
trap
sheet
of
the
Connecticut
Valley
Triassic
area.
Here
the
main
portion
of
the
sheet
is
composed
of
a
gray
holocrystalline
diabase
of
fine
to
medium
grain
not
unlike
the
intrusive
traps
which
form
East
and
West
Rocks
near
New
Haven.
In
the
northernmost
of
the
four
quarries
there
occur,
included
in
the
normal
rock,
irregular
areas
of
a
gabbroid
rock
very
unusual
in
appearance
for
an
extrusive
lava.
While
studying
the
occurrence
of
datolite
and
other
secondary
minerals
in
these
quarries,
the
writer's
attention
was
attracted
to
these
very
coarse-grained
phases
of
the
sheet
and
they
were
examined
in
some
detail.
At
first
it
was
believed
that
they
represented
a
later
intrusion
which
had
chanced
to
penetrate
the
previously
existing
flow,
but
a
study
of
the
relations
of
the
coarse
material
to
the
surrounding
diabase
of
normal
grain
led
to
the
abandonment
of
this
view.
The
contact
with
the
surrounding
rock
is
sharp,
the
transition
from
coarse
to
fine
material
being
accomplished
in
a
distance
of
an
inch
or
less.
The
coarse-grained
gabbro
forms
irregular
rounded
areas
often
many
yards
in
diameter
in
the
finer-grained
diabase.
In
the
hand
specimen
this
coarse
rock
shows
broad
blades
of
bronzy
greenish-
black
pyroxene
which
reach
an
inch
or
more
in
length,
imbedded
in
a
coarse
granular
aggregate
of
pyroxene
and
greenish
feldspar.
Under
the
microscope
the
rock
is
seen
to
be
composed
of
large
'Published
by
permission
of
the
Secretary
of
the
Smithsonian
Institution.
579
58o
EARL
V
.
SHANNON
crystals
of
pyroxene
and
plagioclase
in
a
subordinate
fine-grained
ground-mass
composed
largely
of
needle-like
crystals
of
plagioclase
with
interstitial
augite
in
anhedral
patches
now
largely
altered
to
a
greenish-brown
chlorite
which
is
probably
diabantite.i
The
large
pyroxene
phenocrysts
are
rather
pale
greenish-brown
in
color,
with
well-developed
cleavage.
They
are
unaltered
and
are
in
part
transparent
and
in
part
turbid
from
the
presence
of
an
opaque
brownish
pigment.
They
are
not
commonly
simple
but
are
bladed
aggregates
made
up
of
a
number
of
irregular
interlocking
crystals
not
quite
in
parallel
position
so
that
they
do
not
all
extinguish
simultaneously.
In
composition
this
pyroxene
is
ordinary
augite.
The
plagioclase
phenocrysts
are
characterized
by
rather
narrow
twinning
lamellae
and
are
near-basic
andesine
in
composition.
The
most
striking
microscopic
feature
of
the
rock
is
the
presence
of
broad
patches
of
a
graphic
intergrowth
of
plagioclase
and
augite
in
fixed
orientation,
giving
remarkable
pegmatitic
textures
entirely
analogous
to
the
quartz-feldspar
aggregates
of
graphic
granite.
In
places
a
phenocryst
of
plagioclase
terminates
in
a
branching
fernlike
intergrowth
of
plagioclase
and
augite.
A
question
naturally
arises
as
to
the
origin
of
such
coarse
forms
in
an
intrusive
flow
less
than
a
hundred
feet
in
thickness.
Ordi-
narily
such
surface
flows
cool
too
rapidly
to
permit
the
growth
of
large
crystal
individuals.
The
mode
of
occurrence
of
this
coarse
phase
indicates
that
the
large
crystals
of
plagioclase
and
augite
were
formed
after
the
lava
had
reached
its
present
position
and
precludes
the
supposition
that
it
was
intruded
into
the
extrusive
sheet
after
the
sheet
had
been
buried
by
later
sedimentation.
Emerson
2
has
described
similar
coarse
forms
from
this
trap
sheet
further
north
associated
with
mudstone,
pitchstone,
etc.,
formed
by
the
molten
lava
coming
in
contact
with
water
and
mud.
Strangely
enough,
he
attributes
the
coarseness
of
grain
of
the
plumose
diabase
to
sudden
cooling
or
quenching
of
the
molten
'Earl
V.
Shannon,
"Diabantite,
Stilpnomelane,
and
Chalcodite
from
Westfield,
Massachusetts,"
Proc.
U.S.
Nat.
illus.
In
print.
B.
2
K.
Emerson,
"Plumose
Diabase
and
Palognite
from
the
Holyoke
Trap
Sheet,"
Bull.
Geol.
Soc.
Amer
.,XVI
(1905),
91.
GABBROID
DIABASE
IN
WESTFIELD
581
material
by
the
water.
This
is
quite
contrary
to
our
knowledge
of
the
effect
of
the
rate
of
cooling
upon
the
grain
of
igneous
rocks.
The
writer
after
some
study
has
reached
the
conclusion
that
the
coarseness
of
crystallization
of
the
gabbroid
diabase
is
due
to
absorption
of
confined
water
by
the
still
molten
portion
of
the
flow,
with
the
formation
of
miniature
pegmatite
chambers.
Quite
certainly
here
as
elsewhere
the
lava
of
this
flow
encountered
moist
mud
flats
and
ponds
of
water.
The
coarse
gabbro
occurs
for
the
most
part
near
but
not
at
the
top
of
the
flow.
If
we
may
con-
ceive
the
top
of
the
flow
as
forming
here
a
resistant
crust
due
to
early
solidification,
the
water
from
below
heated
to
a
high
tempera-
ture
and
ascending
through
the
still
molten lava
must
come
to
rest
beneath
the
previously
solidified
crust
and
might
readily
be
con-
ceived
to
mix
with
the
still
molten
material
to
form
a
magma
rich
in
volatile
constituents
and
thus
capable
of
remaining
fluid
for
longer
periods
and
at
lower
temperatures
than
when
in
its
original
dry
state.
This
would
permit
the
formation
of
coarse-
grained
pegmatitic
textures
like
those
here
described.
Emerson
has
shown
in
the
paper
cited
that
water
did
ascend
through
the
flow
and
that
such
water
in
some
instances
caused
explosions
rup-
turing
the
upper
crust.
It
is
easy
to
understand
how
pressures
could
obtain
in
a
pegmatitic
chamber
of
the
sort
indicated
sufficient
to
rupture
the
roof
of
solid
diabase
after
the
formation
of
large
plumose
crystals
of
augite
was
well
advanced.
The
remaining
molten
material
would
be
chilled
following
the
explosion
and
yield
perlitic
glasses
associated
with
plumose
diabases
exactly
in
the
manner
described
by
Emerson.