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Determination of Tectonic Setting of Sandstone-Mudstone Suites Using $SiO_{2}$ Content and$K_{2}O/Na_{2}O$ RatioAuthor(s): B. P. Roser and R. J. KorschReviewed work(s):Source: The Journal of Geology, Vol. 94, No. 5 (Sep., 1986), pp. 635-650Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/30078330 .
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VOLUME 94
T H
JOURN L
O
GEOLOGY
September 1986
DETERMINATION
OF
TECTONIC
SETTING OF
SANDSTONE-MUDSTONE
SUITES
USING
SiO2
CONTENT
AND K20/Na20
RATIO'
B.
P.
ROSER2
AND
R.
J.
KORSCH3
Department f
Geology,
Victoria
University
of
Wellington,
Private
Bag,
Wellington,
New
Zealand
ABSTRACT
Several
previous
studies
have
shown
that
sandstones
rom
different
ectonic
settings
possess
characteris-
tic
chemistry,
particularly
iO2
content
and
K20/Na20
ratio. Systematic
variationof
these
variableswith
change
in
grain
size
in
New Zealand
greywackes
shows that
such
discrimination
an
be
extended
to
the
finer-grained
members
(siltstones,
mudstones)
of
sedimentary
equences.
Compilation
f
publisheddata
from
ancient sedimentarysuites defines
passive margin
(PM),
active
continental
margin
(ACM),
and
oceanic
island
arc
(ARC)
ectonic
setting
fields
on
a
simple
bivariate
K20/Na20-Si02
plot. These
fields
are
confirmed
by
published
analyses
of
modern
sands and
muds
of
known
tectonic
setting,
which
also
reveal
distinctive
sand-mud rends
in
sediments
from the
forearc environment f
island
arcs,
where
muds
show
greater
ratios
and
SiO2
han
the
sands.
Results
gained
from
New Zealand
erranes
are
in
good
agreement
withpublished
paleoenvironmental
nterpretations.
Quartzose
Greenland
errane
edimentsare of
PM
type,
and
quartz-intermediate
amples from
the
Torlesse,
Haast Schist,
and
Miocene
terranes
are
classifiedas
ACM.Quartz-poor olcanogenic andstones rom heCaples,Maitai,andMurihikuerranes allintheARC
field, and
argillites
plot
at
larger
K20/Na20
and
SiO2
near or
in
the
ACM
field, supporting
proposed
arc
settings.
The
New
Zealand
data
llustrate
well
the
effects
of
grain
size, petrologic
volution,
and
maturation
on
the
chemistry
of
sediments,
and
overall
results
demonstrate he
value
of
sampling
he
entire
range
of
grain
sizes
when
chemical
discrimination
s
to
be
attempted.
The chemical
approach
s
a
useful
complement
to
petrographic
nalysis,
and
the
two
methods
combined
are
a
powerful
ool
for
examination
f
provenance
and
determination
f
tectonic
setting.
INTRODUCTION
In
the
last
two
decades
evolution
of plate
tectonic
theory
and
detailed petrographic
studies of sandstones from sedimentary se-
quences
has led
to
the development
of
a
num-
ber
of
detrital
modal discriminants
aimed
at
determination
of
tectonic
setting of
ancient
basins
(Crook
1974;
Dickinson
and
Suczek
1979;
Dickinson
et
al.
1983). Such
studies
'
Manuscript
eceived
August
28,
1985;
revised
December
27,
1985.
2
Presentaddress:
Dept.
of
Geology,
University
of
Otago,
P.O.
Box
56, Dunedin,
New
Zealand.
3 Presentaddress:Bureauof MineralResources,
G.P.O.
Box
378,
Canberra,
A.C.T.
2601,
Australia.
[JOURNAL
F
GEOLOGY,
986,
vol.
94, p.
635-650]
©
1986
by
The
University
of
Chicago.
All
rights
reserved.
0022-1376/86/9405-003$1.00
have
been
complemented
by
similar
work
on
modern
sediments
of
known plate
tectonic
setting
(e.g.
Valloni
and
Maynard
1981;
Pot-
ter
1984;
Yerino and Maynard
1984),
and
characteristic
modal
compositions
for sand-
stones of
differing
tectonic
environments
are
now
reasonably
well
established.
The
nature
of
the
petrographic
method
nor-
mally
restricts
its
application
to
sandstones,
and
so interbedded
argillite-mudrock
mem-
bers
of
sedimentary
sequences
have
not
been
widely
used
for
determination
of provenance
an'd
ectonic
setting
(Blatt
1985).
This
restric-
tion does
not
apply
to
bulk chemical
data.
The geochemistry of sedimentary rocks
reflects
predominantly
the
nature
and
propor-
tion
of
their detrital
components
and
hence
their
provenance. Even
though
diagenesis
may
alter
original
chemistry,
changes
are
themselves
related to plate
tectonic
environ-
635
NUMBER
5
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B. P.
ROSER
AND
R.
J.
KORSCH
ment
(Siever
1979),
and
bulk
composition
should
still
reflect
tectonic
setting and
so
en-
able
development
of chemically-based
dis-
criminants
to
supplement
the
petrographic
approach.
Despite
a
long
literature
history,
chemical
discrimination
lags
behind that
of-
fered by petrography.
Central
to a
number
of
previous
studies
(Middleton 1960;
Crook
1974;
Schwab
1975;
Maynard
et
al.
1982;
Bhatia 1983)
is
the rec-
ognition of characteristic
K20/Na20
ratios
and
SiO2
contents
of
sandstones
from
con-
trasting
tectonic
settings.
This
paper
reex-
amines
the
significance
of
these parameters
in
sandstones,
and
extends their
application to
the
finer-grained
members
of
turbidite
suites.
K20/Na20
AND Sio2 IN
SANDSTONES
Middleton
(1960)
examined
the
chemistry
of
sandstones
in
terms
of
the
geosynclinal
models
of
the
time
and
found
that eugeosyn-
clinal
greywackes
were distinctive,
with
K20/
Na20 ratios of
<
1,
in
contrast
to
ratios of
>
1
in
samples
from
more
stable settings.
Crook
(1974)
divided
greywackes
into
three
classes
and
assigned each
to
a
major
plate
tectonic
environment.
Crook's
groups
are:
1) Quartz-poor greywackes, with <15%
framework
quartz, average
58%
volatile-free
SiO2,
and
K20/Na2000AB;l.
They
are
of
basic
volcanic
provenance
in
magmatic
island
arcs.
2)
Quartz-intermediate;
15-65% quartz,
average
68-74%
SiO2, and
K20/Na20<l.
Provenance
is
mixed,
and
rocks
of
this class
are
typical
of
evolved
active
continental
mar-
gins.
3)
Quartz-rich; >65%
quartz,
average
89%
SiO2,
and
K20/Na20>1.
They
are
deposited
at
passive
continental
margins
and
in
plate
interiors.
The
above
classification
was
based
on
ap-
parent
breaks
in
the
frequency
distribution
of
QFR
framework compositions
of
328
Phanerozoic
greywackes.
Later work
in
both
ancient
and
modern
sediments
has shown
that these
breaks
may
not
be
real
or
may
oc-
cur
elsewhere
(Korsch
1978;
Valloni
and
Maynard
1981),
but
in principle
the
division
remains a useful starting point for chemical
discrimination.
Schwab (1975)
tested
Crook's proposal
with
more
data
and
supported
the
model,
but
commented
that
more
data from
modern
sands
of
known
tectonic
setting
were
needed
for
the
results
to
be
conclusive.
This
was
partly
met
by Maynard
et
al.
(1982),
with
data
from
five
major
tectonic
settings:
passive
margin
(trailing
edge),
continental
margin
arc,
strike-slip
margin,
backarc
and
forearc.
They found
that
passive margin
sediments
were chemically distinct from those of the ac-
tive
settings,
among
which
only
the
sands
from
forearc
basins
of
island
arcs
were
dis-
tinctive.
Compositions
of
the
remaining ac-
tive
settings
overlapped
considerably.
Pas-
sive
margin
sands
of
Maynard
et
al.
(1982)
average 77.9%
SiO2
and
K20/Na20
1.05,
whereas
forearc
sands
equivalent
to
Crook's
quartz-poor
class
average 61.5%
Si02
and
K20/Na20
0.37.
The
more
complex
and
evolved settings (continental margin arc,
strike-slip,
backarc)
show
SiO2
and
K20/
Na20
intermediate
between the
above
(69.5%,
0.72;
67.8%,
0.74;
and
68.8%,
0.55
respectively).
These
data largely
support
Crook's
chemical
classification.
COMPOSITIONAL
VARIATION
WITH
GRAIN
SIZE
Common to
the
work
of
Crook
(1974),
Schwab
(1975), and
Maynard
et
al.
(1982) is
the
reduction
of
individual
chemical
analyses
to grouped average values. The use by
petrographers of
samples of
restricted grain
size
range
is,
however,
recognition
of
the
ef-
fects of
grain
size
on
the
modal composition
of
sediments (Ingersoll
et
al.
1984).
Chemical
composition
will
also be affected, and
so the
results
of any
geochemical
discrimination will
be
influenced
by
the
grain
size
of
the samples
analyzed (e.g.,
Roser and
Korsch
1985).
Data
from
Mesozoic
sediments
of the
Torlesse
ter-
rane
of
New
Zealand
suggest
that
this
compli-
cation may
in
fact
be
used
to
advantage.
The
Torlesse sediments
consist
of monot-
onous
sequences
of
indurated,
interbedded
greywacke
and
argillite, with
subordinate
conglomerate
and
tectonic
inclusions of
oceanic
metabasites
and
pelagic
sediments.
Sandstone
petrography
indicates
a
domi-
nantly
plutonic-metamorphic
provenance
with
a
lesser
volcanic and sedimentary
com-
ponent,
consistent
with
derivation
from
a
continental magmatic arc source. This is sup-
ported
by
modal data
from
MacKinnon
(1983)
and
Korsch (1984)
applied
to
the
tectonic
dis-
criminant
plot
of
Dickinson
et al.
(1983),
which indicates
a
transitional
to
dissected
arc
provenance.
Petrographically
the
grey-
636
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TECTONIC
SETTING
OF SANDSTONE-MUDSTONE
SUITES
wackes
belong to
the
quartz-intermediate
class
of
Crook
(1974).
The
Torlesse
is
inter-
preted
as
an
accretionary
terrane formed
at
the
New
Zealand
subduction
margin, fed by
sediment
from
the
Antarctic portion
of
Gond-
wana
(MacKinnon
1983;
Korsch
and
Well-
man in press).
Rowe
(1980)
and
Roser
(1983)
found
that
modal
and
chemical
compositions in
Torlesse
greywackes
and
argillites
form
systematic
continua closely
dependent
on
grain size.
As
mean
grain
size
decreases
in
the
transition
from
sandstone
to argillite,
modal
quartz,
feldspar
and
lithic fragments
decline in
abun-
dance,
and
modal
matrix
and
phyllosilicate
correspondingly
increase.
This
affects
bulk
chemistry
strongly.
SiO2
and
Na20
decrease
regularly
from
sandstone to
argillite,
and
K20
increases.
These
changes
cause
a
progressive
decline
in
K20/Na20
with
increasing
SiO2
(fig.
1).
Si02 rises from
near
60% to
over
75%
as
grain
size
increases
from
argillite
to sand-
stone,
and
K20/Na20 falls
from
over
3.0 in
argillites
to
as
low
as
0.4
in
sandstones. Sev-
eral
sandstones
show
K20/Na20
ratios above
the 1.0 value
limit of
Crook's
quartz-
intermediate
class,
and some
overlap
occurs
between argillites and the finest grained sand-
stones.
The above
features
illustrate
the
effect
of
grain
size
on K20/Na20
systematics
and
the
problems
it
can
cause
in
chemically-based
discrimination. More
importantly,
the
sys-
tematic
variation
of
K20/Na20
and
SiO2
in
the
example
suggests that the
chemistry
of
argillites
and
fine
sandstones
may
be
as
dis-
tinctive as
their
companion sands.
By
demon-
strating
similar
trends
in
other
suites,
K20/
Na20-SiO2 classification can be extended to
include
the
finer-grained
members
of
sedi-
mentary
units.
K20/Na20-sio2 RELATIONS
IN
ANCIENT
SEDIMENTS
AND
THEIR TECTONIC SETTING
Literature
analyses
of sandstones
and
ar-
gillites/shales
from ancient
sedimentary
se-
quences
of
inferred tectonic
setting
have
been
used
to
establish a tectonic
classifica-
tion based on SiO2
content and
K20/Na20
ratio.
Individual
data
are
used
rather
than
av-
erages
so
that
trends can
be
highlighted.
Most
of
the
studies used
include
data
for
both
sand-
stones
and
argillites,
and
some
also
give
modal framework
compositions. Division
has
60
65
o
70
75
FIG.
1.-K20/Na-SiO2
relations
in grey-
wackes
(filled
circles)
and
argillites
(open
circles)
from
he Torlesse
errane,
Wellington, ew
Zea-
land.
Data
from
Rowe
(1980),
Roser
(1983)and
Roser
(unpub.
data),
recalculated
to
100%
LOI-
free.
N
=
79.
been
made
nto
three
first-order
ectonic
cate-
gories, broadly
similar
to
those
of
Crook
(1974).
It
is stressed
here
that
within
these
divisions several depositional settings are
possible.
Work
in progress is
directed
to-
wards
defining
subdivisions
within
individual
categories.
The
categories
are:
1)
Passive Continental
Margin
(PM).-
Mineralogically
mature
(quartz-rich)
sedi-
ments
deposited
in
plate
interiors
at
stable
continental
margins
or
intracratonic
basins.
Represented
by
(a)
Ordovician
and
Silurian
greywackes
and
shales
from
Australia
that
are
recycled quartz-rich
sediments
derived
from older adjacent continental
terrains
(Wy-
born
and
Chappell
1983),
and
(b)
the Robert-
son
Bay Group,
Antarctica
(Harrington
et
al.
1967;
Nathan
1976),
which
consists
of
sub-
marine
fan
turbidites
derived
mainly
from
a
mature
continental
source
(Wright
1981).
This
category
is
equivalent
to the
trailing-
edge
tectonic
setting
of
Maynard
et
al.
(1982).
Hence
sediments
in
this category
were
de-
rived
from
stable continental areas and
were
deposited in sites away from active plate
boundaries.
2)
Active
Continental
Margin
(ACM).-
Quartz-intermediate
sediments
derived
from
tectonically
active
continental
margins
on
or
adjacent
to active
plate
boundaries.
Repre-
637
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PM-basins
on
continental
crust, and basins
associated
with
ocean
floor
spreading,
failed
rifts
and
Atlantic-type
continental margins;
(2)
ACM-subduction-related
basins, conti-
nental
collision
basins,
and
pull-apart
basins
associated
with
strike-slip
fault
zones;
and
(3)
ARC-subduction-related basins.
To
enable
direct
comparison,
all
analyses
have
been
recalculated
to
100%
volatile-free.
When
plotted on a
binary
K20/Na20-Si02
di-
agram
(fig. 2) data
for
the
three
tectonic
set-
tings
fall into
distinct
fields.
ACM
data
show
a
clear decline in K20/Na20
with
increasing
SiO2
(fig.
2),
comparable with that
exhibited
by
the
trend
from
argillite
to
sandstone
in
the
Torlesse
terrane
(fig.
1).
A similar,
but more
scattered, trend
occurs
in the
passive
margin
samples,
even
though
their
lithology
cannot
fully
be differentiated
here,
as
no distinction
was
made between
sandstone
and
shale
by
Wyborn
and
Chappell
(1983). ARC
sand-
stones
plot
together at
low
K20/Na2O
and
SiO2,
distinct from the
more
evolved
ACM
rocks.
Within
the arc
group
the
virtually
non-
quartzose
Baldwin
Formation
samples
plot
FIG.
2.-Tectonic
discrimination diagram
for
sandstones
and
argillites, derived from
published
data recalculated
o
100%
olatile-free.
PM
=
Pas-
sive
margin;
ACM
=
active
continental
margin;
ARC
=
oceanic island
arc
margin.
Dividing
ines
between the
fields
are
placed
by
eye
at
apparent
breaks.
Sources
of
data
are given
in
the
text.
Sym-
bols-PM:
Filled
triangles-undifferentiated
Aus-
traliangreywackesand
shales,
plus
Robertson
Bay
Group
greywackes;
open
triangles-Robertson
Bay shales.
N
=
35.
ACM: Filled
circles-
Franciscan, Santa Ynez, and Kodiak greywackes
and
arkoses;
open
circles-Franciscan
argillites
and Santa
Ynez
shales.
N
=
76.
ARC:
Dia-
monds-U
yak/Cape
Current
greywackes;
squares-Baldwin
Formation
greywackes.
N =
24.
B.
P.
ROSER
AND
R.
J.
KORSCH
sented
by rocks
from
(a)
the
Franciscan
Com-
plex,
California
(Bailey
et
al.
1964;
Taliaferro
1943)
and
Kodiak Formation,
Alaska
(Con-
nelly
1978), which
are
quartzo-feldspathic
continental-derived
trench deposits
incorpo-
rated
into
an
accretionary
wedge, and
(b)
the
Santa Ynez Mountains, California (Van de
Kamp
et
al.
1976)
which
were
deposited
at
a
complicated
continental
margin where
both
subduction
and
strike-slip processes
were
ac-
tive.
Hence
this
category
includes
complex
active
margins
including
material
derived
from
continental
margin
magmatic
arcs
(and
deposited
in
a
variety
of
basin
settings
includ-
ing
trench, forearc,
intraarc,
and
backarc)
and material
derived
from uplifted areas
asso-
ciated
with
strike-slip
faults
and
deposited in
pull-apart
basins.
Petrologically evolved
or
deeply
dissected
magmatic island
arcs
could
also
be
included
in
this
category.
3)
Oceanic
Island Arc (ARC).-Quartz-
poor volcanogenic sediments
derived
from
oceanic
island
arcs. Represented
by
(a)
the
Baldwin
Formation,
Australia
(Chappell
1968)
which
are
forearc basin
sediments
de-
rived from
an
andesitic
island
arc
source,
and
(b)
the Uyak
and
Cape
Current
greywackes,
Alaska (Connelly 1978), which were derived
from
an
andesitic
source and
deposited
in
a
trench
adjacent to
an
active
volcanic
arc.
Connelly found
an
average
17%
framework
quartz in
the
Uyak
greywackes
and therefore
classified
them
as
active continental
margin
type but
observed that
their
composition
verged
on
those
of
oceanic
island
arc
grey-
wackes.
In
view
of
their
lesser
quartz
content
and
average
K20/Na2O
ratio
of
0.25,
they are
here
placed
in
the
ARC
category
as
an
upper
boundary. Hence sediments in this category
were
derived
from an
island
arc
source
and
were
deposited
in
a
variety
of
settings
includ-
ing
forearc,
intraarc, and backarc
basins
and
trenches.
The
categories
defined
above
reflect
the
compositions
of
rocks
in the
source
areas.
Care is
needed
when
assessing
the
tectonic
setting
of
sediments
deposited
in
basins
re-
lated
to
active
plate
boundaries
because
these
basins
(trench,
forearc, intraarc,
backarc)
can
be related to
either
a
continental margin
magmatic
arc
(ACM)
or
an
island
arc
(ARC).
Reading
(1982)
defined five
types
of
sedimen-
tary basins
related
to plate
tectonic
setting.
In
terms
of
our
categories,
these
basins
are:
(1)
638
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TECTONIC
SETTING
OF
SANDSTONE-MUDSTONE
SUITES
apart
from
the
transitional
Uyak-Cape
Cur-
rent
rocks.
No
argillite
data
are
available
in
Chappell
1968) or
Connelly
(1978).
The
data
suggest
that
the
chemistry
of
both
sandstones
and
argillites
can
indeed
be used
for
tectonic
discrimination.
Positioning
of
datafromlower left to upperrighton the dis-
criminant
plot
(fig.
2)
is
first
dependent
on
the
nature
of volcanism,
extent of
plutonism
and
erosion
level,
and
then the
effect of
mineralogical
maturation
through
sediment
recycling.
In the
PM
and ACM fields varia-
tion
from upper
eft
to
lower
right
s
related
o
grain
size.
Given
the
complexity
of
these con-
trols
on
sediment
composition,
and the
vari-
ety
of
hybrid
tectonic
settings possible
(e.g. strike-slipmargin,backarc),transitions
across
field
boundariesmust be
expected.
MODERN
SEDIMENTS
Any
discriminants
aimed
at
determination
of
tectonic
setting
are
based
soundly
only
if
supported
y
data
from
modem
equivalents
f
known
environment,
a
point
recognized by
Crook (1974),
Schwab
(1975),
and numerous
petrographic
studies.
The
modern
sediment
data
of
Maynard
et
al.
(1982)
are
invaluable
for that purposehere, as data are given for
muds
overlying many
of the
sands
used
in
their
petrographic
work.
Maynard
et
al.
(1982)
found
similar
chemical
distinctions
could be
made
among
their
muds
as
for their
sands. Trailing-edge
and
forearc muds
were
distinctive chemically,
but
compositions
of
continental
margin
arc,
strike-slip,
and
back-
arc
muds
overlapped
considerably.
Maynard
et
al.
(1982) examined
the
mud
compositions
as
averages
separate
from
the
companion
sands. From the above discussion of grain
size
effects,
however,
it
is
advantageous
to
combine
the
data
by
treating
sand-mud
oup-
lets from
single
sites
as
representative
f
bulk
compositional
trend.
The
results
of
such treatmentare given
in
figure
3,
where
sand and mud
of
each
couplet
are
joined
by
tie
lines.
The
trailing-edge
am-
ples (PM
type) trend
across
the
ACM-PM
boundary,
and the
muds
are
mostly
classified
as ACM(fig.3A).Examination f the individ-
ual
oxide
data
shows
that
considerableCaO
(up
to
18.9%)
s
present
n
some
samples,
es-
pecially
the
muds.
Presumably
much
of
this
represents
biogenic
CaCO3
and
the
dilution
effect
from
this
will
affect
results.
The
data
cannot
be
recalculated
arbonate
ree,
as
CO,
values
are
lacking,
so
the
trailing-edge
nal-
yses have
been
recalculated
to
100%CaO-
free instead.
This
was
also done
for
the
data
of
figure
2 and
the
field
boundaries
adjusted
accordingly
on
figure 3B.
Recalculation
movesmostdatapointsintothe PMfield,and
nearlyall
muds have
higher
K20/Na20
ratios
and
lower
SiO2
than
their
companion
sands
(fig.
3B).
Hence
the
trailing-edge
ediments
of
Valloni
and Maynard
t
al.
(1982)
support
he
PM
field delimited
on
figure
2.
CaO
contents
of
the
active margin
amples
do
not
appear
anomalous,
and
the
data
there-
fore
have
not
been
recalculated.
Leading-
edge
sands
(equivalent
to
ACM)
plot
in
the
ACMfield(fig. 3C), andsand-mudpairstrend
as
do
the
suites
in
figure
2.
Strike-slip
margin
sediments
also
fall
in
the
ACM
field
(fig.
3D).
Obviously
this
need
not
be the
case, but
de-
pends
on the
nature
of
the
terranes
jux-
taposedat such
a
margin,
and
the
results here
are
probably
due
to most
of
the
samples
being
from
the
California
trike-slip
margin.
Plots
of
forearc
(fig.
3E)
and
backarc
sam-
ples
(fig.
3F)
are
particularly
interesting.
Forearc
sands
fall
mostly
in
the
ARC
field,
confirmingts validity,but companionmuds
plot
at
higher
SiO2
and K20/NaO, giving
a
sand-mud
rend
perpendicular
o
that
of
the
ACM
and
PM
fields. No
explanation
is
ad-
vanced at
present,
but
it
seems
that
controls
on
compositional
rends
in
arc
sediments
dif-
fer
from
those
in
more
evolved
settings.
Confirmation of
the anomalous
trend
in
forearc
sediments is
given
by
the
backarc
data (fig.
3F),
which
form
two differently
trending
sets.
The
first
set
fall
in the
ACM
field, and sand-mud rends(chain-dashed ie
lines)
parallel
he boundaries.
The
second
set
(solid
tie
lines)
plot in
the
ARC
and
ACM
fields,
but
trend
perpendicular
o the first,
similar o
the
forearc
samples
(fig.
3E).
This
pattern
is
explained
by
the
location
of
the
sample
sites
given
by Valloni and
Maynard
(1981).
ARC-trending
amplepairsare
mostly
proximal o arcs in
their
backarc
basins,
and
so
are
most
influenced
by
that
source,
whereasACM-trending airsare moredistal
and
reflect
greater
continental
nfluence.
Distinctive
orearc
chemistry
s
also
exhib-
ited
by
recent
data
from
Crook et
al.
(1984)
for
sediments
from
the
Solomon
Islands
arc.
Samples
from
forearc
settings plot
low
in
the
639
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TECTONIC SETTING
OF
SANDSTONE-MUDSTONE
SUITES
A
FORE-ARC
ACM
ARC
50
60
70 80
)
INTER-ARC
ACM
ARC
50 60
70
80
SiO2
FIG.
4.-Modern
sediments
from
the
Solomon
Islands
arc:
volatile-free
carbonate-corrected
ata
from
Crook
et
al.
(1984).
Filled
circles
are
sands
(4
phimediangrainsize andcoarser)andopencircles
muds.
Tie-lines
oin
sand-mud uites
at
single
sites;
grain size
decreases in
the
direction
arrowed.
(A)
Forearc:
trench,
forearc basin,
and
forearc
ridge
settings.
N =
23.
(B)
Interarcbasins.
N
=
12.
permit
recognition
of
backarc
basins.
If
sand-
stone-argillite
sample
pairs
from
any
ancient
sedimentary
sequence
show
two
trends, as in
figure
3F,
deposition
in a
backarc
setting
could
be
suspected.
The arc
data
of
Valloni
and
Maynard
(1981)
and
Crook
et
al.
(1984)
emphasize,
however,
that
sediments
depos-
ited
proximal
to
active
arcs
will
possess
very
similar
compositions,
whether
they
are
in
trench,
forearc,
intraarc,
or
backarc
situa-
tions.
Only
where
the
basin
behind
the
arc
is
bounded
on
the
other side
by
a continental
block
can distinctive
backarc
compositions
result.
Sediments
deposited
on
oceanic
lithosphere
behind
truly
oceanic
island
arcs
will only exhibit arc chemistry, and in this
situation
forearc
and
backarc
cannot
be
dif-
ferentiated
readily.
Another
illustration of
the
complexities
introduced
by
the geometry of
plate tectonic elements
is
given
by
the
Argen-
tine
beach
sand
QFL
data
of
Potter
(1984).
There
the
narrowness of
Patagonia
results
in
leading
edge
QFL compositions
on
the pas-
sive
margin side
of
the
continent,
and
Potter
reported
that
identification
of
tectonic
setting
using
sandstone
mineralogy
alone
could
be
erroneous
unless regional
aspects were
also
considered. This also applies to use of
geochemistry.
APPLICATION TO
NEW
ZEALAND
SEDIMENTARY
TERRANES
The
geology of New
Zealand
is
complex,
but has
been
well
summarized
in
a
number
of
papers
describing
the
geological
evolution
of
the
region
(Blake et
al. 1974;
Coombs
et al.
1976;
Sporli
1978;
Korsch
and Wellman
in
press). These studies are based on a large
geologic
literature.
Over
the
last
decade
con-
siderable
petrographic data
have
also
become
available,
and
tectonic
settings
for
many
ter-
ranes
are
now
well
established.
PM,
ACM,
and
ARC
settings
are
all represented,
and
classifications
are
based
on criteria
such as
tectonic
relationships
and petrography
which
are
independent
of
the
geochemistry
pre-
sented below.
Sufficient chemical
data
are
available
from
seven
terranes
(fig.
5)
to
apply
the K20/Na20-Si02 discriminant.
1)
Greenland
Terrane.-The
Ordovician
Greenland
terrane
(Greenland
and
Waiuta
Groups)
consists of
alternating
argillites
and
quartz-rich
greywackes.
Sandstone
frame-
work
compositions
average Q80FsRI2(Laird
1972),
and
individual
data
would
plot
in
the
craton
and
recycled
orogen fields
of Dickin-
son
et
al. (1983). Nathan
(1976)
considered
Greenland
Group
quartz polycyclic,
recycled
from
a
widespread quartzose sedimentary or
metasedimentary
continental
source
(PM).
This
is
supported
by the
discriminant
plot
(fig.
6),
where
data
lie
almost
entirely
within
the
PM
field.
A
relatively
narrow linear
trend
from
argillite to
sandstone is
present.
2)
Torlesse
Terrane.-These
sediments
and
their
inferred
tectonic
setting
(ACM)
are
described above.
Analyses
of
a
large
number
of
samples
collected
over
an outcrop
length
of
760
km fall
within
the
ACM
field
(fig.
7),
compatible with the quartz-intermediate na-
ture of
the
sandstones
and
the
depositional
models
of
MacKinnon
(1983)
and Korsch
and
Wellman (in
press).
Strong linear
trend
is
ap-
parent
from
argillite to
sandstone,
as
ex-
pected
from
figure
1.
Data
plotted
here
repre-
641
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B. P.
ROSER
AND R.
J.
KORSCH
FIG.
5.-Present-day
distribution
of some
major
geologic
terranes
in
New
Zealand.
Assignments
o
tectonic
categories
are
after
the
references
cited in
the text.
GREENLAND
ACM
ARC
60
70
80 90
SiO,
FIG.6.-Greenland
terrane. Open
triangles
are
argillites,
filled
triangles
greywackes.
Data
from
Morgan
1908),Reed
(1957),
Nathan 1976),
Eggers
(1978),
and
Roser
(unpub.
data).
N
=
47.
sent
all
five
petrofacies
of
MacKinnon (1983),
and the
spread of
the data
is
predictable
from
the
variations
in
modal
composition
that he
described.
The
Torlesse
data
plot
in
the
same
position
as
the
Franciscan
samples
in
figure
2,
emphasizing
the
similarity
between these
two frequently
compared terranes.
TORLESSE
PM
ARC
60
70
80
90
Si02
FIG.
.-Torlesse
terrane.
Open circles
are
argil-
lites,
filled
circles
greywackes.
Data
from
Reed
(1957),
Rowe
(1980),
Roser
(1983),
and
unpublished
analyses
by
Korsch
36)
and
Roser
(169).
N
=
253.
3)
Haast
Schist
Terrane.-The
Haast
Schist
terrane
consists
of
regionally meta-
morphosed
psammitic
and
pelitic
schists,
and
has
gradational
boundaries
with the
adjacent
quartzofeldspathic
Torlesse
(ACM)
and
vol-
canogenic Caples
(ARC) terranes. Metamor-
phic
grade
ranges
from
lower
greenschist
to
642
Greenland
PM
Torlesse
-ACM
Haast
Schist
ACM/ARC
Caples-ARC
Maitai -ARC
Murihiku -ARC
ECNI
Miocene-ACM
SOUTH
/
Brook
St.
Dun
Mt.
100
km
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TECTONIC
SETTING
OF
SANDSTONE-MUDSTONE SUITES
the
oligoclase
zone
of
the
amphibolite
facies
(Landis and
Bishop 1972), and
consequently
original
detrital
mineralogy
has
been
largely
or
completely
destroyed.
Published
analyses
appear to
be
metamorphosed
Torlesse
sedi-
ments,
as
the
data
occupy
a
comparable posi-
tion
in
the
ACM
field and
display a similar
pelitic-psammitic trend
(fig.
8).
4) Caples
Terrane.-This
terrane
consists
of
three
portions,
comprising
the
Caples
Group
of
Otago, equivalents in
the
Pelorus
Group
in
Marlborough,
and
the
Waipapa
Group
of
North
Island. The
sediments
are
poorly-bedded
lithic-volcanogenic
grey-
wackes and
argillites,
with
subordinate
metabasites
and
pelagic sediments.
Deriva-
tion is considered to have been from the
Brook
Street
volcanic
arc
located
to
the
west,
and
deposition
occurred
in
a
trench
environ-
ment
as
a
submarine
fan
complex
or
in
trench-slope
basins, with
subsequent incor-
poration
into
an
accretionary prism
(Turnbull
1980;
MacKinnon
1983;
Korsch
and
Wellman
in
press).
Hence
this
terrane
is
an
ARC
category.
Turnbull
(1979)
divided part
of
the
Otago
section
of
the
terrane
into
five
formations
with
four
distinctive
QFL
framework
compo-
sitions. These are the Kays Creek (average
QsF20L75), Bold
Peak
(QioFisL75),
Upper
Peak
(Q30F20L50),
nd
Momus
and
Mt.
Camp-
bell
Formations
(together
Q35FisLSo).
Turn-
bull's Kays
Creek and
Bold
Peak
data
plot in
the
undissected arc
field of
the
QFL
plot
of
Dickinson
et
al.
(1983),
and
Upper
Peak
and
Momus-Mt.
Campbell
in
the
transitional
arc
and recycled
orogen
fields. New
chemical
60
70
80
90
SiO2
FIc.
8.-Haast
Schist
terrane.Open
riangles
re
pelitic
schists,
filled
triangles
psammitic
schists.
Data from
Williamson 1939),
Mason (1962),
and
Grapes
et
al. (1982).
N
=
51.
data for
most
of
Turnbull's
sandstones
match
the
patterns
shown
by
his
petrography
and
give a
good
illustration
of
the
effects
of
provenance
on
the K20/Na20-Si02
plot.
Data
for
the
Kays
Creek
Formation plot
low in
the
ARC
field,
consistent
with
its
low
quartz
con-
tent and andesitic derivation, whereas Bold
Peak
data
group
higher
in
the
ARC
field,
reflecting
a
greater
dacitic
and
plutonic
in-
fluence
(fig.
9A).
Momus-Mt.
Campbell
sam-
ples
fall well in
the
ACM
field.
These forma-
tions
are
relatively
quartzose
and
contain
plutonic
debris in
addition
to
greater
felsic
volcanic
detritus
(Turnbull
1979). Upper
Peak
analyses
spread
from
the
ARC-ACM
join
into
the
ACM
field,
reflecting
the
varied
source of Bold Peak-Momus composition
proposed
for
this
formation
by
Turnbull.
Limited
petrographic
data
for matrix-rich
greywackes
from
the Pelorus section
of
the
Caples
terrane
(Vitaliano
1968) average
Qi5F28RS7.
Similar
data
for
medium- to
coarse-grained
sandstones
from
the
Waipapa
segment
(Mayer 1969;
Skinner
1972)
show
greater
variation
and
average
Q26F40R34
nd
Q11F22R67.
lthough
the data
cannot
be
recast
to
approximate
QFL parameters
and so
be
applied to Dickinson et al.'s discriminant
plot,
Pelorus
and
Waipapa
sandstones
proba-
bly
represent
all
three
stages
of
arc
evolution.
In
addition
to
being more
feldspathic,
May-
er's
samples
also
have
greater
proportions
of
nonvolcanic to volcanic rock
fragments
than
do
those of
Skinner
(1972)
and
Turnbull
(1979).
These
features
suggest
that the
arc
system
providing sediment
to the
Waipapa
segment
was more
mature
than
that
in
the
southern
sector.
This
is supported
by
the
chemistry
of
Pelorus
and
Waipapa
sediments.
Pelorus greywackes
occupy the
upper
ARC
field
but
spread
to low
values
as shown by
Caples
data
(fig. 9B).
Waipapa
greywackes
from
the
Auckland-North
Auckland penin-
sula,
which
includes
the
study
site
of
Mayer
(1969),
cluster
uniformly
at
higher
K20/Na20
and
Si02
at
the
ARC-ACM
join
(fig.
9C),
sug-
gestive
of
a more
mature
provenance.
Another
significant
feature
of
the
Pelorus
and Waipapa chemical data is that argillites
plot
at
greater
ratios
and
SiO2
than
do the
greywackes.
This
is
the same
pattern
as
that
exhibited
by modern
forearc
sediments
(figs.
3 and
4)
and
confirms
an ARC
setting
for the
Caples
terrane.
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B.
P.
ROSER AND
R.
J.
KORSCH
@
CAPLES
60
70
80
90
)
PELORUS
60
70
o80
9
©
WAIPAPA
60
70
80
90
Si02
FIG. 9.-Caples terrane.
(A)
Caples
Group,
southernSouth Island. Invertedopen triangles-
Kays
Creek
Formation
K); filled
triangles-Bold
Peak
Formation
(B);
open
circle-Upper
Peak
Formation
U);
filled
squares-Momus/Mt.
Camp-
bell
Formations (M).
(Roser,
unpub.
data;
N
=
39).
(B)
Pelorus
Group,
northern
South
Island.
Open
squares
argillites,
illed squares
greywackes.
Data
from Vitaliano
(1968)
and Korsch
(unpub.
data).
N
=
44.
(C)
Waipapa
Group,
North
Island.
Symbols
as
in (B).
Data from
Elliot
(1968),
Reid
(1982),
Wood
(1976),
and
Roser
(unpub.
data).
N
=
38.
5) Maitai
Terrane.-Maitai
terrane sedi-
ments
are
mainly
grey,
greenish,
and red
vol-
canogenic
sandstones
and
siltstones,
along
with
conglomerate,
breccia,
and limestone.
Sandstones
are
of
high-Al
basalt
and
basaltic
andesite
provenance derived
from
the
Brook
Street arc and were deposited in a forearc
basin
environment (Coombs
et
al.
1976;
Landis
1980;
Korsch
and
Wellman in
press).
Systematic
QFL
data
are
lacking,
but
a
de-
scription
by
Landis
(1980)
stresses
the quartz
and
K-feldspar
deficient
(absent
or
trace
only), highly
lithic,
volcanogenic
nature
of
most
Maitai
sandstones.
Only
one
part
(Tramway-Annear
Formations)
is more
quartzose
(10-25%
Q,
Landis
1980),
but
this
is
unusual,
and
the
Maitai
terrane
as
a
whole
is
a
good
example
of
arc
sedimentation.
This
is
confirmed
by
the
sediment
chemistry.
With
few
exceptions,
sandstones
plot
low in
the
ARC
field
at
<58% Si02
(fig.
10).
Siltstones
and
argillites
display
the
diagnostic
arc
trend
in
plotting
at
greater
K20/Na20
and
SiO2,
as
also seen
for
the
related
Caples
terrane.
6)
Murihiku
Terrane.
Triassic-Jurassic
Murihiku
terrane sediments
are
a
thick
se-
quence
(10
km)
of
quartz-poor
volcanogenic
sandstones and mudstones, along with minor
tuffs
and
conglomerates.
They are
considered
to
have
been
derived
from
an
active
but ma-
ture
volcanic
arc
and deposited
in a
forearc
basin
environment
inshore
of
the
depositional
site
of
the
Caples terrane (Coombs
et al.
1976;
MacKinnon 1983).
Recalculation
by
MacKinnon
(1980) of
available
petrographic
data
gave an average
composition
of
Q2F25L73
for
the
lowermost
North
Range
Group
(Lower-Middle
Triassic),
compared
with
Q12F40L48
for
the
Middle-Upper
Triassic
Taringatura
Group
and
the
Jurassic
part
of
the
sequence. These
compositions
represent
undissected and transitional
arc prove-
nances,
respectively,
according
to
the
plot
of
Dickinson
et
al.
(1983),
and
hence
this
is
an
ARC terrane.
Provenance
was
mainly
andesitic
through
to
the
Middle
Triassic,
when
it
became
more
acidic
until
late
Triassic,
returning
to
andesitic
detritus
through the
Jurassic (Boles
1974). The
changes
are
mirrored
by
swings
in sandstone
SiO2
contents,
from
52-59%,
to
64-72%
and
returning
to
<60%
(Boles
1974;
Coombs
et
al.
1976).
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TECTONIC
SETTING
OF
SANDSTONE-MUDSTONE
SUITES
50
60
70
80
90
Si02
FIG.
10.-Maitai terrane.
Filled
squares
are
sandstones,
open
squares
siltstones,
and
argillites.
Data
from
Landis
(1980)
and Korsch
(unpub.
data).
N
=
24.
The
two
sandstones
plotting
n
the
ACM
field
are
from
the
Tramway
Formation,
one of only
two relativelyquartzofeldspathicormationsn the
Maitai (Landis
1980). Data plotted
from
Landis
(1980)are
his
CO,-freeanalyses.
Note
shift
of
hori-
zontal
axis.
The trends
described
by
Boles
are
apparent
on
K20/Na20-Si02 plots
and
are
emphasized
by
the
addition
of
new
data
(fig.
11).
An-
desitic
North
Range
Group
sandstones
clus-
ter
in
the
ARC
field,
and
Taringatura
samples
spread
into
the
ACM field
as
rhyolitic
detritus
becomes more abundant. Jurassic sands
spread
back deep
into
the
arc
field, illustrat-
ing
the
return
to
andesitic
provenance.
The
few
data
available
for
Murihiku
siltstones
also
reflect
the
changes
in
provenance.
Those
of
the North
Range
Group
follow
the arc
pat-
tern
in
plotting
at
greater
ratios
and
SiO2
than
the
sands,
whereas
Taringatura
siltstones
are
more
similar
to the
ACM
trend,
possibly
reflecting
a
greater
non-volcanic
contribu-
tion.
7)
Miocene Sediments
from the
East
Coast
Region,
North
Island.-Thick
sequences
of
Neogene
flysch,
massive siltstone-mudstone,
and
lesser
amounts
of tuff,
of
ACM
setting,
crop
out
along
the
eastern
coast
of
North
Is-
land,
forming part
of
the
larger
terrane de-
scribed
as the
East
Coast
Deformed
Belt.
Miocene
sequences
were
deposited
as
a
series
of
inner-slope
forearc
basins
in
the
Hikurangi
subduction
trench, with
sediment
derived from the westerly axial range (Tor-
lesse terrane)
and
volcanogenic
detritus
from
the
acid-intermediate Coromandel
volcanic
arc
(Van
der
Lingen
and
Pettinga
1980).
Al-
though
no systematic
QFL
data are
available,
SiO2
FIG.
I1.-Murihiku
terrane,
South
Island.
(A)
Triassic.
North Range
Group
triangles)
nd
Tarin-
gatura
Group
(squares).
Open
symbols
are
silt-
stones,
filled
symbols
sandstones.
Data
from
Boles
(1974)
and
Roser
(unpub.
data). N
=
77.
(B) Jur-
assic sandstones
(diamonds).
Data
sources
as
above.
N
=
33.
the
dual
source
terrane
is
supported
by
gen-
eral
petrographic
descriptions
of sandstone
by Ghent
and
Henderson
(1966).
Chemistry of
Middle
to
Upper
Miocene
sandstones
and
mudstones
from
the
northern
part
of
the
terrane
cannot
be
plotted
directly
on the
K20/Na20-SiO02
diagram,
as
consider-
able
amounts
of
calcareous
cement
occur
in
many
sandstones
and
mudstones contain
abundant
microfossils.
Some
mudstones
con-
tain
up
to 20%
CaO,
much
of
which
is
biogenic.
Consequently
all data
has
been
re-
calculated
to
100%
CaO
and LOI-free
and
plotted
with
similarly
adjusted
field
bound-
aries. Recalculated
data
plot
well
in
the
ACM
field
and
show
a
tight grain
size
trend
(fig.
12A).
Interbedded
rhyolitic
tephras plot
with
the sands, and the data overall fall in a similar
position
to
Torlesse
sediments
(fig.
7).
These
features
suggest
that
the
chemistry
of the
East
Coast
sediments can
be
derived
by
mix-
ing
of
detritus
from
the
two
sources.
645
MURIHIKU
(A
Triassic
PM
60
70
80
90
MURIHIKU
B
Jurassic
60
70 80
90
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B.
P.
ROSER
AND
R.
J.
KORSCH
ECNI
MIOCENE
A
North (CaO-free)
ACM
ARC
60
70 80
90
)
South
ACM
ARC
60
70
80
90
SiO2
FIG.
12.-Miocene
sediments,
East
Coast North
Island
(ECNI).
(A)
Northern
sector.
Open
cir-
cles-siltstones and mudstones; filled circles--
sandstones:
triangles-tuffs.
All data
recalculated
to
100%
CaO-free,field
boundaries
adjusted
as
in
figure
3B.
N
=
112
Roser
unpub.data).
(B) South-
ern
sector
turbidites.
Symbols
as
in
(A).
N
=
47
(Korsch
and
Roser unpub. data).
Lower
Miocene
turbidites
from
the
south-
ern part
are
not
calcareous,
but
the
coarser
sands
have
been
cemented
by silica.
This
causes
data
points
to
trend
into
the
PM
field
(fig.
12B), and a
better
indication
of their
provenance is given by mudstones and fine
sands,
which
are
not
so
affected.
DISCUSSION
Results
for
the
New
Zealand
terranes
agree
well
with
published tectonic
interpretations
and,
together
with
the
modern
sediment
data,
provide
confirmation of
the
K20/Na20-Si02
model
and
the
validity
of
its
extension
to
finer-grained
members
of sedimentary
suites.
Patterns
in
the
New
Zealand
arc
terranes
are
particularly
important
in
that
they
verify
the
distinctive
sand-mud
arc
trend from
modern
sediments and
illustrate
well
the
changes
in
sediment
chemistry
produced
by
petrologic
evolution
and/or
erosion
level
in
arc
systems.
A
similar
plot
to
that
used
here
was
used
by
Maynard et
al.
(1982)
in
their
study of
modern
sediment
chemistry.
They
used
SiO2/A1203
ratio
instead
of
SiO2
alone
and
presented
sand and
mud
data for
differing
settings
on
separate
diagrams,
plotting
mean values
and
one standard deviation error bars (Maynard
et
al.
1982,
figs.
9
and
10).
Only
forearc
and
trailing-edge
environments
were distinctive.
Considering
that
SiO2 and
A1203
vary
anti-
pathetically
in
many
ancient
sedimentary
suites,
grain
size
effects
will
also
be apparent
on
plots
using
Si0O2/Al203n
conjunction
with
K20/Na2O.
This
is
demonstrated
by a
subset
of
the
New
Zealand
data, and
fields
similar
to
those
of
the
K20/Na20-SiO2
plot
can
be
dis-
tinguished
(fig.
13).
PM-type
Greenland
ter-
rane
greywackes
have
higher Si02/A1203
than
argillites,
and
this
trend
is
repeated by
the
fields
for
TE
sands
and
muds
from
Maynard
et al.
(1982).
An
even tighter
grainsize
trend
is
exhibited
by
the
ACM
Torlesse
data.
ARC
sands
have
lower
Si02/A1203
but
are
separ-
able
into
two
categories:
A1
(<10%
Q,
basaltic and
andesitic
detritus)
and
A2 (higher
Q,
acidic
volcanic
detritus). Both
compare
with the
FA
sand
field
of
Maynard et
al.
(1982). For simplicity arc argillites have not
been
plotted,
but they
show
the
same
pattern
as
on
the
K20/Na20Oplots, with
displacement
to higher
ratios
than companion
sandstones.
Incorporation
of
Al203
into
the
discriminant
narrows midrange
variation
and widens it
to-
ward
the PM
field
as
Si
and
Al
become more
fractionated
by
recycling
and
the
grain
size
effect,
resulting
in
a fan-shaped
distribution.
Although
use
of
Si0O2/A1203emoves
the need
for
recalculation
of
data
for
samples
affected
by biogenic carbonate, the overall results are
the same
as
those
gained from
the
simpler
K20/Na20-SiO2
plot.
The
differentiation of
tectonic environment
made possible
here
by the
use
of
data
for
both
sandstones
and
argillites
emphasizes
the
im-
portance and
advantage
of
sampling
the full
grain size
range
in
sedimentary
terranes, es-
pecially if
geochemically-based
discrimina-
tion
is to
be attempted.
This
is particularly
relevant
in
ARC
settings
where
the
chemistry
of some
sandstones
may
approach
that
of
more evolved
ACM
sands.
Even
though the
success of
the
chemical
approach
here
suggests
that
chemistry
alone
could
be
used
for
tectonic
discrimination
in
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TECTONIC
SETTING
OF
SANDSTONE-MUDSTONE
SUITES
0.01
0.1
1
10
100
K20/Na20
FIG. 13.-SiO/Al031-K20/Na2O
relations
of
contrastingNew
Zealand
ediments.
A1
=
arc set-
ting,basalticandandesiticdetritus;A2 = evolved
arc
setting,
felsitic-plutonic
detritus;
ACM
=
ac-
tive
continental
margin;
PM
=
passive
margin.
Di-
viding
lines
are
placed
by
eye
at
apparentbreaks;
dashedAi-A2
boundary
mphasizes
ransitional
a-
ture. Dotted
lines
are approximateone standard
deviation
fields
for
modern
trailing-edge
sands
(TS),
trailing-edgemuds
(TM)
and
forearc
sands
(FS)
from
Maynard
t
al.
(1982).
New
Zealanddata
are
subsets
from
previousfigures.
Open
and
filled
triangles
are
argillites and
greywackes from
the
PM-type
Greenland
errane
fig.
5),
N
=
47;
open
and
filled
circles
ACM
Torlesse
argillites
and
grey-
wackes(fig.6), N = 109;open
diamonds
A2 sand-
stones
from
the
Taringatura
Group
(fig.
O1A),
p-
per
Peak
and
Momus/Mt.Campbell
Fmtns.
(fig.
8A)
and
WaipapaGroup
(fig.
8C),
N =
67; filled
diamondsare
A, sandstones
rom
the North
Range
Group
(fig.
10A),
Bold
Peak
and
Kays
Creek
Fmtns.
(fig.
8A)
and
the
Maitai
errane
fig.
9),
N
=
59.
place
of
petrographic
analysis,
we
want to
emphasize
here
that
the
two
methods
are
complementary
and
are
best
combined
if
pos-
sible. Compared
with
modern
analytical
methods
(e.g.,
AAS,
XRFS,
INAA,
ICP)
which
are
now
routine,
rapid, and
permit
col-
lection
of
large
datasets, petrographic
analy-
sis
remains
a time-consuming
and
skilled
pro-
cedure.
The provenance
information
from
petrography
is
unique,
however,
particularly
with
respect
to
the petrologic
nature
of
de-
trital
lithics. The
chemical data
may
be
more
easily gained, but its interpretation is eased if
petrographic
data
are
available.
The com-
plexities
of
the New
Zealand arc terranes
(Caples,
Maitai,
and
Murihiku)
illustrate
the
importance
of
using
both
approaches,
in com-
bination.
It
is
in
situations
where
sandstone
mineral-
ogy
has
been obscured by
diagenesis
or
metamorphism,
or
where
sequences are
dom-
inated
by
fine-grained
members
that
the
chemical
method
alone
is
most
valuable.
In
this
respect
the
use
of
Na and
K
is
not ideal,
as their immobility under these conditions
may
be suspect.
Some
of
the
results of
this
study,
however,
suggest
that
characteristic
K20/Na20O
ratios
have not
been
greatly
dis-
turbed
by
postdepositional
processes.
De-
spite
relatively intense regional
metamor-
phism,
samples
from the
Haast Schist
terrane
plot
in
about
the
same
place
as
their
less
metamorphosed
(prehnite-pumpellyite
facies)
Torlesse
precursors
(figs.
7
and
8).
Heavily
cemented Miocene sandstones from the
ECNI
show
identical
ratios
to
poorly
lithified,
uncemented
ones at
the same
lo-
calities.
Furthermore, Maynard
et
al.
(1982)
found that
ratios
of
ancient
greywackes
re-
sembled
those
of
modern
counterparts
and
concluded
that
no diagenetic
effects
were
necessary to
explain
the
chemistry.
They
also
interpreted greater
K20
of
ancient
shales
compared
to
modern
muds
as
a
detrital
rather
than
diagenetic
feature.
In the long term, it would be preferable for
geochemical
discriminants
for
sediments to
be based
on
elements
immobile
in secondary
processes (e.g.,
Ti,
P, Nb,
Y, Zr),
as
used
in
metabasite
discrimination diagrams
(e.g.
Pearce
and Cann
1973). Use of
many
im-
mobile
elements, however,
is
hampered by
residence
in
high
density
accessory
minerals
such
as zircon and
apatite,
which
may
not
be
evenly
distributed
throughout
beds,
so
char-
acteristic geochemical
signatures
may
be
difficult
to
determine.
Suitable
calibration
datasets
are
also
scarce
at
present,
as
com-
prehensive
major and
trace
element data
for
both
sandstones
and
argillites
from
suitable
terranes
are few.
Another
approach
is
to
in-
clude
more
elements
in
the
discriminant,
such
as
in the
discriminant
function
analysis
scheme
for
sandstones
put
forward
by
Bhatia
(1983).
Any
new
attempts
to
develop
such
schemes should
be
based
on
large
datasets
covering complete grain size spectra, to ac-
commodate
the
variations
demonstrated
in
this study and
by
Roser
and
Korsch
(1985).
In
the
interim,
the
K20/Na20-Si02
plot
of-
fers
a
simple
but
effective
discriminant.
Us-
ing
only
three
elements
permits
the
use
of
647
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B.
P.
ROSER AND
R. J.
KORSCH
partial
analyses,
a
useful
feature
if
an
analyt-
ical
method
such
as AAS
is all
that
is
avail-
able,
or
if
the
discriminant
is
to
be
used
as
a
regional
screening
tool.
If
chemical charac-
teristics
within
a restricted area
of
a
terrane
are
established
and
used
in
conjunction
with
petrographic data, the study area could be
quickly
extended using
the
chemical parame-
ters
alone.
Any
major
provenance
change de-
tected
by
shifts
in
the
chemistry
could
be
verified
by follow-up
petrographic
examina-
tion.
In this
and
any
other
application
it is
important
to
treat data
individually,
rather
than
group
data
as
terrane
averages,
as
this
would
obscure
any
spatial
or
temporal
varia-
tions (e.g., Caples
and
Murihiku
terranes).
Equally important in any sampling regime in
suspected
arc
environments
is
the
collection
of
sandstone-argillite
sample
pairs
at
single
sites.
Collection
of
either
lithology
alone
may
not
permit
a unique
classification
or
detection
of
provenance
maturation.
Perhaps
the
best
reason
for using
Si,
Na,
and K
data
alone
is
that
their
variations in
sediments
are
relatively
well
known
and are
largely controlled
by
major framework
con-
stituents,
as
recognized
by
the
work
of
Mid-
dleton (1960) and Crook (1974). Discrimina-
tion
based
on these
elements
therefore rests
on a
firm
petrographic
foundation
and
so is
a
useful
working
model for
the
present.
CONCLUSIONS
The
chemistry
of fine-grained members
of
ancient
sedimentary
suites
from
differing
tec-
tonic
settings
is
as
distinctive
as
companion
sands.
The
reflection of
provenance
in
the
chemistry
allows
effective
discrimination
into
three
broad
categories (PM, ACM, ARC)
to
be
made
simply
using
K20/Na20-SiO2 rela-
tions,
although
transitions
between
the
envi-
ronments
are
to
be
expected.
Data
for
mod-
ern
sediments
support
the
model
and
the
general
discriminant
fields
proposed, and
re-
veal
that
sand-mud
pairs
in
forearc
basin
set-
tings
are
distinctive
from
those
in more
evolved
environments (PM,
ACM).
A large
volume of data from New Zealand terranes
of well-established
tectonic
setting
and
provenance
offer further confirmation
of
the
discriminant
and
the
distinctive
forearc
com-
positions.
They
also
illustrate
well
the
effects
of
petrologic evolution
in
arc
systems
on
the
chemistry
of
associated
sediments
and
the
transitions between
simplistic
tectonic
set-
tings
that
must
inevitably
result.
The
effects
of
grain
size,
petrologic
evolu-
tion, and sediment maturation emphasize the
importance
of
sampling
the
full
range
in
lithology of
any
terrane
and
of integrating
chemical
studies
with
detailed
petrographic
analysis.
The
two
approaches are
com-
plementary
and
are
useful
tools
for
the
inter-
pretation
of ancient sedimentary
sequences
when
combined with
regional studies.
ACKNOWLEDGMENTS.-Financial
upport
was provided by a N.Z. University Grants
Committee Postgraduate
Scholarship
(BPR)
and
several
internal
research
grants
from
Vic-
toria
University
of
Wellington. The
manu-
script
was
prepared during
the
tenure
of
a
NZUGC
Postdoctoral Fellowship
at the Uni-
versity
of
Otago
(BPR).
Unpublished
XRF
analyses
used
were made
in the
Analytical
Facility,
VUW,
with
the
help
of
K.
Palmer
and H.
Roe.
Our
thanks
to T.
Watanabe,
J.
A.
Gamble,
A.
R.
Pyne and
P. A.
Morris
for helpful discussion, to
C.
A.
Landis
for
comments
on
the manuscript,
and
to
G.
deV.
Klein
for
critical
review.
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