CHAPTER 6. GEOPHYSICAL RESPONSES OF KNOWN ROCK TYPES 72

CHAPTER 6, GEOPHYSICAL RESPONSES OF I(NOWN ROCI( TYPES 66

CHAPTER 6. CHAPTER 6. GEOPHYSICAL RESPONSES OF KNOWN ROCK TYPES 72

ble

for

the lower susceptibilities

in

samples which apparently contained macroscopic maguetite grains. The magnetite was probably martitized

to

haematite.

Basement anomalies

in the Mt. Lofty

Ranges

fo¡m two arcs: the

^{southern one}

is

formed

by the Mt.

Compass

Inlier

^{and anomalies}

BS1-8S3. The

Houghton

Inlier,

Warren

Inlier

and

u,oo-"li"s ^B54-856

form the northern arc.

While

BS1-BS4 are shallow, BS5 and

856

range in depth

from

1.5

to 3km.

The northern arc is offset

to

the west of the southern arc and they are separated

by

a

fault

(Section 5.2.1).

The northern

portion

of the

Mt.

Compass

Inlier is

^covered

by

CRA magnetic

data.

On the

total

magnetic field contour maps, the inliers can be easily related to magnetic highs. The inlier is elongaie

in

shape and bisected by a right-lateral

fault.

^{The vertical} magnetic gradient contour map highlights the variation

in

magnetic properties

within

the

inlier.

Magnetic lows have ^been .orr"lu,t"d.

with

epidote-diorite assemblages and

with

the two occulrences of gold mineralization' Susceptibility measurements on outcrops ^of the Warren Inlier identified coarse-grained ^micaceous schists as strongly

magnetic. In the

^{case of}

the

northern

part of the Mt.

Compass

Inlier,

the mica-schists are magnetic while the epidote-diorite is

not'

6.2 Igneous rocks

Foden et at. (inpress) have grouped the different granites and other igneous rocks found within

theareaintopre-,syn-,and.post-tectonicrocks. SomeoftheseareshowninFigures6.land6.2.

The pre-tectonic group incluãe the Truro Volcanics and amphibolites and dolerites

in

the Cooke

Hill

^{and Tungkillo} at"us ( Tepkosheet

-

Gatehouse, 1988a). Syn-tectonic granitoids are comrnorr and widespread and include the granites

at

Palmer,

Mt. crawford,

Reedy creek and Encounter Bay and the Rathjen and Tanunda Creek Gneiss. The post-tectonic ^phase

of

Ordovician ^felsic magmatism was associated

with

mafrc dyke emplacement. Post-tectonic intrusives include ^the

fo[ãwing: Murray Bridge

^Granites,

Mannum Granite,

Sedan

and

Long

Ridge

Glanites, ^the Black

HiIl Norite. The ,*u,r*, of

sub-alkaline, dolerite dykes, mostly oriented NW-SB ^found cross-cutting

the

Reedy

creek

Granodiorite

and diorite, and the Mannum Granite

^{are also} considered by Foden et al.(in press)

to

be post-tectonic'

6.2.L Granites and granitic gneisses

Most of the above-mentioned igneous rocks have characteristic magnetic and radiometric ^signa- tures an¿ have been classified as belonging

to

the magnetite-series

or ilmenite

series based on

their

opaque mineralogy and magnetic properties (Table 6'1)'

A

similar classification ^was made by Webster and Scheibner (1984) of the New England Gran- itoids on the basis of aeromagnetic

interpretation.

Magnetite-series granitoids (e'g. Walcha ^Road

Adamellite)

were related

to

well-defined magnetic highs while

the

magnetic effect of ilmenite- series granitoids (e.g. Mole Granite) resulted

in

a depression of

total

magnetic field contours.

The syn-tectonic granites ^vary in their

geophysical

properties. The

^{Palmer Granite,} Reedy Creek

Granodiorite, Rathjen

and Tanunda Creek Gneiss are magnetic, contain ^mag- netitå, and

their

average magnetic susceptibilities are well

in

excess of 500

x

10-6 ^cgs clualifying them as magnetite-series granitoids. Compared

to

the surrounding metasediments, they a'ppear

radioactive ihough

their radioactivity

must be due

to

increased uranium

and/or thotium.

^The amount

of

potassium present

in the

granites (Mancktelow, 1979) was compared

with that

in metased.iments (Table

2.7)

^{and was found}

to

be

not

significantly different'

35'30'S

35',15'S

t3B'30'E FLEURIEU PENINSULA

KANGAROO ISLAND

¡-Granil€

W¡lloughby cranits

a

Granite

ENCOUNTER BAY

. Encounter Bay

Figure 6.2: Location diagr¿ìm

for

Eucounter Ray Granite and Cape lVilloughby _Granite.

CHAPTER 6.

GEOPHYSICAL RESPONSES OF /INOWIÙ

ROCI{

^{TYPES 73} The

palmer Granite

(magnetic anomaly

PG)

has been described

by \Mhite

et aI. (1967) as being either fine-grained

or

coarse-grained,

both

variants being of

granitic

composition

with u,.."rrory

magnetite.

Pyrite

and rare molybdenite have been detected

in

^the fine-grained type'

The

mapped contact

oi th" granite with the

metasediments can

be

easily traced

ftom

both

,"rorrrug*tics

^and

radiometrics. The

^{present outcrop}

pattern

shows

it to be

an oval-sha'ped body

with

strike length ⁵ km and trending

NNW.

The northern

part

of the granite is dominated by the coarse-graio"ã

typ"

^{and is} more magnetic and radiometric than the southern part, which is

dtminated uy ttre fine-grained

type.

Shear zones trending roughly NE ^are related to albitization of the granità and the albitized ^zones can be clearly identified ^as lows on the radiometric irnage.

Magneìic modelling ^{suggests a} westerly dipping body. From its geochemistry, Mancktelorv (i979)

.orr.lod", that ^the

Palmer Granite is

l-type.

Oxidation ratios are well above 50 (Appendix F and Table 6.1) and

this too

would suggest

that it is

^a magnetite-series

granite

and therefore I-type.

The

Rathjen

Gneiss (magnetic anomaly RG) is ^a highly foliated, sheet-like body of strongly deformed

granite with a

prominent north-trending

lineation (White, 1966b).

^{Outcrops gave}

consistently high susceptibility readings and euhedral magnetite ^was obsetved in polished blocks.

An

^oxidation

ratio ^of

³⁷ was computed

from an

analysis presented

in Rattigan

and lVegener (1951). NRM measurements on one sample ^{gave a} very low

Q

^Gneiss

àu,r, be separated

into

an eastern and ^a western block which

ar

^{tion in}

lithology and grain size. This contact trends

NNW

and is

par

^adigan

(1ggg)"inte.prãtud an igneous origin

for

the Rathjen Gneiss based on

the

^presence of xenoliths and the igneous nature of the contact

with

the adjacent

biotite

schists.

The eastern

part

is magnetic and radiometric. The northern closure has been modelled ^{as a} south-plunging syncline,

with

a depth extent of ⁸⁰⁰ metres. The syncline axis can be extended

right

through

the

gneiss though several

NE

trending

faults

offset

the axis.

^These

^faults

^have

be'en identifled by breaks

in

magnetic and radiometric trends and are parallel

to

^{a,nd sometimes}

coincide

with

a

joint

system describe,il by

White

(1956).

In

contrast the western

part

appears less

magnetic and

radiometric.

Outcïops are few and no longer

exhibit the

characteristic lineation and appear

to

contain less

biotite.

Since most research has concentrated on

the

eastern part,

it is

possible

that

^either

^the

^western

^{part is} not true

Rathjen Gneiss

or

else

it is

a variant of

the

material found on

the

eastern

side.

Magnetic susceptibilities are much lower,

of

the order of 300

x

10-6 cgs. The

,,tail"

_{of the gneiss is a}

thin

near-vertical sheet outcropping between the migmatites

anJthe

palmer Granite.

Its

narrow

width,

less than 25 m, together

with

the intense magnetic signatures of the surrounding rocks prevented any study of

its

geophysical propelties'

The Springton Fault separates magnetic Rathjen Gneiss from less magnetic Rathjen Gneiss.

Just northwest

of the

Rathjen Gneiss

the

Springton

Fault

has

vertically

displaced

a

magnetic source. This magnetic source has an arcuate outline and may represent a subsurface extension of the Rathjen Gneiss (Plate 5).

Granitic

^{gneisses east}

of Mt. Kitchenet are

collectively

known

as

the Tanunda Creek Gneiss.

Chinner (1955) recognized three types ^of gneiss: the predominant type, ^a quartz-biotite- feldspar ^gneiss which

for*,

around 90% ofthe gneiss outcrop, is ^a buff coloured, mediurn-glained

rock with

pronounced

foliation

and

lineation.

Occasional "potash-deficient" ^{gneisses occur as} narrow bands

within the first type.

^The

third type

contains hornblende,

biotite,

feldspal and

qtartzand

crops out as a narrow band. which can ^be followed for several miles. Chinner

(op' cit')

zuggested

that

^{these gneisses were} the product of

the

metamorphism

of

sedimentary material.

The Tanunda Creek Gìeiss is similar to the Rathjen Gneiss

(White,

1956; Offier, 1966)

fol

^rvhich Madigan (1988) and Foden et at.

(in

^{press) favour} an igneous origin.

The granite ^gneisses of the Tanunda Creek Gneiss cause both magnetic (anomaly

TCG)

and

CHAPTER 6. GEOPHYSICAL

^{RESPONSES OF}

I(NOWN ROCI(

^{TYPES 74} radiometric anomalies. ^Together,

the

gneisses

form

an elongate body, roughly

4km long

and

trending

^NS,

the

^average

width

^being

800m. The

predominant

type, a

potassium-rich rock, shows signs of potassium metasomatism. Abundant potassium minerals, mainly microcline, and

allanite,logether with

the excellent quality of the outcrop of these tocks, are probably the main causes for the high radiometric signature. Accessory magnetite has been detected

in

these rocks (Chinner,

op.

cli.), and since

the

rocks appear

to dip

steeply towards

the

east,

their

geometry

iì

similar

to that

of a wide dyke. Magnetic modelling of the edges confi,rms the easterly dips.

If it

is igneous

in

origin, then

its

magnetic properties together

with

its high oxidation

ratio

qualify the Tanunda Creek Gneiss to be a magnetite-series granìte'

The

Mt. Crawford Granite

^{Gneiss is} the only granite

in

the

Mt. Lofty

Rangeslvhich ^is seen

in

contact

with

Adelaide Supergroup rocks

(Mills,

1973).

All

other granites are

in

contact

with

Kanmantoo Group rocks. Quality of aeromagnetic coverage over the granite gneiss was too regional

in

nature

to

study

its

magnetic lesponse.

The

Reedy Creek Granodiorite

^{is a} distinctive, white, coarse-grained, almost porphyritic

rock. Major

minerals include oligoclase, plagioclase,

with

lesser quartz, hornblende,

biotite

and variable microcline. Accessories

of

sphene, zircon, epidote and magnetite are common (White- head, 1gZ5). Fine-grained

diorite

occurs ^as elongate bodies and bands

within

the granodiorite and. as a massive dyke .ross-cutting the migmatites west of the granodiorite. The diorite intrudes the granodiorite though they

exhibit

good compositional

continuity

^(Moeller, 1980) suggesting

they

developed

from

related magmas (Foden

et al., in press).

Aeromagnetic

data,

magnetic susceptibility measurements, oxidation

ratio

(37.69) and the pïesence of magnetite indicate

that

the Reedy Creek Granodiorite belongs

to

the magnetite-series. Regional aeromagnetic interpre-

tation

indicates a much larger body which is now under Caenozoic covel'

The Encounter Bay ^Granites

^crop

^{out at}

^Cape lVilloughby,

on the

southern coast of Kangaroo

Island,

and,

on the

south coast

of

Fleurieu Peninsula

in

Encounter

Bay'

They fall

within

the group ofsyn-tectonic ^{granites because they are believed} to have intruded the younger Kanmantoo Group ^sediments

prior to the

culmination of the

first

^{folding event} (Milnes et al',

lg77). The major

^granite

type is a

medium

to

coarse-grained

biotite

granite which contains opalescent

blue quartz.

The Encounter

Bay

Granites have probably become contaminatecl by digestion

of

Kanmantoo Group metasedimentary

rocks.

Magnetic anomalies have

not

been associated

with

these granites. Albitized granite at Rosetta Head returned very low susceptibility values

but

^the

,rrugn"ii.

properties of the group as a whole have

not

been investigated.

The syn-tectonic granites described above are mineralogica'lly and geochemically

l-type

^(Fo- den eú

al.r\n

press). The Monarto Granite, though also considered

to

be syn-tectonic,

fits

into

the

S-type

classifiátion

^{(Hoesni, 1985).}

It is

a medium-grained

light

grey granite and thougli Mancktelow (1g7g) reported magnetite

within it, all

magnetic susceptibility measurements ^on samples and outcrops gave

uniformly

low values. The granite also contains primary muscovite which sets

it

apart from the palmer _and Murray Bridge granites and is consistent with

its

cla'ssi-

fication as an S-type granite, although field relationships indicate

that

the Palmer and Monarto Granite are of similar age (Mancktelow, 1979). The magnetic ^response over the Monarto Granite is dissimilar

to that

ovãr the magnetite-series granitoids. The

north

Monarto Granite ^does not affect

the

magnetic response much though

the

south Monarto Granite ^depresses the fielcl and disrupts the

pattern.

Between the north and south Monarto Granite, ^a magnetic high is ^observed which also disturbs the normal gradient in the area. Gravity measurements (Lewis,

A. M.,

1985) indicated ^a high over the north and south granites

with

a slight low between them. Lewis,

A.

M.

(1g8b) interpÃted. two granite bodies separated

by

a

thin

veneer of granite overlying magnetic metasedimentary rocks of the Kanmantoo Group'

Granite stocks near Rockleigh,

in

the Springton area and

in

the south

l(arinya

Syncline ^are