Roser 1986

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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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7/25/2019 Roser 1986


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

7/25/2019 Roser 1986


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

7/25/2019 Roser 1986


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

7/25/2019 Roser 1986


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

7/25/2019 Roser 1986


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

7/25/2019 Roser 1986


7/25/2019 Roser 1986


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

7/25/2019 Roser 1986


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

7/25/2019 Roser 1986


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.

643

7/25/2019 Roser 1986


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).

644

7/25/2019 Roser 1986


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

7/25/2019 Roser 1986


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

646

7/25/2019 Roser 1986


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

7/25/2019 Roser 1986


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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